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u? Joseph Black
FhUad.Fiib.hy  CWatthefi^vilTcy idoj 
LECTURES
ON THE
elements of Ctjmustvp,
DELIVERED
^£/M^^^
IN THE UNIVERSITY OF .EBINBURQH;
                BY THE LATE




   PROFESSOR OF CHEMISTRY IN THAT UNIVERSITY,

PHYSICIAN TO HIS MAJESTY FOR SCOTLAND, MEMBER OF THE ROYAL
   SOCIETY OF EDINBURGH, OF THE ROYAL ACADEMY OF SCIENCES
      AT TARIS, AND THE IMPERIAL ACADEMY OF SCI-
            ENCES AT ST. PETERSBURGH.   y * /
7     '
*V
PUBLISHED FROM HIS MANUSCRIPTS,
        JOHN ROBISON, LLD.
PROFESSOR OF NATURAL PHILOSOPHY IN THE UNIVER-
          SITY OF EDINBURGH.
FIRST AMERICAN FROM THE LAST LONDON EDITIONV*'
V? -   VOL. I.
■V--   v
PHILADELPHIA:
   PRINTED FOR MATHEW CAREY, NO. 122, MARKET STREET.
SOLD BY BIRCH ^5* SMALL, S.F.BRADFORD, AND JACOB JOHNSON
  PHILADELPHIA; BRISBAN C5* BRANNAN, THOMSON IS" HART, AND
  T. & J. SWORDS, NEW-YORK ; BEERS AND HOWE, NEW-HAVEN ;
  ETHERIDGE t5" BUSS, AND THOMAS & ANDREWS, BOSTON.
1807. 
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           CHARACTER  OF BLACK'S LECTURES.

     ((THE name of Dr. Black will proliably be remembered as long as
 the science of Chemistry exists.  His twogreat discoveries of latent heat,
 and of the cause of that difference observable between the properties of
 the mild  alkalies and alkaline earths, an(( of these substances, when in a
 caustic state, must be acknowledged, by ill philosophers, as having com-
 municated the impulse, and  pointed out/he way to the splendid investi-
 gations'of modern Chemistry.   These claims on the remembrance of
 posterity, could never have been set asicfe, even if the presetu publication
 had not taken place.  Still it is in a higi degree satisfactory to possess a
 record of them in the words of  their aithor....more especially as we by
 this means become acquainted with themanner, and may form some faint
 idea of the effect produced by the leciures  of this celebrated teacher.
 Professor Robison, the former  pupil and intimate friend of Dr. Black,
 was entrusted, by his executors, with tie arduous and delicate office of
 revising the loose manuscript notes, Iran which Dr. Black delivered his
 lectures,  and reducing them to a state iroper for publication. Tbe docu-
, ments of Dr. Black's fame could not ha e been committed to abler hands.
 The volumes before us exhibit  a very accurate representation, not only
 of the opinions, but, we doubt not, of ti,e very words of the author; while
 the notes, which the editor has supplied, from the stores of his own know-
 ledge, confer an additional value on tht work.
     " A  large, we will not say  a disproportionate  share of the work, is
 devoted to the illustration of the aithor's own  immortal discoveries,
 which are relateti with great minutejess, and in a most  engaging man-
 ner.   And it is particularly satisfactory to behold on all occasions a most
 happy exemption from jealousy of hi felloW-labourers in the inexhausti-
 ble mine  of experimental knowledge and the most scrupulous equity in
 assigning the fame of great discoveres to the rightful claimants."
    Aikin's Armual Review^ and History of Literature for 1803 p. 924.

     " In  editing these posthumous llbours of pne of the greatest philoso-
 phers which this  country ever produced,  professor Robison  has con-
 ferred on the scientific world a  mosj valuable obligation.  The lectures
 of Dr. Black, as a system of chemcal instruction, possess very peculiar
 merits—and may fairly be admitteqto contain the most accessible stores
 of information which persons ignonnt of the science can at present com-
 mand. They are delivered  as miujh as possible in the analytical mode.
 They take  for granted no  previous acquaintance with  science in the
 learner—and they require less perjiaps than any other work, the assis-
 tance of apparatus.
     " The  two grand discoveries 3f Dr. Black are those of latent heat,
 and the nature of alkaline earths a'jd fixed air.  The present publication
 contains the only history vie  have .of the former, and a more copious ac-
 count of die latter than that which the author published during his life.
     '' Mr. Robison's notes confer great additional value on this publica-
 tion.  They illustrate the history of Dr. Black's discoveries, and con-
 tain discussions upon  various points of modern chemistry ni" the greatest
 importance."                     Monthly Magazine^  vol. 16,p. 623.

     " In bringing forward the present work, Dr. Robison has perform-
 ed an  acceptable service to the world, and an agreeable and honourable
 duty to the memory of an illustrious friend, whose good opinion andcon-
                            degv'e.  To do justice to his reputation as
                                   A 

. discoverer,  to nuLc^J* liable qualities of^his  mind, ^ t.
civc a faithful transcr.pt of tl osc^»r^ wh  lau.,,ble views by which
the advancement *  Chemica 1 *nee, a« *c  ^ ^ .^ preg
uic ^^ry>^»i.Uv;'f^y\e^^mMh satisfaction in reflecting
vcmr,te the  u"^kl  ■;\,|;u;1 /leo  die most distinguished, and, at
tWatiUe character ..nd^n^e        unassuming philosophers of
the siroe time, one ->t the miJ*™°£\,     blic  in a  manner  which is
the last rcytury, are now laid texo.e  «« Pu™£    industry and learn-
equally creditab'e to his memon, and «>*^^ x,o/<> 42)/, 187.
ing of the editor."                  J      y





x™ on  ™ladmitted  by the b*t modern'enquirers.  The inductive
^ZuXriff^^ thepremises correctly stated, and-the con-

 ^^ir^^nfZ^  system of both ancient and mo-
 dern Cheni^n, in a form and vith  advantages far  superior to those
 ■which anv other work has offered                               _  ,.
     -™me  apology may benecesary for our extended  account of this
 work.  The dnractcr  of Dr.BLck must furnish this apology.   And
 when  our readers  reflect that tlu first germ of the modern system of
 ClumMrv waS animated by hislabmrs and ingenuity;  that with a degree
 of inclement equalled only by his muteness, the first hints were pursued
 with a vigour of inductive reasoning without a parallel or  an  example,
 except in  the optics, and without a ;opy but in the labours of Cavendish
 and Kinvan-the apology will be easily accepted."                •
                   Critical Reviev,  Series Third, vol. l,p. 86 tf284.

     " It gives us peculiar pleasure o view the publication of these lec-
 tures-and still more to find  them <c!ited by a person who enjoyed the
 personal friendship of  Dr. Black, aid whose genius is under the control
 of tl>e same degree of  philosophical caution as that of the  author.  We
  here  behold a veteran  professor of distinguished  talents, who may  be
  justly styled the father of modern ciemistry, uninfluenced by  the super-
  cilious dogmatism of the French scv-ol.  We  see him  adding-th? dis-
  coveries of the moderns to the origiial structure of the  older chf mists,
  and carefully avoiding those alteration which were only  the offspring of
  vanity, or of the systematic confusioi which was introduced by the em-
  pirical politicians of that time."        British Critic, vol. 23, fi. 645.
      The high estimation in which we hold the Lectures of Dr. Black,
  induces us cordially to recommend  rtv m to  the  friends* of" •chemical
  science.
      Declining any invidious discussiovr, or comparisons respecting the
'  dixovenes of their illustrious author, with some fancifully ascribed to
  earlier chemists, cr arrogated by his contemporaries—We believe that
  lucid arrangement, .trength of argumen', and excellence of experimen-
  tal illustration, render them more instructive  to the student and  more
  gratify ing  to the  proficient than most oti er publications in that depart-
  ment of physical enquiry.
                                    JAMES REYNOLDS,  M.D.
                                    ADAM SEYBERT, M. D.
                                    B.^T,\HTQN. M. 1),
      PhilackVjhia, Dec. i:th,  1S06.  ^^ 
V
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rhUod.Fiuh.by Mattlu-w  c'ai-cy    7807.
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EXPLANATION  OF THE  PLATES.
A few figures being added to those  expressly alluded tQ
    in the Lectures, the following short explanation
            of them was thought necessary.
                        PLATE II.

Fig. A represents a solution glass ; the long taper shape of which
       permits agitation of the mixture, without risk of spilling :
       it also prevents the loss of small drops which are sometimes
       thrown  out by effervescence, which would  in  some nice
       cases of assay, &c. derange the calculations.

     B, with the vessel B below it, is the old circulating apparatus.
       The ends of the spouts, which come from each  side of the
       capital B, are inserted and luted into the two short tubes pro-
       jecting  from the body or cucurbit B.

     C is the Hessian, and D the Ipsian or black-lead crucible.

     E is the ordinary form of a retort, of glass, or earthen ware.

     F represents a tubulated,or stoppered retort, with its stopper/

     G is a separatory, for pouring off the clear part of a solution,
       r.». mivtnrp. which has any feculent matter floating above. 
6
Fig. H  represents a cucurbit, with its capital //.  Its form is very
        nearly that of the common still.  To promote the conden-
        sation, the capital should be of a considerable diameter.
        Observe that the outer rim of the capital should be  lower
        than the hole h in the middle or throat; otherwise, the
        vapour which condenses on the roof, and trickles down its
        inside slope, would fall back again into the body H. But the
        rim being lower, the condensed fluid collects there, and from
       thence goes into the spout, and runs down into the receiver.
       The form of a capital is not unlike a mushroom.   Observe
       also, that the throat of the capital should go into the neck of
       the body, and not encompass it.  If the neck fits into the
       throat, the condensed fluid lies on the luting of the joint, and
      is contaminated  by it.

     I is the most approved shape of a receiver.  A taper spout i
      is sometimes joined to the  remote end, to be inserted into
      the neck of another receiver, when the surface of one  is not
      deemed sufficient for a speedy condensation.  The spout is
      sometimes joined laterally, as at i; by which the fluid ob-
      tained by condensation  runs off into  another vessel, which
      may be speedily changed when  any  change  of product  is
      expected.

    L represents one of the intermediate vessels of Woulfe's dis-
      tilling apparatus.  The tube 1 reaches  nearly to the bottom.
      The other tube m opens into the space above  the water in
      this vessel, while its other extremity dips almost to the bot-
      tom of the next vessel, 8cc.

   M is a sketch of the essential parts of what is  called the pneu-
      matic apparatus.  The tube a, being properly bent, introduces
      the gas into the water or mercury which filled the jar M in-
      verted into water or mercury.  The gas rises up through
      the fluid, and occupies the upper part b, the fluid subsiding a
      little on the entrance of every bubble of gas.  Its elasticity
      does not balance the pressure of the atmosphere, till the sur-
      face of the fluid within the jar is on the same level with the
      surface in the cistern c.
is a perspective  view of a muffle.  The projections over
each of the  lateral apertures, are for preventing the coals 
ri 
I  ^
■jfeet  10 [Jiches..
msi§§mmmk	
	
	
 
7
        with which it is surrounded from tumbling into the muffle
        as the fuel subsides.

Fig. O is the section of a cupel made of bone-ashes.

     P represents a safety pipe, for  distillations, in which the
        vapours frequently change their elasticity, and are some-
        times reabsorbed by the matter in the retort.  One end is
        ground like a stopper; and fits a hole in the top of the re-
        tort, or other convenient part of the apparatus.   When the
        vapours tend to burst the vessels, they press on the surface
        a of the fluid contained in  the ball, and  cause  it to rise
        through the upright tube into the cup at the top,—thus act-
        ing against its weight.  Should the elasticity still increase,
        the gas escapes, by rising in bubbles through the fluid. That
        this may  always happen, it is necessary that the capacity of
        the cup be sufficient to hold all the liquor, which, at the be-
        ginning, filled about one-half of the ball below.   Then all
        risk of bursting  the vessels is avoided; and yet a conti-
        nual waste of vapour by the vent hole commonly used, is
        not incurred.

           On  the other hand, when  a reabsorption commences in
        the retort, which would sometimes bring  back cold matter
        from the receiver, and instantly split the retort, the  atmos-
        phere  will press on the fluid in the vertical pipe, and rise in
        bubbles into the ball, from which it will pass into the vessels,
        without risk, except in cases where vapours are produced,
        which will take fire when mixed with air.
                         PLATE  III.

  The only figure in this plate that is not explained in the course of
Lectures is Jig. A, No. 1, No. 2, No. 3.
  They are sections of an experimental furnace by Mr. Macquer,
which is in great  estimation at  Paris.   It is made  of pottery, or
earthen ware, set on a base of brick-work.        •

No. 1. is a longitudinal section along the middle of its breadth.  A is
      the ash pit, B the ribs of the grate, C the fire place, D its door,
      ™~i„ ♦„ i:& „o- ~~a „,„.,«. bo vciy  well fittcdt  E is the lhroat 
a
       of the furnace, through which the flame rushes into the labo-
       ratory F, where the arched roof rises considerably.   In its
       highest part there is a door at the side,  represented by the
       dotted line.  The materials to be scorified, or otherwise treat-
       ed in this reverberatory,  are put in at this door, and the state
       of the process may be  seen from time to time.   There is a
       smaller aperture on the opposite side, even with the floor, or
       but a very little higher.   The scoriae are driven^nto this pas-
       sage by the bellows, in certain processes.   The arch of the re-
      verberatory is much lower at G, where it communicates  with
      the upright vent or chimney H.

No. 2. is a plan, or rather a horizontal section.

No. 3. is a  cross section.   F is the fire place: and G is the arch of
      the reverberatory.

    The numbers  on various parts of these  three figures are the
dimensions, at those places, in inches. 
TO JAMES  WATT, ESQ
OF HEATHFIELD, NEAR  BIRMINGHAM.
    DEAR SIR,
       BY placing your name in the front of this edition
of the Lectures of our excellent Master, I think that 1
pay my best respects to his memory, and also do a service
to the Public.  By thus turning the reader's attention to
Dr. Black's most illustrious Pupil,  I remind him of the
important  services derived from his discoveries:  For
surely nothing  in modern times has made such an addition
to the power of man as you have done by your improve-
ments on  the steam  engine, which you profess to owe to
the instructions and  information you received from  Dr.
Black,

      When I contemplate the unparalleled state of pros-
perity of the British empire,  resulting from the skill,
spirit, and activity  of its inhabitants, and reflect on the
imperious  call, now  upon us,  for still greater exertions,
that  we may maintain ourselves in  this our envied pre-
eminence,  1 feel it my duty to hold forth every incitement
that can animate  to this honorable  emulation.   1 shew
the Reader,  in your example, that there is  no preemi- 
                        [   iv  ]
 nence in scientific attainment which he may not hope to
 reach, by rigidly  adhering to the sober plan of experi-
 mental inquiry,  so constantly inculcated by Dr. Black ;
 and turning a deaf ear to all the fascinating promises of
 splendid theories.  The spark,  which I thus throw out,
 idhy jfjbance to light among suitable  materials,....some
 felices animse, quibus  hasc cognoscere  curse est,....
 minds  perhaps unconscious  of their own powers.  Even
 yours might have lain dormant,  had not Dr. Black dis-
 covered its latent fire.                  #

      I acknowledge, that, J have also another inducement.
 J wish to be known as a person  who enjoyed a share in
 tic good opinion, and the uninterrupted friendship of two
 of the most eminent philosophers and most worthy men of
 the age and nation.

      That you may long continue to enjoy your well earned
 honors, happy in the society of your friends,  and  still
 increasing the means of the national prosperity by your
 inventions, is the earnest wish of

              Dear Sir,
                   Your affectionate Friend,
                        And obedient humble Servant,
                                     J.  ROBISON.
Edinburgh, April 7,  1803. 
SUBSCRIBERS' NAMES.
Branch T. Archer.
James Agnew, Trenton.
George Armroyd.
George B. Burrows, Philadelphia.
Hezekiah Belknap, Princeton.
Tiberius Jefferson Bryant, Philadelphia.
David Bertron.
Mordecai Y. Bryant.
Morgan Browne, Chester Town, Maryland.
Richard Brown, Alexandria, District of Columbia
William Brent, jun. Washington City.
James  Black, Philadelphia.
Edward Burd, Philadelphia.
Samuel Betton, jun. Germantown.
Binny and Ronaldson, Philadelphia.
Birch and Small, two hundred copies.
Samuel F. Bradford, fifty copies.
Beers and Howe, New Haven,  twenty five copies.
Joseph Ball, Philadelphia.
Benjamin Ballard, Virginia.
John Y. Bryant, Philadelphia.
Benj. Smith  Barton,  m. d. Professor  of Materia
     Medica, Natural  History and Botany, in the
     University of Pennsylvania.
James C. Bronaugh, Virginia.
Samuel Bleight.
 T. I. and M. Y. Bryant, Philadelphia.
 Joseph Cloud, Philadelphia.
 Turner Carmac, Philadelphia.
 Dr. Joseph Curry, Pittsburgh.
 William Currie, m. d. Philadelphia.
 J. Church.
 John A. Cuthbert, Princeton.
 Bernard Dornin, New York, six copies. 
(  ii  )
  James Dougal, Milton, Susquehannah.
  William P. Dewees, Philadelphia.
  J. W. Dyott.
  Thomas H. Dawson.
  Etheridge and Bliss, Boston, twelve copies.
  John Elliott and Sons, Philadelphia.
  Dr. John Floyd, Louisville, Kentucky.
  Gideon C. Forsyth, Wheeling, Pennsylvania.
  Seth B. Foster, Philadelphia.
  Samuel  Fahnestock, Lancaster.
 Stephen Le Ferrand, Princeton.
 John W. Farron, Charleston.
 William Grayson,  Winchester, Virginia.
 John Griscom, Burlington.
 Dr. James Gallaher, Philadelphia.
 Calvin Gould.
 Samuel P. Griffitts, m.  d.
 E. Griffiths.
 Christopher Hughes, Baltimore.
 Jasper Hand, Lancaster.
 John Harrison, Philadelphia.
 Joshua Humphreys.
 John Hall, Flemington,  New Jersey.
 Robert Harris, sen. Philadelphia.
 James Humphreys.
 George Hunter,
 Z. Hoefman.
 J. Hutchinson.
 Benjamin Hicks, New York.

 Jacob Johnson, Philadelphia, fifty copies.
Joseph Jones.
Thomas C. James,  Philadelphia.

Hugh Kennedy, Hagerstown, Maryland.
Wartman Kuhn, Philadelphia. 
(  in   )
William Kneass,
Elly Kitchin.
Charles Lukins.
William Magruder, Baltimore,
Henry Marim, Philadelphia.
Dr. William M'Nevin, New York.
John Meers, Philadelphia.
William M'Dougall, do.
David G. Mitchell, Pennsylvania
Michael Merrill.
A. M'Kenney.
John M'Leod, Alexandria.
James Mease, Philadelphia.
Charles Meredith,
Samuel Moore,
J. Mathieu.
G. Moore, Lancaster, Pennsylvania.
A. May.
Robert H. Nicholls, Washington City.
Francis Nichols, Philadelphia.
Stephen North,
Jeremiah Norgrave.

John Ott, Georgetown, District of Columbia.
Dr.  John C. Otto, Philadelphia.
Joseph  K. Potts, Philadelphia.
John Phillips.
Philip S. Physick, Philadelphia.
James Proudfit.
Thomas M. Potter, Trenton.
John Redman, m. d. Philadelphia.
Benjamin Rush, m. d.
P. K. Rogers, m. d.
James Reynolds.
J. C. Rousseau. 
(  iv   )
  E. H. Smith.
  Thomas and'John Swords, New York, twelve copies.
  Joseph Strong, Philadelphia.
  John Speakman,    do.
  Adam Seybert, m. d. do.
  Isaac A. Smith.
  Benjamin Smith, Lexington, Kentucky.
  William Shaw, m. d.
  J. Stuart.
  Nathaniel W. Sample, jun. Strasburgh Village.
  James Sloan, jun. Baltimore.
 Gustavus H. Scot, Washington.
 Isaac A. Smith, Princeton.
 John M. Scot.
 Wright C. Stanly.
 George A. Z.  Smith, Charleston.
 Henry G. Tucker, Winchester, Virginia.
 Thompson and Hart, New York, twenty-five copies,
 Stephen Thorn, Granville, New York.
 David Thomas, Hilltown Buck's county.
 Dr. Samuel  Tucker, Burlington, New Jersey.
 Allen Thomas.
 Thomas and'Afldrews, Boston, twenty copies.
 Peregrine Wroth, Chestertown, Maryland.
 Beale M. Worthington, Maryland.
 John White, Philadelphia.
 Charles J. Wister,
 Joshua M. Wallace, Burlington.
 Dr. Peter Wendall, Albany.
 Robert Wittbank, Lewistown
 John White,
 Frederick Wolbert.
Godfrey Welser.
Samuel Whetherill, jun,
Job Wilson.
John Weiden. 
THE  EDITOR'S PREFACE.
     THAT I engaged to revise and prepare for publication
the prelections of this eminent  Professor, may appear pre-
sumptuous, and to require some apology.   Chemistry is a
science of such immense extent, so multifarious, so abstruse
in its principles, and intricate in  their combination and mutu-
al dependence, that to pretend to appreciate, or, if necessary,
to alter any thing written by Dr. Black, requires no common
sagacity, and a degree of information not to  be  looked for in
one  who is not professedly a Chemist.  And it seems a task
too great for any person sufficiently occupied in  official duties
of a very different nature.   I acknowledge the  justice of the
charge.
   But I trust that when the reader, and particularly those
who have had the happiness of  listening to the  prelections of
this  excellent teacher, is informed in what manner this task
fell  into my hands,  the appearances of unwarrantable  pre-
sumption will be considerably lessened, and that my endea-
vors to perform it in  a suitable manner will be received  with
some indulgence.
   Immediately after the decease of my worthy preceptor, col-
league, and friend, his executors notified their intention  of
publishing an edition of his lectures  from  his  manuscript
notes.  This was done  in order to  prevent a publication,
which, they were informed, was  intended from notes taken  by
his students, and gradually improved by corrections and addi-
tions of successive years.   Copies of such notes were in very
general circulation about the College.  But the very best  of
them are so imperfect, have been the work of persons so little
acquainted with general learning, and are so full of mistakes,
and  very inadequate representations of Dr. Black's senti-
ments, that  the exjcutors wished to prevent the unfavorable
impression that s^n a publication would make pf the know- 
VI
PREFACE.
 ledge and talents  of  their departed friend.  Mr. Archibald
 Geddes, manager of the extensive glass-works at Leith, who
 had long lived in  the most  intimate and  cordial friendship
 with Dr.  Black, and in whose attachment to his memory and
 reputation the executors had the greatest confidence,  men-
 tioned me as a person whom their friend had thought well ac-
 quainted with chemistry, and  who had been in  the habits of
 intimacy with him  in the period of his greatest exertions ; and
 moreover, as one attached to  Dr. Black by every tie of res-
 pect, obligation, and affection, and therefore likely to be parti-
 cularly anxious to support his claims to eminence, by a careful
 and accurate  exhibition of his sentiments and doctrines.  This
 suggestion appeared reasonable, and he was desired to intimate
 their wish to  me.
  When the proposal was made to me, it startled me ;  but it
 pleased me.   It was very gratifying thus to have the last and
 the best opportunity of paying my respects  to  the memory of
 my excellent  friend.   I was indeed attached to  Dr. Black by
 every honorable tie.  I owed him much ;  and the reader will,
 I hope, forgrVe me if I mention here the origin of my acquin-
 tance with this celebrated man. It is a piece of self-indulgence,
 though not foreign  to my present purpose, as it shews the ear-
 ly opportunities I enjoyed of acquiring a thorough knowledge
 of his studies and character.
  My acquaintance with him began at Glasgow in 1758  I
 being then a student in that  LTniversity;  and  it began  in a
 nay  which marked the distinguished amiableness of his dis-
 position and behaviour.   It was at the house of one  of the
 Professors, to whom I was telling the great entertainment 1
 had received from the lectures of Dr. Robert Dick, Professor
of Natural Philosophy, and  how much I admired him as a
I? .rarer.   Dr. Black joined  in the commendation, and then
addressing himself to  me, questioned me a good deal about
 -Natural Philosophy, so as to perceive what were the peculiar
-J-ts of my  attention.  His advices relative to my favourite
study were so impressive, and given in  a manner so unaffect
eciiy  serious and ki,d, that they are still ^  frcsh in     mind
-if of yesterday's date.   I was  a Strang to  hh., L not 
PREFACE.
VII
even his pupil; and he was- prompted to take that pains with
me, solely, by the way in which he heard me speaking of the
lectures of one whom he loved and esteemed.  Gently and
gracefully checking my disposition to form theories, he warn-
ed me to suspect all theories  whatever,  pressed on me the
necessity of improving in mathematical knowledge, and gave
me Newton's Optics to read, advising me to make that book
the model  of all my studies, and to reject, even without ex-
amination, every hypothetical explanation, as a mere waste of
time and ingenuity.   I am conscious that it was to these ad-
vices, so impressively, because  so,kindly bestowed, that I owe
any ability that I may now possess  for scientific  attainments :
For he set me  into a path which I fear I should never have
chosen for myself.
   Our acquaintance was soon  interrupted by my leaving col-
lege.   When I returned to Glasgow, after four years absence,
I became  Dr. Black's scholar.   He was then  engaged in his
speculations about what he termed  latent heat.   I renewed   »
my acquaintance with hjm,  and with Mr. James Watt, the
celebrated engineer.   He had already heard two courses of
the  Professor's lectures,  and  was completely master of the
subject.   Dr. Black was pleased to admit me1 to the greatest
intimacy of acquaintance, and few days passed in which I was
not in company with these eminent philosophers, both of them
in the prime of their scientific  energy, and the period of then-
keenest research.   In such  society, I must have been stupid
not to improve; and this is the time of my life that I recollect
with the greatest satisfaction.
   When  Dr.  Black was removed to Edinburgh in 17"66, he
recommended  me to the  University of Glasgow  as his suc-
cessor in the chemical lectureship.   I exerted my utmost en-
 deavors not to be altogether  undeserving of  this honor;....   -f
 and I am happy to think that these endeavors obtained his
 approbation, of which he gave me  the most convincing proof,
 by recommending me in the warmest terms to the Patrons of
 the University of Edinburgh,  as well qualified to fill the Pro-
 fessorship of Natural Philosophy, vacant by the death of Mr.
 Russel, his near relation; a recommendation which, doubt- 
Vlli
PREFACE.
 less,  had ample  weight in deciding the competition m my
 favor.  I became Dr. Black's colleague in 1774, and  ever
 since that time have been honoured by his uninterrupted inti-
 macy and friendship.   Si  quid est in me ingenu, quod sentto
 quam sit exiguum, nut si hujusce rei ratio aliqua, ab optimarum
 artium studiis ac disciplind profecta, a qua ego nullum confiteor
 atatis mece tempus abhorruisse, earum rerum omnium hie Aulus
 Licinius fructum  a me  repetere proph suo jure debet.   Nam
 quoad longissime potest mens mea respicere  spatium preeteriti
 temporis, etpueritice memoriam recordari ultimam, inde  usque
 repetens, hum video mihi principem, et ad suscipicndam, et ad
 ingryliendam rationem horum studiorum extitisse.*
  The proposal, then upon the part of Dr. Black's Trustees
and Executors, for charging me with this important duty to
his memory,  was not without  some very reasonable induce-
 ments.  But every circumstance which rendered this proposal
 acceptable to me,  contributed to point out to me another per-
 son, more fit for the task, and more likely to execute it as it
 ought to be done.  This was Mr. Watt of Birmingham :....at-
 tached to  Dr. Black by the same ties, having had superior op-
 portunities of knowing the whole train of his thoughts, and far
 more ablf to do  justice to  his merits, because  profoundly
 versed in chemical science.   I advised this choice,  and Mr.
 Watt was applied to; but his occupations were too incessant,
 and too  serious, to allow him to think of such an avocation.
  I had no reason for declining the task, but the very power-
 ful one  of bad health, and the fear of its growing worse, and
 my being thereby rendered unable  to fulfil my engagements.
 It was, however, strongly  pressed upon me;  so that,  after
 some fear and hesitation, I agreed to the proposal, on condi-
 tion of  having Mr. Watt's  assistance,  which  the Executors
 requested and obtained,  as far as could be obtained by a cor-
 respondence of letters,  which  Mr.  Watt  entered into with
 great cheerfulness and zeal.
   But I had not sufficiently weighed the burden which I had
 taken on my shoulders.  I had been informed that Dr. Black
 had, for two or three years before his death, occupied himself
                   * Cicero pro ArcliU Poeta. 
PREFACE.                     ix
in the  revisal of the notes of his  lectures,  and had brought
them into very good order.  Two or three of them, which
I looked into, in order to  form a judgment of  my task, cor-
responded with this account.....and the engagement was  en-
tered into.   This was in  January  18,00,  while I was  occu-
pied with my own college duty, so that, I  could do nothing in
the affair till the May following.  When  I  then entered se-
riously on the task, I found that the notes were (with the ex-
ception of perhaps a score of lectures) in the same imperfect
condition tha> they had been in from the beginning, consist-.
ing, entirely  of single leaves of  paper in octavo, full of era-
sions, interlinings,  and  alterations of every kind; so that, in
many places, it was not very certain  which of several notes
was to be chosen.   They were often in  such a state, that I
could not give them to my amanuensis to  be transcribed; and
the only thing that could be done was for me to dictate from
them.  I took this method, as the only security for obtaining
a fair transcript.   This process necessadj- consumed a great
deal of time before I got to the end.   It was  only then that I
could  form a judgment of the performance  ; for,  as I was
going on, almost decyphering, my attention was wholly  en-
grossed by the lines before me, and I had scarcely any notion
of a page of it, taken together.
  I now found a difficulty of another kind.   Throughout the
whole series of Lectures, wherever the subject was very plain
and obvious,the manuscript contained merely a memorandum,
from which Dr. Black had lectured extempore ; in many pla-
ces, a reference was made to something standing on the  table,
or something going forward in the furnaces.  All those blanks
were to< be filled up, before I could say that I had made out
even a rough draught of the lectures. This was done, and then
it only remained to make some alterations in the modes of exr
pression, to cancel allusions to a former day's  lecture, and
other circumstances of this kind, which  were not suitable to
 the appearance in the form of a  book.   In a few places, I
 found  myself considerably at a loss to ascertain the author's
meaning, when the reference was very slight, often in a note
with the pencil.   I mention all these circumstances,  to ac-
    VOL.  I.                    B 
.v                     HREFACE.

count for the seeming delay in the publication.  No  doubt,
my proceeding was slower than it would have been had I been
in good health ;  but the additional delay on this account has
npt been considerable.  I had  the assistance of a very fair co-
py of notes, taken by a student, or rather manufactured by the
comparison of many such notes.   Copies of this kind were to
be purchased for four or five guineas.   This copy belonged to
Dr.  Black, and he had made many alterations and insertions
of whole pages with his own hand.  It was of considerable
service to  me for filling up the  blanks above mentioned.  Be-
sides the notes which Dr. Black had before him while he lec-
tured, and which  were all put into separate  parcels, each of
which contained a lecture, there are other small parcels, titled
with the different articles of  the course, and containing notes
and memorandums of experiments, quotations from authors,
speculations and conjectures on interesting facts  or opinions.
From these also  I was frequently enabled to supply what Dr.
Black had said in thfcecture.
   With such helps, I trust that I have omitted nothing  of any
importance, and have every where expressed Dr.  Black's sen-
timents with accuracy.  This is always done in his own words,
exct-pt in  the  cases already mentioned, where I filled up a
blank in the manuscript.   Even in those cases, if the words of
the above  mentioned notes  taken in the class expressed the
subject with distinctness, I took them, in preference to any in-
sertion of my own, as probably not greatly differing from Dr.
Black's discourse.  Where  I  had no such help, I question
not but that the difference between Dr. Black's manner of ex-
pressing himself, and mine,  will be perceived by the gentle-
men who  had the pleasure of hearing him.  I am sensible
that his language had a perspicuous simplicity which I  cannot
attain.
   I have twice  taken the liberty of making a change in the
arrangement of an article.   Dr. Black had considered the pij.
rophori, such as the phosphorus of alum, of Baldwin, t\e BoL
ognan stone, fcV. under the  same title with the phosphorus of
 ■trine, solely, I presume,- because they are usually  called by the
 ■:a:ne name, ph^kcru,.  I have pl^d them at the end of 
PREFACE.
\\
the article charcoal, because  they all contain charcoal, and
cannot be well understood till the properties of sulphur and of
charcoal have been explained.   For the same reason, a small
part of the account *f the diamond is separated from the  rest,
and inserted in the article charcoal.  The other case is the ac-
count of the observations and experiments of M'Bride, Lane,
Percival, and those of Priestley, Cavendish, Scheele and La-
voisier, subsequent  to  the  establishment of the theory  of
quicklime.  (See vol.  II. page 89, &c.) These particulars
stood in an order somewhat different  from what is followed
here; and I trust that the change will be thought to render
the account -more connected and perspicuous..  Indeed, the ,
whole of the article Alkaline Earths is not in a form that gave
satisfaction to the author; and there lie among these papers se-
veral projects for changing both the arrangement and the  man-
ner of treating the  subject.,  Dr. Black had  alwajs paid too
much deference  to the futile objections of Professor Meyer,
and the partisans of his acidum pingue; and had continued to
trf-at those objections as still  requiring a refutation, after the
theory of acidum pingue was altogether exploded.   He  gives
sufficient indication, in these memorandums, of the manner in
which he intended to consider the subject; and 1 should wil-
lingly  have made  the  alteration,  had  I  not thought  it too
great a liberty to insert a composition of my own, in the  place
of any thing written by Dr.  Black.
   The  memorandum*on the medicinal preparations of  mer-
cury are so extremely slight and imperfect, that, ignorant as I
am of medicine  and pharmacy, I could not venture to make
any use of them. The memorandums on the chemical analysis ,
of animal and vegitable substances are not in condition fit for
publication, not being at all accommodated to the present state
of chemical science.  Should it appear that this publication is
favorably received by the public, an appendix will follow, in the
same form, in which these two articles will be properly treated.
The  doctrine of fermentation (vinous,  acetous, and putrefac-
tive) will make a principal article in that work, seeing that
they are of the very first importance, both to the artist and to
the philosopher. 
xu
PREFACE.
   The discoveries which have been made  within  these  last
 thirtv years, and which have produced so great a change m che-
 mical science, were adopted by Dr. Black, and introduced in-
 to his lectures, in  proportion as he saw »hem well  supported
 and  confirmed.  This, however, taking place by degrees, has
 occasioned frequent changes in his lectures,  and  I am not cer-
 tain that he has been fully decided as to the way in which he
 should introduce the new doctrines of combustion  and acidi-
 fication.  His papers contain  several plans  to  this purpose.
 One of them, which seems to  have had the  preference in his
 estimation, is to bring the whole gradually into view, in the
 history of the nitric acid, in all its various relations.   This
 method would have been  extremely perspicuous and convin-
 cing, and had I had sufficient materials of Dr. Black's composi-
 tion for the beginning of this  history, I should perhaps have
 followed it out, although it would have required  almost anew
 writing of the whole particular doctrines of  chemistry.   But
 this not having been supplied to me, I had not  sufficient au-
 thority for making such  a  change.   As  it stands in these
 volumes, the doctrines are all  fully explained, and   supported
 by topics of argument  that are quite easy and familiar;  and
 this is done in a manner that gives full confidence.   The only
 deficiency that I perceive is in the incidental way  in which the
 composition of the volatile alkali is introduced ;  and perhaps
 also the peculiarities of the oxygenated muriatic acid.   Had
 manganese been first considered merely as a substance affor-
 ding vital air, the  oxygenated form of the  acid would have
"been early established, and this would have afforded solid ar-
 guments for some other abstruse points of doctrine.
  A discourse, given at the first meeting in each course of lec-
 tures, has been omitted entirely, as having  little or no connec-
 tion with their  appearance as a book ;  and a short history of
 chemistry has also been left out, as too general to be very in-
 teresting or instructive.
  On the whole, what follows in the text of this  work,  is the
discourse  of  Dr. Black;  as  much  as possible in his own
words, and, in every passage, a faithful exhibition of his sen-
timents and opinions.   I have added some notes, chieflv rela- 
                        PREFACE.                  xiii

 live to the author's peculiar doctrines, and his claims to ori-
 ginality and priority ; sometimes illustrative of the text; and,
 on a few occasions,  contributing,  I hope, to tlie reader's ac-
 quiring just and philosophical notions of the subject.   I hope
 that they will be found pertinent, and, in general, that my en-
 deavours to make the labours of my venerated master useful,
 as he earnestly wished them, and to appear not unworthy of  ^
 himself,  will be indulgently received by a candid public,  to
 whose judgment they are now submitted  with all becoming
 deference and respect.
  When we have received much pleasure or instruction from    '
 the writings of any person, we are apt to feel some attachment
 to the man to whom we have been thus  obliged, and some in-
 terest in any thing that personally concerns  him ;....we wish
 to be better acquainted with him....to know who  he is....what
 were his usual occupations....his fortunes....his general man-
 ners.  If we have often listened to  him as a  public speaker,
 we  cannot help forming to ourselves a  notion of his  temper
 and dispositions from the tones of his voice, and the expres-
 sion of his countenance ; and we figure  to ourselves how this
 person would behave as a companion, a friend, or an inmate.
 If his  public appearance has been conciliating, or eminently
 pleasing, we wish to converse with him in the familiar inter-
 course of life to witness him in his domestic occupations, and
 to share  in his amusements.  I think  that I may venture  to
 say that these are the wishes of all who have  listened to this
 most engaging lecturer....who have remarked the  pleasing
 smile, which began to form  on his countenance, when he was
 about to exhibit or relate any thing that he considered as pe-
 culiarly interesting.   They will hold themselves  as obliged
 to any  man who  will bring them  more closely  into  society
 with this amiable person, by giving  some account of his or-
 dinary habits, his studies, his pursuits, and even his fortunes.
  I should be happy  that it were in my power to satisfy these
 natural wishes by some  discriminating description  of Dr.
 Black,  by some account of his life,  his character, and man-
ners. This would not only please, but (as I apprehend) would
 instruct, by exhibiting a  fair example of what is amiable and
 worthy. 
   xiv                   PREFACE.

      But I am too sensible quid Jerre recusent, quid valeant hu-
   meri, to think mvself equal to this task....to give ,uch a pic-
   ture  that a competent judge shall say,  " This is Dr. Black."
   I will, however, for the  satisfaction of his pupils, and as a sort
   of dutv to my departed friend,  mention such circumstances
   of his' life and situation, as  have come to  my knowledge,
   especially  such as have some relation to this publication of his
   official discourses.  For this purpose, I shall use the freedom
   to avail myself of the information contained in a paper read
   to the Koyal Society of Edinburgh, (of which Dr. Black was
   a member), by his ^ear relation,  Dr.  Adam Ferguson, Pro-
   fessor of Mathematics in the  University, and well known in
   the republic of letters, by works of the  very first rank.
     Dr. Joseph Black was born in France, on the banks of the
   Garonne, in the year 1728.* His father, Mr. John Black, was
   a native of Belfast in Ireland, but of a Scotch family, which
   had'been  some time settled  there.  Mr. Black resided, for
   the most part at Bordeaux,  where he carried  on the wine
   trade.  He married a daughter of Mr. Robert Gordon,  of
   the family of Hilhead in Aberdeenshire, who was also engag-
   ed in the same trade at  Bordeaux, where he was very suc-
   cessful, and had the satisfaction of clearing the  family estate
   in Scotland'pf the incumbrances which had been increasing
   for some generations.
     The mother of Dr. Black, and the mother of Mr. James
   Russel, Professor of Natural Philosophy in the  Universitv of
   Edinburgh, were sisters;  and the  mother of Dr. Adam Fer-
   guson was their aunt, a circumstance which  was the origin,
   though not the cement, of a friendship subsisting  between
   them  through  life.   Their  ostensible  connection  became
 Jfcuch closer by Dr. Ferguson's marrying a daughter of  Dr.
 iTOack's sister,  a young lady  who had  a  great deal of her
   uncle's elegance of mind and manners.
     Mr. Black of Bordeaux was a gentleman of the most amia-
*  ble manners, candid and liberal in his  sentiments, and of no

     # In the Inaugural Dissertation, published when he received the de-ree of
   Doctor of Medicine, he stiles himself Galla Hibermu.
                                        American Editor. 
PREFACE.
xv
common information.  These features of his character, and
particularly the strength of his attachments, and the warmth
of his heart, appear in the strongest manner, in a series of let-
ters  to his son, which have been preserved by him with the
nicest care.
  So much worth had not escaped the discerning eye of the
great Montesquieu, one of the presidents of the court of jus-
tice  in that province.  This illustrious magistrate and excel-
lent  man honored Mr. Black with a friendship  and intimacy
altogether uncommon; of which his descendents are, to this
day, justly proud.  They preserve  letters, and fragments of
correspondence, between the  president and their ancestor, as
they would titles of honour descending in their family.  On a
paper wrapped round a bundle of such letters,  I find the fol-
lowing note in the hand-writing of Dr. Black:
  "  My father was honored with  president  Montesquieu's
*' friendship, on account of his good character and his virtues.
" He had no ambition to be very rich, but was cheerful and
" contented,  benevolent and liberal minded,  industrious and
" prudent in business, of the strictest  probity  and honor,
" very temperate and regular in his manner of life.   He, and
" my mother, who was equally domestic, educated thirteen of
" their children, eight sons, and five daughters, who all grew
" up to be men and  women,  and settled  in different places.
" My mother taught her children to read English, there being
" no school for that purpose at Bordeaux."
   The  great partiality of the president Montesquieu for the
constitutional government of  these  kingdoms is well known.
I think it not unlikely that he derived much of his informa-
tion about many  things peculiar to our  fortunate  situation
from the British gentlemen whom he might see at Bordeaux.
Here he could make  his inquiries at his leisure, and here he
was certain of being listened to  with  attention and respect.
IJis stay in England was short and hurried ; too many objects
were forced on his attention at once.   This however was  not
his chief inducement  to an  intimate  acquaintance  with  the
family of Mr. Black.  With a great simplicity of heart and
manners, this illustrious person had a glowing sense of modest 
XV1
PRE F AOL.
 worth.  His private memorandums, lately published, seem
 trulv to express his genuine sentiments and choice in the en-
 joyment of life.  Though solicited by every thing that could
 encourage a man of honorable ambition, he quitted the capi-
 tal in the height of his reputation for professional talents, and
 even quitted his high office in the province, that he might be
 fully master of his  time,  and choose his society.   He lived
 almost constantly at his country seat in the neighbourhood ot
 Bordeaux.
   I  am sorry that I did not look  more narrowly into the Pre-
 sident's correspondence with Mr.  Black.   I can  only recol-
 lect  the being delighted with  the impression which the sight
 of so much  worth and domestic happiness  made on this ex-
 cellent judge of  human nature.    Dr. Ferguson  mentions a
 letter,  written by him, when he heard of Mr. Black's inten-
 tion  to leave Bordeaux.   In this letter, among other expres-
 sions of kindness,  are the following: " I  cannot reconcile
 " myself to the thoughts of your leaving Bordeaux.  I  lose
 " the most agreeable pleasure  that I had, that of seeing you
" often, and forgetting  myself with you."  I remember also
 another letter, in  which were nearly the following words: " I
" rejoice to hear of the good health of all your family;  and I
" endeavour to make your satisfaction solace me for the loss
" of  those tranquil hours  which I enjoyed in  the midst of
u my friends, contemplating their  happiness and their  vir-
" tues."
  I could not refuse myself the pleasure of recording these
few  particulars,  descriptive of an amiade  family, which so
 warmly engaged  the notice and  the heart of this illustrious
 friend.   But they  are not  without their  use,  when they
 shew the charm  of domestic virtue,  and  the  unspeakable
 advantages of good habits in the outset of life.  Mr.  Black
 lived to see that his paternal care had not been in vain.
  Long  before  Mr.  Black retired from business,  his  son
 Joseph was sent home to Belfast, that he might have the edu-
 cation of  a British subject.  This was in the year 1740  he
 being then twelve years of age. After the ordinary  instruction
 in a grammar school, &c. he was sent, in 1746, to continue his 
                       PREFACE.                   xvii

 education at college, in the University of Glasgow.  I have
 no account  of those  youthful studies, but I  presume that
 he had  employed  his  time  to good purpose.  I infer this,
 partly, from some passages in his father's letters to him, ex-
 pressive of his great satisfaction with the accounts  which he
 had received from  others, of his progress in his studies ; and
 partly, from the very 'perspicuous  manner in which I have
 heard Dr. Black state the distinctions between the theories of
 ethics which had  been taught at Glasgow by  Dr.  Francis
 Hutcheson,  and, after  him,  by Dr. Adam Smith.   Physical
 science, however, had  chiefly attracted his attention ;  and he
 was a favorite pupil of the  professor  of  natural philosphy,
 Dr. Robert Dick, and  the intimate companion of his son and
 successor.  This young professor was of a character peculiar-
 ly suited to Dr.  Black's taste,  having the clearest conception
 and soundest judgment, a manly steadiness of opinions and
 conduct, accompanied by a modesty that was very uncommon.
 When he succeeded his father in 1751, he became the delight
 of his students, and was, indeed, the most perspicuous and in-
 structive lecturer I ever heard.   Yet Dr. Black informed me
 that he was unhappy in the thoughts of not being sufficiently
 qualified for the office, and wished to resign it.   He was carri-
 ed off by a fever in 1757,  Dr. Black always spoke of him in
 terms of the highest respect for his talents^and his great worth,
 and I could observe that he was always pleased when I made
 him the subject of conversation.  Dr. Dick had been the cho-
 sen friend of his youth.
   Being required by his father to make choice of a profession,
 Mr.  Black  preferred that of medicine,  as the most suited to
 the  general habits of his studies, not foreseeing, during the
 happy gaiety of  youth, h^ much he would suffer by an-
 xious solicitude and fears m the practice of this noble art.
   It  was fortunate  for  Dr.  Black that, when he  began his
 medical studies at Glasgow, the celebrated Dr. William Cul-
 len had just entered on  his great career, was become consci-
ous of his own strength, and saw  the unoccupied field of phi-
losophical chemistry open before him.   It had  been treated
hitherto only asf> very curious and useful art, which was in>
    vol.  i.                  c 
 xvai                  PREFACE.

 deed  susceptible of much improvement by means of rational
 inquirv and discussion.  But Cullen saw  in it a vast depart-
 ment of the science of nature, which must be founded on prin-
 ciples as immutable as the laws of mechanism, and which may
 be one day formed into a great system of doctrines, of vari-
 ous degrees of subordination and dependence. He was  deter-
 mined to attempt this mighty task, and  promised himself
 great  reputation by" its accomplishment.  Nor was he alto-
 gether disappointed.   He quickly  succeeded in taking che-
 mistry jut  of the hands of the  artists, the metallurgists, and
 pharmaceutists, and exhibited it as a liberal science, the study
 of a gentleman.  He  carried  into his medical  lectures the
 same ideas of a great  system of nature, and made  his pupils
 perceive  something of that affinity by which, as Cicero finely
 observes, all the sciences arc connected, tendering to each other
 a mutual illustration  and  assistance.   His  pupils • became
 zealous chemists as  well as refined  physiologists.  Young
 Black was particularly delighted with a view which accorded
 so happily with  those enlarged habits of thought which he had
 acquired ; and his great bias to this study was soon perceived
 by  Dr. Cullcn.   No  professor took a more lively interest in
 the progress of an emulous student than Dr.  Cullen.   It was
 his  delight to encourage  and assist their efforts, and  there-
 fore he was not long in attaching Mr. Black to himself, in the
 most  intimate co-operation ; insomuch, that  the latter was
<|£gnsidered as his assistant in all his operations ; and his experi-
 ments were frequently adduced in the lecture, as good autho-
 3 ity.   Thus began a mutual confidence and friendship,  which
 did honor both to the  professor and his pupil, and was always
 mentioned by the latter with gratitude and respect.
   Our young philosopher had la'Alown a very comprehen-
 sive and serious plan  for the  conMict  of his  studies.'   This
 appears  by a number of a note-Books found amoung his pa*-"
 pers.  There are some in which' he seems to have inserted
 every thing as it took his fancy, in medicine, chemistry, juris-
 prudence, or matters of  taste  ; and I find others into  which
 he has transferred the same things, but has distributed the
  according to thuir scientific connections.  *i short, he b-
                                          /
m 
PREFACE.
X:<£
  kept  a  journal  and leger of  his studies,  and has posted
  his books like  a merchant.    I  have  looked  over  these
  memorandums with some care, and have there seen the first
  germs of those discoveries which have at last produced such a
  complete revolution in chemical science.   What particularly
  struck me, was the steadiness with which he advanced in any
  path  of knowledge,....nulla retrorsumF  Things  are inserted
  for the first time, from some present impression of their singu-
  larity or importance, but without any allusion to their connec-
  tions.  When a thing of the same kind,  is mentioned  again,
  there is generally a reference back to its fellow ;  and thus the
  most insulated facts  often acquired a  connecaon which gave
  them scientific importance.
    By these references I got the*order in which things had oc-
  curred, (for there are very few dates) and a pretty certain know-
  ledge of the years when he made the observation^.  For,  iu
  what I call his day-books, mention is sometimes made of oc
 *currences that were perfectly known.to me, as I had lived in the
  place from my infancy, and was well acquainted with every
  thing that passed.  In the oldest parcels  of these notes, I
  fmd queries  respecting the nature of cooling mixtures, and
  the cold produced by liquefaction ; but it is not till some time
•'After, I think not before the year 175^, that I can date any
  observation relative  to fixed air.  Yet this was the first of his
  investigations that he completed, by establishing it as a general
  law of nature. I do not imagine that Mr. Black's researches at
  this time (or perhaps at any time) have been keen orpertinacious.
  This  could not accord with the native gentleness of his mind;
  but his  canceptions being distinct, and his judgment sound,
  his progress in scientific research, if slow, wasjsteady, and his
  acquisitions were solid. Perhaps this moderation and sobriety
  of thought was his happiest disposition, and the most condu-
  cive  to his improvement.   I am convinced that it was from
  experience that he was led to caution his pupils  so  earnestly,
 to check the first  incitements of high expectations, and never
  allaw  their fancy to be warmed by the brilliant appearance of
 some  general view, which promised at the first  glance to ex-
 plain a multitude of phenomena. 
XX
PREFACE. .
  This calmness of procedure appears  in another  circum-
stance.  The College Registers shew that Mr.  Black might
have offered himself as a candidate for a doctor s  degree,
near three years earlier than he actually did.   When he was
graduated in 1754, he had not availed himself of his  medical
standing in  the University of  Glasgow,  but  took the course
prescribed by tne rules of that University.
  Dr.  Black went to Edinburgh, to finish his medical studies,
in 1750, or  1751.   There he lived  with his  cousin-german,
Mr. James Russel, Professor of Natural Philosophy in the
University.   Mr. Russel was really a philosopher.   No man
saw more  clearly the great scale of Nature, as it is diversified
by tne,powers of mechanism, chemical affinity,  and the prin-
ciples of growth, life, sentiment, and intellect: and  I  think he
was the first who ascertained  with  some precision  the  true
pale of Natural Philosophy, and-gave  a truly philosophical
table of its  various parts. He  was not more enlarged  in his*
general views, than he was ingenious in his examination ot
subordinate parts;  and he was most happy  in  the  talent rjf-
presenting things to the mind  in  a simple and  familiar man!
ner.   11  such a society, Mr. Black  must have passed his time
both agreeablv and profitably.
   It was the good fortune of chemical science, that at  this very
 time,  the opinions  of professors were divided concerning the
 manner in which certain lithontriptic medicines, and"pa'rticu-
 larly lime-water, acted in alleviating the excruciating pains of
 the stone and gravel. The students usually partake of differences
 of opinion, and are thereby animated to more serious study, and
 science gains by their emulation. This was a subject quite suited
 to the taste of young Mr. Black, one of Dr. Cullen's most zea-
 lous and  intelligent chemical scholars.   It was indeed a most
 interesting subject, both to the chemist and  the physician.
 All the medicines  which were then in  vogue, as solvents of
 the calculous concretion, resembled more or less the lapis in.
 fernalis,  and the common ley of the soap  boilers, two sub-
  stances  so terribly acrimonious, that in  a very short tirfie,
  they  will reduce the  firmest and most  solid parts of the ani-
  mal body to a mere pulp. Therefore, while they were powerful 
PREFACE,                    xxi
lithontriptics, they were hazardous medicines, if in unskilful
hands.   All of them seem to derive their efficacy from quick-
lime, and. this derives its power  from fire.  Its  wonderful
property of becoming intensely hot, and even sometimes igni-
ted,  when moderately wetted with water, had long engaged
the attention of the chemists.  It was, therefore, very natural
for them to ascribe its power to igneous matte* imbibed from
the fire, retained in the lime, and communicated  by it to alka-
lis, and other substances, which it renders so powerfully acrid.
Hence, undoubtedly,  arose the denomination of causticity,
given to the quality so induced.   I  see that Mr.  Black had
entertained the opinion, that caustic alkalis acquired igneous
matter  from quicklime.  In one  memorandum, he  hints at
some way of catching this matter as  it escapes from  lime,
while  it becomes  mild by exposure to the air;....but on the
opposite blank page is  written, " Nothing escape's,....the cup
" rises  considerably by absorbing air." A few pages after
this, he compares the loss  of weight sustained  by an ounce of
chalk, when calcined, with its loss when dissolved in  spirit of
salt.  Immediately after, a medical case  is mentioned, which I
know  to  have occurred in November 1752.   From this  it
would appear, that he had before this time  suspected the real
nature  of these substances.   He had then prosecuted his in-
quiry with vigour; the experiments with magnesia are soon
mentioned.
   These laid open the whole mystery, as appears  by one other
memorandum.  u When  I precipitate  lime by a common
" alkali, there is no effervescence.  The air quits the alkali for
" the lime, but it is not lime any longer, but c. c. c.   It  now
" effervesces, which good lime will not."  He had now dis-
covered that the  terrible acrimony  of those  powerful  sub-
stances is their native property, and not any igneous matter
derived from  the lime, and by the lime from the fire.  He
had discovered that a cubic  inch of marble consisted of about
half its weight of pure  lime, and as much air  as would fill a
vessel  holding six wine  gallonr,  and that it was rendered
tasteless and mild by this addition, in the same manner as oil
of vitriol is rendered tasteless and mild in the form of alabas-
ter, by its combination with  calcareous earth. 
XX11
fREFACE.
   What a multitude of important consequences now present
 themselves to the mind of an intelligent chemist and physi-
 cian ? I am inclined to think, that it was at this time that the
 animating hope of scientific reputation first dawned on the
 mind of  this young philosopher.   His experiments  on the
 lithrontriptic  medicines,  by shewing him the true nature of
 quicklime, had in one glance shewed him what causticity is,
 and to what substances it belongs, and how to induce it, or re-
 move it, or direct its activity at pleasure.   This was a subject
 even  more interestiug to the physicians than to the chemist.
 It had divided  the opinions in both departments,  and given
 rise to many mysterious notions concerning the nature of fire
 (and favorites,  because mysterious) derived from the  very
remarkable properties  of quicklime.   Our  notions are now
altogether reversed.  Lime imparts nothing;  it only removes
 from  substances, naturally caustic,  that air which  renders
 them mild; and by this addition it becomes mild or inactive.
 These mysterious notions are now exploded as mere fancies ;
 and great simplicity is now perceived in those  operations of
 nature,  which  formerly appeared  very  intricate  and  ab-
 struse.    And  it  is  evident that  the  practice of physic
 must derive unspeakable advantage from all this informa-
 tion.
 It is surely a  dull mind that will not be animated by such a
 prospect.  I presume that Mr. Black felt its genial influence ;
 and I suppose thatTiaving fixed on  this for the subject of his
 inaugural essay, he deferred application for  a degree, till his
 doctrine should be established beyond the possibility  of con-
 tradiction, by a train of decisive experiments.  Thus did an
 honorable ambition happily accord  with his native modera-
 tion of character.
   The inaugural essay, and the precise  time of its appearance
in public,  were  fortunate circumstances for science.  At this
very time, Dr.  Cullen was removed to  Edinburgh, and there
was a vacancy in the chemical chair at  Glasgow.  On whom
 rould it be bestowed with so much propriety, as on such  an
 alumnus,of the University,....on one who had distinguished
 •m^lf, both as a chemist, and as an excellent reasoner ?  For 
PREFACE.
xxiii
I hesitate not to  say,  that excepting the optics of Newton,
then  is not a finer model for philosophical investigation, than
the  essay on  magnesia and quicklime.  He was appointed
professor of anatomy and lecturer on chemistry in the Uni-
versity of Glasgow, in 1756.  Had this vacancy not happened,
it might probably have been the lot of Dr. Black to practise
medicine in some provincial town of Britain or  Ireland; and
thus occupied, the serious concern which he took in the cases
under his treatment would have absorbed his whole attention,
and might perhaps have overpowered his feeble constitution.
It was therefore equally fortunate for himself and for the pub-
lic,  that a situation now presented itself, which allowed him
to dedicate his talents  chiefly to the  cultivation of chemistry,
his favorite science.
  Mere I  must not omit a circumstance told me by his  bro-
ther.   When Dr. Black took his degree in medicine, he sent
some  copies of his essay to his father in Bordeaux.  A copy
was given by the old gentleman to  his  friend  the president
Montesquieu, who, after a few days,  called on Mr. Black and
said to him," Mr. Black, my very  good friend,  I rejoice wi'th
" you;  your son will be the honor of your name and of your
" family." That sagacious philosopher saw, with the first glance,
the door opened to a field of research, altogether novel, and of
unknown extent.   What could be  more singular than to find
so subtile a substance as air existing in the form of a  hard
stone, and its presence accompanied by  such a  change in the
properties of that stone ? What bounds could reasonably  be
set to the imagination, in supposing that other aereal fluids, as
remarkable in their properties,  might exist in a solid form in
many other bodies, which at present attract no notice, because
of our ignorance of their nature and their composition ? Here
was a new instrument put into our hands, and a new  mode of
investigation suggested;  and it appeared unquestionable that
many and great discoveries must be made in this new track of
inquiry,  indeed, I have often  wondered that Dr. Black was
not more incited to proceed in this  track,  which he had so
clearly laid open.  This must have  proceeded from the multi -
plicity of  new duties which crowded on him at once, in con. 
XXIV
PREFACE.
sequence of his  department in  the  University of Glasgow.
His first appointment was to the professorship of anatomy, and
the lectureship on chemistry.  He did not consider himself
as so well qualified to be useful in the former branch of medi-
cal studv, but was determined to do his utmost.  Soon after,
however, he made arrangements with the professor of medi-
cine, and, with the concurrence  of the University, the pro-
fessors exchanged their tasks.
   His lectures, therefore, on the Institutes of Medicine, were
his chief task.  They gave the greatest satisfaction, by their
perspicuity and simplicity, and by the cautious moderation of
all his general doctrines.   It required, however, all this per-
spicuity,  and all  this neatness in the manner of exhibiting
simple truths, to create a relish for this great moderation and
caution, after the brilliant prospects of systematic knowledge,to
which the students had been accustomed from the Doctor's ce-
lebrated predecessor. But Dr. Black had no wish to form a med-
ical school, which should be distinguished by some all-compre-
hending  doctrine.  He contented himself with giving a clear
and systematic account of as much of physiology as he thought
founded on good principles, and a short sketch of such gene-
ral doctrines as were maintained by eminent authors, but per-
haps on a less firm foundation.  Without  this, he said that
his students could not read their writings, which, in other re-
spects, were highly valuable. He then endeavoured to deduce
a  few canons of  medical  practice; and concluded with cer-
tain  rules, founded  on successful practice only, but not so
deducible from the  previously delivered  principles of physi-
ology ; observing that we should not despair of being able, on
some future  day, to  proceed in the opposite direction,  de-
ducing the first principles entirely from the practice.  It does
not appear, however, that he had ever satisfied himself with
his method of treating those subjects. * He did not encourage
conversation on  those topics; and  there  are no remains of
his medical lectures to be  found  among his papers.   I owe
the account now given of them to a respectable surgeon in
Glasgow, who attended these lectures in the two last years of
his Professorship in that University. 
PREFACE.                   xxv
  My acquaintance with Dr. Black gave me full opportunity
of seeing, from his extreme anxiety about his parents, that he
was deeply impressed with a sense of the immense  import-
ance of the healing art.  I cannot doubt,  therefore,  but that
his thoughts were  much and most seriously employed on his
medical  lectures ;  and I am confident  that the less he was
satisfied with the limited portion of knowledge he could com-
municate to his studenjaw the more earnestly would he strive
to increase it.
  It is in this way  that I account to myself for the remarkable
fact,  that, although  Dr. Black had opened such a new and
boundless field of chemical research,  which promised  so
much, both of knowledge and of fame, and in which we see,
by the progress of some very slovenly adventurers, that it
was  extremely easy to discover objects, both new, and won-
derful, and  important;  that  notwithstanding all this, he  did
not immediately engage with ardor and perseverance in this
race of discovery and of honor.    No doubt, his  duty as Pro-
fessor of Chemistry forced him  to divide his attention, and
probably other tracks of chemical enquiry might also  hold out
very tempting objects.  But still  nothing can more clearly
shew Dr. Black's calm and unambitious character.
  But all  this was very unfortunate, in my opinion, for the
world.   What a difference  there  would have been  between
the  patient, judicious,  and progressive investigation of Dr.
Black, and  the hasty, wavering, and  often slovenly expert
menting of some manufacturers in science, whose wish to get
first to market with every thing was represented by  them as
proceeding  from public spirit, while the endeavors of others
to correct their own errors, to arrange and methodize their
materials, and thus to advance securely, though slowly, in the
great path of philosophical discovery, was attributed  to a nar-
row-minded pride, or  the selfish vanity of being accounted
the  author of a system.   But,  est  modus in rebus.....It must
be owned that Dr. Blacjc was too little  animated by  his own
success,....too insensible to the real value of literary fame,
and  to the notice  taken by the public of his discoveries, and
hot sufficiently excited to the vigorous prosecution of them.
    VOL. I.                   D 
xxvi                 PREFACE.
 His theory of quicklime and causticity was soon known to the
 German chemists, and met with strong opposition.  Myste-
 rious doctrines concerning the intimate nature of fire were
 very prevalent in  the German schools, and were very various.
 Their notions of  the causticity of alkaline substances  always
 involved some of these doctrines.   This gave rise, therefore,
 to a variety of objections to a doctrine which took  this  matter
 out  of the hands of the  pyrologista^iltogether.   The most
 formidable opponent was  Dr.  Meyer of Osnaburg, who had
 published a  considerable volume on  quicklime, in which he
 professed to explain all the phenomena by the action of  acidum
pingue, formed in the lime during calcination,  and consisting
 of igneous, matter in a  certain inexplicable combination with
 other substances.   There  is  scarcely to be seen  a book in
 which there is such a number of injudicious experiments, and
 unskilful attempts to reason from them. Yet this performance
 of Professor Meyer gave  Dr. Black considerable uneasiness ;i
 and, for several years, he was at the pains to refute all  his ar-
 guments, and all his objections to the theory given  by himself,
 but without adding a single experiment to those by which he
 had already established it.  Yet the obvious and simple ex-
 periment, of making the matter expelled from marble by a
 calcining heat pass into a solution of caustic alkali, and  render
 it effervescent and mild, would have ended all disputes. This
 was done by Jaquin, at  Vienna, in 1767, and at once silenced
 all the German chemists, as the experiment of Well, in which
 he calcined marble by a burning glass, put an end  to Meyer's
 theory.   But  Dr. Black  always expressed  a  dislike  to  the
 bringing forward a multitude  of experiments of the same
 kind, saying  that  he felt  his  own  confidence  more forcibly
 won by one judicious and simple experiment than by any num-
 ber of examples of inferior evidence.
  Dr. Black's reception at Glasgow by  the University was in
 the highest degree  encouraging.  His  former conduct there
 as a student had not only done him credit in his classes  but
 had conciliated the affection of the Professors to a very great
 degree.   When he returned to his Alma Mater Academia
 as a Professor, he was immediately connected in the strictest 
PREFACE.
xxvit
  friendship with the celebrated Dr. Adam Smith; a friendship
  which became more and more intimate and confidential through
  the whole of their lives.   A certain simplicity of character,
  with an incorruptible integrity, which was acutely sensible to
  the smallest indelicacy or incorrectness, was instantly seen by
  each of these friends in the character of the other, and riveted
  the band of their union.  Dr.  Smith used to say that no man
  had less nonsense in his  head  than Dr. Black; and he often
  acknowledged himself obliged to him for setting him right in
  his judgment of character,  confessing that he himself was apt
  to form  his opinion too generally from a single feature. Indeed,
  were  I  to say what intellectual talent Dr.  Black possessed in
  the most uncommon degree, I think I should  say that it was
  his judgment of human character, and a talent which  he had
  of expressing his opinion  in a single short phrase, which fixed
  it  in the mind, never to  be forgotten.   Dr.  Smith's pictures
 of men had not always this precise similitude, he being more
 apt to decide hastily of character; and he was not unfrequently
 mistaken in the judgments he formed on a short  acquaint-
 ance.
   When I returned to College in 1763, I found Dr. Black in
 high reputation as a professor, and a favorite  physician of that
 large and active city.  Indeed his sweetness of manner, which
 the dullest eye must have perceived to be free from all studious
 endeavor to please, apd  the evident concern which he took
 in the cases under his care, could not but  make him a most
 welcome visitor  in every family.  His  countenance, at that
 time of  life, was  equally  engaging as his  manners were at-
 tractive, so that I do not  wonder that, in the general  popu-
 larity of his character, he  was in particular a favorite with the
 ladies.   I could not but remark that they regarded themselves
 as honored by the  attentions of  Dr.  Black ; for these were not
 indiscriminately bestowed, but  exclusively  paid to those who
 evinced a superiority in mental accomplishments, or propriety
 of demeanor, and in grace and elegance of manners.
  But I am forgetting the professor and philosopher.  It was
at this time, between the years 1759 and 1763, that he brought
to maturity those speculations concerning the combination of 
 xxviii                   PREFACE.

 heat or fire with the substance of tangible matter, which had
 longoccupied his thoughts occasionally  The simple experi-
 ments and familiar observations by which he Uemonstnwed
 the fixation (I may call it) of heat in bodies when it melts or
 evaporates the m, renderthe inference so palpable and obvious,
 that one is disposed to wonder that it had not been made long
 before.  But it is really not so obvious, and it requires atten-
 tive reflection, to conceive distinctly the procedure of nature.
 When I lift a piece of wood out of some 'jjsx or vessel, where
 every thing has been kept extremely cold, I feel it cold in my
 hand.   If I lift out of  the same box a piece of lead, it feels
 colder still ; and a piece of ice from the same place feels coider
than either.   The first suggestion of sense is, that I receive
 cold from the wood ;....more from the lead ;.. .and that the
 ice proves a source of cold till it be all melted.  But the person
 who is habituated to the consideration of things of this nature
 makes an inference which is precisely the contrary to all this.
 Such a person infers that the wood takes a little heat from his
 hand, and is soon warmed so much as to take no more.   The
 lead takes more heat from him before it be as  much satiated;
and the ice continues o rob him of heat as fast as in the first
 moment, and thereTore feels equally cold  till  all be melted.
 Dr. Black made  this inference.  He  had also some vague
 notion that the heat so received by the ice, during its conver-
 sion into water, was not lost, but was contained in  the water.
 His chief inducement to think so, was  a curious observation
 of Fahrenheit, recorded by Dr. Boerhaave, namely, that water
 would sometimes grow  considerably colder than melting snow,
 without freezing, and would freeze in a  moment, if disturbed,
 and in the act of freezing emitted many degrees of heat.
   But how was this conjecture  to be confirmed or refuted ?
 Dr. Black hoped  to do this  by comparing the time  of rais-
 ing a pound of water one degree in its  temperature, with the
 time required for melting a pound of ice, both being supposed
 to receive the heat equally fast.  And  on  the other hand, by
 comparing the time of  depressing the temperature of a pound
                                      iperature of a pound
                                    __ssary for freezing it
completely, he should learn how much heat emerged during
!!«!!™™\  S,reC\ r.lth  the time n^^ry for freezing it 
•PREFACE.
xxix
 the congelation.  If the conjecture be just, as much heat must
 be observed to come out of the pound of water in freezing as
 were lost in melting a pound of ice.   This thought occurred
 to him in the summer season :  and as there was no ice-house
 then in Glasgow, he waited with  impatience for the winter;
 and in December 1761, he made  the decisive experiment, in
 a large hall adjoining to his college rooms, expending on the
 ice during its liquefaction, and obtaining from the water du-
 ring its congelation, as much  heat as would have raised the
 water somewhat more  than 140 degrees in its temperature.
   But I must observe here, that this experiment, so anxiously
 longed for by Dr.  Black, only  served to  inform him how
 much heat was thus absorbed by the ice, retained by the water
 while fluid, and emitted by it in the act of freezing.   But he
 had already full conviction of the truth of the doctrine, by-
 reflecting  on the observations of every day of frost or thaw^
 Since a fine winter  day of sunshine did not at once clear the
 hills of snow, nor a frosty night  suddenly cover  the ponds
 with a thick  cake of ice, Dr.  Black was already convinced
 that much heat was absorbed and^ fixed in the water which
 slowly trickled from the wreaths of snow,; and on the other
 hand, t*hat much heat emerged from it while it was as slowly
 changing into ice.  For, during a thaw, a  thermometer will
 always sink when removed from the air into"melting  snow;
 and during severe frost, it will rise when plunged into freez-
 ing water.  Therefore, in the first case, the snow is receiving
 heat, and in the last, the water is allowing it to emerge again.
 These were most unquestionable inferences, from observations
 the most familiar;  and they now appear most obvious and
 easy:  yet, before Dr.  Black, no person seems to  have made
 them.   Fahrenheit, Boerhaave, Mairan, De Luc,  and all the
 inquisitive meteorologists of the two preceding  centuries,
 though incessantly  contemplating and employing the same
 facts in their disquisitions, never mention having  had  such a
 thought; nor is a trace of it to be seen in the laborious collec-
tions of that unwearied  compiler,  Professor Muschenbroeck.
 It is the undivided property of my ingenious and  acute pre-
 ceptor. 
 xxx                   PREFACE.

   Philosophers had long been accustomed to consider the ther-
 mometer as the surest means for detecting the presence of heat
 or fire in bodies, and they distrusted all others.   Yet this in-
 strument gave no indication of the presence of these 145  de-
 grees of heat in the water.  Dr. Black therefore said that the
 heat is concealed in the water..../a^; and he briefly expressed
 this fact, by calling it  concealed or latent heat.  The epithet
 expressed purely and accurately  the very .circumstance  he
 wished to express, and he could not have pitched on one more
 proper. Yet even this unexceptionable epithet was sometimes
 misunderstood;  and  latent heat was spoken of as  something
 different from other heat.   But this  proceeded from mere in-
 attention.
   Dr.  Black having established  this discovery in the most
incontrovertible manner, by simple and decisive experiments,
drew  up an account of the whole investigation, and the doc-
trine, which he founded on  it, and read it to a literary society
which  met every Friday in the Faculty room of the college,
consisting of the members of the  University,  and several
gentlemen of the city, who had a relish for  philosophy  and
literature.  This was done  April 23, 1762, as appears by the
 registers of the society.
   Dr.  Black quickly perceived the vast importance of  this
 discover)7; and took a pleasure in laying before his  students a
view of the extensive and beneficial  effects of this habitude of
heat in  the economy of nature.  He made them  remark how
by this  means there  was accumulated, during the summer
 reason,  a vast magazine  of heat,  which, by gradually emerg-
ing, during congelation, from the water which covers the face
of the earth,  serves to temper the deadly cold of winter. Were
it  not for this quantity of  heat,  amounting to 145 degrees,
 which emerges from every particle of water as it freezes, and
 which diffuses itself through the  atmosphere, the  sun would
 no sooner go a few degrees to the'south of the equator, than
 we should feel all the horrors of winter.
   His thoughts on this combination  of heat were not confined
 to the simple case of water, but extended to every phenomenon
 • f congelation and liquefaction, not even excepting the changes 
PREFACE.
xxxi
 vhich are effected by the functions of animal and vegetable  &?■*.
 life.  He conceived the accretion of solid matter as a source    **
 of a part at least of the warmth of animals.
   His thoughts running in this manner over every phenome-
 non of composition and resolution of heat, he had long found
 reason to suspect the legitimacy of the measures of heat given
 by  the  thermometer.   He had  made experiments on some
 bodies which change their mode of aggregation from solid to
 fluid, and the  contrary, not all at once, but by imperceptible
 degrees; and  had found that, in the whole of this change,
 they  were absorbing heat.   Resin and sealing wax are vf ry
 clear examples of this.  A  thermometer plunged into resin
 in very  thin fusion, and compared with another standing in
 oil in the same vessel, shewed, from the  very beginning, a
 very  different  progress  of refrigeration.   We are not certain
 that  something of the same kind does not happen in water
 and other fluids.  They may have different degrees of fluidi-,
 ty in  different temperatures,  although we have no way of dis-
 covering them, so fluid do they seem in all  temperatures.
   Such were the surmises which made Dr. Black think it ne-
 cessary to examine the  scale of the thermometer, in order to
 learn whether equal differences  of expansion corresponded to
 equal additions or abstractions of heat.   He made a series of
 experiments on this subject, and read an account of their re-
 sult in the literary  society above mentioned,  on the 28th of
 March, 1760.
  The result of this inquiry was, that equal additions or ab-
 stractions of heat produced equal variations of bulk in the
 liquor of the thermometers employed by him,  and therefore
that the scale of expansion was also a scale of heat.  Dr. Black
did not know, at that time, that the celebrated mathematician,
Dr. Brooke Taylor, author of the Method of Increments^  had
had the same doubts respecting the thermometric scale,  and
had examined it by the very same experiments.
  These surmises and doubts about  the truth of the thermo-
metrical indications, arose entirely from the notion which  was
floating  in his  mind about liquefying heat,  and from the par-
tial and incomplete  nature of his occasional experiments on 
xxxii
PREFACE.
 melting and congealing bodies.  Much about  the same time,
 Mr.  De  Luc entertained similar suspicions ;  but they pro-
 ceeded from considerations altogether different;  from doubts
 about the equableness of expansion by  equal variations of
 temperature  in short, from  the  same doubts thai had oc-
 curred long  before  to Brooke Taylor,  and to  Rhcnalciinl
 These were the thoughts of a philosopher  interested only in
 an instrument of research.  But the result of his scrupulous
 examination of thermometers was most unexpected and im-
 portant ;  for, without this information, naturalists were liable
 to enormous mistakes in their judgments of temperature.  Dr.
 Black's surmises about the thermometric scale were those ot
 a chemist, studying the nature of fluidity.
   There is such an analogy between the cessation of thermo-
 metric expansion,  during the liquefaction of ice, and during
 the conversion of  water  into steam, that Dr. Black had no
 sooner explained the first of those anomalies,  than he felt in
 his own mind that all his former conjectures  about a variety
 of phenomena in the  boiling, and even in the gentle evapora-
 tion of fluids, were well founded; and he was persuaded that
 in the same manner as ice, in liquefaction,  requires the  com-
bination of a great quantity of heat, in order  to form water,
so water, in order  to  its conversion  into  steam,, also requires
another combination with heat, in  an  unknown  proportion.
When he considered the slow production of steam, notwith-
standing the continued heat of glowing fuel in contact with
the vessel....the scalding power of steam....and the great htat
raised in  the refrigeratory of a still....he was so much con-
 vinced of the perfect  similarity of Nature's procedure in both
 cases, that he taught  this docrine, in his lectures in 1761, be-
 fore he had made a single experiment on the subject;  and he
 explained with great facility  of argument,  many phenomena
 of nature  which result from this vaporifc combination of heat.
 This  must not be considered as unwarrantable or hypotheti-
 cal.   It resulted from a careful study of those facts which the
 operations of nature continually presented to him. He saw no
 occasion  for more experience for establishing the fundamen-
 tal proposition.   I have some notes taken in the class this 
                                                      /
                      PREFACE.                  xxxiii

 session, by a nobleman  eminent for his science and learning,
 by which it appears that Dr. Black had brought his thoughts
 on this subject to full maturity, and that nothing was wanting
 but a set of plain experiments, to ascertain the precise quanti-
 ty of heat which was combined in steam, in a state not indi-
 cated  by a thermometer,  and therefore latent, in the same
 sense that the liquefying heat is latent in water.
   Whoever thinks seriously on the many interesting objects
 which chemistry presents on every hand to the man of philo-
 sophical curiosity....how many things appear, of which we can
 give no  account, and yet are  of extensive influence1 in many
 departments of the science....and who reflects on the feelings
 of an ingenuous and honorable mind, engaged by duty to give
 instruction on all these particulars....will not wonder that Dr.
 Black did not immediately support his doctrine by an appara-
 tus of experiments, which should fix every  point, and leave
 nothing  to be added by  others to a discovery so new, so cu-
 rious,  so important,  and  so  intimately connected with  his
 theory of  liquefaction.   But  Dr.  Black was. not  a trader in
 science....nor had he any strong incitement Trom literary am-
 bition, to make him  neglect, either his  system of lectures,
 every one  of which required his earnest study, or his patients,
 whose cases at all times filled him with anxiety and solicitude.
 It was late before he had made such experiments  as satisfied
 him,  in  respect to the precise quantity of the heat latent in
 steam....not till the summer of 1764.  But he had not rested
 all this while  satisfied with mere conjectures.  He thought
 that what we  may observe every day is sufficient for settling
 the main questions; for we know that  the temperature of a
 fluid rises by  every addition  of heat, till it begins to boil....
 after which it rises no more, let it boil ever so violently. We
 know that steam, though so powerfully scalding, is  no  hotter
 by a thermometer than  boiling  water.   Therefore the heat
which enters the water while boiling, is either lost altogether,
or is concealed from the thermometer....latent in the steam.
 And lastly, the  observed  scalding power of steam, and the
heat which it imparts to the worm tub of a still,  are sufficient
proofs that this heat is really contained in the steam, and may
    VOL.  I. j                  E 
xxxiv                 PREFACE.
 be brought out of it again by reconverting it into boiling hot
 water.  Dr. Black had verified all this, and narrated his ex-
 periments in his lectures.  The experiments were of the most
 simple kind,  and the inference from them most obvious and
 satisfactory.   He observed that whatever time was employed
 to heat water  from its ordinary temperature  (about 50°),  to
 the boiling temperature (212°), the same fire must be applied
 five times as long, in order to convert it all into steam. Hence
 he might  infer that the steam had carried off 810 degrees  of
 heat.  He found, on the  other hand,  that a pound of water
 passing along the worm of a still in the form of steam, com-
 municated 20 degrees of heat to 40 pounds of water in the
 worm tub.  Hence he inferred that the steam had given out
 800 degrees of heat.
   Fortunately for Dr. Black, and for the world, he had now
 gotten a pupil  who was as keenly interested  in this scientific
 question as the Professor.  This was Mr. James  Watt, then
 employed  in fitting up the  instruments in the M'Farlane  Ob-
 servatory  of the University  ; a philosopher,  in the most ex-
 alted sense of  the word, who never could be satisfied with a
 conjectural knowledge of  any subject,  and who grudged no
 labour or study to acquire certainty in his researches.   He
 chanced to have  in  his  hands, for  repairs,  a  model of
 Newcomers steam-engine,  belonging to the Natural Philo-
 sophy Class, and  was delighted with the opportunity which
 this small machine gave him for trying experiments connect-
 ed with the theory of ebullition, which he had just learned
 from Dr. Black.  These he prosecuted  in a most happy train
 of success, and did not stop, till his steam engine was render-
 ed more like the most docile of animals, than a frame of life-
less matter; so that, while  its power  is competent to the lift-
ing a house from its place, a child of ten years old shall, with
a touch of his hand, make it go fast or slow, forwards or back-
wards, and act either powerfully or feebly.  This gentleman
attached to Dr. Black by  every tie of respect, esteem,  and
affection, supplied him wtth proofs and illustrations in abund-
ance, of all the points on which the professor wanted informa-
tion.  These were  always recited in the class, with the most 
                      PREFACE.                  xxxv

cordial  acknowledgement of obligation to Mr. Watt.  Mr.
Irvine also, a young student of medicine, of a quick apprehen-
sion,  sound, understanding,  and particularly disposed to con-
sider every thing mathematically, was at this tin.e a hearer of
Dr. Black's lectures, and greatlv captivated with chemical sci-
ence.  He engaged, with .great pleasure and zerl, in all ex-
aminations which seemed to interest the Professor, and par-
ticularly such as would admit of mathematical consideration ;
thermometrical experiments on the scale of heat;....on the
connection between expansion and variation of teinperature ;
....on the measures of heat, &c. ;....all these were fields ot re-
search altogether to his mind.  Hefsupplied Dr. Black with a
a vast number of experimerits*on the*equilibrium of heat, on the
specific heats of different substances, and on the continued ab-
sorption end fixation of heat by glass, sealing-wax, resin, and
other substances, which gradually become more fluid. The re-
gister of these experiments are in my possession, and are similar
to those which now fill many p'ages of the  Memoirs of the fo-
reign Academies. I think it my duty now to call upon his son,
who, I am informed, inherits much of his  father's philosophi-
cal spirit and ingenuity, to look over his papers, and see whe-
ther any of them have been put into a state fit for public view,
being confident that the  studies  of such a man as Dr. Irvine
must have been extremely ingenious and important.
   It was with Mr. Irvine's  assistance that Dr. Black made
his first experiments for measuring the latent heat of steam.
He found it to be as much as would raise seven or eight hun-
dred times as much water, one  degree in its temperature by
Fahrenheit's scale ;  which fact Dr. Black expressed by say-
ing that steam contained 700 or 800 degreessof heat latent
in it.  Afterwards, with the assistance of Mr. Watt, and bet-
ter apparatus, he found that the  latent heat of steam was not
less  than 850,  and sometimes  very  considerably exceeded
this, being so much the more as the  pressure of the atmos-
phere diminished.
  Thus was established another law of nature, of most exten-
sive and important influence in the train of changes  that are
going on around us.   Here we observe  another combination 
xxxvi                PREFACE.

of h,at or fir?, .he * agent by whose operation all these
changes are aff.ceo.   Heat, or the cau-  h£ «™
to p., on a real form, aod ,. ™ ^ mL matter may be
•*as a mere condition or state, into wmcn
brought; as noise or sound is known to »^e«^^
tainlndulaung or  tremulous  motion of air, or other elastic
mattf .  But we now see heat susceptible of fixation, of being
accumulated in bodies,  and, as it were laid by, nil we have
occasion for it; and we are as  certain of get mg the stored-up
heat out of the  steam or the water, by changing  them into
watcT or ice, as we are certain of getting out of our drawers
the things we laid up in them.
   The influence of this last" combination of heat is much more
extensive than appears in the experiments by which its reality
was estabhVncd. Dr. Black discovered that this absorption and
accumulation takes place, not  only in the boiling of all fluids,
and all conversions of matter  into strongly elastic steams, but
also in every case  of evaporation,  even the most  gentle and
 unperceived.   When the hand is dipped  into warm water,
 and then held up in the air till "the film of water adhering to
 it is dried off, we feel it remarkably colder than the other hand,
 exposed to the same air.   If  we dip one  finger into a glass of
 water, and another of the same hand into a glass of str.ng
 spirit of wine, and hold up the hand in the  air, the finger dip-
 ped into the spirit  is the  first dry, and till  it be dry, it feels
 remarkably colder than the other; but  now, the  other con-
 tinues the colder of the two, till it also be perfectly dry.
    Here v.-^e can also trace another magnificent train of changes,
 which are nicelv accommodated to the wants of the inhabit-
 ants of this globe.  In the equatorial regions, the oppressive
 heat of tne sun is  prevented from a destructive accumulation
 by copious evaporation.   The waters, stored with their vapo-
 rific heat, are thus carried aloft into the atmosphere, till the
 rarest of the vapour reaches  the very cold regions of the air,
 which immediately forms a small portion of it into a fleecy
 cloud.  This also further tempers  the scorching  heat by its
  opacity, performing the acceptable office of a screen.   From
  thence, the clouds are carried to the inland countries, to form 
PREFACE.
xxxvii
the sources in the mountains, which are to supply the number-
less streams that water the fields.  And, by the steady opera-
tion of causes which are tolerably uniform, the greater part of
the vapours pass on to the circumpolar regions, there to de-,
scend in rains and dews;  and in this beneficent  conversion
into rain by the cold of  those regions, each particle of steam
gives up the 700 or 800 degrees of heat which were latent in
it.  These are  immediatelv diffused, and soften the rigor of
those less comfortable climates.
   Surely then, these two chemical laws of nature are curious,
of extensive influence, and of mighty importance.  The dis-
covery, and the satisfactory establishment of them, were tides
to fame and honor,  and the name of Dr. Black should hjfe
now been familiar among the philosophers of Europe.  About
this he gave himself little concern, and was  perfectly satisfied
when he saw  that his  pupils understood  the doctrine as de-
livered in the lectures.   One thing indeed gave  him much
satisfaction.  Mr. Watt, whose worth  of heart was now as
well known to Dr.  Black as his excellent understanding, and
who was become his intimate friend, had  obtained his Majes-
ty's patent for the improvement which he had  made on  the
steam-engine, by his judicious application of Dr.  Black's in-
structions, and was  now in the straight  road  to  riches and
fame.   Dr. Black would scarcely have been more gratified,
had those advantages  accrued to himself.  Their joint studies
had brought to light many unnoticed properties of vapours ;
and I believe that both the friends  considered this period of
successful investigation  as  among the most fertile of enjoy-
ment of any part of their lives.   I had the pleasure of witness-
ing some  of their inquiries, and sharing iivthe knowledge re-
sulting from them ;  and to me  also this period is matter of
the most pleasing recollection.
   Meantime, Mr. Watt's engine became very gene*-lly known
through Europe. Its immense superiority, in respect* power
and economy, offered to the busy part of society a most cer-
tain and powerful  first mover for all machinery; and thus
attracted the attention of all those engaged  in the great busi-
ness of making money.  It was this, more than all  the love of 
xxxviii                PREFACE.
 knowledge, so boldly claimed by the eighteenth century, that
 spread the knowledge of the doctrine of latent heat, and the
 name of Dr. Bfack. Chemists, mechanicians, meteorologists,
 manufacturers, all took an interest in it; and publications and
 plagiarisms multiplied apace. These were totally disregarded
 by the unambitious author, who, in the meanwhile, was happy
 in the success of his  friend, and in the thoughts of having
 exerted his own talents so  usefully for the public.
    Dr.  Black  continued in  the  university  of Glasgow from
 1756 to 1766, much respected as an eminent professor, much
 employed as an able and most attentive physician, and much
 b^oved as an amiable and accomplished gentleman, and happy
'i|pthe enjoyment  of  a small but a select society of friends.
 Often, however, have I seen how oppressive his medical du-
 ties were to his spirits, when he  saw that all his efforts did not
 alleviate the sufferings of the distressed.  When his clear friend
 Dr. Dick was carried off, Dr. Black's distress was indeed ex-
 ceedingly great, particularly, as he thought that another mode
 of treatment might have been more successful.
    In the mean time, his reputation as a chemical philosopher
 was every day increasing; and pupils from foreign countries
 carried  home with them the peculiar doctrines of his courses;
 and fix^P#.r and latent heat began to be spoken of among the
 naturalists on the Continent.  The progress however of this
 diffusion of knowledge must have been slow, had things  con-
 tinued in the same train.  But in 1766  Dr. Cullen, chemical
 professor in Edinburgh, was appointed professor of medicine,
 and thus a vacancy was made in the chemical chair of that
 tJniversity. There was but one wish with respect to a successor.
 Indeed, when the^acancy  happened in  1756, by the death of
 Dr. Plummer, tm reputation of young Black was so high, as
 a person notdnly ingenious and  inventive, but singularly cor-
 rect anrllfl&teal in his manner  of thinking and writing, that,
 had tmf"JFoice  depended  on the university, the newly gra-
 duated  physician  would have  been professor of  chemistry.
 He had now, in 1766, greatly added to his claim of merit,"by
 his more important discovery of the procedure  of nature  in
 producing fluidity and vapor ;  and he had acquired the  high 
PREFACE.                  xxxix
    esteem of alh, by the singular moderation and scrupulous cau-
    tion which marked "all his researches.
       Such'a man was of the highest value to a celebrated seminary
    of learning.   Ingenious men, of a fertile and lively imagina-
    tion, are but too apt to give a loose to their fancy, in forming
    wide-grasping theories, and dressing them  out in  specious
    attire.  The young student, ardent and  credulous, is dazz&d
    by what appears a strong and wide-spreading light, no£e-
    marking that perhaps it is not  the  natural emanation froWa
    luminary,  but is artificially collected  by  mirrors  andglassjs;
t   or that what he takes for real objects are  only the shadowy
    representations by a magic lanthorn.  To this, in a great mea-
    sure, may we ascribe the continual flux of theory which may
    be observed  in, all  universities.   Yet  the consequences to
    science are most unfortunate. Not only do the precious years
    of youth and of mental energy pass on without solid instruc-
    tion, but also the most  unfortunate of all habits  is acquired,
    that of considering the extensive and plausible application of.
    a theory to the explanation of phenomena as a valid proof of
    its truth. But, on the other handj the lectures of such a teacher
    as Dr. Black,, never  permitting  this play of fancy, and even
    rarely  introducing conjecture, would be safe lessons for inge-
    nuous  youth.  The affirmations of the  professor maybe trusted
    as matter of experience, and the student  will acquire  betimes
    the habit of never proceeding, in research of  any kind, with-
    out sounding the channel as he advances.
      Dr.  Black, was  appointed to the chemical  chair at Edin-
   burg, to the general satisfaction of the public $  but the univer-
   sity of Glasgow thence sustained an irreparable loss.   In this
   new scene, his  talents were more conspicuous, and more ex-
   tensively useful.  He saw this ; and while he could not but be
   highly  gratified  by the great concourse of pupils, which the
   reputation of the medical colleges of Edinburgh brought to
   his lectures, his mind was forcibly  impressed by the impor-
   tance of his duties as their teacher.  This  had an effect, of
   which it is  difficult to say whether  it  has been fortunate for
   the public or not.  Dr. Black now formed the  firm resolution
   of directing his whole study to the improvement of his scholars 
xl                   PREFACE.
in the elementary knowledge of chemistry.  He saw too many
ofjnem with a very scanty stock of previous learning,  tie
h$ many from the workshop of the manufacturer, who had
ntfne at all; and he saw that the number of such hearers must
increase  with the  increasing activity and  prosperity  of the
   intry : and these appeared to him as by no  means the least
   Want part of his auditory.  To engage  the attention of
   -pupils, and to be perfectly understood by the most ilhte-
   F was therefore  considered by Dr. Black as h.s most sa-
   1 duty.  Plain  doctrines, therefore, taught in the plainest
 . Jner, must employ his chief study.  That no help may be
wanting, all  must be  illustrated by suitable  experiments, by
the exhibitions of specimens, and the  management of chemi-
cal ^processes.   Nice and   abstruse  philosqphical opinions
would not interest such hearers ; and any doctrines inculcated
in a refined manner, and referring to elaborate disquisitions
of others, would not be understood by the major part of an
audience of  young persons,  as  yet  only beginning their
studies.
   To this resolution Dr. Black rigidly adhered,  endeavor-
ing every year to make his courses more plain and familiar^
and illustrating them by a greater Variety-of  examples in the
way of  experiment.   No man could perform  these more
neatly and successfully.  They were always ingeniously and
judiciously contrived, clearly establishing the point in view,
and never more than sufficed for this purpose.  While he
scorned the quackery of a showman, the simplicity, neatness,
and elegance,  with which they were performed, were truly
admirable.   Indeed, the simplex munditiis'* stamped every
thing that he did.  I think it was  the  unperceived operation
of this impression that made Dr. Black's lectures such  a treat
to all his scholars.   They were not only instructed, but (they
knew not how) delighted;  and without  any effort to  please,
but solely by the  natural emanation of a gentle  and  elegant
mind, cooperating, indeed, with a most perspicuous exhibi-
 tion of his sentiments, Dr.  Black became a favorite lecturer;
 and many were induced, by the report of his students, to at-
 tend his courses, without having any particular relish for che- 
PREFACE.                    xli
 mical knowledge, but merely in  order to be pleased.  This,
 however, contributed greatly  to the extending the knowledge
 of chemistry ; and it became a fashionable part of the accom-
 plishment of a gentleman.
   In the mean time, the path which had been opened by Dr.
 Black to a new province of chemical research began to be fre-
 quented by men of science in various parts of Europe.   It
 was not only a most unexpected and curious thing to find that
 a matter so solid and impenetrable as marble could appear in
 the form of air, and this air be again put into our hands in the
 form  of marble ; but this new acquaintance had properties
 which forcibly called for the most serious attention.  This air
 can be«poured from one jar into another, like as much water;
 and when it is poured out on a candle, or even on a fire in suf-
 ficient quantity, they are extinguished in an instant, as if water
 had been poured on them.  But further, should a man take
 one full inspiration of this air, this inspiration will be his last;
 he expires without a groan.   In short, this is the deadly va-
 pour which has often produced fatal effects in our mines.  Be-
 ing much heavier than common air, it glides downwards,  col-
 lects in the drifts and lower parts of the mine, and sometimes «
 is so copious as to fill them, and even to rise  to a considerable
 height in the shaft.   The unfortunate miner, let down  by a
 rope to his work, as soon as his head gets under the surface
 of this fluid, drops off without any  warning, and is dashed to
 pieces, or suffocated by this choke-damp.   But we have now
 learned its property of  extinguishing flame; and  it is usual,
 before the workmen go down, to let down a  choffer  of burn-
 ing coals, which  are extinguished as soon as they enter  the
 fixed air.  I may add, that it is this fluid that has long drawn
 the curious traveller to the Grotto del Cane in Italy, so called
because a dog falls down dead as soon as he reaches the mid-
 dle of the grotto.
  On the other hand, it has been discovered that this very air,
so fatal, when applied to the nerves of the breathing organs,
is most salutary, when received into the stomach; and, as an
external application, it is most powerful in healing ulcerated
wounds, cleansing foul sores, and in general, counteracting all
   vol.  i.           ,         i" 
xlii
PREFACE.
 tendency to gangrene or putrefaction in the disorganised parts
 of the animal frame.   Water impregnated with this air allevi-
 ates one of the most  excruciating of human sufferings, the
 pain of the stone or gravel.
  It has  also been discovered that this air, so destructive and
 so salutary, is forming in vast quantities every moment around
 us.   Dr.  Black discovered that the breathing of animals chan-
 ges common air into fjb:ed air; and that this change is accom-
panied bv the emersion of heat; which emersion seems to be
the  principal source jk the heat generated in the bodies of all
breathing animalsjjrAnother most copious source of fixed
air is the  combustton of nine-tenths of all inflammable bodies.
Every thing, which, in the course of burning, suffers the change
which we call charring, changes common air into fixed air by
burning in it.  In  this way does it sometimes happen  that
persons have been found lifeless, who  have  been shut up in a
close room with a charcoal fire.   Lastly, another abundant
source of fixed air is the working of  fermenting  liqours, as
they are  ripening into wine or other intoxicating beverages.
 The froth  and foam continually rising from such liquids is
one of the purest kinds of fixed air.
  Surrounded, then, as we are by sources which are continu-
ally pouring in upon us this powerful substance, it behoves us
 to be on our guard;  and we are highly obliged to him  who
 gave us the means of detecting, a method of removing it, and
 methods for procuring it, when we would avail ourselves of its
 salutary powers.  Somewhat of all this had been known be-
 fore 1756.   The ingenious researches and experiments of Dr.
 Hales had occasionally called our attention to some of the
 modes of the production of fixed air; but they appeared as sin-
 gularities, insulated facts not connected  in the general  eco-
 nomy of  nature, or, at least, the connection was not observed.
 But now, having learned a sure and easy test of the presence
 of this kind of air, and understanding some  of the  means by
which  nature  hold- it accumulated in such vast abundance
 and some of the methods for setting it at liberty, so as to ob-
 tain it by itstlf, in, circumstances which lay  it open  to o ir ex-
 amination, .the chemists were  enabled to detect it in almost 
                       PREFACE.                   xliii

 every body that could be presented to them, and  were busy
 in scrutinizing every substance with this view.  They were
 particularly curious to examine every elastic eruption that
 they observed, on account of its resemblance  to those  erup-
 tions in which fixed  air is extricated.  The effervescence of
 metals during their solution  in acids resembles so much the
 effervescence of acids and alkalis,....the frothy ebullition  of
 some putrescent mixtures resembles so much the fermentation
 of wines and worts, that curiosty led imrtrediattdy to the ex-
 amination of the elastic matter which was extricated on those
 occasions.  These airs, however, were found altogether dif-
 ferent from fixed air;  but this circumstance only fired the cu-
 riosity of the inquirers so much the more, and incited them to
 multiply experiments, and examine every body, in order to
 extricate or to create some elastic matter for a new subject of
 experiment.
   Thus arose a new species of chemistry, chit fly conversant
 with a rial fluids, having an apparatus and manner of proceed-
 ing altogether peculiar to itself, and so unlike all that we are hi-
 therto acquainted with, that ^very thing may be said to be big
 with curiosity and with novelty.^ This department of chemi-
 cal science got the  name of-rPNEUMATic*Chemistry.
   Of all  those who were  occupied in these rtstarches, the
 most eminent were Dr. Priestley and Dr. Scheele. Dr. Priest-
 ley, by the number and variety of his experiments,  and the
 substances he  discovered; and Dr. Scheele, by the ingenuity,
 and the unwearied  patience with which he examined the in-
 dividual novelty which engaged his attention, and  the  saga-
 city with which he contrived his experiments, so as to* lead
 him with certainty  to  some important result,  to be added to
 our former stock of chemical knowledge.  These two philojl
 sophers, unknown  tJ each other, discovered  the  same  sub?*
 stances ;  substances which  were acting the most important
 parts in the great operations of nature. Both of them discov-
 ered that a real fluid which alone sustains the life of breath-
 ing animals, and which alone supports inflammation and  com-
 bustion ; for which reasons it has been called vital air and em-
pyreal air* Both of them also discovered another species of 
xliv                   PREFACE.
 air, much more abundant, and indeed proved by Scheele to
 form the chief portion of our atmosphere.  He called it Jou
 or putrid air, having discovered it first in the putrescent fer-
 mentations.   Dr. Priestley procured it from much less offen-
 sive  materials, and  called  it phlogisticated air.   It  extin-
 guishes life and fire as certainly as the fixed air of Dr. Black
 does, but is quite a different, substance:  Scheele first demon-
 strated that our atmosphere is a mixture, but not a compound,
 of this  and  of vital air.   Dr. Priestley also discovered some
 of the means  by which nature removes all the taints  which
 are occasioned in the atmosphere by the breathing of animals,
 the burning of fuels, and the fermentation of bodies, shewing
 that these corruptions are combinations of vital air with other
 elastic  matters, and that all of them are again decompounded
 in the process of vegetation, so that the plants restore to us the
 vital air in its original purity ; thus  accomplishing one of the
 grand and beneficent circles of natural operations.
    Such  discoveries  necessarily gave a dignity to pneumatic
 chemistry,  which sets it very high in the rank of natural sci-
 ences.  It is.no lonjg;egtconfinedi||^the study of those proper-
 ties of bodies which vnake'tftfcm ftie^ubjects of human art, by
 which they are worked, up  for our* purposes.   We are now
 admitfce'cf into the' laboraWy of nature herself, and instructed
 in some of- those great  processes by which the author of this
 fair  worid makes it a habitable  place,  and  a never-failing
 source of life and enjoyment, by a circle of beneficent changes,
 in whk& the same materials are  made the means of enjoy-
 mejitJ^^iccessive races of inhabitants, and  are  again and
 aganvpr,esented in their original purity and usefulness.
    So captivating to every mind of sensibility, it was no won-
    r that pneumatic  chemistry became  a  very general study,
.^and engaged the eager attention of the most accomplished in
  acquirements, and the most eminent in the ranks of society.
  In Germany, in Italy,  in Britain, in France, it found cultiva-
 tors in every class of  society.   The a'^real fluids having now
  become as familiar, and as easily  managed,  as the tangible
  substances, which we are accustomed to hammer, to grind, to
  dksolvv. aud distil, they were mixed Und subjected to all the
 
                       PREFACE.                    xlv

torturing degrees of heat, and passed from  retort to retort,
and, in short, were examined in  every way that imagination
could suggest.   Such a pertinacious scrutiny of nature could
not fail of bringing many things to light.   The  honorable
Mr. Cavendish has discovered that aquafortis consists of two
kinds of air;  of vital air, the support of life and of fire, and
mephitic air,* which extinguishes both.   And he discovered
that water, which,  since the first dawn of natural philosophy,
has been considered as an element,  is also composed of vital
air, and of that air which sometimes takes fire in our coal pits,f
and lays all waste by its explosion.   Others have discovered
that salt of hartshorn consits of inflammable and mephitic air;
nay, that fixed air itself consists  of diamond dissolved in vi-
tal air.
  In the midst of this ardor of research, and this rich harvest
of discovery,  Mr.  Lavoisier appeared,  and took  an active
share ;....not hunting after new  substances, he considered
those already known, with more  sagacity than the  multitude
busy in the chace.  He thought that the  chemical relations of
various substances had been  mistaken by all; that we  hold
many bodies as simple, of which we can shew the composi-
tion, and those to be compounded which are really more  sim-
ple.   Thus, sulphur, which the chemists, ever since the  days
of Stahl, have supposed to consist of vitriolic acid, and that
matter  which  imparts inflammability to bodies,  was proved
by Lavoisier to be more simple  than the acid, and that this
acid was in fact composed of  sulphur  and vital air.   He
proved  that in the phenomenon which  we  call  combustion
or inflammation, the only tiling  of which we are  absolutely
certain is the combination of the  inflammable body with  vital
air ; and that,  by separating this air from it again, the  body
regains its primitive form, and is again  inflammable, that is,
again capable of uniting, in a particular way, with vital air.
Reflecting now on the two cases in which Dr. Black had dis-
covered a combination of tangible matter *.vith fire, in such a
way as not  to  be discovered by the temperature,  but only by
the liquid or the vaporous form which it causes the substance
         * Or azotio gas.            t Kyui.'i^nous gras. 
xlvi
PREFACE.
 to assume, he asserted the reality of a third  combination of
 tangible matter with heat, to be added to those discovered by
 Dr. Black : a combination, which was not to  be changed by
 the  mere, contact of a sufficient quantity of any cold matter,
 but require^ the contact of another substance, properly related
 to vital aifin  the way of chemical affinity.  Heat combined
 in this manner renders a fluid a real, or permanently elastic,
 and  no longer condensible like watery vapors.   These  com-
 pounds he denominated gases.  Of this kind  are  all the airs
 lately discovered.   Lastly, as. the chief point of his doctrines,
 he affirms that the  light and heat  which appear in the  com-
 bustion, are ingredients of the vital air, detached from it, and
 from it alone,  when its  ponderable  part  combines  with the
body that we call inflammable.
  The doctrine  is not altogether new.   Dr. Robert Hooke,
 one of the first members, and the brightest ornaments of the*
 Royal  Society of London, published the same  doctrine in his
 Micrography,  1G65; and Dr. Mayhow of Oxford entertained
 opinions extremely  similar ;....Rey, a French chemist of that
 time, had similar conjectures.  But by some  unaccountable
 fatality, these publications were forgotten.
  This doctrine concerning combustion,  the great, the  cha-
 racteristic phenomenon of chemical nature, has at last receiv-
 ed almost universal adoption, though not till after considera-
 ble hesitation and opposition; and it has made  a complete re-
 volution in chemical science.
  But it is thought that the cultivation of pneumatic chemistry
has given us even more important information ; and it promises
 acquisitions of knowledge of a still more elevated rank ; it pro-
mists admission to the more mysterious operations of nature
 in the functions of vegetable and animal life.  For, by the pneu-
 matic anah sis, to which organized  bodies have been subjected
 by some of the eminent chemists of the present  day, it appears
 that they consist of a very small number of simple substances,
 capable of the a\real form, but existing in the plant or animal
 in a liquid or a concrete state, in consequence of being differ-
 ently combined or  associated. * The  laws of many of those
 affinities have been discovered; and it is thought that we per- 
PREFACE.                 xlrii
 ceive, in many cases, how, by a change of temperature, or by
 presenting  another  ingredient,  these combinations change,
 new forms  are assumed, and the distinguishing products of
 animal and vegetable bodies appear.  It is thought that we
 perceive,  in some cases, how the functibns of the organized
 body produce this variation of temperature, or this change of
 situation among the ingredients, which occasions the chemical
 combination observed.
   On the other hand, we have now got some very distinct in-
 dications of the internal procedure of nature in those sponta-
 neous  fermentations which take place in the contents of  ani-
 mal and vegetable bodies independent of their vital functions
 and which at last destroy them, or reduce them to brute unor-
 ganized matter.  These changes are thought td proceed from
 small variations of temperature or position, which, by chang-
 ing the mutual forces of attraction, destroy that equilibrium of
 force which  maintains things in their present condition.   The
 particles change their partners, (so to express myself)  and in
 their new combinations, present to our view substances which
 were not existing before in the animal or vegetable matter.
 Thus does the sweet juice of the grape give us wine,  which
 did  not exist before....and this gives us inflammable spirits,
 which  did not  exist in  the wine, or gives us sour, mawkish,
 or foetid liquors or fumes,  none  of which  had any previous
 existence.
   Justly,  therefore, have I said  that chemistry has risen  to a
 high and unexpected rank in the  scale of science, important to
 society, by the vast additions which it has made to the  power
 of man, and precious to the philosopher,  by the connections
 which  it has brought into view between  the different agents
 in the grand  circle  of  natural  operations which  constitute
 growth, life, decay, and  final destruction.  I think myself
 equally entitled to say, that it was the two discoveries of Dr.
 Black....fixed air, and combined  heat, which gave the incite-
 ment, pointed out the road, and furnished the chief helps for
 pursuing it.
   It is reasonable to suppose that he took  an active part in
those keen researches, which have thus occupied the attention
of the  philosophers.  But alas!...." his lot forbade."   His 
xlviii
PREFACE.
  constitution  had  always  been exceedingly  delicate.   The
  slightest cold, the most trifling approach to repletion, imme-
  diately affected his breast, occasioned feverishntss, and, if con-
  tinued for two or three days, brought on  a spitting of blood.
  In this situation, nothing restored him to ease but relaxation
  of thought and gentle exercise.  The sedentary life to which
  study confined him was manifestly hurtful; and he never  al-
  lowed himself to indulge in any intense thinking, or puzzling
  research, without finding these  complaints sensibly increased.
    Thus situated,  Dr. Black was obliged to be contented as
  the  spectator  of the  successful labours of others.   So  com-
 pletely trammelled was he in this respect, that although his
 friends saw others disingenuous enough to avail themselves of
 the novelties announced by Dr. Black in his lectures, without
 acknowledging the  obligation,  and  were  thence  afraid  that
 their friend's claim  of originality and priority  might become
 doubtful; and although they repeatedly urged him to  publish
 an account of what he had done, this remained unaccomplish-
 ed to the last.   Dr. Black often began the task ; but  was  so
 nice in his notions of the manner in  which it should be exe-
 cuted, that the pains he  took in forming a  plan of the work
 never failed to  affect his health,  and  oblige  him to desist.  Of
 this I saw a most distinct instance,  when  his  dissatisfaction
 with the artful  conduct of  Mr. Lavoisier provoked him  to
 make an unusual exertion.*
  * I embrace this earliest opportunity that occurs to correct a mistake which
I have made in page     of Vol. II. where I say, " Nor is he named in those
" passages of the earlier dissertations where the character and properties of
" fixed air, and of mild and caustic  alkalis,  are treated of."  The words
" of the earlier dissertations," should not have been there.  The observation
relates only to the joint  memoirs of Lavoisier and La Place,  published by
the Royal Academy of Sciences.   Mr. Lavoisier, in his first publication of
Opuscles Physiques  et Chymiques, in 1774:, gives a brief account of all that
had been published or taught concerning quicklime, and fixed air, and effer-
vescence. And Dr. Black's performance has its share of his attention, and
his final approbation.  But I must say,  that it is with none of those expres-
sions of esteem and respect which Mr. Lavoisier professes to have always
entertained for the author.  Dr.  Black, at the  time that he was offended
with  Mr. Lavoisier's insincerity, knew perfectly that he had taken  notice of
his doctrines concerning quicklime, &c. in that early  dissertation:  but
the ground of offence was recent, and gross. 
                       PREFACE.                   xlix

   Dr. Black, therefore, devoted his whole time and attention
 to the communications which his pupils had a right to expect
 from him.   Moderate  in all his wishes, he was never anxious
 to bring  himself into view, unless the occasion  required his
 appearance.  His reputation naturally engaged him in an ex-
 tensive correspondence, he being often appealed to as a judge,
 and often consulted as a philosopher.  On  such  occasions,
 when he  could give his opinion without being obstrusive, (a
 thing which he detested) he was ever ready to communicate
 it, and to give everv useful  information....which he did with
 frankness and sincerity,  and with  the  most unaffected mo-
 desty.
   As to the manner in which Dr. Black acquitted  himself in.
 his public character of a professor, I need only  say that none
 contributed more largely to establish,  and' support, and in-
 crease, the high character which the University of Edinburgh
 has acquired.  His talent for communicating knowledge was
 not less eminent than for observation and inference from what
 he saw.   He soon became one of the  principal ornaments of
 the University;  and his lectures were attended by an  audi-
 ence which continued increasing from  year to year, for more
 than thirty years. It could not be otherwise.  His personal
 appearance and manner were those of a gentleman, and pecu-
 liarly pleasing.   His voice  in lecturing was low, but fine;
 and his articulation so distinct that he was perfectly well heard
 by an audience consisting  of several hundreds.  His discourse
 was so plain, and perspicuous, his  illustration by experiment
 so apposite, that his sentiments on  any  subject never could be
 mistaken,  even by the most illiterate; and his  instructions
 were so clear of all hypothesis or conjecture, that the hearer
 rested on  his conclusions with a confidence scarcely exceeded
 in matters of his  own experience.
   I have  already observed, that the strong sense which Dr.
 Black entertained of the duties of his professional situation,
precluded all extensive medical practise, which otherwise he
had every talent,  and every accomplishment fitted to insure*
He restricted his attendance as a physician  to a few families
of intimate and  respected  fritnds.   He was, however, " a
   VOL.  I.                   g 
1                    PREFACE.
" ph) sician of good repute,  in a place where  the character
" of a physician implies no common degree of liberality, pro-
" prietv and dignity of  manners,  as well as of learning and
u skill."
  Averse, by disposition, from ostenation,  or any inclina-
tion to obtrude his opinions, on the public, Dr. Black peculiarly
disliked appearing as an author.  His dissertation,  De  Acido
a cibis  orto, et de Magnesia, was a work  of duty, being his In-
augural Thesis.   His  Experiments on Magnesia, Quicklime,
and other alkaline substances,  printed soon after,  was almost
indispensably necessary for putting on a proper foundation
what was only indicated in his inaugural dissertation.   His
Observations on the more ready freezing of water that has been
boiled, published in the Philosophical  Transactions of  Lon-
don in 1774,  was  also called for;  and his Analysis of the
waters of some boiling springs in Iceland, made  at the request
of his friend T. J. Stanley, Esq. was read to the Royal Socie-
ty of Edinburgh, and published by the Council.  Dr.   Black
was perhaps fastidiously nice in his notions of a philosophical
performance, and too severe in his observations on the hurri-
ed  publications of some chemists, which he used to call slo-
venly, and to consider as literary manufacture for profit.   But
surely, ever)- man who gives the public a new  and important
fact, confers a public benefit; and when he adds his own re-
flections and opinions, he  only shews what have been his own
incitements to exertion.   Few persons are so insensible  to or-
dinary propriety,  as to make pretensions to authority;  and if
 they did, it would be disregarded, while  the philosopher would
 avail himself of  the  information communicated.   Had Dr.
 Priestly and  Dr.  Sch.de  been as fastidious as Dr. Black, we
 might at  this day have been still ignorant of  the chief doc-
 trines of  chemical philosophy.  But such  was  Dr. Black's
 aversion to all hypothesis and conjecture in any experimental
 science, that he  could not endure the title  of  a system to be
 given to  any body of chemical doctrines yet published; and
- he  did not call his own discourses Lectures on  Chemistry, but
 Lectures  on the Effects of Heat and Mixture; so  far did he
 think  all his endeavours were from forming a  system   of the 
                        PREFACE.                     li

  chemical department of science. In the last years of his life, he
  was convinced of the propriety of this scrupulousness, by the
  precipitancy with which he saw young men, who had scarcely
  left the forms of the school, publishing in all quarters of Eu-
  rope.   Intoxicated, as  it were, with the large draughts of in-
  formation afforded by  pneumatic chemistry, they think them-
  selves adequate to the task of giving a system of this almost
  boundless science,....a  system which shall not leave  a phe-
  nomenon unexplained;  arfd  they obtrude  these their crude
  conceptions on the public with most unbecoming confidence
  and authority.  He saw the public  pleased with this manner
  of proceeding, and far from being scrupulous about the solidi-
  ty of the  foundation, provided the structure be shapely and
  extensive.  He dreaded the consequences of this passion for
  theories.   He therefore resolved to abide scrupulously  by
  his first plan, which he  had adopted in the  hour of calm re-
  flection, and had  modelled as much  as he could on the rules
 of philosophising, so  warmly inculcated, and so scrupulously
 followed by the illustrious Newton.  Scheele, Bergmann, and
  Berthollet, were the chemkts of later times whom he thought
 most highly of, as Margraaf and Crammer were most admired
 by him among those of older date.   He corresponded occasi-
 onally with Seguin, and with Crell, who had been his pupil; but
 did not encourage much  intercourse of this kind, having found
 that his informations sometimes appeared in print as  the in-
 vestigations of the publishers.  He could not be engaged to
 transmit any essay to the Royal Academy of Sciences  at Pa-
 ris,  or the Imperial Academy  at St.  Petersburgh, of both of
 which he had been elected a foreign associate.
   Such was Dr.  Black,  considered  as a public man I wish
 that I  could as easily describe him in his private capacity,....at
 home, or in society,....as an acquaintance,  or a friend:  but
 this  requires a talent to which I have no pretensions.   It is
 not a very  difficult matter to draw a figure, which shall shew
with abundant accuracy, any peculiarity of a man's dress, and
perhaps even give somewhat of his air; but the delicate strokes
which mark his features, and shall make us know the man, re-
quire the hand of a painter. 
lii
PREFACE.
   I have already observed, that when I was first acquainted
 with  Dr.  Black, his aspect was comel>  and interesting.   As
 he advanced in years, his countenance continued to preserve
 that pleasing expression of inward satisfaction, which, by giv-
 ing ease to the beholder, never fails to  please.  His manner
 was perfectly easy,  and unaffected, and graceful.  He was of
 most  easy approach, affable and readily entered into conversa-
 tion, whedier serious or trivial.   Hio  mind being abundantly
 furnished with matter,  his conversation was at ail times perti-
 nenc and agreeable: for Dr.  Black's acquirements  were not
 merely those of a man of science.   He was a stranger to none
 of the elegant accomplishments of life,   tie  therefore easily
 fell into any topic of conversation, and  supported his part in
 it respectably.  He had a fine or accurate musical ear, and a
 voice which would obey it in the most perfect manner; for he
 sung, and  performed on the flute, with great taste and feeling;
 and could sing a plain air at sight,  which many  instrumental
 performers  cannot  do.  But  this was  science.  Dr.  Black
 was a very  intelligent judge of musical  composition;  ana I
 never heard any person express  sp intelligibly the character-
 istic differences of some of the national musics of Europe.  I
 speak of  Dr. Black as I knew him at Glasgow :  after his
 coming to Edinburgh, he gave up most of those amusements.
 Without having studied drawing, he had acquired a consider-
able power of expression  with his pencil, both in figures and
 and in landscape.  He  was peculiarly happy in expressing the
passions;  and seemed, in this respect, to have the talent of a
history painter.  H - had not had any opportunities of becom-
 ing a  connoisseur; but his opinion of a  piece of painting, or
sculpture, was respected by good judges.  Figure, indeed,
 of every kind, attracted his attention ;....in architecture, furni-
 ture, ornament of every sort,  it was never a matter of indiffe-
 rence. Even a retort, or a crucible, was to his eye an example
 of beauty or deformity.   His memorandum books are full of
 studies (may I call them)of this sort;  and there is one draw-
 ing of an  iron-furnace,  fitted up with rough unhewn timber,
 that is finished with great beauty, and would not disgrace the
 hand  of a Woollet.  Naturally,  therefore, th? voung  ladies
i 
PREFACE.                      liii
were proud  of Dr. Black's approbation of their taste in mat-
ters of ornament.  These are not indifferent things ; they are
features of an elegant mind, and  they account for some part
of that satisfaction and pleasure which persons of all different
habits and pursuits felt in Dr.  Black's company and  conver-
sation.
  I think that I could frequently discover what was  the  cir-
cumstance of form, &c.  in which Dr.  Black perceived or
sought for beauty,....it was some suitableness or propriety;
and he has often pointed it out to me, in things where I never
 hould have looked for it.  Yet I saw that he was ingenious-
ly in the right.   I may almost say that the love of propriety-
was the leading sentiment of Dr.  Black's mind.   This  was
the first standard to which he appealed in all his judgements;
and I believe he endeavoured  to make it the directing prin-
ciple of his  conduct.   Happy is  the man whose moderation
of  pursuits  leaves this sentiment in possession of much  au-
thority.  Seldom  are our judgments greatly  wrong  on  this
question ; but we too seldom listen to them.
  Dr. Black had the strongest claim to the  appellation of a
man of propriety and correctness.  His friend Dr.  Fergu-
son knew him  well, and  can delineate his moral  features in-
finitely better than I can.   Dr. Ferguson says of him,....
  " As Dr. Black had never  any thing  for  ostentation,  he
" was, at all times, precisely what the occasion required,  and
" no more.   Much as he was engaged  in the details of his
" public station, and chemical  exhibitions, his chambers were
u never seen lumbered with books and papers, or specimens
" of mineralogy, &c. or the apparatus of experiments.  Nor did
" any one see Dr. Black  hurried at one time to recover mat-
u ter which  had been improperly neglected on a former occa-
" sion.  Every thing being done  in its proper  season  and
" place, he ever seemed to have leisure in store ;  and he  was
" ready to receive his friend or acquaintance, and to take his
" part with cheerfulness  in any cdnversation that occurred.
" And, let  me .remark,  that no  one ever with more  ease to
" himself refrained from professional discussions  of any sort,
" or conversation in which he  was acknowledged superior.... 
 liv                     PREFACE.

 " or with less self-denial, in mixed company, left the subject
 " of conversation to be chosen by others."  Yet was he far
 from maintaining a silence  indicating either  indifference  or
 neglect; on the contrary, he loved to promote social conver-
 sation  by every  cheerful thought that  occurred.   " Many
 "years member of a society of noblemen  and gentlemen of
 "the first rank, of judges,  lawyers,  military men, and pro-
 « fessed men of letters, he kept his place with the most easy
 «* propriety, having knowledge sufficient for giving  him an
 " interest in the conversation of each, and  for taking a res-
" pectable share  in  it, without  exhibiting  any peculiarities
" arising from his more accustomed habits of thought." This
society, and another small evening party, or club of gentle-
men, more professedly scholars, were the only public compa-
nies which his delicate health permitted him to frequent.
   But  Dr. Black had neither the temper nor the feelings of a
recluse.  He loved society, and felt himself beloved in it. In
coming to Edinburgh, he had the happiness of rejoining his
friend, Mr. Adam Smith, whose society and  friendship had
 given him so much delight  in Glasgow.  The same promi-
nent feature of character, " perfect singleness of heart," void
of all guile, attached him  warmly  to Mr. David Hume; and
the attachment was mutual and  equally confidential.  His
relation Dr. Adam Ferguson, Mr. John  Home, Dr. Alexan-
 der Carlvle, and one or two other  gentlemen of talents and of
elegant accomplishments,  were his chief society in  his hours
of relaxation.   His professional  eminence was not the bond
of this  acquaintance ;  for  their attachment to him, and to each
other, arose from their experience of ingenuity, and candor,
 and good ta3te, rather than from any similarityof studies, or
agreement of opinions.
   But  there were others  of  Dr.  Black's  intimate friends, to
whom  his philosophical  talents,  and more particularly his
 chemical and geological knowledge, were powerful sources of
attachment.  Mr. Clerk of Elden, and his brother Sir George,
 Dr. Roebuck, and Dr. James Hutton, were ever  as ready to
receive  information from him as he could  be  to impart it;
and in  their society  he  could indulge  in  his profession?.! 
PREFACE.                     lv
studies.   To the last of these gentlemen Dr. Black was most
affectionately attached;  and in respect to habits of intimacy,
Dr. Hutton should perhaps have been placed first on this list*
a He made up in physical speculation all that was wanting in
" any of the rest of his acquaintance.  Yet  would it be diffi-
" cult to  say whether the characters of Dr. Black and Dr.
" Hutton, so often seen together, were  most to be remarked
" for resemblance or contrast.   Both profound in physical
" science  ; both rigid  adherents to fact, in exclusion of  all
" hypothesis, or the most specious conjecture:  both  of con-
" summate  humanity and candor.  Dr.  Black was  serious,
" but not morose.   Dr. Hutton playful, without  petulance.
" The one was always on solid ground;  and of him it might
" be said, nil molitur inepte.   The other,  whether for plea-
" santry,  or serious reflection, could be in  the air, speculate
" beyond the laws of nature,  or their  phenomina, and treat
" the common ndtion of body, extended and impenetrable, as
" a vulgar error."  But, with all this diversity of relish, the
friends were united by mutual respect for the talents of each
other, and the most implicit confidence in each other's inte-
grity and worth.   Dr. Hutton was the only person now near
him, to whom Dr. Black imparted every speculation  in che-
mical science, and who knew all his literary labours.  Seldom
were the friends asunder for two days together.  Mr. Watt
was now  at a distance, in Birmingham; but they kept up a
close and philosophical correspondence, in which all their
speculations  and projects were  known to each other.  Soon
after his coming to Edinburgh, theDoctor got another pupil,
Mr. Archibald Geddes, managejmp the glass-works at Leith,
who soon engaged his Professor^tattention by the readiness
and propriety with which he applied to the improvement of
his manufacture the instructions which he received in the lec-
ture.  Farther acquaintance shewed  more to esteem and at-
tach ; and it terminated in the most intimate and confidential
friendship.   From this friend no circumstance of Dr.  Black's
former life or his present condition was withheld;  and to his
assistance he had recourse in every thing that affected either
his fortune or his comfort.
• 
Ivi
PREFACE.
   In this society Dr. Black passed his days calmly, but cheer-
 fully, respected and  beloved,  and  conscious  that  he was
 worthy of this regard.   Though unmarried, his house was
 not unoccupied ; for the uniformity of his single life was often
 agreeably chequered by the  welcome visits of his numerous
 friends, the descendants of the respectable pair at Bordeaux.
 In these domestic scenes he  appeared to the best advantage,
 for h« enjoyed them most.   Large companies were not to his
 taste.  Parties of men of learning, he thought,  had seldom
 much of conviviality;  this requires conversation of more ge-
 neral interest, and  where the affections are rather  engaged
 than the  understanding.
  My narration now draws towards a close.   The infirmities
 of advanced life now bearing more heavily on a feeble consti-
 tution, gradually curtailed those hours of walking and gentle
 exercise which had always been necessary  for  Dr.  Black's
 ease.  Company and conversation began to fatigue ;  he went
 less  abroad, and was  visited only by his intimate  friends.
 His  duty at College now  became too heavy a task, and he got
 an assistant, who took a share of the lectures,  and relieved
 him  from the fatigue of the experiments.  But,  at length, /
 even this was  more  than  enough for his diminished strength,
 and he was obliged to give over lecturing altogether.  But all
this  gradual decline, and occasional  short  periods of more
 than common indisposition,  made no change in the gentle
 cheerfulness of his disposition and manners.  A friend was
 always received with  unaffected welcome;  and  whenever
 he found himself engaging  more  in  conversation than was
 consistent with his health, he said so ;  and said it in a manner
 so pleasing, that his guest was only the more induced  to re-
 peat his visit.
  I cannot conclude what should be said of this amiable per-
 son so well as  in the words of Dr. Ferguson, who says,
  " The life of Dr. Black was not less distinguished  by cor-
 " rectness and propriety  of conduct,  than by ingenious rea-
 " soning and  scientific research.  For he  carried  into  his
" private affairs the  same order and good conduct that he em-
 " ployed in his philosophical studies, and in  his  professional 
PREFACE.                     lvii
" duties.   And he reaped through  life the benefit of his at-
" tention to this particular, in the ease of his  circumstances,
" and in the power which it gave him on occasion to assist his
" friends, or to contribute to the promoting of any public con-
" venience.
  " From those, indeed, with whom mere remissness or ne-
" gleet is allowed to pass for generosity,  Dr. Black may have
" been  thought too attentive to the  increase  of his fortune.
u But they did not know him*; and if they were required to
" substantiate the  charge, the  proofs  which they would ad-
" duce, would be found indistinguishable from the real effects
" of sound reason and good sense.  His expences were in-
" deed regulated, but in no way unbecoming his station. His
" house was spacious; and his table, at  which he never im-
" properly declined any company,  was plentiful and elegant,
" ....rather above than below his condition.  His contribu-
" tions for all public purposes were liberal, and like a gentle-
" man; and his purse was open to assist  his friend.f  Much
"of his practice as a physician arose from his previous con-
" nection with the patient as a  friend ; and he was as assidu-
" ous where he would not accept,  or  could not expect a fee,
" as in the most lucrative part of his profession."
   I have already said that Dr. Black's constitution never was
robust, and that, as he advanced in years, it became gradually
more  delicate and frail; so that every cold he caught occa-
sioned some degree of spitting of blood.  Yet he seemed to
have this unfortunate disposition of body almost under com-
 mand, so that he never allowed it to proceed far, or to occa-
 sion any  distressing illness; and he thus spun his thread of
 life to  the last fibre ;  and even  this does not seem to have
 broken, but merely to have ended.  "  He guarded  against
 " illness, by restricting himself to  a moderate,  or  I  should
  » About the time  that  he left Glasgow, he lost three-fourths of all the
fruits of his labours, by the failure of the  house  where he had lodged his
money.  He foresaw this failure for two years; yet no  man ever  observed
ihe smallest appearance of fretfulness, or any alteration of his behaviour to
the person by whom he was to suffer so severely.
  •j- I could give more than one or two instances in niiich a great part of his
 lovtune was at risk for his friend.
     VOL. I.                   H 
Iviii                   PREFACE.
 " rather call it, an abstemious diet; and he met  his increas-
 " ing infirmities with a proportional increase of attention and
 " care,....regulating his  food and exercise by the measure of
 " his strength.   It is wonderful with what skill and  success
 " he thus made the most of a feeble constitution,  by thus pre-
 " venting the access of disease from abroad.   He  enjoyed a
 " health which was feeble indeed, but scarcely  interrupted,
 " and a mind ever undisturbed, in the  calm and cheerful use
 " of all his faculties.   A life so prolonged had the advantage
 " of present  ease,  and  the prospect,  when the  just  period
 " should arrive, of a calm dissolution."....His only apprehen-
 sion  was that of a long continued sick-bed ; and this perhaps
 less from anv selfish feeling,  than from the  humane conside-
 ration of the trouble  and distress occasioned to  attending
 friends; and never was this modest and generous wish more
 completely gratified.   " On the 26th Nov. 1799, and in the
 " seventy-first year of his age, he expired, without any con-
 " vulsion,  shock, or stupor, to  announce or retard  the ap-
 " proach of death.   Being at table, with his usual fare, some
 " bread, a  few prunes, and a measured quantity of milk,  di-
 " luted with water, and  having the cup in his hand  when the
 " last stroke of his  pulse was to be given,  he had  set it down
 " on his knees, which were joined together, and kept it steady
 " with his hand,  in  the manner of a person perfectly at ease ;
 " and in this  attitude  expired, without spilling  a drop, and
 " without a writhe in his countenance ;  as if  an  experiment
 " had been required to shew  to his friends  the facility with
 " which he departed."  His  servant opened the door to tell
 him that some one had left his name, but getting no answer,
 stepped about half way towards him, and seeing him sitting
 in that easy posture, supporting  his bason of milk with one
 hand, he thought that he had dropped asleep, which  he had
 sometimes  seen  happen after his meals.   He went back, and
 shut the door;  but before he got down stairs, some anxietv
 which he  could not account for,  made  him return and look
again at his master.  Even then, he was  satisfied, after coming
pretty near him, and turned to go away ; but again returned,
and coding quite close to him, he found him without life. 
PREFACE.                   lis
  " So ended a life, which had passed in the most correct ap-
" plication of reason and good sense to all the objects of pur."
" suit which providence had prescribed to  his lot ;" with
many topics of agreeable recollection, and few things to ruffle
his thoughts.  He had long enjoyed the tender and affection-
ate regard of parents whom he loved, honored, and revered ;
with the delightful  consciousness of being a dutiful son, and
being cherished as such ;....one of a  family  remarkable for
sweetness of disposition and manners, he had lived with his
brothers and sisters in terms  of mutual love and attachment.
He had never lost a  friend,  but by the stroke of mortality,
and he felt himself worthy of that constancy and regard. He
had  followed  a profession altogether to his  taste; and had
followed it in a manner, and  with a success, which procured
him the esteem and respect of all competent judges, and set
his name among the most eminent, and he was conscious that
his reputation was not unmerited; and with a success, in res-
pect of emolument, which secured the respect, even of the
ignorant; which gave him the command  of every rational
gratification, and enabled him to add greatly to the  comforts
of the numerous  descendents of his worthy parents,....heirs,
not only of their name, but likewise of their unambitious mo-
deration, and amiable  simplicity of character.   " The fortune
" that he left shewed  how much he had profited  by the order
" and just arrangement which he had ever maintaned in his
" affairs, amounting  almost  to the  double  of what any one
" thought that his income and his frugality could have amass-
" ed.   The whole was disposed of by will, in a most acjcu-
" rate and satisfactory manner, parcelled into shares, accord-
" ing to the degree in which  each individual was'the object
" of his care and solicitude."

      His saltern accumulem  donis, et fungar inani
      Munere..............
                                          N 
 
LECTURES
           ON

CHEMISTRY.
         OF  CHEMISTRY IN GENERAL.

     CHEMISTRY, like all other sciences, has arisen from
the reflections of ingenious men on the general facts which
occur in the practice of the various arts of common life.   It
will not greatly conduce,  gentlemen, to your progress in the
science, to  trace chemistry from its first humble appearance
in the practice of the more ingenious artists and manufactu-
rers, till, by a copious  collection and judicious arrangement
and comparison of those practices, men still more ingenious
and  speculative deduced  certain fixed laws of material na-
ture, according to which all the chemical phenomena pro-
ceeded, and thus have raised this department of study to the
rank and character of a science, or system of general doc-
trines, expressive of those laws of nature, and so arranged,
according to  the principles of sound logic, as to  carry evi-
dence and  conviction  into its various branches, and to fur-
nish maxims and precepts for successfully directing the prac-
tice in a great  variety of the useful arts.   This, though not
unusual in  the beginning of such a course of lectures, cannot,
I imagine, be attended with any considerble advantage, be-
cause, to persons altogether unacquainted with the chemical
facts, and even with the substance in the hands of the chemist,
such a narration would be little more than the mentioning of 
<2
OF CHEMISTRY IN GENERAL.
 many names, and telling you something of each, of which you
 could not form any distinct#onception.
    I  hold it better, therefore, to turn your whole attention to
 the present state of chemistry, and give you such a description
 of it as shall correspond with our present attainments in the
 science, fully comprehending them all,  while it shall exclude
 every thing foreign from our proper study.   I  shall give you
 a description or definition of it.  This being somewhat pecu-
 liar, I must beg your indulgence, while I  give my reasons for
 being dissatisfied with the definitions which have been given
 of chemistry by others; and I  trust that you  will not think
 your time misemployed in  attending to them, because, in sta-
 ting my objections to the definitions given by others, I shall
 be h-d to point out the essential differences between chemistry
 and oh:r branches of knowledge with which it has been con-
 founded.
   In the first place then, one manifest impropriety in the grea..
 ter part of the definitions of chemistry  is, that their authors
 represent chemistry as an art.  This error is to be seen in the
 definitions given by authors of the highest name, for such surely
 were Boerhaave and Stahl.  Yet it is plain, from  the writings
 of those authors,  that their idea of chemistry did not corres-
 pond with such a denomination.  They  were probably led in-
 to this mistake by confining  their attention  too much to the
 chemical books which  appeared before  the beginning of this
 century.  The greater number of those books had, indeed, for
 their* principal object, a chemical art....the art  of pharmacy;
 and, although they bore the title of courses of chemistry, they
 contained little more than  the detail of processes, or  rules
 whereby a great number and variety  of  substances were pre-
 pared for the uses of medicine, with occasional  services  to
 other arts, particularly metallurgy.  At  the same time, also
those commonly known by  the name of chemists were chiefly
or solely, artists, employed in making  certain chemical pre-
parations  and products,  according to  rules and directions
which they had learned to observe.   But surely such  persons,
 confiningthemselves to the exercise  of an art, and, perhaps,
 describing and teaching this art in  the most judicious and 
          OF CHEMISTRY IN GENERAL.         3

complete manner, are still upon a footing with all other che-
mical artists,  such as brewers, distillers,  dyers, and many
others ; and, if we choose to apply the term chemist to them,
we must find some other term to distinguish from  these ar-
tists such  men as Sir Isaac Newton, Mr. Boyle,  Cavendish,
Priestley,  Boerhaave, Scheele,  Bergmann,  Lavoiser, and
others, who have improved our science.  The public now per
ceive the distinction, and in  some measure make it, by calling
the chemical artists Trading Chemists.
   From what I have now said, you may perceive something
of the distinction which I think necessary to keep in view be-
tween art  and science, between the  artist  and the man of
knowledge, or the philosopher.  The man of knowledge, the
philosopher, is he who studies and acquires knowledge in or-   -
der to improve his own mind; and with a desire of extending
the department of knowledge to which he turns his attention,
or to render it useful to the  world, by discoveries, or by in-
ventions, which may be the foundation of new arts, or of im-
provements in those already  established.   Excited by one or
more of these motives, the  philosopher  employs himself in
acquiring knowledge and in communicating it.   The artist
only executes and practises  what the  philosopher or maji of
invention  has discovered or  contrived, while the  business of
the trader is to  retail the productions of  the artist, exchange
some of them for others, and  transport them to distant  places
for that purpose.
  I may illustrate this distinction between the man of science
and  the artist still more by an  example or  two.  While Sir
Isaac Newton was employed in his experimental inquiry con-
cerning the nature and laws of light, and was led by his  ge-
nius from one beautiful  discovery to another, until he pro-
duced the admirable' work which he has left us on this  sub-
ject, he was acting the part of the philosopher.   His  obser-
vations and discoveries induced him to believe that, by reason
of the nature of light, and the manner in  which it is affected
by transparent refracting mediums, the telescopes in use were
capable  of only a very limited degree of magnifying power.
But the manner  in  which it  was reflected exhibited no such 
4        OF CHEMISTRY  IN  GENERAL.
obstacles to amplification.  He therefore proposed that teles-
copes should be attempted, which should perform their effect
by reflection; by which he saw that a much more perfect and
distinct image might be formed.   He went still farther: he
tried different mixtures of metals, to learn which would form
the best composition for the mirrors which must be employed
in these telescopes instead of the glass lenses hitherto  em-
ployed.  He even tried different ways of grinding and polish-
ing those mirrors,  in order to learn how to give them the
most perfect figure and the most exquisite polish,  so  as to
reflect the brightest and most perfect image.  After having
thus completed all  the improvements  he had projected, al-
though he published directions for making a reflecting teles-
cope in perfection, he was, in all this, still acting the part of
the philosopher.   The person who merely put in practice the
directions given by Newton,  following implicitly the rules
which he had laid down,  was  the  humble  artist.   Sir Isaac
Newton, for want of good workmen, acted this part also, in
all its detail, being unable to obtain a good reflecting telescope
in any other way than that of making it himself, according
to his own rules.
  In like manner, we find numerous operators who, either
with  their own hands, or by the hands of others whom  they
employ, exercise the various branches of the valuable art of
pottery.   These persons, by an" apprenticeship,  or otherwise,
have  learned how to choose and to mix the proper materials ;
how to form the vessels;  to apply the glazing and other de-
corations ; and lastly, how to give the proper degree of fire
to  consolidate and  finish the ware.  These are all artists
while they only exert in practice the skill they have acquired,
whether by  communication from others, or by efforts of their
own ingenuity.  But if there be a  Wedgewood among them,
who takes pleasure in attaining more extensive  knowledge of
the subject, who, by comparing the practice of other  potters
with his own, and by making new trials, and varying the com-
position, the glazing, the firing, and other parts of the process,
 endeavors to make improvements upon the art, or to under-
 stand it  better than before ;  such a person, in my opinion, is 
            DEFINITION  OF  CHEMISTRY.         5

 a philosopher, or  a man of thought,  study* and  invention.
 Even in medicine, the same distinction may, I think, be very
 properly made.  The physician who only practises  what he ■
 has learned, treats his patient as we  say secundum artem, and
 gives himself no further trouble, should be reckoned an artist:
 but, when he bestows uncommon attention and study  upon
 ■the diseases he has occasion to treat, endeavors to understand
 them  better  than  ordinary, or to  improve the  method of
 curing them, he, in so far, is certainly one of the most useful
 philosophers....a medical philosopher.*
   It may be objected, perhaps,  that I use  some freedom with
 common language  in  this manner of applying the term philo-
 sopher, when I  do not confine it,  as  is commonly done, to
 men of great learning and retirement, but apply it to any man
 who endeavors  to  acquire knowledge, or thinks  and  reasons
 upon any useful subject.   In this sense, the term,  it may be
 said,  will apply  to  a plain farmer, if he  only studies the con-
 struction of his plough, and how far it is  adapted to produce
 in the best manner the effect for which it  is intended,  and
 perhaps endeavors to improve it.   And, in so far as  he does
 this, I have no scruple to reckon him a philosopher ;  a rustic
 one, he may possibly  be thought,  but a more useful one than
 many who think the  title indisputably theirs.   Men of great
 learning  and  retirement often contribute little or nothing to
 the  progress  of improvement.   They  spend  their time in
 learning and admiring the inventions of others, without  ever

  • Does not the distinction between the philosopher and th$ artist consist
 purely in this : That the latter employs with success certain processes and
 manipulations with the subjects of his art, without being able to give any
 other reason for his proceeding but that he was so directed to act, and has
 always  found these precepts completely effective ? And, in communicating
 this art to another person,  he merely inculcates certain precepts which, he
 tells him, will never fail; while the philosopher perceives, by his knowledge
 of the laws of nature, how every operation is efficient, and must, in confor-
 mity to those laws, be followed by the desired effect, and by no other.   The k
 artist may, like Wedgewood, improve his art greatly, by dint of many trials,
 either made at random, or by reflecting on his former practice : but the phi-
losopher can improve the art, and even invent new arts, by proceeding from
fixed principles and applying them with propriety; and he sees clearly the
reason of every effect in the nature of the means employed     editor.
    VOL.  I.                      I 
6         DEFINITION OF CHEMISTRY.
 proposing a new thought of thejr  own, or ever discovering
 one useful power in nature.
   If this distinction, therefore, between science and art be al-
 lowed me, I would not define the system of knowledge which
 I propose to comprehend in these  lectures, by saying that it
 is an art.  An  art is a set of rules and directions for the use
 of an artist, who, by practising them, is enabled to obtain cer-
 tain productions which are the object of the art.  A science
 is a body, or system of knowledge; and chemistry, as now
 generally studied and taught, is undoubtedly a science, which,
 though it has given origin  to numerous arts, is distinct from
 them all, as you will more easily perceive when we are farther
 advanced in this course.
   But, setting aside this impropriety in some  definitions of
 chemistry, what has been added  to distinguish it from  other
 branches of knowledge is not less exceptionable.   The object
 of it is commonly said to be the resolution, or  division, of
 compounded bodies into the principles of which they are com-
 posed, and the production of new compounds, by combining
 bodies  which were  before in a separate state.   This will be
 found to be the meaning of Dr. Stahl's definition, though de-
 livered in terms that are a little abstruse.  And the same dis-
 tinction is expressed more clearly in Mr. Macquer's, which,
 as being one of the latest, may be taken as an example.* -
   But, when this  manner of defining chemistry is better con-
 sidered, it will be found  improper  upon several  accounts ;....
 as, first, in a great number of the experiments or operations
 which the chemist performs, he neither divides compounded'
 bodies into their principles, nor combines others which were
 separated  before, nor has he either of these ends in his view
 in many of his inquiries.  To  form an idea of the  sort of
 knowledge which he possesses, and of the manner in which
 his  mind  is commonly employed, we need only to read the
 chemical history  of any particular substance, as  contained in
 the  ordinary systematical books ;  or we may look to the ac-

<T* La Chymie est une science dont l'objet est de connoitre la nature,' et les
 proprieties des tons les corps, par leurs analyses et leurs combinaisons..
 Di:t, de Chyinix. 
            DEFINITION OF  CHEMISTRY.         7

  count of some late chemical f nquiry into the nature of a new-
  ly discovered fossil, or other object, which had not been much
  examined before.   If we observe what particulars have drawn
  the chemist's attention,  and what points he has been anxious
  to inquire about, \v,e find that  he  first examined its external
  appearance, in order to form  some judgment  of its nature,
  by  comparing it with a number of other bodies he already
 knows.   After examining its external appearance, he straight-
 way inquires whether or not it resembles thtm in other  res-
 pects.   He puts it into the  fire, to  learn if it be inflammable,
 or whether its color, texture, weight, or other  qualities, can
 thus be  changed; whether  it melts or assumes the  form of
 vapor,  and at  what  particular degree of  heat  it undergoes
 these changes.   He  mixes" it with a variety of  other bodies,
 solid or fluid, in order to learn  whether or not it dissolves in
 the fluids, or any of them, and if it does, whether any changes
 of color, odor,  or other qualities, are thus  induced.  And
 many of these last trials  are  diversified by the application of
 heat at the same time.   Now, I aver that we sh/mld give a
 very improper account of his employment and object on this
 occasion, if we define it by saying that it is only to compound
 the fossil with other bodies, or  to separate the ingredients of
 which it is composed.  When he examines the facility where-
 with it  is melted, or converted into vapor; the particular
 manner in which it burns, if it be an  inflammable body ; the
 changes of color, texture, consistency, odor, or other qualities,
 induced in it by the multiplicity of  his experiments....he cer-
 tainly is not studying to decompound the subject of  his in-
 quire, or to combine it with other bodies.  If it should hap-
 pen, during the course of his research, that any appearances
 present themselves which give him reason to think that it is a
compounded body,  separable  into substances  of different
kinds, he will investigate its nature in this respect also, and
will apply various means which he  knows to be  effectual  in
separating the ingredients of other bodies.  But although he
should thus find it to be a compounded substance, and be able
to seperate the different ingredients of  which it is composed,
he will consider this discovery as only  a part of the chemical 
 8        DEFINITION OF CHEMISTRY.

 history of this new object, the nature of which he was thus in-
 vestigating.
   Dr. Boerhaave appears  to have perceived the  impropriety
 of the definitions which had been  attempted by others, and
 proposed one in which he gave a new distinction, or character,
 of chemistry.   We find his idea of chemistry the most clear-
 ly expressed in some passages of his  Methodus Studii Medici
 and of his Elementa Chemice.  He aims at it too in the defini-
 tion  which he gives in  the  beginning of  his elements;  but
 that definition is expressed in such general terms, that it can-
 not be understood by itself.  In his Methodus Studii  Medici
 we find  his meaning  fully expressed.   Recommending the
 study of chemistry to the physician, he says that it is a principal,
 or rather the most important branch of the  study of  nature j
 and he characterises it by adding, that, whereas what is com-
 monly called natural philosophy is  the study of the more ge-
 neral qualities and affections of matter, such as external form,
 number, bulk,  weight, motion, rest, chemistry is the  investi-
 gation and sj,udy of the more  particular qualities, the quali-
 ties peculiar to  each distinct kind of matter, many of which
 qualities  are of the greatest importance to mankind : such
 as the polarity of the magnet, and its power to attract iron and
 steel; the disposition  of steel to  receive the  same  powers
from the  touch of the magnet; the exploding quality  of gun-
powder ;  the softness and  flexibility of lead; the hardness
 and strength of iron;  the density and durability of gold ; the
 poisonous quality of arsenic ; the nourishing quality of wheat
 and other grains ; the sweetness of sugar  and of honey ; and
 innumerable others.  These are all qualities which are found
 in those  particular bodies, but not in  bodies in general; and
 Dr. Boerhaave thought it to be the characteristic of chemistry
 that it was the science which treated of these particular quali-
 ties of bodies.*  I  find myself, however, under the necessity
 of dissenting from the Doctor upon this point for the follow-
 ing reasons.

  * See Method. Studii Medici, 1751, page 133.  Elementa Chemise, Paris,
l"o3, page 45. 
           DEFINITION OF CHEMISTRY.         9

   If it be made the distinction of chemistry, that it is employ-
 ed in considering the particular qualities of bodies, in opposi-
. tion to those general ones which belong equally to all kinds of
 matter, a great many things will be comprehended among the
 objects of this science which in reality do not belong to it,
 but to other.sciences: and the bounds  of it w^Jl  therefore be
 extended in some respects a great deal too far.   To give ex-
 amples, we may begin with some of those very qualitj*| which
 Dr. Boerhaave has quoted as objects of chemistry, wof the
 chemist's attention.  The attracting and polar qualities of the
 magnet and of steel, when rightly prepared.  These are qua-'
 lities particularly belonging to these bodies;  and, for this
 reason, Dr. Boerhaave has cited them as qualities, the study"
 and knowledge of which  employed the attention of the  che-
 mist  more than of any other philosopher, and which he could
 best  explain.   But it is certain that he has done this  impro-
 perly.  The chemists have not paid any particular  attention
 to the polarity of the  magnet, or the nattireyof its attractive
 power.  They avail  themselves of this power indeed when
 they wish to purify iron filings ; and they take some notice of
 these qualities when they treat  of that particular  ore of iron
 called loadstone, or magnet.  But  they have studied the  na-
 ture £>f this stone, not on account of these qualities,  but as it
 is an ore of iron.  The books, in which you will  find the at-
 tracting qualities and polarity of the magnet, and of magneti-
 cal steel, most fully considered, are  not the books of chemistry,
 but the books of experimental or mechanical philosophy. The
 same in ay be  said of the electrical qualities of the tourmalin,
 the study of which belongs to the  electrician ;  as also of the
qualities of Iceland crystal with respect to light, to which the
attention of the optician is directed,  not that of the chemist.
But besides these, we have still a long list of particular quali-
 ties to add, the study of which certainly does not belong to
the chemist.  These  are the virtues of  all the remedies em-
ployed by physicians to act on the body;  the purgative quality
of rhubarb, the emetic one of ipecacuanha, and so  on  of all
the rest.  These  are  certainly particular qualities ; but no
person wil!\say that it is the chemist who studies them, or 
10        DEFINITION OF CHEMISTRY.
  ought to study them the most.  It is the physician who ought
  to be best acquainted with them;  and the study of them has
  always been* considered as a necessary branch of his science.
  Dr.  Boerhaave has been so little aware of this, that he has
  mentioned some of those very qualities, by which different sub-
  stances act on the human body, as objects of chemical study ;
  such as, the poisonous quality of arsenic.   " Sed chemia  in-
  " veni|**kr  sua experimenta vim  deleteream inesse huic ar-
  " senic^"   But surely this is without foundation.   I will be
  bold to say that there is no chemical experiment from  which
  it could be iniVrred that arsenic is poisonous.
     Thus  it appears that  Dr. Boerhaave'.s  definition  takes in
  many things which do not belong to this science.  I will fur-
  ther aver, that it leaves out many things which do really be-
  long to it.   The various  kinds of  matter have many resem-
  blances to one and other in tru'ir disposition to be affected  by
  lieat.  They have general qualities, or a general nature, with
  regard to heat, which become objects of the chemist's atten-
  tion  and study, but which Dr. Boerhaave excludes by his de-
  finition,  They have  also a few general qualities, which they
  show in their manner of  mixing with  one  another,  and
  which are undoubtedly objects of the chemist's attention, but
  which are also thus excluded bv Dr. Boerhaave.        •
    I  find myself, therefore,  under  the necessity of rejecting
  this  definition, or distinction of  chemistry, and all others I
  have hitherto met with.  Even Mr. Fourcroy's, though lately
  proposed, does not appear to be well chosen or imagined.  He
  defines chcmis'ry " to be a science which teaches the know-
  " ledge of the intimate and reciprocal action of all the bodies
  " in  nature on one another."  To this definition it ma^ be ob-
  jected, that it requires much explanation.  The intimate and
  reciprocal  action  are  terms which cannot be readily under-
  stood.   They would themselves need new  definitions to ex-
  plain them,  and to limit their meaning,  and they might be the
  subjects of much disputation.*
   * When motion is given and taken away, by the collision of bodies, they
  certainly act intimately and reciprocally on each other: yet the study of this
  action is foreign to chemistry.  Some further restriction is therefore neces-
■Asarj- ere this definition be suitable to our study. 
           DEFINITION OF CHEMISTRY.        11

   Since I find reason, therefore, to reject the definitions of
 others, it is incumbent on me to offer one which  may be free
 from the objections which I have stated against them.   And
 this I shall venture to attempt in these words:
   Chemistry is the science or study of those effects and quali-
 ties of  matter which are discovered by mixing bodies variously
 together, or applying them to  one another with a view to mix-
 ture, and by exposing them to different degrees of heat, alone,
 or in mixture with one another, in order to enlarge our know.
 ledge of nature, and to promote the useful arts.
   Or, in fewer words,  That the chemist studies  the effects pro-
 duccd by heat and by mixture, in all bodies, or mixtures of bo-
 dies, natural or artificial, and studies them with a view to the
 improvement of arts,  and the knowledge of nature.*
   This, in my  opinion, takes in all that is proper to chemis-
 try, and inseparable from it, and at the same time leaves out
 every thing that does not belong to the science.
   It may, perhaps, be thought to  leave out a  great deal  too
 much ;  and  strangers to it may find difficulty in conceiving
 how any great variety of curious or useful knowledge  can be
 comprehended under this definition.
   But  when we shall have had opportunities  to enter more
 fully into our subject, I have no doubt of its appearing clear,
 that not only an attention, to the effects of heat and mixture, with
 the  view I mentioned in the definition, is the characteristic of
 the  chemical philosopher, but that the study and science thus
 defined Jead to an inexhaustible fund of interesting or useful
 discoveries.
   To form some idea of the views which this science opens,
 and the objects which it presents to the mind, we  must  reflect
 a little upon the  subject of heat.  Whatever that is which  we
 call  heat, and in whatever manner produced, it is certainly the
 chief material principle of activity in nature.   Upon its gen-
 tle action, as confined  within the bounds prescribed to it in

  * May it be thus defined' Chemistry is the study of the effects of heat and
 mixture, with the view of discovering their general and subordinate laws, and
of improving the useful arts.
                                                  EDITOR. 
 12        DEFINITION  OF CHEMISTRY.

 this part of the universe, depend the production and preser-
 vation of all animal and vegetable life.   Take away heat, to a
 certain degree, and they must all perish : a total stop would
 be put to all the operations of nature.   But beside this exten-
 sive influence, by  which it  supports action and life in this
 great system of beings, the manner in which the various par-
 ticular kinds of matter are affected by the different degrees of
 heat, is a subject  of inquiry which we shall find most lertile of
 surprising and useful discoveries
   Nor are  the  effects of mixture  less various or extensive
than those of  heat.  It is a field of inquiry  to which we can-
not see any bounds.
  But, further, in order to apprehend better the nature and ex-
tent of this science, it is necessary to observe, that the chemist
does not confine his attention to the phenomena which he dis-
 covers himself by intended  experiments: the knowledge  he
has  acquired  directs his attention  to all similar phenomena
 and operations which happen spontaneously, or in the course
 of nature, and enables him to reason better concerning these
 than any other person.  Thus, various mixtures of subtile ex-
halations are oftentimes formed in the air, some of which pro-
 duce meteors of different kinds, which draw our attention by
 their striking appearance.  Others, though not perceptible to
the sight, or other senses, become but too  manifest by their
pernicious or fatal effects on various animals.
  A perpetual succession of mixtures and combinations is also
going on in the waters of this globe, especially those of the
ocean, and those which flow through the hidden veins and ca-
verns of the earth, where their qualities are often influenced
also by the effects of subterranean heat.
  Even the solid parts of the globe are undergoing constant
 changes of their  mixture and composition, which give  occa-
sion to new productions.
  And, in animals and vegetables, their nourishment and
growth, and the production of the different fluid and solid sub-
stances of which their bodies consist, depend very much upon
 combinations of water with other matters, or upon changes of
mixtures and combinations which had been formed before. 
DEFINITION OF CHEMISTRY.        13
   On all these subjects, therefore, anJyrjj£jJU^Rt---«lqp4pst on
account of the  knowledge he  has acquired of the effects of
mixture and heat, is the best judge.   He  is more  ready to
understand them, and to reason upon them, than another per*
son.  And this has been one cause of the difficulty of giving a
proper definition of chemistry.  Some of those who tried it
thought it was necessary to comprehend in their definition all
those subjects concerning  which the chemists had attempted
to reason; and that all the  qualities and phenomena which
they had endeavored to explain were  proper and  necessary
objects of chemistry.  But this was surely a very great error j
for although chemical experiments  have thrown some light
on the digestion of food in the stomach, we must not there-
fore consider the study and knowledge of the digestion of our
food as an article which belongs  in particular to chemistry :
the branch of science to which it especially belongs is the stu-
dy of medicine.  Some of the  chemists have pretended to ex-
plain  the virtues of the most of the remedies  employed by
physicians, supposing them to depend on certain proportions
which they contained of the  imaginary principles, salt, sul-
phur, water, earth, and others.   Must we on this account ad-
mit that the study of the virtues of remedies does not belong
to the physician but to the chemist?   They also attempted to
explain  the phenomena of thunder ahd lightning, by supposing
that nitrous and other vapors were elevated into the atmos-
phere, and acted there on one another as we see them act in
exploding compositions.   Shall we, therefore,  consider the
study of these  meteors as a necessary part of chemical study
or knowledge, although later discoveries have shewn that the
study of them belongs most particularly to the electrician ?
   The discoveries of the geometer have enabled him to ex-
plain many things in mechanics, in Optics, in astronomy, and
in the structure of the bodies of animals ; but we are not, there-
fore, to  say that all these  branches  of knowledge  belohg to
geometry,  and make a proper part  of it.  The only study
which belongs in particular to the  geometer is that of the pro-
perties and relations of lines,  figures,  and  quantities.  The
knowledge he acquires, by the study of these, proves a source
   VOL. I.                  'KL 
           DEFINITION OF CHEMISTRY.

froir^wlih^hTv^reTfv^hany  clear  explications  of obscure
points, and the solution of the most intricate questions in other
sciences ; but all these other sciences, as they are distinct from
one another, are likewise so from the study which principally
occupies and characterises the geometer.
   Chemistry, therefore, must be understood to have the same
relation to many o her branches of knowledge that  geometry
has.   It supplies principles by which  many  otherwise dark
and* intricate  points in these  other sciences  are clearly ex-
plained ;  and  thus  throws much  light on many of the great
operations of nature.  But if we desire to form  a just judg-
ment of the nature  and  limits of this  science, we  must not
consider these applications of.it to the support and illustration
of others, as essential parts of chemistry, or as  parts  which
must be comprised in its definition.  In defining this science,
we must  confine our attention to  that body of knowledge
which principally occupies and engages the  chemist,  and by
the means of which he is enabled to throw light upon other
sciences more or less allied to  his own.*
   What is above stated, will, I hope serve to convey a pre-
cise idea of the extent  and nature of  chemistry, and at the
same  time show that the  chemist does not confine his atten-
tion to mere facts, but that his  study is speculative aud philo-

  • Is it not by a strict attention to the limits allowed by this definition  that
we most clearly discover, in the organiaed bodies of this globe, principles of
mutual relation and action altogether different from  any discovered in the
purely chemical phenomena ? The hemlock and the pea spring from their
respective seeds in the same water; the alum and the nitre shoot from their
respective crystals in the same brine  ; and both go on increasing with con-
siderable similarity.  But, in the first example, a certain germ is necessary,
and it grows by intus-susception, and subsequent assimilation of heterogene-
ous matter, and protrusion of anew substance.   In the second, any particle
whatever of each of the two salts may be the incipient crystal; and the ap-
parent shooting is not a growth, nor a protrusion, nor the production of a
new substance, but is an apposition from without of a substance already pre-
sent in the surrounding brine.  The first is effected by a principle of growth,
or vegetable life, producing two  perfectly distinct  individuals.   The second
exhibits no such principle;  and the  result is only a part of a mass, which
may increase till it contain all that exists of its  kind in this globe.   The mu-
 tual relations and law a of action a"re altogether  disparate and unlike.
                                                        EDITOR. 
          DEFINITION OF CHEMISTRY.        15

sophical science, proceeding like all other such sciences, on
the relation of cause and effect.
   If it be questioned, in the next place, upon what foundation
it  has been  considered as so extensively useful, we could
easily give a list of the  most useful arts which have had their
origin from the observation of the  effects of heat and mixture
In various bodies.   And as many of these arts  employ  for
their chief materials substances the most familiar to the che-
mist, and the nature of which he knows better than any other
person, it must be perceived that  there is no science more
connected with the useful arts,  or of greater  consequence to
their improvement.
   In order, however, to see this in a clearer light, let us take
a cursory view of the chemical history of vegetables in gene-
ral,  and observe what particulars have been attended to, and
how far the study of these has been useful.   In the first place,
by an attentive examination of vegetables, or vegetable sub-
stances, we have learned that some abound with resin, others
with gum;  some contain sweet juices, or sugar, and others
•astringent, or bitter, or acid matter; in others we find oils of
different kinds ; and in others coloring or tinging materials,
which can be transferred to wool, or linen, or other subjects of
art;  and many contain substances which have medicinal effi-
cacy.   All of  them are either extracted or prepared for  use
by operations  which are  entirely or mostly chemical,  and
which are therefore the foundation and support of a variety
of useful arts.
   By applying water to vegetables, with a view to mixture,
the chemist has learned that commonly something is  extract-
ed and  dissolved by the water;  and the application of heat to
this water has shown that  it commonly  dissolves more than
without it.   By exposing this infusion or decoction to a gen-
tle heat, in open vessels, he has found that the water thus gra-
dually exhales and leaves the whole,  or most of what it  had
dissolved, in the form of a solid or tenacious mass, applicable
to the purposes of medicine ;  and  a variety of others, accord-
ing to the nature of the vegetable.  While the water  exhales,
he has often observed an odorous matter to arise with the vapor. 
  16        DEFINITION  OF CHEMISTRY.

  This has suggested to him the contrivance of vessels and in-
  struments by which this vapor might be preserved and  con-
  densed again In cold in th" apparatus for distillation.  He has
  then discovered that the odor resides in a subtile and volatileoil,
  which is thus separated and preserved ;  and the various kinds
  of which are useful in different arts,  as well as in medicine.
    The same operations, repeated with spirit of wine, instead
  of water, afford a  variety of other products, many of which
  are equally useful.
    Bv the simple application of fire,  the vegetable is burnt to
 nshes, the volatile parts  being either dissipated or consumed.
 These ashes, being mixed with water, are dissolved in  part,
 and form a clear ley.  When this clear solution, or ley, is de-
 canted off, and exposed  to heat in open vessels, the water ex-
 hales, and a salt remains behind, which  has  many useful  qua-
 lities.  Being n.ixed with sand, and exposed along with it to
 a violent heat, it produces glass, and is  an indispensable  arti-
 cle in its composition.   Upon this invention again  depend rhe
 arts of enamelling, and glazing all kinds of earthen ware, the
 construction of mirrors, telescopes, microscopes, thermome-
 ters, and other philosophical  instruments, besides the  easy
 manufacture of vessels, the most useful and elegant, for many
 of  the purposes  of common   life.   The same  salt properly
 mixed with oils, unites with them, and forms soap, and is die
 most active  ingredient in that useful  compound.   It is  like-
 wise, both by itself, and in form of soap, an important material
 in the hu of bleaching linen,  and is often used in the art of
 dying, which is entirely chemical.
  When the vegetable is exposed to the action of heat, diffe-
rent kinds of steam, or vapor, are  observed  to arise from it
before it takes fire.  This has  suggested the  condensation of
these steams ; and this again gave origin  to the art of making
tar, and to what is called the chemical analysis of vegetables,
by  which, and by the  further action of fire  upon them   we
learn a most curious and wonderful truth,  which, however
agrees perfectly with what is  observed in  attending to their
growth and nourishment.  I mean that they are all composed
of the same materials  or principles,  and these  very few h» 
          DEFINITION OF CHEMISTRY.        17

number.   Notwithstanding  the  immense  variety among
them, or their parts,  in point of form, color,  strength or
hardness, smell, taste, medicinal efficacy, and other qualities,
they can all be resolved into a very few principles, or elemen-
tary substances, which are  too simple to be supposed disgui-
sed, and from whose various proportion, combination, and ar-
rangement,  therefore, the  whole of  this amazing  variety is
produced.
   Instead of this outline of the chemical  history of vegetables,
we might have found an example  equally  good for our pur-
pose in the history of the metals, the  extraction of which from
their ores, the refinement and separation from one another,
the giving them different degrees of  hardness, elasticity, and
other properties,  by which they become fit for  many useful
and important purposes, are all examples of the effects of mix-
ture and heat.  But to enter fully into any  one of these arti-
cles would carry us at present a great deal too fan
   It may perhaps be thought that I have said too much in re-
presenting chemistry as the study which  has given occasion to
the invention and improvement of  so  many  arts; and that
many of those arts have not received their origin or improve-
ment so much from chemists as from other persons, who, un-
acquainted with chemistry in general, but being employed in
some chemical art,  and hoping to  find their interest in its
improvement, engaged themselves in  inquiries and new trials,
which conducted them  to some useful discovery.  But to this
I wouldlsmswer with what I said before, that the nature of a
science, and what belongs to it, do not depend on an arbitrary
name, or on the extent to which it is known and cultivated by
any particular person,  or at any particular time.  It was cer-
tainly a part of chemistry which these artists were cultivating,
although they might perhaps be unacquainted with the rest of
the science,  and did not know that the study  and research in
which they  were employed belonged to  chemistry.   I can
imagine a person living in a part of the world where the name
of chemistry is totally unknown, but who, by his genius, taste,
and industry,  in making experiments, might acquire a great
part of the knowledge by which a chemist is at present distin- 
 18        DEFINITION  OF CHEMISTRY.

 guished among us.   His discoveries would not be the letsS a
 part of chemical knowledge that he did not know the name
 of this science; nor is it to be doubted that he would deserve
 the  name of a chemist, and  would even be considered as a
 very extraordinary chemical genius.
   It is perhaps true  that a greater number of improvements
in arts have been invented by ingenious men who were artists
themselves, than by  general and merely philosophical chem-
ists; but this is not surprising.  The number of philosophi-
cal chemists is small, when commpared with that of the che-
mical artists.   And there are other reasons why an ingenious
 and  inquisitive artist may often  discover  improvements in
his art, especially such as are obvious  and easy.   The art to
which he applies engrosses  the whole of his  attention;  and
his interest constantly pushes him on to  attempt improve-
ments : that he may vie with his rivals, and better his condi-
tion.   He therefore becomes, in fact, a chemical philosopher,
as well as an artist, with this advantage, that his whole study
is directed to one point, while the attention of the general and
philosophical chemist is divided among all.
   The condition of an artist, therefore, is, in some respects
more favorable to the discovery  of  some improvements;
but it would  be  incomparably more so, were he to acquire
some general knowledge of  the sciences connected with his
art.   His field of knowledge would thus become  more ample
and his views more extended ; and his invention would have
a larger stock to employ itself on, while, at the same time
the general principles of which  he  would become  master
would enable him to contrive new trials with better prospect
of success, and to understand them more thoroughly when he
had made them.
   From  this it is plain, that an  acquaintance with the gene-
ral principles of chemistry would be of the greatest use and
importance to many  ingenious and inquisitive artists, whose
art is wholly or in part chemical.
   These few hints  will, I hope, be sufficient to answer the
purpose I have at  present in view, which  is to engage your
attention in the study of chemistry.  And, as I have now also 
            DEFINITION OF CHEMISTRY.         Id

 explained to you my idea of the nature and extent of this sci-
 ence, you  are fully prepared'to enter upon it, and to  under-
 stand  the  propriety of the plan which is to  be observed  in
 this course, and the great lines of  which  I shall now lay be-
 fore you.
   In the first  place, the principal division  of our subject shall
 be into the more general and the more particular doctrines of
 chemistry
   Under the  division of more  general doctrines shall be deli-
 vered,
   1. An account of the more general or universal effects  of
 heat.
   2. The more general observations and discoveries relating
 to mixture.
   3. An account of the chemical apparatus  or instruments, and
 the  manner of using them, or the chemical operations.
   Under the division of the more particular doctrines will be
 given
   A particular account of all the most remarkable bodies, or
 kinds of matter, which the chemists have studied ; which shall
 be distributed into a number of classes, and considered in that
 order which,  in my opinion, is best suited to their being easi-
 ly understood and remembered.*
   And lastly,  while we thus give  an  extensive view of the
 science of  chemistry,  we shall not  neglect the application  of
 it to the illustration  and improvement of pharmacy and other
 chemical arts.t This is the second great branch of my pro-

  * I trust that you will find that the account which I shall give of their pro-
perties is not merely a vast collection of individual facts, each of which must
be remembered in itself, but that your own reflections  will naturally form
those fac's into general groupes or parcels, each individual of a class having
a common character, which being  recollected,  they are  all remembered.
By this voluntary  process of your own thoughts,  you will find yourselves, at
the end of our course, professors of the knowledge  ofi many extensive laws
of mrure, which  regulate all the chemical operations on this globe.  This
will be-the  body of science, founded on observation and  experiment, which
constitutes  the reward of your attention and study.
  f The  application of Chemistry to arts and manufactures has heretofore
been  too much  neglected by  scientific  writers.  The illustrious author
'•f these  Lectures v-xz  not inattentive to  its importance, and the present 
 20         DEFINITION OF  CHEMISTRY.

 posed plan. It is plain that this will be in our power.   F°J" «
 science be the discovery of the laws of nature, the knowkc;5e
 of those laws will enable us to foresee what will be the result of
 any process, and must point out to us, in all cases, the means,
 and the  best  means, for producing any  desired  chemical
 effect:  and here does our science repay, with a liberality un-
 paralleled in any other science,  all her  former obligations  to
 the arts of life.  From them did she borrow the many facts
 which excited  her  to speculate ;  and  her occupation  has  at
 last enabled her to repay her debts  with large interest, while
 she has grown  rich in knowledge almost beyond hope.

edition, it is hoped will be found, in addition, to contain some particulars on
this subject not entirely uninteresting.  Dr. Black's  Lectures were always
attended  by a considerable number of ingenious artists....an example worthy
of adoption.
                                             AMSR.  EDITOR. 
GENERAL  DOCTRINES  OF CHEMISTRY.
PART I.
GENERAL  EFFECTS OF  HEAT.
                   INTRODUCTION.

             OF HEAT IN GENERAL.

     THAT this extensive  subject may be treated in a pro-
 fitable manner, I propose
   1st. To ascertain what I mean by the word heat in these
 lectures.
   Idly. To explain the meaning of the term cold, and ascer-
 tain the real difference between cold and heat.
   3dly. To mention some of  the attempts which have been
 made to discover the nature of heat, or to form  an idea of
 what may be the immediate cause of it.
   4th, and lastly, I shall begin  to describe the sensible effects
 produced by heat on the bodies to which it is communicated.
   Any  person who  reflects on the  ideas which we annex to
 the word heat will perceive that this  word is used for two
 meanings, or to express two different things.  It either means
a sensation excited in our organs, or a certain quality, affec-
tion, or  condition, of the bodies around us,  by which they
excite in us  that sensation.   The  word is used in the first
sense when we say, we feel heat; in the second when we say,
   vol. I.                  h 
vi-J           OF  HEAT IN GENERAL.

there is heat in the fire, or in a hot stone.  There cannot be
a sensation of heat in the fire, or in the hot stone, but the
matter of the fire, or of the stone, is in a state or condition by
which it excites in us the sensation of heat.
   Now, in beginning to treat of heat and its effects, I propose
to use the word  in this  second sense only, or as expressing
that state, condition, or quality of matter, by which it excites
in us the sensation of heat.  This idea of heat will be modi-
fied a little, and extended as we proceed, but the meaning of
the word will continue at bottom the same, and the reason of
the modification will be easily perceived.            t
  All the experience we have relating to this  quality or affec-
tion  of matter shews, that it is the  most communicable from
one body to another of any quality that we know.  Hot bodies
cannot be placed in the  contact or neighbourhood of colder
ones, without communicating to these a part of their heat.
  When a  lump of hot iron is  taken out of the fire, how  can
we prevent it from communicating its heat to the surround-
ing  matter ?   Lay it  on the ground, or on  a stone, it very
quickly  communicates to them a part  of  its  heat;  lay it on
wood, or any other vegetable or animal matter, it heats  them
in a very short time to such a degree' as to set them on fire;
let it be suspended in the air by a wire, a little  attention will
soon  convince us that it communicates heat very fast to the
air in contact with it.
  Thus heat is perpetually communicated  from hotter bodies
to the colder around them,  and, while it passes from the one
to the other, it penetrates all kinds of matter without excep-
tion  : density and compactness are no obstacle to its progress :
it appears to pass even faster into dense bodies, in most cases,
than into rare ones; but the rare and the dense are all affect-
ed by it, and transmit it to others: Even the  vacuum formed
b\ the air-pump is pervaded by it.  Sir Isaac Newton first
discovered this by an experiment.   He  suspended an instru-
ment for measuring heat in a large glass vessel, and exhausted
the air,  and suspending at the  same time another similar in-
strument in another glass vessel, equal to the  former, but not
exhausted, he perceived that the one was affected by the va- 
OF HEAT IN GENERAL.           23
 nations of heat as well  as the other.   (Newton's  Optics,
 Query 18th.)
   •Much more lately some experiments on the same subject
 were made by the celebrated  Dr.  Franklin and some of his
 friends at Paris.  They suspended a hot body under the ex-
 hausted receiver of an air-pump,  and another similar body,
 equally hot, in the air of the  room near the air-pump, and
 these bodies being such as to shew exactly the variations of
 heat that happened in them, it was  perceived that both of
 them  gradually lost a part of their heat, until they were re-
 duced to, the temperature of the room in which the  experi-
 ment was made, but that the one  which hung in the  air lost
 its heat faster than the one which was suspended in vacuo.
   The thermometers fell from 60° (Reamur).

                                         IN THE AIR.
                                            in 7 min.
                                             22
                                             29
                                             63
 The times of cooling are  nearly in the  proportion of 5 to 2.
 This is further confirmed by a set of similar experiments,
 made  byJSir Benjamin  Thompson.  (Phil.  Trans,  for the
 year 1786.)
   Sir  Isaac  Newton thought that such experiments  gave a
 proof that the vacuum of an air-pu^ip is not perfect, but that
 there is in it some subtile matter by which the heat is trans-
 mitted.   This opinion probably was founded on a very gene-
 ral association in our minds, between the ideas of heat and
 matter; for, when we thinffof heat, we always conceive it as
 residing in some kind of matter; or possibly this notion of
 Sir Isaac  might be founded on some opinion which he had
 formed concerning the nature of heat.
   There is great reason, however, independently of this expe-
riment, for believing that the vacuum of m^tir-pump is not a
perfect vacuum, and for thinking that  trere is always some
subtile  matter, or vapor, present in  it;  but I can easily  ima-
gine, and we shall afterwards see abundant reason to believe,
	IN VACUO.
to 50°	in 17 min.
37	54
30	85
20	167
 
24               HEAT AND  COLD.
that heat may be communicated, or pass through a vacuum, or
a space emptv of all other matter.                     m
   In this manner, therefore,  and upon all occasions without
exception, is heat communicated from hotter bodies to colder
ones, when they are in contact, or in the vicinity of one another;
and the communication goes on until the  bodies are reduced
to an equal temperature, indicating an equilibrium of heat
with one another.
   When we consider this communication of heat  from  hot
bodies to colder ones, the first question which may  naturally
occur to our mind, is, In what manner have these two bodies
acted,  the one on the other,  on this occasion ?  Has one of
them  lost something,  which the  other  has gained?  And
which of them has lost, or which has received ?
   The vulgar opinion is, that  the hot body has lost something
which  has been  added to the other.   And those who have
attempted to reason  more profoundly on the nature of heat,
have agreed with the multitude on this point; and have sup-
posed that heat is a positive quality, and depends, either upon
an exceedingly subtile and active matter,  introduced into the
pores of bodies, or upon a tremor or vibration excited among
ihv-ir  particles, or  perhaps among  the particles  of a peculiar
sub^tam e present in all bodies ; which subtile matter, or  tre-
mulous motion, they have supposed to be communicated from
the hot body to  the  colder, agreeably to our general experi-
ence of the communicatiomof matter or of motion.
   But although many philosophers have thus agreed with the
indistinct notion of the vulgar concerning heat,  that it is a
positive quality, or an active  power residing in the hot body,
and by  which it acts on the cold one; some of them have not
been  altogether consistent in this  opinion.  They  have  not
adhered to it,  with respect to all the  various cases in which
bodies of different temperatures act one on the other.   They
have supposed  that, in some cases, the  colder body is the
active mass, or co^^is the active matter ; and that the warm-
er body is the  passlv! subject which is acted upon, or  into
which something is introduced.  When a mass  of ice, for ex-
ample,  or a lump of very cold iron, is laid on the warm hand,
% 
HEAT AND COLD.
instead of heat being communicated from the warm hand to
the icft, or cold iron, they have  supposed that there is in the
ice, or cold iron, a multitude of minute particles, which they
call particles of frost, or frigorific particles, and which have a
tendency to pass from the very cold bodies into any  others
that are less  cold;  and that many of the effects,  or  conse-
quences of cold, particularly the  freezing of fluids, depend on
the action of these  frigorific particles.  They call them Spi-
cule, or little darts, imagining that this  form will explain the
acutely painful sensation, and some other  effects of« intense
cold.
   This, however, is the groundless work of imagination.
   To form a  well-grounded judgment on  this  subject,  we
must begin by laying aside  all  prejudices  and  suppositions
concerning the nature of heat and cold, and then propose to
ourselves this simple question.  From whence do these two
seemingly distinct  qualities of  bodies originally  proceed;
Where are the sources of heat and cold ? It will immediately
occur, that heat has a manifest  source, or  cause,  in the sun
and in fires.   The sun is evidently the principal, and perhaps
ultimately, the only source  of the heat diffused through this
globe.   When the sun shines, we feel  that it warms  us,
and we cannot miss to observe that every thing else is  warm-
ed around us.   It is also plain that those seasons are the hot-
test, during which it shines  the most, as well as those climates
which are  the most  directly exposed to its light.   When
the sun disappears, the heat abates, and abates the more the
longer his influence is intercepted.   We  must therefore ac-
knowledge the sun  as a manifest cause, acting on all the mat-
ter around us, and introducing something into it, or bringing:
it into a  condition which is not its most spontaneous state.
We cannot therefore avoid considering this new condition or
heat, thus induced in t!v: matter around us, as a positive qua-
lity, or real affection,  of which,  the sun is the primary cause,
and which is afterwards communicated from those bodies, thus
first affected to others.                                 *
   But, after  having formed this conclusion with  regard to
heat, where shall we find  any primary cause or fountain of 
 26               HEAT AND COLD.

 cold? I am ignorant of any general occasion or cause of co  ,
 except the absence or diminished action of the sun, or winds
 blowing from those regions on which his light has the weak.
 est power.  I therefore  see no reason for considering cold as
 any thing but a diminution of heat.   The  frigorific atoms,
 and particles of frost, which have been supposed to be brought
 by the cold winds, are altogether imaginary.  We have  not
 the smallest evidence of their existence, and none of the phe-
 nomena, on account of which they have been supposed to ex-
 ist, require such  a fiction in order to their being explained.
  Some persons, however, may perhaps still find it difficult
to divest themselves entirely of the prejudice, that in certain
cases, cold acts in a positive manner.   Such persons may per-
haps appeal to our  feelings, which give us a striking proof of
the reality  of cold as well as of heat.   When we touch a lump
of ice, we feel distinctly that it has a quality of coldness, as
 well as that hot iron has the quality of heat.
  But let  us examine what  we mean by this quality of cold-
ness.  We mean a quality, or condition by which the ice pro-
duces a disagreeable  sensation  in the hand which touches it;
to which sensation  we give the name  of cold, and consider it
as contrary to, heat, and to be as much a reality.   So far we
 are  right.   The  sensation of cold in our organs is no doubt as
real a feeling as the sensation of heat.   But if we thence con-
 clude that it must be produced by an active or positive cause,
 an  emanation from the  ice  into our  organs, or in any other
 way than by a diminution of  heat, we form a hasty judgment.
 Of this  we may  be convinced by several experiments.  We
 can, for instance,  take a quantity of water, and reduce it to
 such a state that it will  appear warm  to  one  person, and cold
 to another, and  neither warm  nor cold to a third; the first
 person must be  prepared for the experiment by bathing his
 hand in cold water immediately before ; the  second, by bath-
 ing his hand in hot water, or by a feverish heat in his blood;
 and the hand of the third person must be in its ordinary na-
 tural state, while the water with which these experiments are
 made is of lukewarm temperature.  Even to the same person,
 such water might be made to appear warm, when felt with one 
HEAT AND COLD.               27
hand, and cold, when felt with the other.   We are therefore
under  the necessity of concluding from these facts, that our
sensations of heat and cold do not depend on two different
active  causes, or positive qualities in those bodies which ex-
cite these sensations, but upon certain differences of heat be-
tween  those bodies and our organs.  And, in general, every-
body appears hot  or warm on being touched, which is more
heated than the hand, and communicates heat to it; and every
body which  is less heated than  the hand, and which draws
heat  from the hand which touches it, appears cold, or is said
to be cold.  The sensation is in some cases agreeable, and. in
others disagreeable, according to its intensity, and the state of
our organs;  but it proceeds always from the  same cause, the
communication  of heat  from other bodies.«to our organs, or
 from our organs to them.  What can we more^reasonably ex-
 pect than that the sensation  produced by the*intro-susception.
of the cause  of  heat, whatever that may be,  will be different
from the sensation  that accompanies its  emission from our
 bodies ? The sensations  of hunger and repletion are equally
 distinct.
   Besides the uneasiness produced by the touch of very cold
 bodies,  the freezing of water has induced many to believe the
 existence of frigorific particles.  Water, they imagined to be
 naturally, or essentially fluid, and to have its fluidity in con-
 sequence of the round  figure and fine polish of its particles;
 and they thought that to give it solidity, some powerful agent
 must be employed, which can pervert it from its natural state.
 They have  therefore  supposed  the existence of -frigorific
 atoms, of  angular, pointed, and wedge-like forms, which, be-
 ing introduced among  those of the water, entangle, and fix
 them one with  another.
    But the whole of thisstoo is imagination and fiction.  We
 have  not the least proof that the particles of water are round,
 or any  good reason for  imagining that they  have that form.
 An assemblage of  small  round bodies, however smooth or
 polished, would not have the properties which are well known
 in water ; and  the supposition, that fluidity is a natural or es-
 sential quality  of water,  is a great mistake, occasioned by our
 ^eing  it  in these parts of  the world much  more frequently 
 28                HEAT AND COLD.

 fluid than solid.  In some other parts of the world, its most
 common or natural state is a state of solidity ; there are parts
 of the globe in which it rarely or never is seen fluid ; and the
 one or  the other  state  of  the substance, as  of all  other
 bodies, depends on the degree of heat to which  it is exposed.
 Pure ice never melts but when  we attempt to heat it above a
 certain degree;  and if we cool pure water to the same degree,
 or below it, we are sure to  see  it sooner or later completely
 congealed.
   On  these two facts alone, however, the sensation we have
 of cold, and the freezing of water, has been commonly found-
 ed the belief of the existence of frigorific atoms,  among  the
 greater number of those who have thought proper to adopt
 S*uch an opinion. .
   But some of them have been influenced also by the effect.
 of salts upon ice or snow.  Many  experiments  have shewn,
 that  certain salts, or strong saline liquors, if they be added to
 ice or snow, occasion these last to melt very quickly, and, at
 the  same time, to become much colder; in consequence of
 which, this mixture of ice and salts is employed occasionally
 for freezing many liquids which  cannot be frozen by ordi-
 wvv colds.   The liquid which is  to be frozen is  put into a
 vessel, and this vessel is plunged into the mixture of ice and
 salt.
   These, and a few other facts which we shall afterwards con-
 sider, are enumerated by Professor Muschenbroek, among
 the reasons which he gives for his belief of the existence of fri-
 gorific or congealing particles ; but they are not a good foun-
 dation for such an opinion; and  we shall in the  sequel have
 an opportunity to explain these facts, without having recourse
 to such a supposition.
   We  have, therefore, reason to conclude, that when bodies
 unequally heated are approached to one another, it is always
the warmer or less  cold body which acts on the other,  and
communicates to it a real something, which  we call  heat.
Coldness  is only the absence or  deficiency of heat.  It is  the
state the most proper to common matter; the state which it
would assume were it left to itself, and were it not affected by
any external cause.  Heat is plainly something extraneous to 
            COLD....NATURE OF HEAT.         29

 it:  It is either something superadded to common matter, or
 some alteration of it from its most spontaneous state.
   Having arrived at  this conclusion, it may perhaps be re-
 quired of me, in the next place, to  express  more distincdy
 this something;  to give a full description or definition of what
 I mean by the word, heat in matter.
   This, however, is a demand which I cannot satisfy entirely.
 Yet I shall mention by and by, the supposition relating to this
 subject,  which  appears to me the  most probable.  But our
 knowledge of heat  is not brought to that state  of perfection
 that might enable us to propose with confidence a theory of
 heat, or assign an immediate cause for  it.   Some ingenious
 attempts have been made in this part of our subject, but none
 of them have been sufficient to explain the whole of it.  This
 however  should not give us much uneasiness.   It is not the
 immediate manner  of acting, dependent on the ultimate na-
ture of this peculiar substance, or the particular  condition  of
common matter, that  we are most interested in ;  we are far
 removed  as  yet from  that extent of chemical knowledge,
which makes this a necessary step of farther improvement.
 We have still before us an abundant field  of research in the
 various general facts or  laws of action, which constitute the
 real objects of pure chemical  science, namely, the distinctive
 characters of bodies, as affected by heat and mixture.   And,
 I  apprehend, that it is only when we have nearly completed
 this catalogue, that we  shall have a sufficient number of re-
 sembling  facts to lead us to a clear knowledge of the manner
 of  acting peculiar to this  substance, or this modification  of
matter ; and, when we have at last*attained it, I  presume that
the discovery will not be chemical but mechanical.  It would,
however,  be unpardonable, to pass without notice, some of the
most ingenious  attempts which have had a certain currency
among the philosophical chemists.
   The first attempt I think was made by Lord Verulam ; next
after him, Mr. Boyle gave several dissertations on heat;  and
Dr.  Boerhaave,  in his  lectures on chemistry, endeavored to
prosecute  the subject still farther, and  to improve  on the two
former authors.
    VOL.  I.                   M 
CO             NATURE OF  HEAT.

  Lord Verulam's attempt may be seen in his treatise De for-
ma Calidi, which he offers to the public as a model of the pro-
per manner of prosecuting investigations in natural philoso-
phy.  In this treatise he enumerates all the principal facts
then known relating to heat, or to the production of heat, and
endeavors, after  a cautious and  mature consideration of
these, to form some well founded opinion of its cause.
  The only conclusion,  however, that he is able to draw from
the whole of his facts, is a very general  one, viz.  that  heat is
motion.
  This conclusion is founded chiefly on the consideration of
several means by which heat is  produced, or made to appear,
in bodies ;  as the percussion of iron, the friction of  solid bo-
dies, the collision of flint and steel.
  The  first of these examples  is a practice to which  black-
smiths have sometimes recourse  for kindling a fire ;  they take
a rod of soft iron, half an inch or  less in thickness, and laying
the end of it upon their anvil, they turn and strike  that end
very quickly on its different sides, with smart blows of  a ham-
mer.   It very soon becomes red hot, and can be employed to
kindle shavings of wood, or other very combustible matter.
   The heat producible by the strong friction of solid bodies,
occurs often in some parts of heavy machinery, when  proper
care is not taken to diminish that friction as much as possible,
by the interposition of lubricating substances; as in the axles
of  wheels that are  heavy themselves, or  heavily  loaded.
Thick  forests  are said to have taken  fire sometimes by the
friction of branches  against one another in stormy weather.
 And savages, in different parts  of the world, have recourse to
 the friction of pieces of wood for kindling their fires.  A pro-
 per opportunity  will afterwards occur  for considering this
 manner of producing heat, with some attention.
   The third example above adduced in the collision  of flint
 and steel, is universally known.
   In all these  examples, heat is produced or made  to appear
 buddenlv, in bodies  which have not received it in the usual
 way of communication from others, and the only cause of its 
      \
             COLD....NATURE  OF HEAT.         31

 production  is a mechanical force or impulse, or mechanical
 violence.
   It was, therefore, very natural  for Lord Verulam  to form
 his conclusion, as the most usual: nay, perhaps the sole effect
 of mechanical force or impulse, applied to a body, is to  produce
 some sort of motion of that body.  This eminent philosopher
 has had a great number of followers on this subject.
   But his opinion has been adopted with two different modi-
 fications.
   The greater number of  the English philosophers supposed
 this motion to  be in the small particles of the heated bodies,
 and  imagine that it  is a rapid tremor, or vibration of these
 particles among one another.  Mr. Macquer also, and Mons.
 Fourcroy, both incline, or did incline, to this opinion.   I
 acknowledge that I cannot form to myself a conception of this
 internal tremor, that has  any  tendency to  explain, even the
 more simple effects of heat, or those phenomena which indi-
 cate its presence in a  body ; and  I think that Lord Verulam
 and his followers have been contented with very slight resem-
 blances indeed, between those most simple effects of heat, and
 the legitimate  consequences of a tremulous motion.   I also
 see many cases, in which intense heat is produced in this way,
 where I am certain that the internal tremor is incomparably
 less than in other cases of percussion, similar in  all other
 respects.  Thus  the blows, which make  a piece of soft iron
 intensely hot, produce no heat in a similar piece of very elastic
 steel.
   But the greater number of French and German philoso-
 phers, and Dr.  Boerhaave, have supposed that the motion in
 which heat consists is not a tremor, or  vibration of the particles
 of the hot body itself, but of the particles of a subtile, highly
 elastic, and  penetrating fluid  matter, which  is contained in
 the pores of hot bodies, or  interposed among their particles:
a  matter, which  they imagine to be  diffused through the
whole universe, pervading with ease the densest bodies;  a
matter, which some suppose, when  modified in different ways,
produces light,  and the phenomena of electricity. 
32              NATURE  OF HEAT.

  But neither of these suppositions were fully and accurate >
consi ere a o.  th.ir authors, or applied  to  explain the whole
of ill. facts and phenomena relating to heat.   They did not,
therefore, supply us with a proper theory or explication of the
na ure of heat.
  A more ingenious  attempt  has lately been made, the first
outlines of which, so far as I  know, were  given by the late
Dr. Cleghorn, in  his  inaugural dissertation,  published  here
on the subject of heat. He supposes, that heat depends on the
abundance of that subtile fluid elastic matter, which  had been
imagined before by other philosophers to be present  in every
part  of the universe, and to be the cause of heat.  But  these
other philosophers had assumed, or supposed  one  property
only belonging to this subtile matter, viz. its great  elasticity,
or the strong repellency of its  particles for one  another;
whereas, Dr. Cleghorn supposed it possessed another property
also, that is, a strong  attraction for the  particles of the  otner
kinds of  matter in nature, which have in general more or less
attraction for one another.  He supposes, that the common
grosser kinds of matter consist of attracting particles, or  parti-
cles  which have a strong attraction for one another,  and for
the  matter of heat;  while the subtile elastic matter  of heat
Is self-rspelling matter, the particles of which  have a strong
repulsion for one another, while they are attracted by the  other
kinds of matter, and that with different degrees of force.
   This opinion, or suppbsition, can be applied to explain  many
of the remarkable facts relating to heat; and it is conformable
to those experiments of Dr.  Franklin, and of  Sir  Benjamin
Thompson, quoted above.  For, wherever there is but a very
srmll quantity of common matter, as in the vacuum of an air-
pump, there we may expect to find the matter of heat exces-
 sively rarefied, in consequence of its own very great elasticity
 and self-repellencv, which, in  this case, is little counteracted
 by the attraction of other matter.
    \ cold body,  therefore,  placed  in  such a vacuum, is
 supplied more slowly with heat, or with  the matter  of heat,
 than when placed in contact with common matter in a denser
 state, which, by its attraction for the matter of heat, condenses 
COLD....NATURE  OF HEAT.
33
a much greater quantity of it into the same space.   And a hot
body, placed in such a vacuum, will retain its heat longer than
in ordinary circumstances, in consequence of the scarcity of
common matter in contact with it, by the attraction of which,
its heat would be drawn off more quickly than if there were no
other matter present but the matter of heat.
  Such an idea  of the nature of heat  is, therefore, the most
probable of am that I know; and an ingenious attempt to make
use of it has been published by Dr. Higgins, in his book on
vegetable acid, and other subjects.   It  is, however, altogether
a suppostion ; and I  cannot at present make you understand
the application of this theory, or the manner in  which it has
been formed;  the greater number of you not being yet ac-
quainted with the  effects of heat,  and the different pheno-
mena  which this theorv is meant to explain, nor  with  some
discoveries  which preceded  this theory,  and gave occasion
to it.
  Our first business must, therefore, necessarily be, to study
the facts belonging to cur subject, and to atten.i to the manner
in whLh heat enters various bodies,  or is communicated from
one to another, together wii-h the consequences of its entrance,
that is, the effects that it produces on the bodies.
   These particulars,  when  considered  with attention, will
lead  us to  some more adequate knowledge and information
upon  the subject....which again will  enable you  to examine
and understand  the attempts that have been made to explain
it,  and put you in  the  way to  form  a  judgment of their va-
lidity.
   When we  attend  to the effects produced by heat  in the
bodies to which it is communicated, we see that they are very
various in the different kinds of matter.   But there are some
effects which are produced in all kinds^ or  in a  great variety
of  bodies, in  a similar manner or  with such inconsiderable
variations, that  the similarity of its action is sufficiently evi-
dent.   This is  true,  especially with  regard to the simpler
kinds of matter, such as water, salts, stones, metals, air, and
many others.   These similar effects, produced  by heat upon
such  bodies of  the more  simple kind, may  therefore be con- 
34
NATURE OF HEAT.
sidered as the  general effects of heat; and thus distinguish-
ed from many  which it produces on certain particular bodies
only.
  These general effects  of heat are, expansion,  fluidity,
VAPOR*, IGNITION,  or  INCANDESCENCE, and  INFLAMMA-
TION, Or COMBUSTION. 
[  35  ]
             SECT. I....OF EXPANSION.

   BY expansion is meant an enlargement of the bulk of bo-
dies, which may be observed when their heat is increased;
while, on the contrary, there is a corresponding contraction
when it is diminished.
   Of this effect of heat, we have the most extensive experi-
ence, with respect to all the simpler kinds of matter in nature.
The only seeming exceptions, are a very  few bodies, which,
while we vary their heat to make it rise a little above, or fall a
little below a certain temperature, suffer, in appearance, some
irregular variations of their bulk, which do not agree with the
general rule ; but these irregularities  are special to those bo-
dies, and to those particular variations of  their heat.  When
we expose the same bodies to equal variations of heat, but at
a higher or  lower temperature, they are affected in the gene-
ral manner.
   It may, therefore, be announced as one  of the most general  "■
facts in chemistry, that all bodies are expanded by heat, and
contracted by cold.
   It is not necessary to ^satisfy you of this by a great number
of experiments.  You may be assured of the general fact, and
I shall merely assist you to conceive it  more  distinctly, by a
few  examples.   To choose these the more properly,  we may
remark, that matter appears  to us always  under three forms,
or under forms  intermediate  between some of these  three.
We have it  either, first, solid and hard in different degrees;
or, secondly, in the form of a fluid like water, oil, quicksilver,
or the like ; or,  thirdly,  we have it in the form of an elastic
fluid, or vapor, like air or the steam of water.  To see an ex-
ample of the expansion of solid and hard matter by heat, we
may take a  cylinder of iron or brass, one inch in diameter,
and six or eight  inches long,  and exactly  equal in thickness
from  one end to the other, the ends being also flat or square
with the sides ;  we must also provide a flat ruler of iron, with
a round hole in it, that is one  inch in diameter^ and that will 
 36                  EXPANSION.

 barely allow the cylinder to pass throughjt; and  the  same
 ruler must have two projecting parts on the edge of it at a dis-
 tance from one  another, which is exactly equal to the length
 of the cvlinder  while it is  cold.  The cylinder being  now
 made red hot, and  tried in  this state, will  be found too thick
 to pass through  the holes, and too long to fall  in between the
 two projecting parts of the ruler;  but, being again cooled, it
 will be found contracted to its former dimensions.
   An example of the expansion of fluid matter may be had
 by putting some water, or oil, or spirit of wine, into a round
or oval glass, which has a long and slender neck, and by filling
the whole body  of the glass only.   If the  glass  be then
warmed bv setting it  in hot water, while the heat penetrates
into il, and into the  fluid which it contains, this fluid will swell
and  rise in the neck ; and if the  heat be  again  abstracted
by means  of cold water, the fluid will return  to  its former
bulk.
   The third experiment'may  be made with air,  confined in
a  bladder, and in such  quantity, that the bladder, while  the
air is cold, shall not be fully distended  by it.  If the air thus
confined be gradually warmed, by warming the bladder before
a lire, it will expand and blow up the bladder until it be fully
distended; and being  again cooled, it will be sure to return to
its former flaccid state.
   Such expansions  and  contractions, therefore, are always a
 consequence of the variations of heat in bodies.  In whatever
 temperature of heat we  make the experiment, if a body is
 ;.iade still hotter, it is sure to expand, or, if it be made cooler
 than that temperature, it is sure to  contract.   If, for example,
 instead of heating the iron or brass, it  be made much colder
 than at first, with ice or snow, a nice  mensuration of its di-
 mensions would shew that it had  contracted, instead of  ex-
 panding ;  and after  it  is made red hot, and thereby expanded,
 were we to make it still much hotter, it would suffer a still
 greater expansion,  more or less, proportional to the heat it
 had received.
   The expansion produced by heat must, therefore,'be under-
 wood to take place  in all bodies, on every occasion when their 
                     EXPANSION.                   oi

heat is increased, excepting only a few particular cases, to be
mentioned presently.
  There is, however, this difference, when  the experiments
are made with different kinds  of matter, that they do not
agree together with respect to the quantity of the  expansion
or contraction which they  suffer from the same  increase or
diminution of their heat.   In solid bodies it is  in general
small, and not perceptible  without nice mensuration of their
dimensions.  In  fluids it is more obvious, as is seen in water;
and in elastic  fluids it is still more remarkable.   But in the
different species  of solids, fluids, or elastjc  fluids, it is ex,
ceedingly different, nor has any circumstance that explains this
variety been yet  discovered.  We cannot,  therefore, form a
judgment of the rate according  to which any particular kind
of matter expands, except by making experiments, to compare
it with others in  this particular.  A number of experiments
have accordingly  been made by different authors, the  most
accurate of which are those published by Mr. Ellicot in the
Phil. Trans, vol. 39, and those by Mr. Smeaton, Phil. Trans.
48,  and some by Mr. Berthoud, Essai  sur I'Horlogerie, se-
cond edition, Paris 1786.                                (
   Before we proceed further, it may be here remarked that
this effect of heat occasions  some bodies to crack, and  break
in pieces, when they are  suddenly heated  or cooled.   The
substances most  liable to this accident are such as are other-
wise brittle,  or which have not either flexibility  or a  strong
cohesion of their parts: such are sulphur, glass, and earthen-
ware, and even cast-iron, which, though  it has a strong cohe-
sion of its parts, is liable to split, by reason of its want of flex-
ibility.  The manner in which this effect is produced by ex.
pansion is sufficiently evident.   It is liable to happen in these
bodies when the heat is applied  suddenly, and to  some parts
of them  only, and when the parts to which the heat is applied
are not excessively thin.
   When, for example, heat is  applied suddenly to  a part of
a glass vessel that is not very thin, the heat expands that sur-
face of the glass to'which it is applied, and continues to expand
it more, biflore it can  penetrate to the other surface :  as thft 
 J8                   EXPANSION.

 last is not  yet expanded, the glass is necessarily strained, or
 stretched, as if a force were  applied to alter its form;  and
 although it withstands this force to a certain degree, it cannot
 long withstand  it, on account of its brittleness: it is therefore
 split or broken.  And the same must happen to bodies of this
 kind,  when, after being heated, they are suddenly cooled.   In
 i^oth cases the fissure always begins in the coldest side.
  If such bodies were flexible, it is evident that their flexi-
 bility  would preserve them from breaking when  suddenly
 heated or cooled, as they would easily bear that small change
 of their form, which would be induced by the unequal expan-
sion or contraction of  their parts; and  accordingly, other
metals which are flexible are not liable to this accident, as cast-
iron is.   But glass, though very valuable to the chemists by
 nam  excellent qualities, is so liable to it, that they often suffer
inconvenience and losses by this defect in glass vessels.   The
way to avoid these inconveniences and losses, as much as pos-
sible,  will be pointed out hereafter, in describing the chemical
 vessels and other parts of the apparatus.
  To other artists, the expansion of bodies by heat has proved
useful on some  occasions.  When it is necessary, for example,
to bind pieces of work very strongly with iron bands or hoops,
such as carriage-wheels, or very large vessels employed by
the brewer, or other artists, the purpose is  easily attained by
taking advantage of this power of heat.   The  iron-hoops arc
made  red hot, and driven on-suddenly, while  thus extended
 and widened by their expansion, and then they are suddenly
 cooled by throwing cold water on them: this makes them con-
 tract again and bind tlTe work with such very great force, that,
 on man)'occasions, they make a deep print in it.
  Another set of artists finding that this power of heat affect-
 ed some nice machines which they are employed in construct-
 ing, and prevented these from having the degree of perfection
 desired in their effect,'have invented contrivances  by which
 expansion is made to counteract itself, cr to remedy those
 very defects which  it occasioned in such machines.  These
 artists are  the  clockmakers and  watchmakers.   It was  per-
 ^lv,d that  the going of common clocks and watches is very 
EXPANSION.                 3&
sensibly affected by the variations of heat, occasioning the ex-
pansion or contraction of some of their parts which are intend-
ed to regulate their motion.  In the common clock, the rate
of its going is regulated by the length of its pendulum. When
the pendulum is shortened, the clock  goes faster;  when it is
lengthened, it goes slower;  and as it  must necessarily be
lengthened by heat, and shortened by cold, a common clock,
that  goes at a proper rate in a moderate temperature of heat,
will  go too slow in a warmer one,  and too fast in a colder.
This has  been remedied very  ingeniously by different con-
trivances, of one of which I shall here give the most simple
idea.
   Suppose that we have two metals, one of which expands or
contracts just three times as much as the other, by the same
variation of heat.  They may, in the following manner, be
employed in the construction of a pendulum, that shall neither
be lengthened by heat, nor shortened by cold.
   From the point of expansion A (fig. 1.) a rod or thick wire,
A B, of the less expansible  metal, must hang down a certain
length. At the lower end it must have a stud, or cross piece,
B C,  strongly  fastened, and projecting a little  to  one side.
On  the projecting part, C, of this cross piece, must be erected
a pillar,  C D, of the more expansible metal.   To the top of
this pillar, another cross and projecting piece, D E, must be
strongly fastened;  and, from this last, must again hang down
another rod or wire, E  F, of the first metal, having the ball of
the  pendulum at its  extremity.  And now,  if the height of
the  pillar C D, be one-third of the length of the two rods taken
together, the pendulum can neither be lengthened by heat nor
shortened  by cold.  For, by the expansion of the pillar, the
pendulum  is shortened, or the ball is raised nearer to the point
of suspension, because the upper end D of the pillar is more
raised by its expansion, than the lower end C is depressed by
the  expansion of A B ; and, on the other hand by its contrac-
tion, the pendulum is lengthened, or the ball is lowered; but,
while  this happens, the two rods, by their expansion or con-
traction, produce a contrary effect; and the quantity of expan-
sion or contraction is  the same  in the rods that it is in the 
40                  EXPANSION.
pillar, the greater length  in  the  rods  compensating  tor the
greater expansibility in the pillar.  The consequence there-
fore must be, that the length of the pendulum, that is, the
distance between the point of suspension and the ball, cannot
be varied by heat or cold.  Accordingly, the clocks made for
the use of astronomers, have pendulums constructed upon this
principle, in which  pillars of the more expansible metals are
employed to counteract the  expansion  of  the other parts of
the pendulum-rod.
   I already remarked, that there are a few  seeming excep-
tions from  the general fact  of the expansion  of bodies by
heat, and their contraction by cold.  These are now to be
taken notice of.
  The most obvious and remarkable example of  such an ex-
ception occurs in water.  This  fluid, in passing through all
the variations of heat, between the greatest  which it can bear,
and a cold approaching to that  by which it is frozen, is ex-
panded or contracted like other fluids. But, during its change
from  the state of a fluid to that of ice, instead of contracting
by  *he diminution of its heat, it suffers a remarkable expan-
sion, becoming more bulky b\ about one-eighth, and that with
a force which is almost irresistible.   It is  common to see
bottles burst by the freezing of water within them.  The ice,
being first formed in the neck of the bottle, shuts  it up close ;
and the rest of the water, while  it afterwards freezes, is sure
to burst the bottle by its expansion.
  But experiments have shewn that  this expansion of freezing
water is performed with a force much greater than that neces-
sary for the bursting of bottles.   Mr. Boyle found that water,
confined by a moveable  plug or stopper, in a strong brass tube,
three  inches in diameter, lifted, while  it froze, a weight of
74 lb. with which the stopper was loaded (History of Cold).
Huygens burst  an  old cannon  by freezing water confined
within it (Du  Hamel Hist, de l'Acad. Roy. 1.  i.  § 2. ch.  i.)
The academicians of  Florence  burst  a small hollow  brass
globe, the cavity of which was one inch in diameter, by filling
it with water, which was afterwards frozen: and Professor
Muschenbroeck has computed that the force necessary to the 
EXPANSION.                    41
bursting of this ball must have been equal to a pressure of 27720
lbs weight.  In the progress of this experiment, however, it
appeared that the expanding force of this quantity of freezing
water had not  power  to burst a stronger ball (Muschenb.
Tentam. Florentina, p. 135).  See experiments on the expan-
sive force of freezing water, made by  Major  Williams at
Quebec, in  the years 1784  and 1785,  Phil. Trans, of Edin.
vol. ii.
   This strong expansive force in freezing water explains
many things that happen in  frosjt,  or are consequences of it;
such as the  bursting of water-pipes, the raising of pavement,
the increase of fertility in the soil, and the splitting of trees,
and even of rocks in  some cases.   The gradual decay and
demolition  of neglected buildings also, and of the more ele-
vated and rocky parts of the earth's surface, are promoted by
the same  cause.   The bursting of water-pipes happens when
they are too much exposed to  the cold, or  not sufficiently
covered with earth, or other matter, to preserve the water in
them from  freezing.   The  raising or loosening of the stones
in the pavement, is occasioned by the freezing of the humidity
contained in the sand or earth in which the  stones are im-
bedded.  The increase  of fertility in the  soil is a conse-
quence of the disunion of all the parts of it, which  are liable
to  cohere too  strongly together,  and  to give  it a  degree  of
toughness and  density not penetrable by the fine fibres of the
roots of plants ; but, while the humidity which is in such soil
is changed into ice, being  every  where interposed between
the particles of earth, it disunites these by its expansion,  and,
when the thaw comes, the whole mass is much less coherent
 and more easily penetrable.
   The other effects of frost  above enumerated, some of which
 are  principally observed  in  very cold countries, all depend on
 the  same circumstance,  the expansion of freezing water,  or
 watery fluids, in the pores,  or crevices, or natural cavities, of
various bodie's.
   As  this  expansion  of freezing water has  long been  per-
 ceived, some attempts have been  made to explain it, or jjgfr 
42                 EXPANSION.
 discover the cause of it.   It appears at present to depend o»
 two causes.
   One of these is the extrication of the air which water con*
 tains, combined with it,  in a dense non-elastic state.  This
 air can be  extricated from the water, by the use of  the air.
 pump, and by heat, and, when thus separated, always assumes
 the  elastic form of common air.  We can also perceive that
 it is separated from the water during its congelation.   While
 water is congealing, numerous bubbles of elastic air are form,
 ed within it,  some of which rise to the surface, and  escape,
 so long as it  is not entirely covered with ice; but those pro-
 duced after this happens,  are entangled in the ice, and form
 those numerous cavities commonlv  seen  in  it.  A   mass of
 ice, therefore,  with these  cavities in it, must  be more bulky
than the mass of water from which it was made.  And this,
accordingly,  is one cause of the expansion of freezing water
 in ordinary  circumstances.  But experiments have  shewn
 that it does not depend on this cause alone.   Water has been
 deprived of its air, by the air-pump, and byvheat, as much as
 possible, and yet, when frozen,  was sensibly expanded.   Mr.
 Mairan, who wrote a treatise on ice, is of opinion, therefore,
 that another  cause contributes  to the expansion of water  in
 freezing: and this other cause  which he endeavors to ascer-
 tain, is a strong tendency of the parts of the water, to  arrange
 themselves into ranks and lines,  which cross one another  at
 angles of 60 and  120 degrees.   He  proves the existence of
 such a  tendency in the parts of water, while they  concrete
 into ice, by many examples of it.   It may  be perceived in
 water freezing in a common bason,  or other such vessel; the
 ice first formed, is in oblong, slender, pointed concretions, at
 the surface of the wa'er, and adhering to the side of the bason
 by one end.   While  these increase in length and size, others
 shoot  out from the edges  of them,  at the angles above men-
 tioned ; and  from these a  third order, proceeding from the
 sides of the second, may perhaps be observed.  Other similar
 concretions,  or thin  plates of ice, are also  formed, which
 point downwards ;  and the water, after being traversed  by a
 multitude  of these spicular concretions  and plates of ice 
EXPANSION.                   43
comes at  last to be totally frozen.   Now these appearances
shew plainly, that the parts of the water are disposed to  co-
here together in one particular manner, and  to assume one
arrangement in preference  to others, when they concrete.
Were they equally disposed to unite in every way,  and  in
every direction, the ice first  formed  would be a smooth crust
of equal thickness, adhering to the sides or internal surface
of the bason ;  and, in the farther progress of the congelation,
this crust of ice would  increase in thickness, until the whole
water were  changed into ice.  Another proof of this tenden-
cy of the parts of water to arrange themselves in a particular
manner, may be had by examining the flakes of snow, while
it falls in very cold weather.   These are evidently formed by
the concretion of the parts  of water into ice in  the  atmos-
phere, where there is no obstacle to prevent their concreting
together, in  any manner to which they are the most disposed.
When the weather is so cold that we can examine these flakes
of snow before any part of them melt, we find them generally
composed of a number of fine needles, or spicular concretions,
sometimer*; irregularly collected together, but, at other times,
joined into regular plane figures, resembling a star  of six rays,
the angles between which are  exactly equal.   On many oc-
casions, the rays of these  starry  figures have  had  small
branches  issuing from their sides,  at  angles equal to those
which the rays themselves formed with one  another, and
have been otherwise compounded and varied (Muschenbroeck
Phys. de Meteoris).   Some  authors have concluded, from
these  figures, that such snow was not formed  from  pure
watery  vapors, but from vapors or clouds which contained an
admixture of saline, or other particles, which gave the dispo-
sition to form such figures.  But this opinion is not consistent
with the experiments that have been made with snow, which,
<r\ hen collected pure, and melted into water, gives  a water
purer than  any other natural water.   These forms of the
snow  must, therefore, be  the effect of a property natural
to pure water, and they  are  most  regular when the  snow
falls  in  very cold weather, and with little or no wind.   The
atmosphere  being  then calm,  does not hinder the water,  or 
44                  EXPANSION.

watery vapors that are in it, to concrete in the manner to
which they are the most disposed.
  To understand more distincdy how the parts of water may
have this tendency to concrete in this particular manner, we
may imagine that it depends on some degree of polarity, with
which  the attraction for one another is attended.   Ihtreis
no doubt that the immediate cause  of their concretion is their
attraction for one another ; and if we suppose that this attraction
is modified by some sort of polarity, which disposes the parts
of the water to cohere together,  by some of their sides in pre-
ference  to  others, they will naturally form rectilineal figures
and spicular masses, joined together at certain angles, such as
would be formed by a great number of little balls of cork, if
a small steel magnet were thrust into each of  them, and the
whole of them  thrown into a tub of water.   But, whatever
maybe thought of this supposition, there is no room to doubt,
that one  reoson of the swelling of water, when it is changed
into ice, is, that the parts of it enter suddenly into  a mode of
arrangement and connection, different from that of the parts of
the  water in its liquid state, and which require*', more space,
in the proportion of about nine to eight.
  I must here observe that this exception to the general contrac-
tion of simple substances, by the diminution of their heat, is
not confined to  the mere passage of water into the  solid form
of ice, but  begins to be observed while the water is yet a small
degree warmer than ice.  It ceases to  contract,  and while
cooling down to the temperature of ice, it expands aboutyyYjth
of its bulk.  This was first observed (I think) by Mr, Baume of
the  French Academy,  and is  mentioned by him  in the ac-
counts which he gives of his hydrometer, and of distillation.
Mr. de Luc has examined this matter with stiM more atten-
tion ; and  Count Rumford has  pointed out, with much inge-
nuity, some important consequences of this singularity, in the
great operations of nature.  (See  Count Rumford's essay6,
vii. 281,  &c.
  The only  other  examples  of exception  from the  general
fact of expansion by heat have  been observed  in fusible iron,
and one or two other metals ;  and they  are very  similar to 
EXPANSION.                  45
 that of water above described.  These metals are expanded
 by heat, and* contracted by cold, in every variation of heat to
 which they are commonly exposed.   But when we heat them
 so much as to make them melt, the metal, while  melting, in-
 stead of being expanded bv the heat then communicated to it,
 is sensibly contracted, and forms a fluid denser thfcn  the solid
 was immediately before;  a consequence and test of which is,
 that any small portion of the metal that yet remains unmelted,
 swims at the surface of the melted  metal, as ice  does in wa.
 ter.   This singularity is observed in iron, antimony, and one
 or two more of the Brittle metals.   In these several cases,
 certain other appearances present themselves similar to those
 observed in the case of water.  The parts of the metal evi-
 dently  assume a particular arrangement, while they are con-
 gealing, in consequence of which they concrete  into a num-
 ber of oblong, spicular, orangular masses, or branched figures,
 very  conspicuous in antimony, bismuth, and zinc, when they
 are broken, and perceptible in cast-iron also.   We may there-
 fore suppose that the attraction by which their parts are made
 to cohere is also attended with some degree of polarity, which
 forces them to assume a determined relative position, «nd a
 new and more bulky arrangement,  in passing from a state of
 fluidity to that of solidity.
   Having  now described the few exceptions from the gene-
 ral fact of the expansion of bodies by heat, and their contrac-
 tion by cold, it is proper to  mention a remarkable circum-
 stance with which it is attended.   The increase  of bulk pro-
 duced by heat is in  many cases so considerable, that it might
 naturally produce a  supposition that the weight of the body is
 also increased.   But,  when we examine the fact by experi-
 ment, we find it quite otherwise.
   Some experiments for this  purpose are circumstantially re-
lated by Dr. Boerhaave in his Elements of Chemistry.  They
were made with a mass of iron, of the weight of  five pounds,
and repeated afterwards with other metals, which being weigh-
ed red hot, and after they were again cold, never shewed any
variation of their weight,  but what was so trifling and uncer-
tain, that the Doctor concludes there was not in truth any varia-
    vol.  i.                   © 
40                  EXPANSION.
tion, but only small appearances of it, proceeding from errors'
or mismanagement of the balance.
   Professor Muschenbroeck, however, was not satisfied with
these experiments of Dr. Boerhaave.   He was strongly, in-
clined  to  believe that heat  is  ponderous,  or  produced  by  a
ponderous substance,  ft appears also that Mr. Buffon thought
he could shew by experiments that a body is  heavier when  it
is hot  than when it is cold.   But similar experiments being
made more lately in England, by Dr. Roebuck, (Phil. Trans.
vol. 66.) and also by Mr. Whitehurst (ibid.), who is distin-
guished by his accuracy  and ingenuity in the  construction of
very nice and delicate balances, they disagreed  entirely with
those reported to have been made by Mons. Buffon.  Dr .Roe-
buck made his experiments with the most scrupulous accuracy,
and on considerable masses of iron, iron-scales, copper, and sil-
ver ; and Mr Whitehurst made his with a small mass  of gold,
weighed with an exceedingly nice and delicate balance. In
all these experiments the bodies appeared heavier cold than
when they were hot; and one reason for this is obvious : the
hot body when placed in one of the scales of the balance, heat-
ed and rarefied the air over that scale, which  being therefore
less pressed down by the air over it  than the  other scale, this
last preponderated.  More  recent experiments, by Dr. For-
dyce,  in which this cause of error was ingeniously avoided,
concur to shew that bodies become heavier, though in a very
small degree only, not by the increase, but by the diminution
of their heat.  These experiments  are to be  considered in
the sequel.
   It is therefore evident,  that if heat depends on the  pre-
sence of a subtile matter introduced into  bodies, this matter
has not any perceptible degree of gravitation.
   But it  would be  improper  to omit the  mention of one
other  fact  which  Muschenbroeck and  others thought  to
be a strong proof of the opinion that heat  is a gravitating
substance.   This fact was discovered in  making  experi-
ments with the metallic substances.   Many  of these,  when
exposed to the action of  heat for some time, are gradu-
ally  calcined, or  changed into a calx,  which,  so long a^  it 
EXPANSION.                   47
 remains in that state,  is  an earthy-like- substance, which
 has none  of  the  metallic properties.  And  the  fact above
 alluded  to  is, that  the weight of the calx  is greater  than
 that of the metal from which it was produced: 100 pounds
 of lead yield  110 or 112 pounds  of calx, and other metals
 give a still greater increase  of weight in their  calxes.
   But this increase  of  weight in a metallic substance by. cal-
 cination is no proof that heat is ponderous.   When the me-
 tallic calx is removed from the fire, the heat leaves it as  it
 would leave any  other matter.   It  must therefore be some-
 thing else than heat which gives this increase of weight; and
 later discoveries have well ascertained that it is air*, a quan-
 tity of which is attracted and fixed by the calcining metal, and
 can again  be extricated from it.  No metal  can be calcined
 without the plentiful admission of air to it, while it is expos-
 ed to the action of heat.
   It has not, therefore, been proved, by any experiment, that
 the weight of bodies is increased by their being simply heat-
 ed, or by the presence of heat in them.
   This may be thought very inconsistent with the idea of
 the nature  or cause of heat^which I lately mentioned as the
 most plausible: I mean tneTiotion that heat  depends on the
 abundance of a subtile matter highly elastic, or s'elf-repellent,
 which easily  enters into  all bodies, and penetrates  them
 throughout, being strongly attracted by  the matter of  those
 bodies.
   It must be confessed  that the above fact may be stated as a '
 strong objection against this supposition.
   Some  have  attempted to remove it, by supposing the  mat-
 ter of heat to have  such a very high degree of subtility and
 tenuity, that no quantity of it that  we can  collect together
 can  have  any sensible weight.   Others have thought  it
 might be the  matter which  causes the gravitation  of other
bodies,  and that  it could not be supposed to gravitate  like
them,  but,  on the  contrary, might have an opposite  ten-
dency.

                     " Oxygen or Vital air. 
48                  EXPANSION.
  'These attempts .to "remove the objection are ingenious, but
they  are  not  satisfactory.  We find too much difficulty in
attempting to  comprehend them.   And this has contributed
more than any direct arguments, to confirm  in  their opinion
those, who, with Lord Verulam, assert that heat, or the cause
of our sensation of heat, and all the phenomena which accom-
pany that sensation, is not a material substance, transferable
from one  body to  another, but a mere state or condition, in
which the matter of all  bodies may be found.   Yet, notwith- -
standing this  difficulty, I  imagine  that,  as  we  proceed, you
will find yourselves more and more impressed with the belief
that heat is the effect of a peculiar substance.
  The non-ponderosity  of heat being, however, ascertained
by all the experiments hitherto made, it is necessarily attend.
ed with this obvious consequence,  that the density of bodies,
in comparison  with one another, or what is called their speci-
fic gravity, is considerably affected by the variations of heat.
As  experiments have  shewn that each kind of matter has its
own peculiar rate  of expanding, or differs from others in the
quantity of expansion which it suffers, by a certain variation
of its heat, it  necessarily  follows,  that  when we  increase or
diminish the heat of a  number {•different bodies in a similar
manner, we change the  properties of their densities  to one
another. In the tables of the specific gravity of different bodies,
therefore, these specific gravities are stated as they appear in
a moderate temperature of heat, the degree of which ought
always to be marked  at the head of the table.  In higher or
lower temperatures, it is well known that the specific gravities
of the same  bodies would be differently  proportioned to one
another.
  We have now sufficiently considered Expansion as an effect
of heat, and taken notice of the few exceptions to it.
  Our next employment shall be to explain the nature of the
thermometer, which was invented in conseqtience  of the dis-
coveries already described, and which has proved exceedingly
useful,  both  in chemistry and medicine,  and has enabled
us greatly to enlarge and improve our knowledge of heat. 
EXPANSION....THERMOMETERS.       49
         *   OF  THE  THERMOMETER.^

   The history of this invention is  a little obscure, and the
contrivance of the  first thermometer has  been attributed to
three  or four different persons.  Sanctorio, who  is distin-
guished by the discovery of what is called insensible perspi-
ration, appears, however, to have the best title to it,**  It is
uncertain what purpose he had in view when he contrived the
thermometer which still bears his name ; but it does not ap-
pear that the expanding power of heat had been known, or at-
tended to, before his time ;  and, having observed \t in air,
he thought of employing it as a measure of heat, by the fol-
lowing simple contrivance, (fig.  2.)
   A glass  tube,  open at one end,  is  blown up into a ball
at  the other.   The ball  is warmed, and  the  tube  is  then
set  upright, with its open  end dipped into a %mall cistern
of  any colored liquor.   When  the  ball  is  warmed  by the
approach of a burning coal,  the  air "in it is expanded by the
heat,  or becomes more elastic; a part of it therefore issues
out at the open extremity of the tube,  and rises up through
the colored liquor in bubbles.    Let  the coal be now  re-
moved, and  the ball allowed  to  cool again  to   the  com-
mon temperature of the  place.   While it cools,  the air in
it  loses the additional elasticity which it had  received  from
the heat,  and the  pressure  of  the  atmosphere compresses
it into a smaller  bulk than  that it  had while it was warm,
and causes some of  the colored  liquor to ascend in the
tube.

  * This  title however, is somewhat dubious. Drebel, a  physician at
Alkmaer  in Holland, certainly made many thermometers at this time
(the beginning of the 17th century) and they were very common in Hol-
land, and even in England, before, Sanctorius was known in  these coun-
tries.  Robert Fludd, also, who began to publish  his alchemical writings
in 1597, speaks a great deal about thermometers,  and of many purposes
to which  he  applied them.   Much of this indeed is unintelligible, ov
is stark nonsense...hut  he must have had the conception of such an  in-
strument. 
 50       EXPANSION.... THERMOMETERS.

   A scale of equal degrees,  applied to the whole length of
 the tube, served to divide its  cavity into a number of small
 and equal spaces, and  it is now easy to see  by how many of
 these small and equal spaces the bulk of the air was increased
 in consequence of its being heated,  or diminished in conse-
 quence of its being cooled.  This contrivance, therefore, was
 found to be a real thermometer, or an instrument which shew-
 ed the increase or diminution of heat in the place in which it
 was kept.
  The  power of heat to expand  air  being  thus  perceived,
 a curiosity was soon excited  to try other fluids, as well as
 solid  bodies,  and the result was what has been already de-
 livered.   It was  discovered  that all  bodies  expand  when
 their  heat is increased,  and that other fluids  might be em-
 ployed, as well as air, in the  construction of thermometers.
 It was  even  very soon  perceived that  air  was less fit  for
 this purpose than any other fluid, for this reason, that being
 in this instrument  constantly  subjected to  the pressure  of
 ihe atmosphere,  acting on it  through the open end of the
lube,  it might be contracted or expanded in  bulk, in conse-
quence of variations in that pressure alone, although its heat
continued the same.   This defect in the air thermometer was
perceived by Mr. Boyle, and by the Florentine Academicians,
nearly at  the  same  time,  and  both thought of employing
other fluids.
  Spirit of wine was the first chosen.   It expands or con-
 'racts  much  more  than water by  the  same  variation  of
 its heat, and  is not liable  to freeze like that fluid in violent
 colds,  and it is easily tinged, to make  it more visible in the
tube.                                                  v
  After spirit of  wine had been used for some time, quick,
 iiver was also found to be very  fit  for this  purpose;  and
whichever of these  fluids were chosen, they  were put into a
fclass tube, blown up at one end into a ball, and as much fluid
 • .•ut in as filled the ball and a part of the tube.   By this simple
 contrivance, the  smallest expansions and contractions of the
fluid are  easily  discernible, and easily  divisible into small
parts, or  degrees.  The extremity of  the  tube is closed,  bv 
         EX^NSION....THERMOMETERS.       5=1

by melting the glass, to prevent any loss of the fluid.by evapora-
tion, or any change which might happen in it by the action
of the  air.   Some  authors  had scruples  at first about  this
practice of closing up the  tube :  it was apprehended that the
air confined in the upper part of it would resist the ascent of
the expanding fluid; but  it is easy to expel a  great part, or
the whole of this air, before the tube is closed up; nor  is  it
at all inconvenient to leave a part of it;  experiehce having
shewn  that the  expansion of the  fluid is performed with a
force too strong to be resisted by  the  elastic pressure of
the air, especially when that air is  rarefied by the abstraction
of a  part of it.
   In a thermometer constructed in this manner, the fluid can-
not be  affected by any cause  but heat alone;  and the instru-
ment is therefore free from  the defect of Sanctbrio's ther-
mometer.  But  for a considerable time after it was  brought
to this  state,  it still remained very imperfect.   No method
had  yet been thought df for making thermometers agree with
one  another.   The scale  of  degrees applied to the tube  was
differently constructed and differently applied  in every ther-
mometer and experiments made with one could «not be usefully-
compared with those made with another.   The most impor-
tant  improvement of these instruments, therefore, since they
were first contrived, was the intention of a .method of con-
structing and applying their scales, so as^to make them agree
together when they are exposed to the (sameitemperature of
heat, whatever that be.                     J  „
   This was attempted by different merTlod*s^rnore* or less suc-
cessful, and some of which were practised for a considerable
time in some of the countries of Europey until one was dis-
covered preferable to all  the rest.  This  is now become the
universal rule for the graduation of these instruments.
   It will therefore  be sufficient  here  to explain this method
alone, the advantages of which, in comparison with the others,
are very well stated by Dr. Martin (Martin's Essays on Heat
and  Thermometers.)
   ft  was easily discovered by the experiments of Mr. Boyle, 
 52       EXPANSION....THERMOMEfl:RS.

 and others, that some bodies which are liable to changes of
 their form, in certain variations of  their heat, undergo these
 changes at a certain degree  of  heat, or when  we  attempt to
 increase or diminish their heat above-or below that particu-
 lar degree*.   Therefore, if a thermometer  be applied to
 them, while they are undergoing the change, the fluid in it is
 always reduced to the same state of expansion.  We have an
 example of  this in the freezing of water, and  in the melting
 of ice and snow.  If, for example, I take a thermometer of
 quicksilver, and put it into melting snow, taking care that the
 ball be entirely covered with the snow,  the quicksilver will be
 contracted by the cold, and will descend in the tube ; but it
will soon stop and descend no further, but will continue in the
 same state so long  as any considerable part of the snow re-
 mains unmelted.  1/ I now mark with  a cfiamond, or with a
 small file, that part of the tube  at which the quitjcsilver stop-
 ped, and afterwards  repeat the same  experiment with the
 same thermometer, however often, and at places and times
the most distant, the result will be always exactly the same;
the quicksilver in the melting snow will desend to the same
part of the tube to which it descended the first time, and will
remain there stationary so long as any considerable part of the
snow remains unrAelted.
  This shews, that melting sribw is always equally cold, or has
 the power to reduce the quicksilver of  a thermometer to one
 steady determined,state of expansion or density, which  may
be called the melting-snow expansion of the quicksilver.
  A similar'poweV has'been observed also in boiling waterf.
 If the same thermometer used  in the above experiments, be
 now immersed into boiling water, and the water be kept boil-
 ing around it  for some  time, the quicksilver, heated and ex-
 panded, will ascend  in the tube, but  it will ascend to a certain
 height only ;  however long  or violently the water is boiled, it
 will not ascend higher.   If we  now mark with a diamond or
 file,  that  part  of the tube to which  the quicksilver rose, and

 * This was discovered, with respect to the freezing of water in 1664 by
Dr. Robert Hook.                                   ' .
                                                  EDITOR.
  tTrus a.*o vas the discover of Hook in i^ra
                           "  l • l J^>               EDITOR. 
EXPANSION....THERMOMETERS.     S3
 afterwards repeat the  same experiment, with the same ther-
 mometer, ever so often, in places not very high above the sur-
 face of the sea, the result in this case also will be always the
 same ; the quicksilver will always rise to the same point which
 was marked the first time, or with very, little variations, which
 can be  foreseen, and allowance made for them.   Thus we
 learn, that boiling water, when in the same circumstances^ is
 always equally hot, or has the power to reduce quicksilver to
 another determined and steady state of expansion, which may
 be  called  the  boiling-water expansion of  quicksilver.  And,
 now, having acquired this information, I can repeat the same
 operations with any other thermometer; I can find and mark
 the part of the tube to which the quicksilver descends, when
 reduced to its  melting-snow state of expansion, and the point
 to wich it ascends, when  reduced to its boiling-water state of
 expansion.  The distance between  these two points on the
 tube,  will be very various in different thermometers, on ac-
 count of their different size, and the  different proportions
 which the  balls  and tubes bear to one another; but, being
 marked on each instrument, these two points will enable us to
 perceive when the quicksilver is at any time reduced to either
 of these two determined states of expansion.   And, when we
 have secured these  two corresponding points in every ther-
 mometer,  we can make sure of a third, provided the tube be
 exactly cylindrical, or equally  wide from end to end.  We
 may take the middle point between the two above mentioned ;
 it will stew when the fluid is reduced to a middling state of
 expansion  between the two former;  and, in this manner, we
 can  proceed to  subdivide the  distance between those two
 points into any number of parts or degrees, taking care only
 to divide it into the same number of parts or degrees in every
 thermometer, and to mark or number  these  degrees in the
 same manner in  each.   The  corresponding  degrees in the
 several thermometers will shew the corresponding states of ex-
 pansion in  these different intruments.   And,  if we desire to
 measure or  mark other states of expansion, above or  below
those above  mentioned, we can protract the  scale above or
below the primary points, by adding to it as-many degrees of'
   vol..  f.                   v 
J4      EXPANSION....THERMOMETERS.
 the same size as the tube will hold, and marking or number-
 ing these also in a similar manner in every thermometer.  Or,
 we may choose other fixed points for adjusting these parts of
 the scale, other ways having been found, by which the fluid of
 the thermometer can be reduced to certain determined states
 of  expanson, different from the above; as,  by immersing it
 in boiling quicksilver, or into melting lead or tin, and some
 other metals, by which the  quicksilver  is greatly expanded;
 or, by putting it into a mixture of snow, with certain salts, by
 which it suffers an extraordinary contraction.
  This method for constructing the scales of thermometers,
by reducing the fluid to certain  determined states of expan-
sion, has  great advantages over every other attempted before.
It is easily practised, and as easily with small  thermometers
as with those of a larger size,  which could not be done by
those other methods.  Mr. Boyle first pointed out some  of
the facts on which this method proceeds ; and Sir Isaac New-
ton put it in practice in the 1701 ; and afterwards  Dr.  Hales,
in the 1727, constructed six thermometers  for himself, and
took this method to make them agree; after which, the ar-
tists  who make thermometers practised it with more or less
accuracy.   Dr. Martin, in his essays on heat and thermome-
ters has displayed the advantage of it in the clearest light.
  To insure a perfect correspondence between thermometers,
even by this method, there are several circumstances which
must be attended to, besides those mentioned by Dr.  Martin.
 See  in Phil. Trans, for 1788, an excellent paper on this  sub-
ject by Mr. Cavendish.
  Having thus explained the general principles of the art of
 constructing thermometers, the next question is, whether the
degrees of their scales express,  or point out, equal differences
of heat ?  it is plain,  that the immediate  purpose for which the
scale is applied, is not to measure heat itself, but the expan-
 sion produced by  heat.  The scale of a thermometer divides
 the increments and diminutions  of bulk into a number of small
 and  equal parts, that we may see by how many of these parts
 the bulk of the fluid is increased at one  time, or diminished at
 another; but it remains to be considered, whether these equal 
          EXPANSION....THERMOMETERS.    55

 increments, or diminutions of bulk, he produced by equal in-
 crements, or diminutions of heat.  We can imagine the fact
 to be otherwise, and that in some parts of the scale, a greater
 addition of heat may be required to  produce one degree  of
 expansion, than in other parts.  If a string be strecthed,
 by  suspending a moderate  weight to  it, and we add one
 pound to that weight, we shall  make it a little longer ; but,
 by  adding a second  pound,  we shall  not add as much more
 to the length of the string as the first pound added ; nor will
 a third pound  produce so much effect as  the second pound.
 In like manner, we can imagine, that whent a thermometer
 receives a series of equal additions to its heat, these may not
 produce equal increments of expansion ;  and, therefore, that
 equal  increments of expansion  may require for their pro-
 duction increments of heat very unequal  among themselves.
 This question  has  been  overlooked, or little  attended to,
 by some of the principal writers on thermometers.  It does
 not appear to have occured to Dr. Boerhaave;  and Dr. Martin
 gives very little attention to it.   I began  to attend to it, and
 made an experiment to decide it in the year 1760, and did not
 then know that others had thought of it;  but I soon learned
 that Boyle*, Renaldini  of Padua, Wolfusf, Dr.  Halley, Sir
 Isaac Newton, and Dr. Brook Taylor, had severally given their
 opinions or doubts concerning this question; and some of them
 have described experiments, by which they thought it was de-
 cided.   The most simple and satisfactory  experiment for this
 purpose, is one described by Dr. Brook  Taylor,  in the 32<J
 volume of the Philosophical  Transactions, published in the
 1723.  The experiment  which I made, and communicated to
 the  society at Glasgow, was the same with this;  and Renaldi-
 ni's experiment or process was of the same nature.
  Dr. Taylor's experiment was made by mixing cold and hot
 water together,  by which we can produce various heats, or
temperatures  of heat,  the differences of which from  one
another we can be sure of, independently of any thermometer.
If, for example, we suddenly mix a pound of hot  water with

 1 Boyle, Abr. i. 580.        \ Wolfii Elementa Matheseos, t. i. p. 780 
56      EXPANSION....THERM OMETERS.
a pound of cold, the excess of heat which was in the hot watery
compared with the cold, being now equally divided between it
and the cold water, must produce an excess or strength of heat
in the mixture, just the half of that which existed immediately
before in the hot water.  And, by applying a thermometer to
the hot water first, next to the cold, and lastly to the mixture^
we learn  whether the quicksilver is expanded by  different
heats, in proportion to the differences of these heats.  This
experiment has accordingly  been made, first by Dr. Brook
Taylor in  1723,  next by myself in 1760,  at which  time
I did not know ([as I said just now) that it had been made or
thought of by any other person:  and since  that  time it has
been repeated with much care and attention, and varied in
the contrivance of it, by Mr. De Luc*, and Dr. Crawfordf.
To perform it well, attention must be given to two  principal
particulars :  first, it is necessary that the tube of the thermo-
meter be exactly equal in wideness from end  to end.   This is
ascertained, by introducing as much quicksilver into  it, before
the thermometer is made, as will fill one inch, or one and a
half inch of it, and measuring exactly the length of this  little
column of quicksilver, and  then making it slide gradually
through every part of the tube, and  measuring and marking
down its length in all these different situations.  It is evident,
that  those parts of the tube must be  of equal wideness, in
which the little column of quicksilver is of the  same length,
and such parts only should be employed in  making  thermo.
meters.
   The second object, of necessary attention in performing the
above experiment, is  to make allowance  for the  heating or
cooling of the vessel in which the mixture is made.  If the
warm  water is poured into the cold, the cold vessel in which
die mixture is thus made, will take from the mixture some of
its heat, and make it appear colderthan it should otherwise do.
If, en the contrary, the cold water is poured into the hot, the
hot vessel will impart some  of  its excess of  heat to  the  more
temperate mixture, and  thus raise its heat above the degree
which should be  produced.
     ' De Luc on the Earor.-.ctdr*.            ♦ Crav. ford on HtPf 
          EXPANSION....THEMVIOMETERS.      tf

   To avoid this deception, we mult employ the two vessels of
 the same materials, and of the same size and weight, and then
 by making  the experiment  both the one wayaBkthe other,
 and taking the medium of the results, we shall   E the truth.
   The experiment being made with these^B   Itans, the
 result has  shewn, that  when the thermoB ^^^made of
 quicksilver, the gradual expansions of thi^^raid, "while it is
 heated slowly, from the cold of melting snow to the  heat of
 boiling water, are very nearly proportional to the additions of
 heat by which they are produced.  There is, however, a little
 deviation from the exact proportion; while the heat increases,
 the expansions  become  a little greater than in  proportion to
 the increase or  addition  of  heat.  The equable  or uniform
 increase of heat, therefore, is attended with a small accelera-
 tion of the increase of expansion.  This is much  more  re-
 markable in water, alcohol, and some other fluids, than in
 quicksilver.  In this last fluid, this irregularity is  so incon-
 siderable within the range of heat above mentioned, that it does
 not deserve any notice  in common experiments, and in the
 ordinary use of  thermometers ;  but if it should be necessary^
 on any occasion, to estimate the amount of it in  thermometers
 of quicksilver, and of spirit of wine, it may be collected {roiu
 the experiments of Mr. De Luc, and of Dr. Crawford.
   Here we may remark, that this little disproportion between
 the degrees of expansion and the degrees of heat, is not at all
 inconsistent with that  supposition concerning the nature or
 cause of heat which I formerly mentioned as appearing to me
 the most probable; I mean the supposition that heat depends
 on the  presence or  abundance of an incoaceivably subtile self-
 repelling matter, which is attracted by all other matter of the
 cohering or attracting kind.   For the attraction of  cohesion,
 that is, the attraction which the  particles of a body have for
 one another, like all other attractions, is weakened b)r  distance ;
 and therefore when a body h expanded  by heat, and the dis-
 tance of its particles is a little increased, its attraction of cohe-
rsion being thus a little weakened, a further addition of heat to
 't can easily be imagined to produce more effect in expanding
?■ further;  the distending power of the repellent matter of 
 58      EXPANSIOMP.THERMOMETERS.

 heat being rather less counteracted by the attraction of cohe-
 sion, than it was at the first.
   Such is^fcecision of this important question concerning
 the degreej kermometers.   There is a passage, however,
 in Dr. fjJ  fc's treatise on fire, which is inconsistent with
 what I fl(W fecpresented to be the fact.   He relates, that
 Fahrenheit haWrfe curiosity to mix hot and cold water, in
 order to see what temperature they would  produce.   The
 result is given as matter of curiosity only.   The  importance
 of such an experiment, as a way to verify the scale of heat,
 does not appear to have been thought of;  and his account of
the result is so contrary to the fact, and so absurd in itself, that
it is plain he had misunderstood,Fahrenheit's account of what
happened. He says, that whatever heat is in the colder water,
that heat, and an equal quantity of  the heat of the hot water,
:.s extinguished, and disappears, and that  the excess only of
what is  contained in the  hot above the  cold, remains in the
mixture, and is equally distributed through it.  If this were
true, it would be easy, by adding hot water  to cold, to pro-
duce a mixture colder than  the cold water.  Boiling water,
added  to water a little less hot, would produce a mixture
folder  than ice.  Dr. Boerhaave"s  account of what happens
in this experiment must therefore be disregarded.  We have
dear and distinct accounts from others of what is the real fact,
and I have often made the experiment myself.
  Thus an important point is gained in our knowledge of heat,
 and in the usefulness of thermometers. And, as it is proper, in
 order to  understand the instruments thoroughly, to know the
 imperfections that still attend them, I shall mention what may
 be considered as the remaining imperfections of our thermo-
 meters.
  In the first place, they are  always  more  or less slow in
 shewing the heat of other bodies to which they are applied.
 To understand how this happens, we must remember that the
 thermometer indicates immediately and directly that heat only
 which is in itself, but from  the heat pf  the thermometer we
 judge of that of other bodies to which it is applied.   This
 depends  on the irresistible propensity  of  heat, which  I for. 
        EXPANSION....THERMOMETERS.       5S

 merly mentioned, to be always communicated from any hotter
 hody to the colder around, until  they come to  an equal tem.
 perature with one another.   If the body we wish to examine
 be warmer or colder than the  thermometer, no sooner  are
 they applied to one another than the heat begins to be commu-
 nicated from the one to the other, until both are reduced to an
 equal temperature; and then, by examining the heat of  the
 thermometer, we judge of that of the other body.   As heat,
 however, is not suddenly but gradually communicated in this
 manner, we must wait some time after the application of  the
 thermometer, before we can form this judgment with certain-
 ty.   The way to diminish diis inconvenience, as much as pos-
 sible, is to make our thermometers very simple, and of a small
 size, it being well  known  that  small masses  of matter  are
 heated and cooled much faster than larger ones*.
   A,  second  imperfection attending thermometers  is, that
 the extent of the variations of heat, in the measurement or
 observation of which they can be employed, is limited within
 very "moderate bounds.  One reason of this is, that in strong
 or violent heats, the increments of  expansion  become more
 disproportioned to the increments of heat.  But setting this
 aside, it is well known that the fluid of the thermometer, if it
 were exposed to a heat too strong,  would be converted into
 vapor highly elastic,  which would burst the glass; or if,  on
 the contrary, it were exposed to violent cold, the fluid would
 congeal.  When we attend to these particulars, we find strong
 reasons for preferring quicksilver to all other fluids, for  the
 construction of thermometers; for besides the advantages  at-
 tending it, already taken  notice of, it bears a much stronger
 heat than other fluids, without assuming the form of vapor,
 and it also endures very violent cold before it congeals.  Lint-
 seed oil has also these qualities in a considerable degree, but
 the Viscidity of it renders  it unfit  for the construction of smaM
 thermometers.  Spirit of  wine is still employed for some
 thermometers, on account of its great expansion and contrac-

  * Besides the other qualities of mercury, which make it fitter for a ther
momcter, it greatly cxcells all other fluids in the quickness with which it
attain* the temperature of the jedies to which it is .ip.nlied. 
 iiO       EXPAN5ION....THERMOMETERS.

 tion, it  being almost eight times greater than that of quick-
 silver.  It is the fittest fluid for the measurement of the most
 intense colds ; for, when very pure, it has never been known
 to congeal; and it may also serve very well to shew the varia-
 tions of heat that happen in the atmosphere.   But it is unfit
 for being employed,  when we examine the  heats,  that ap-
 proach to that of boiling water, and those above it.  In such
 heats, the expansions of it become  first excessive, and  after-
 wards the spirit produces vapor, highly elastic, which bursts
 the thermometer.
  The  immediate application and use of thermometers  is,
 therefore, limited to the measurement of a certain small ex,
 tent only of the variations of heat.
  But, if we desire to compare with these degrees to which
the  thermometer  can  be applied, other  more intense  heats
that are far above the reach of  the  instrument, this can  be
 done by a method, of which an example  is given in an inge-
 nious paper on the degrees of heat, by Sir Isaac Newton, in
the  Philosophical Transactions for the year 1701.  He took a
lump of red hot iron  out of  the fire, suddenly exposed it to
a stream of cold air,noAing exactly the beginning of the time
when it  was exposed.  As soon as its heat was somewhat aba-
ted, he observed with attention the  progress of its cooling,  or
how much heat it lost in equal portions of time.  This he did
by the use of an oil thermometer, and also by laying upon the
mass of iron little  bits of different metals, and of mixtures of
 metals, which he knew by previous experiments required cer-
 tain intensities of heat to melt them; and he noted the  times
 when  the heat of the iron was so  much  diminished that it
 ceased to preserve these little masses melted.   Thus, having
 learned  what heat was in the iron at different  periods of the
 whole time it required to cool Uvhe discovered  the  celerity
 and progression with whkh it lost its heat, and consequendy
 could  calculate how much it had lost from the beginning  of
 exposition, or how much its heat at that time exceeded the in-
 ferior heats through which it afterwards passed.
  By such calculations, therefore, we can  form an estimate of
 the  intensity of violent he-.n* to  which" thermometers cannot 
EXPANSION....THERMOMETERS.       61
  be exposed, and thus remedy, in some measure, that defect of
  the instrument, by  which its power is  limited in the  mea-
  surement of heat.  I shall hereafter have occasion to mention
  other methods-by which the intensity of violent heats may be
  measured, or  estimated. . At present I shall further notice,
  that this method, of  Sir Isaac Newton's  is useful  on many
  other occasions, beside the measurement of violent heats.  It
  enables  us to use the thermometer for measuring transitory
  heats.   Notwithstanding the slowrfess of it in pointing out the
  heat of other bodies.   There are many experiments in which
  the heat we  desire to measure exists only for a moment  at
  its highest degree.   When hot and cold water, for example,
  are mixed together,  a middle temperature is instantly produ-
  ced; but the middle temperature continues but for a moment;
  the excess of heat, which the  mixture contains above the heat
  of the surrounding bodies, begins immediately  to be commu-
 nicated to these, and the intensity of it therefore, is very soon
 diminished,....so soon, that there is not time for the thermo-
 meter applied to be raised to that intensity, and therefore to
 indicate it.   But the remedy is easy.   We must note with
 exactness the moment of time at which we know that the heat
 must be at the highest degree, and immediately after, or at
 the same time, apply the thermometer.  The quicksilver will
 rise, but after a little while will cease to rise, being now heat-
 ed to the same degree with the surrounding mixture, the heat
 of which was  abating slowly, while that  of the thermometer
 rapidly increased.  After the  thermometer ceases to rise, we
 must continue our observations.  It will now fall very gradu-
 ally and slowly, accompanying, in its progress of cooling, the
 mixture, or matter, to which it is applied, and will therefore
 indicate the celerity and progression with which such mixture,
 or matter loses heat,   But when we have learned this celerity
 and progression with which the mixture cools, it will be easy to
calculate how much heat it lost from the beginning of the ex-
periment, and, therefore, to estimate what was the intensity
of its greatest heat.  Without this method, invented by  Sir
Isaac Newton, it would be impossible, in many cases, to learn
   vol.. r.                   q. 
u'2
EXPANSION.... THERMOMETERS.
 exactly the full strength of these transient heats; the thermo-
 meter, on account of its slowness, never would indicate their
 full amount.
   Sdly. It may be considered as a third defect of thermome-
 ters, that they do not inform us of the proportions which diffe.
 rent heats bear to one another.  We cannot say that one heat
 is double, or triple of another in point of strength or intensity.
 They inform us only of the differences of heats, or the  excess
 which they surpass one another, or any given degree of heat.
 The reason of this "is, that we are ignorant of the lowest possible
 degree, or beginning of heat.   Some ingenious attempts have
 been made to estimate what it  may be, but they have not pro-
 ved satisfactory.  Our knowledge of the degrees of heat may
 be compared to that  we should have of a chain, the two ends
 of which were hidden from us,  and the middle only exposed to
 our  view.  We might  put distinct marks  on some of the
 links,  and number  the  rest,  according  as  they are nearer
 to, or farther removed  from,  these  principal links ; but not
 knowing the distance of any link from the end of the chain, we
 could not compare them together,  with respect to this dis-
 tance,  or say, that one link was twice as far from the end of
 the chain  zs another.
   The numbers, however, which have been affixed to the de-
 grees of the scales are apt to occasion mistakes on these sub-
jects ;  but these  numbers  must be considered as expressive
 only of the difference,  or  distance, of the several degrees
 from some well known degree, such as that of melting snow,
 or of boiling water.   They have no relation to the lowest
 possible degree  or beginning of heat.
   As the  scale called Fahrenheit's scale, is the one commonly
 used in the British dominions, it may be first described. The
 author of it  did not choose to begin his scale at the cold of
 melting snow, as many others have done.   He knew there
 were more intense colds, and  he chose  to begin at  the most
 intense cold known  at that time, and which could  easily be
 produced  by art.  This degree of cold is produced by mixing
 together  snow  and  sal  ammoniac,  or common  salt.   He 
         EXPANSION..„.THERMOMETERS.       63

 covered his thermometer with a  mixture of  this kind, and
 marked the part of the tube to which the quicksilver descend-
 ed. He next put the thermometer among pure melting snow,
 and marked also the part of the tube at which  the quicksilver
 became stationary in this  situation.   Then, dividing the
 distance between these two points  into thirty-two equal parts,
 he made these the degrees of his scale, and marked them with
 numbers, beginning at the bottom, and increasing upwards:
 and he further protracted the scale with degrees of the same
 size, to the top  of the tube,  marking these with numbers in-
 creasing arithmetically upwards, as far as the scale went.  A
 thermometer, therefore, constructed in this manner, when put
 into  pure melting  snow, does  not point to the beginning of
 the scale, but to the 32d degree.  When exposed to the air of
 this  climate in the most temperate weather of spring or
 autumn, it points to the 48th or 50th  degree.  In our mode-
 rately warm summer air it rises  to  the 64th.  In the warm
 weather of very hot climates  it rises to 80 or 90, or sometimes
 to 100 degrees.  And it points to the last, or near to  it, when
 applied to the warmest parts of the human body, or to blood
 flowing immediately from a vein or an artery; and when it is
 dipped into boiling water, it rises to the degree 212.
   But after this manner of graduating them  had been used
 for some time, it was discovered that  there were colds far
 more intense  than that produced by snow and common salt,
 and that it  was necessary for  measuring these, to make an
 addition to the lower end of the scale.   This was accordingly
 done, in some of those thermometers, by adding a series of
 degrees of  the same size with the former;  but these added
 degrees were marked with  a  series  of numbers increasing
 downwards, and therefore expressing increments of cold, and
 distinguishing from the degrees of the other parts of the scale,
 by being called the  degrees below o in the thermometers that
 are made to contain them.
   These are  the  reasons for the particular  contrivance of
 Fahrenheit's scale*.
  * Wo'fius gives a different account of Fahrenheit's scale.  He chose the
temperature of salt and snov, for the beginning of his scale, and that of 
64      EXPANSION....THERMOMETERS.
   Since it came into use, however, the makers of thermome-
ters, although thev apply this scale, do not find  the point at
which the scale begins, by making a mixture of snow and salt.
They find the points of pure  melting snow, and of boiling
water, and divide  the  interval  into as many  degrees as are
contained between these two points  in  Fahrenheit's  scale,
which are 180;  and, continuing  the  scale down below the
melting snow point,  they begin the numeration 32  degrees
below that point.  Or, if the thermometer be intended for
measuring no greater heats than those of the weather, or the
human body,  the fixed points from which the scale is con-
structed are the  melting-snow point, and  the 96th or 100th
of Fahrenheit's  scale, which is found either by the instru-
ment's being held in the maker's mouth for some time, or by
exact comparison with a standard thermometer.
   Such is the scale of the thermometers generally known and
used  in this country.  Those which have  come into use in
other parts of the world, are divided and numbered in differ-
ent ways, inconsequence of this circumstance, that the instru-
ment, for some  time  after its first invention,  having been
imperfect and ill understood, every person who chose to think,
and to make experiments on  the subject of heat, began by
constructing a thermometer, intended to  be more intelligible
and exact than  those  sold  by the  ordinary  makers.  They
therefore applied to them various scales, divided, and  num-
bered according to some particular method of their own con-
trivance ; and some one of these improved thermometers
came  afterwards, in  each  particular country to supersede
all the rest, and to be established as the one chosen for general
use.
   In France, and some other parts of the continent of Europe,
they make use of Reaumur's thermometer, in which the interval
between the cold of melting snow and the heat of boiling water,

boiling mercury for its termination, and he  divided the interval into 600
equal parts.  This made 32 the  temperature of freezing water, and 212
its boiling temperature.  He also found that each division of this scale was
 -^1^-jOf the bulk of the mercury.
                                                  EDITOR. 
        EXPANSION....THERMOMETERS.       65

is divided into 80 degrees only, the numeration being begun
at the melting-snow point, with a series of numbers increasing
upwards, and another increasing downwards*.  In  Russia,
De Lisle's thermometer is the standard, in which the numera-
tion begins  at the  point  of boiling water,  with a series of
numbers increasing downwards,  and expressing, therefore,
degrees of contraction; and the interval between the  above
point and that of melting snow is  divided into  150 degrees,
which he  supposed to be 10000th parts  of the quicksilver
nearly.  In Sweden, Celsius's is the one established in com-
mon use, in which the interval between the melting-snow point
and that of boiling water, is divided into 100 degrees, the
numeration being  begun at the melting-snow point, with  a
series of numbers increasing upwards.
   But, notwithstanding this variety, it is easy to find the de-
gree  of Fahrenheit's scale, which corresponds  with any de-
gree  in the scales  of these several thermometers.  The two
fixed points, of melting snow, and boiling water,  bring the re-
gulating  points  in every one  of  these scales,  each  degree
between those two  points  has some point of Fahrenheit's scale
which corresponds with it, and the place of which in that scale
 can easily be found by calculation,  or by the use of a diagramf
in  which these several scales are drawn parallel to one another.
 Dr.  Martin  has given an example, containing many thermo-
meters.
   The scales of thermometers being now every where regu-
lated by the principles above explained, the instrument is be-
come very generally useful.  An universal language  is now
established concerning the degrees of heat, or it is very easy
to  translate the language or meaning of one scale into that of
 another.
  * There is, however, a great uncertainty and confusion in the different ac-
counts given by Reaumur of the construction of his thermometers ; snd the
observations which were made by their means cannot be understood with-
out much investigation, in order to discover which of his constructions had
been employed..  For many years past a thermometer is called Reaumur's,
if o be the point of freezing water, and 80 the boiling point....Yet this is very
different from all of his thermometers.
                                                    EDITOP. 
60     EXTENT OF  THE SCALE  OF HEAT.
  IMPROVEMENT OF OUR KNOWLEDGE OF HEAT

        BY MEAN'S OF THE THERMOMETER.

             1st, Extension of our Ideas of Heat.
   THE uninterupted contraction of the thermometer  by a
gradual abstraction of heat, gives us the strongest indication
of the latter being the  cause  of the former.   This is  fre-
quently accompanied by appearances which are altogether un-
like;  yet we do not doubt of their being only different effects
of one cause.  We have no doubt of the freezing of tin being
produced in the same way as the freezing of bees wax, nay, as
the freezing of water; yet  in this  last, heat gives place  to
another sensation, altogether different, namely, the sensation
of cold.  In common life, we never  speak of a diminution of
heat in this case, but of an increase of  cold.   The continued
contraction of the thermometer, and the analogy between the
freezing of tin, of bees wax, and of water, soon induce the be-
lief that all are caused in the same way, by the abstraction of
heat.   Thus are our ideas of the operations of heat very great-
ly enlarged.  We now know that its  influence is so extensive,
and that it is diffused in such quantity through nature, that no
mass  of matter lias ever yet been found so cold as to be total-
ly destitute  of heat; for spirits  of wine have nev^r yet  been
frozen, and we believe that it only requires the diminution of
its heat to produce this effect.  We are sure that it still re-
tained a quantity of heat, and,  possibly, a very great quantity.
This  is proved by covering up  such a cold mass with a  mix-
ture of snow and certain salts, both  of which had been previ-
ously made as cold as the mass.  This mixture becomes in-
tensely  colder than the meterials of it were  immediately be-
fore, as appears by the applicatian of a thermometer to it, and
by the readiness and quickness with which it will freeze man}
fluids that cannot be  frozen by any other means.   It there-
fore reduces the mass involved in it  to the same intense  cold-
ness,  or extracts from this mass a quantity of heat, which wi-
th us learn it had contained before. 
GREAT EXTENSION
  We must, therefore, consider the coldest bodies ever ob-
served in nature as still retaining  a great deal of heat, the
amount of which we cannot define; the beginning, or lowest
possible degree of heat, being yet unknown to us.  Indeed,
when we recollect that heat is continually emanating from sur-
rounding bodies, and that other bodies absorb it so much the
more greedily as they are  colder, we cannot conceive a body
altogether  void of  it,  if  exposed in   the neighborhood  of
others.
  But, to give a clearer view of this subject, it may be proper
here  to mention a few examples of diminutions of heat, or
violent cold, that have  been observed.
  Dr. Boerhaave, in his  elements of chemistry, describes
with  admiration an experiment  made by Fahrenheit.   The
most violent  cold known at the  time  when  this experiment
was made, was that expressed by the beginning of Fahren-
heit's scale, or 32 degrees colder than melting snow;  it is as
much colder  than  melting snow, as melting snow  is colder
than  the air of our summer weather;  and it may be at any
time  produced, by mixing snow or pounded ice with common
salt.  A natural cold, in which the thermometer had fallen to
this degree, had been observed in Iceland, and often in Ger-
many, and some other countries  of the continent of Europe.
The same degree of cold  has also sometimes been observed
in Britain; but it rarely happens, and  never continues  in this
island, except for a few hours, perhaps, of an extremely cold
night in a very hard winter.   It therefore never produces all
its effects ; its duration is not sufficient to let it act with the
whole of its force.   But,  in those places in which a cold of
this degree of intensity continues for some time, it produces
very remarkable effects, such as  splitting timber, by freezing
the juices and humidity that are in the pores of it;  freezing
beer, wine, vinegar, and all other fermented  liquors.   When
stones, or metals, which have been exposed to it, are touched
with the tongue, or the softer parts of  the human body, these
are instantly frozen and mortified, and the principle of life in
them is extinguished.  No animals  can endure such a cold,
except those which are provided by nature with very thick 
68          OF THE  SCALE  OF HEAT.
 and warm furs ; and man is under the necessity of wrapping
 himself in a thick covering of the furs of these animals.   Few
 vegetables are able to withstand its power;  but they are pro-
 tected from it by very deep snow, the fall of which always pre-
 cedes the season of this mortal cold.
   Such is the power of this cold, in those countries in which
 it prevails, during a considerable part of the winter season.
 -  But, in the experiment  related by Dr.  Boerhaave, Fahren.
 heit produced by art, a cold  which was no less than 40 degrees
 lower; it was produced by the action of the strong nitrous
 acid on broken ice, to which it was applied in a very ingenious
 manner, to make ft produce its greatest effect.
   Dr. Boerhaave was astonished at the effect of this  experi-
ment ; but, had he lived a few years  longer, he  would have
heard of degrees of cold observed in nature, which approached
to this, which were equal to  it, or even exceeded it.
   The first example is in the history of a journey to the north
end of the Baltic, by some of the academicians of France, at
the king's expence,  in order to  make  observations and mea-
surements, for deciding the question concerning the figure of
the earth.   These gentlemen wintered under the polar circle,
where, for some days in the middle of winter, the sun does
not rise above the  horizon,  but  only approaches to it,  and
gives a twilight.   In that climate,  they found  it necessary to
use all possible precautions  to  secure themselves from the
dreadful cold which prevailed.   They prevented, as much as
possible,  the  entrance  of  the external air into their apart-
ments ; and if at any time they had occasion to open a win-
dow  or dcor, the humidity of their breath, confined in the air
pf the house, was  condensed and frozen  into  a shower of
snow;  their lungs, when they ventured to breath th,e cold air,
felt as if they were torn asunder ;  and they often heard the
rending of the timber around them by the expansive power of
the frost in its pores.   In  this terrible cold, their best ther-
mometer fill to a degree  equal to the 33d below the begin-
ning of Fahrenheit's scale, or the 65th below the cold of melt-
ing snow; it is as much colder than the degree of  melting snow;
as  this last is  colder than the human bodv in n healthy state. 
               GREAT EXTENSION              69

  But, in other places, and at different times, an intensity of
cold has been observed, in severe winters, still greater than
this, and  fully equal to the cold produced by Fahrenheit's ex-
periment.  Such a degree of cold has been observed at the
British settlements in Hudson's Bay, and in different parts of
the  Russian dominions, especially those that are situated far
to the north,  or distant from the sea,  and exposed to winds
blowing over an extent of northern continent, such as Kamt-
schatka,  Siberia, Petersburgh, and even Moscow. In some of
these places, degrees of cold have been recorded, as occurring
naturally, or produced by art, which even appeared to exceed
by far the 40th degree below o;  such as the colds observed
in Siberia by baron Demidoff*,  in which the mercurial ther-
mometer has many times fallen to the 120th  degree below o,
The cold observed by Professor Pallas at  Kransnojark, in
which his mercurial thermometer fell to 46 degrees below o,
in a regular manner; but being then moved, fell suddenly, with
a start, to the 80th degree, at which it remained a whole day ;
and, in the  course of this day, he congealed some pure quick-
silver, by simply  exposing it in  an open cup to the cold air
(Phil. Trans.) Also  the famous experiments made at Peters-
burgh with snow and nitrous acid, in December 1760, by Pro-
fessor Braun, in which he also congealed quicksilver, and saw
his  mercurial thermometer fall, in some of the  experiments,
to 300,  or 400, or 500, and once to 600 degrees below o of
Fahrenheit's scale.
  But, as soon as  those experiments  were published, it ap-
peared to me perfectly evident,  and I always declared  it as
my opinion, that the cold produced by Professor Braun, in his
mixtures, and that which was observed on some of the other
occasions just now  mentioned, was not near so violent as it ap»
peared to be by the extraordinary descent of the quicksilver
in the thermometer.  From  many  phenomena  described in
those experiments, I found reason to  conclude, that quick-
silver, while it congeals, suffers an excessive and irregular
contraction, much greater than in proportion to  the diminu-
tion of its  heat; that it is the opposite to water in this re-
                       *  Phil. Trans
    VOT.  I                     R 
70         01  THE  SCALE  OI  HEAT.
spect; for water, when congealing, is considerably expanded,
instead of contracting; its  expansion  being a singular  pn?.
nomenon, which does  not  appear in  any other case of the
diminution of its  heat.  But quicksilver, on  the  contrary,
while it congeals,  is very much  contracted,  incomparably
more than in proportion  to  the diminution of its heat, as
measured by  the  thermometer.  This  was evident,  from
many  phenomena  which  appeared  in  these   experiments
of  Professor Braun's, a few of which I shall here  point
out.
  Imo,  In the first place,  when  the mercurial  thermometer,
which was plunged into the mixture of snow and nitrous acid,
fell to those very low degrees, the descent of it was not re-
gular and gradual.  It descended indeed slowly and regular-
ly a considerable way,  in the beginning of each experiment;
but, after this,  its descent was irregular, by a rapid motion,
or  sudden starts of 100 or more degrees at  once ; and this
happened especially  when  the thermometer was  moved or
-disturbed.   Now,  it is well known, that in many fluids,  when
the\ are  sufficiently cooled to make them congeal, the begin-
ning of congelation is promoted  by disturbing or shaking the
fluid  matter.  I therefore concluded, that in the  above ex-
periments,  the quicksilver  in the thermometer  was  first gra-
dually cooled to a sufficient degree,  to prepare it for congeal-
ing;  and that during this diminution of its heat, it contracted
in a slow and gradual manner, and  that  the sudden and vio-
lent starts of contraction which afterwards followed, proceed-
ed, not from the farther diminution of heat, but  from more or
less congelation, which actually happened; for  the Professor
broVe some of these thermometers, and  found the quicksilver
cong- aled within them.
  2£.75, Secondly,  Professor Braun, in  some of these  expe-
riments, congealing some  quicksilver in a  wide  and  short
tube, the upper end of which was left open, saw, that after the
quicksilver began to congeal, the upper  surface of  it became
remarkably  hollow during the  progress of  the congelation.
But the  contraction of the quicksilver which occasioned this
hollowness could not proceed from a proportional diminution
of heat;  for it is well known, and shall be illustrated  in the 
                 GREAT EXTENSION             71

 sequel, that every fluid continues of the same heat, from the
 moment when it begins to congeal,  to that  in which its con-
 gelation is  completed, which generally requires a considera-
 ble time.   The  mass does not become colder, although sur-
 rounded  with colder matter, until the congelation of it is at
 last  completed.   The above excessive  contraction  of the
 quicksilver, therefore,  after it began to congeal,  was  a  con-
 comitant of its  congelation alone, and not of its  becoming
 colder.
   3tio, Thirdly,  To  establish further the idea and  belief of
 this fact with regard to quicksilver, we may remark, that the
 same thing is observable  in other metals, particularly tin and
 lead.  If either  of these  metals be  melted  in a crucible, and
 removed from the fire to  cool slowly, they will soon begin to
 congeal, and, during the progress of their congelation, a con-
 siderable hollow, or a number of hollows, will be formed in
 the upper surface of the metallic mass, although, during the
 time  while these hollows  are formed,  the  congealing metal
 does not become colder j  of which  we may be  satisfied by
 immersing the bulb of a mercurial thermometer into the fluid
 metal, and  keeping it there until the congelation is finished.
 In these metals,  therefore, as well as in quicksilver, congela-
 tion is attended with an excessive contraction.
   The degree of cold necessary for the congelation of quick-
 silver was  not ascertained with precision, untiUater experi-
 ments were made in a more accurate and satisfactory manner,
 with  mixtures similar to  those which Professor  Braun had
 used  in imitation of Fahrenheit:  and these later experiments
 have shewn that the degree of cold produced  in such mixtures
 is not near  so violent as he had supposed  it.
   The first satisfactory proof we had of this was  in some
 experiments made  in the  British settlements in  Hudson's
 Bay,  by Mr. Hutchins,  one of the servants  of the Hudson's
 Bay Company.  His first attempts were made in consequence
of instructions from the Royal Society;  and, as they  had not
been  successful in ascertaining the degree of cold at which
quicksilver begins to congeal, I wrote a short letter, at the
L>c-orc of  a  friend, giving general directions, in a few words, 
72          OF THE SCALE  OF HEAT.

for the proper conduct of the experiments.   In the former
experiments, the mercurial thermometers had been plunged
immediatelv into the cold mixture, as  in  Professor Braun's
trials at Petersburgh, and when the cold became  so great as
to congeal the quicksilver of the thermometer, the excessive
and irregular contraction, which it then underwent, made it
quite unfit for measuring the cold.  I therefore proposed, that
if mercurial thermometers were employed, they should not be
plunged immediately into the cold mixture, but into a quantity
of quicksilver, contained in a small glass, or wide tube, which
glass and quicksilver should be set in the cold mixture. Thus,
while the quicksilver in the wide tube was gradually congeal-
ing, the thermometer itself, placed in the centre of it, would
indicate exactly the degree of cold of the quicksilver con-
gealing around it:  nor could the cold congeal the quicksilver
of the thermometer, so long as any part of the external quick-
silver remained uncongealtd.   A  second set of instructions
were sent the following winter from the Royal Society, very
full, and very accurately drawn up, and new experiments were
made, which were  quite satisfactory.
   And the result was, that the cold, which is sufficient to con-
geal quicksilver, exceeds but very little the  cold which had
been formerly produced by Fahrenheit.  It is the 40th or 41st
degree below o of his scale*, as measured with a mercurial
thermometer, and  the 30th, as measured with a thermometer
made of the purest spirit  of wine, a fluid which does not con-
geal in any degree of cold that has yet been observed.
   This result has been further confirmed by another set of
experiments made at Petersburgh, by Dr. Guthrie, to whom
I likewise gave a few directions to assist him in the conduct
of themf.
   We have reason, therefore, to be perfectly satisfied that the
degree of cold at  which quicksilver begins  to congeal, and
which Is produced by a mixture of snow and strong nitrous
acid properly made,is not more than the 40th or 41st degree
below o of Fahrenheit's scale.  But this degree  is as  much

      • Phil. Trans.  1783.              j j>hil. Trans. 1786-7. 
            DISTRIBUTION OF HEAT.          7S

 colder than melting snow, as melting snow is colder than the
 human blood in a violent fever.
   Even this degree of cold, however, has been since surpassed
 by the successor to Mr. Hutchins at Hudson's Bay.   He was
 supplied with proper  materials, and very full instructions, by
 Mr. Cavendish, in consequence of which he made  a set of
 experiments, by mixing snow  with  vitriolic  acid in different
 states.  He thus produced colds, some of which made the best
 spirit of wine thermometers descend to the 69th degree below
 o of Fahrenheit's scale.
   Thermometers, therefore, by enabling us  to measure and
 compare great aiTd  uncommon diminutions of heat,  have
 assisted us greatly to enlarge, as well as to correct, our ideas
 of heat, to  perceive its extensive  influence, and the  great
 quantity of it which is diffused through nature: for there are
 many reasons for being assured, that the 69th degree  below o
 is far from being the extremity of  cold, or lowest possible
 degree of heat.   When you shall be acquainted with the effects
 of mixture, and with the nature of the materials employed in
 these mixtures, you will not find it difficult to form a judgment
 with respect to this matter,

              2.  Of the Distribution of Heat.
   A  second improvement in  our knowledge of  heat, which
 has been attained by the use of thermometers,  is the  more
 distinct notion we have now  than formerly, of the Distribution
 of heat among different bodies.
   I remarked  formerly, that, even without the help of  ther-
 mometers, we can perceive a tendency of heat to diffuse  itself
 from  any hotter body to  the cooler  around,  until it  be  dis-
 tributed among them, in such a manner that none of them are
 disposed to take  any more heat from the rest.   The heat is
thus brought into a state of  equilibrium.  This  equilibrium
is somewhat curious.   We find that when all  mutual action is
ended, a thermometer, applied  to  any  one  of  the  bodies,
acquires the same degree of  expansion : therefore the  tempe-
rature of them all is thesame, and the equilibrium is univer-
sal.  No previous acquaintance with the  peculiar relation of 
 74          DISTRIBUTION  OF HEAT.

 each to heat could have assured us of this, and we owe the
 discovery entirely to the  thermometer.  We must therefore
 adopt, as one of the most general laws of heat, that " all bodies
 " communicating freely with each other, and exposed to no
 " inequality of external action, acquire the same temperature, as
 " indicated by a thermometer."  All acquire the temperature
 of the surrounding medium.
   By the use of these instruments we have learned, that if we
 take 1000, or more, different kinds of matter, such as metals,
 stones, salts, woods, cork,  feathers, wool, water and a variety
 of other fluids,  although they be all at first of different heats,
let them be placed together in the same room without a fire,
 and into which  the sun  does not shine, the heat will be com-
 municated from the hotter of these bodies to the colder, du,
ring some hours perhaps, or the course of a day, at the end of
which time,  if we apply a  thermometer to them all in succes-
sion, it will  point precisely to  the  same degree.  The heat,
therefore, distributes itself upon this occasion, until  none of
these bodies has a greater  demand  or attraction for heat than
every other of them has ;  in consequence of which, when we
apply a thermometer to them all in succession after the first to
which it is applied has reduced the instrument to its own tem-
perature, none of the rest  are  disposed to increase or dimin-
 ish the quantity of heat which that first one left in it.  This
 is  what has been commonly called an  equal  heat,  or  the
 equality of  heat among different bodies ; I call it the equili-
 brium of heat.   The nature  of this equilibrium was not well
 understood,  until  I pointed out a method of investigating itj
 Dr.  Boerhaave imagined, that  when  it obtains, there  is an
 equal quantity of heat in every equal measure of space, how-
 ever filled up with different bodies ; and professor Muschen-
 broeck expresses his opinion to the same purpose: "  Est
 " enim ignis aequaliter per omnia, non admodum magna, dis-
 '• tributus, ita ut in pede cubico auri et a ris et plumarum, par
 " ignis sit quantitas?*?'  The reason they give for this opinion
 is, that to whichever of those bodies the  thermometer be ap-
 plied, it points to the same degree.
                    * Muschenb. Phisica. 
            DISTRIBUTION  OF HEAT.          7?

  But this is  taking a very hasty view of the  subject.  It is
confounding the quantity of heat in different bodies with its
general strength or intensity, though it is plain that these are
two different things, and should always be distinguished, when
we are thinking of the distribution of heat.   If, for example,.
we have one pound of water  in one vessel, and two pounds in
another, and these two quantities of water are equally hot, as ex-
amined by the thermometer, it is evident, that the two pounds
must contain twice the  quantity of heat that is contained in
one pound.   But, undoubtedly, we can suppose that a cubical
inch of iron may contain more heat than a cubical inch of wood,
heated to the same degree ; and we cannot avoid being convin-
ced of this by daily experience.   Let the iron and wood, for
example, be heated in a bread-oven, to the greatest degree
which the oven commonly gives, and let them then be taken up
and held in the hands of the person who lifts them ; it is certain
that the iron will feel hotter, and will continue for a longer time
to feel hot in the hand which holds it, than the  wood will do;
that is  to say,  the iron will communicate more heat to  the
hand, before it assume the temperature of the hand, than
the wood will  do.  Or, on the  contrary,  let the iron  and
wood be both covered with snow, until the snow has reduced
them to its own temperature, and then let them be both taken
up, and held in the hands ; the iron, in this case, will feel cold-
er than the wood, and will continue for  a longer time to feel
cold in the hand which holds it; that is, it will draw heat fas-
ter, and will continue for a longer time to draw heat from the
hands, before it assume the  temperature of that hand.   It is
therefore plain, that the iron has a greater capacity and attrac-
tion for heat, or the matter  of  heat, that the wood has ;  and
that,  when both of them have been exposed a due time to the
same heating cause, and appear  equally hot by the thermome-
ter, it contains a greater quantity of heat, or of  the matter of
heat, than  the  wood does ;  and, th^bre, in cooling them
both down to the temperature of the ffll^^i greater quantity
of heat comes out of the iron. Or, on thecontrary, if both are
equally cold, as examined by the thermometer, and we, then
make them both  warm to the same degree, the iron takes in
much more heat than the wood. 
 7 6          DISTRIBUTION OF HEAT.

   It was formerly a common supposition, that the quantities
of heat required to increase the heat of different bodies hf
the same number of degrees,  were directly in proportion to
the quantity of matter in each ; and therefore, when the bodies
were of equal size, the quantities of heat were  in proportion
to their  density.   But very  soon  after I began  to think
on this subject, (anno 1760)  I perceived that  this opinion
was a mistake, and that the quantities of  heat which different
kinds of  matter  must receive, to reduce  them to an equili-
brium with one  another, or to raise  their temperature by an
equal number of degrees, are not in proportion to the quantity
of matter in each, but in proportions widely different from this,
and  for which no general principle  or  reason   can  yet be
assigned.  It  will be proper to consult,  on this  subject, the
Comment, de Rebus in Medicina Gestis,  vol. 21, and vol. 26,
containing the valuable experiments of Jo. Carl. Wilcke, ex-
tracted from the Swedish Transactions.   Also  experiments
of Professor Godolin,  in the Nova Acta Reg. Societ. Upsa-
lensis, torn. 5.   This opinion was first suggested to me by an
experiment described by Dr. Boerhaave (Boerhaave Elementa
Chemise, exp. 20, cor. 11.)  After relating the  experiment
which Fahrenheit made at his desire, by  mixing hot and cold
water, he also tells us, that Fahrenheit agitated together quick-
silver and water unequally  heated.   From the Doctor's ac-
count,  it is quite plain, that  quicksilver,  though  it has more
than 13 times the density of water, produced less effect in
heating or  cooling water  to which  it was applied, than an
equal measure of water would  have  produced.   Pie says ex-
pressly, that the quicksilver, whether it was applied hot to
cold water, or cold to hot water,  never produced more effect
in heating  or cooling an equal measure of the water than
would have been produced by water  equally hot or cold with
the quicksilver, and only  two-thirds of  it3 bulk.  He adds,
that it was necessarJj^take three measures of quicksilver to
two of water, in (H^™  produce the same middle tempera-
ture that is produced  by mixing equal mcisores  of hot  anr
cold water. 
            DISTRIBUTION OF  HEAT.         77

   To make this plainer by an example in numbers, let us
 suppose the water to be at the 100th degree of heat, and
 that an equal measure of warm quicksilver at the 150th de-
 gree, is suddenly mixed and  agitated with it.  We know
 that  the middle  temperature between  100  and 150 ; is
 125,  and we know that this middle temperature would be
 produced by mixing the cold water at 100 with an equal
 measure of warm water at 150;  the  heat  of the warm
 water, being lowered by 25 degrees, while that of the cold
 is raised just as much.  But when warm quicksilver is used
 in  piace of warm water, the  temperature of the mixture
 turns out 120 degrees only,  instead of  125.   The quicks
 silver,  therefore, is become  -less  warm by 30 degrees,
 while the water has  become warmer by 20 degrees, only ;
 and yet the quantity of heat which the water has' gained is
 the very same  quantity which the quicksilver  has  lost.
 Tins  shews that the same quantity of the matter of heat
 har. more effect  in heating  quicksilver than in heating  an
 equal measure of water, and therefore that a smaller quan-
 tity of  it is sufficient for increasing  the  sensible heat of
 quicksilver  by the same number of degrees.   The same
 thing appears, whatever way we vary the experiment ; for,
 if the water is the warmer mass and quicksilver  the less
 warm one,  by the above difference,  the temperature pro-
 duced is 130.   The water,  in this case, is  become less
 warm by 20 degrees, while  the heat it has lost, being given
 to the quicksilver, has  made  this  warmer by 30 degrees.
 And lastly, if we take three measures of quicksilver to two
 of water, it is no matter which  of them be the hotter.  The
 temperature produced is always the middle  temperature
 between the two, or 125 degrees,  in the  temperiituj-es al-
 ready mentioned.  Here it is manifest that the same„quan-
 tity of the matter of heat  which makes  two measures of
 water warmer by 25  degrees, is sufficient for making three
 measures of quicksilver warmer by the  same number of
degrees.  Quicksilver, therefore, has Jess capacity for the
 matter of heat than water (if I may be allowed to use this
 expression) has ; it  requires  a smaller quantity of it  to
 raise  its  temperature by the same number of degrees.
                           s 
78         DISTRIBUTION  OF HEAT.
   The inference  which Dr.  Boerhaave  drew from this
 experiment is  very surprising.   Observing that heat is
 not distributed among  different bodies in  proportion  to
 the quantity of matter in each, he concludes that  it is dis-
 tributed in proportion to the space occupied by each body ;
 a  conclusion contradicted  by this  very experiment. Yet
 Muschenbroeck has followed him in this opinion.
   As soon as I understood this experiment in the manner
 I have now explained it, I  found a remarkable agreement
 between it and some experiments made bv  Dr. Martin
 (Essav on the Heating  and Cooling of Bodies) which ap-
 peared at first  very surprising  and unaccountable ;  but,
 being compared with this  one,  may be explained by the
 same  principle.   Dr. Martin  placed  before a good fire,
and at an equal distance from  it, a  quantity of water, and
 an equal bulk or measure of quicksilver, each of them con-
 tained in equal and similar glass vessels, and  each having
 a delicate thermometer  immersed into it.   He then care-
 fully observed the progress, or celerity, with  which  each
of these fluids was heated by the fire, and raised the ther-
 mo;.it ters.   He found,  by  repeated experiments, that the
 quid-   ilver was  warmed by the  fire much faster  than the
 water, almost twice as  fast ;   and  after each  experiment,
 hav ing heated these  two fluids to the same degree, he
 placed them in a  stream of cold air, and  found   that the
 quicksilver was always cooled  much faster than the watei.
 Before these  experiments  were  made, it was  supposed
 that the  quicksilver would require to heat or  cool it a
 longer time than an equal bulk of water, iu the proportion
 of 13  or 14 to one.
   But, from the  view  I have given of Fahrenheit's, or
 Boerhaave's  experiment with  quicksilver and water, the
 above  of  Dr.  Martin's is easily explained.   We need
 only to suppose that the matter of heat, communicated by
 the fire, was  communicated equally to the quicksilver and
 to the  water, but that, as less of it was required for heating
 the quicksilver, than for heating the water, the quicksilver
 necessarily was warmed fastest  of  the two.  And  when
 both,  being equally heated,  were  exposed to the cold air t<-
 cool, the air at  first took their  heat from them equally fast, 
           DISTRIBUTION  OF HEAT.         79

but the quicksilver,  by loosing the same quantity of the
matter of heat that the water lost, was necessarily cooled
to a greater degree ; it therefore became cold much faster.
than the water.  These experiments of Dr. Martin, there-
fore,  agreeing  so well  with  Fahrenheit's  experiment,
plainly shew that quicksilver, notwithstanding its  great
density and weight, requires less heat to heat it, than that
which is  necessary to  heat, by the  same  number of de-
grees, an equal measure  of equally  cold water.  1 he
quicksilver, therefore, may  be  said  to have  less capacity
for the matter of heat.   And we are thus  taught, that,  in
cases in which we may have  occasion to  investigate the
capacity of different bodies  for heat, we can learn it only
by  making experiments.   Some  have accordingly  been
made, both  by myself and others.   Dr. Crawford has
made  a  great number of  very  curious  ones,  and his
Theory of the Heat of Animals is  founded partly on  some
experiments made in this manner, the result of which  is
given in his book on that  subject.
   It appears,  therefore, from the  general result of such
experiments, that  if we had a thousand masses of matter,
©f  the same size and form, but of different materials, and
were to  place them all in the same room, until  they as-
sumed the same temperature ; were we then to introduce
into that room a great  mass of red  hot iron, the heat  of
which, when communicated with all these  different bodies
at the same time, might be sufficient for raising the tempe-
rature of  them all, by 20 degrees  ; the heat thus commu-
nicated from the iron,  although it produced an equal  effect
on  each of these bodies, in raising its temperature by 20
degrees, would not however be equally divided or distri-
buted among them. Some of them would attract and retain
a much greater quantity  of this heat, or matter  of  heat, •
than others ; and the quantity received by  each would not
be  in proportion  to their  densities,  but in proportions
totally unconnected with it;  and perhaps not any two  of
them would receive precisely the same quantity, but  each,
according to its particular capacity, or its  particular  force
of attraction for this matter, would attract and require its
»wn peculiar quantity to  raise its temperature by the 20 
80          DISTRIBUTION  OF HEAT.

degrees, or  to  reduce it to an equilibrium or equality of
saturation with the surrounding bodies.   We must, there-
fore,  conclude  that  different bodies, although  they be of
the same size, or even of the same weight, when they are
reduced to the  same temperature or  degree of heat, what-
ever that be, may contain very different quantities of the
mutter  of heat;  which different  quantities are  necessary
to bring them  to  this  level,  or equilibrium,  with one
another.
   It  may be here remarked,  that the discoveries which
have been made in this way  are very unfavorable to one
of the opinions which have been formed of the nature of
heat.   Many have supposed  that heat is a tremulous,  or
other, motion of the particles of matter, which tremulous
motion they imagined to be communicated from one body
to another.   But, if  this were true,  we must  admit that
the communication would be  in conformity with our gene-
ral experience of the communication of tremulous  motion.
We are not at liberty to feign laws of motion different from
those already admitted, otherwise we can make any suppo-
sition account for any phenomena that we please.  The den-
ser substances  ought surely to be the  most  powerful in
communicating heat to others, or exciting it in them.  The
fact, however, in a great many examples, and yet not in all,
is just the reverse.   Such  an opinion is therefore totally
inconsistent with the phenomna.  I  do not see how this
objection can be evaded.
  When a sufficient number of experiments shall have been
made to ascertain with exactness the capacity of  differ-
ent bodies for the matter of heat, we may try a new method,
different from that of Sir Isaac Newton,  for estimating the
violent  degrees of heat, which are far above the reach of
the thermometer.   Suppose, for example, that a lump of
stone is taken red hot from a strong fire, and plunged into
water, and  that, afcer a little while, when it and the water
are reduced to  the same temperature, the water  is found
warmer than before, by 10degrees.   Suppose also, that we
have learned by other experiments, that,  when heat is com-
municated from this stone to an equal bulk of water, for
•-very one degree by which the water becomes warmer' the 
         COMMUNICATION OF  HEAT.        81

Stone becomes  less warm by two degrees.   Then, if the
stone and water in this experiment were equal in bulk, it is
plain that, while the water was heated by 10 degrees, the
stone must have been cooled 20 degrees'; but if, instead
of their being equal in bulk, the stone had only the fiftieth
part of the bulk of the  water, it must have  been cooled
50 times as much, or was 1000 degrees hotter before it was
plunged  into the  water than  it is now, for  otherwise it
could not have  communicated 10 degrees of heat to fifty
times its bulk of water.
   We have now sufficiently explained what has been learn-
ed  of the distribution of heat among different bodies, the
knowledge of which subject, so far as we have yet advanc-
ed  in it,  has been acquired by the use of thermometers.
 (See Note 1. at the end of the volume.)
      3. Of the  Celerity  of the Communication of Heat.

   A third improvement which has been made in our know-
 ledge of heat by the use of thermometers, is in the inves-
tigation of the  celerity with which it is commucated  from
hotter bodies to colder ones, in different cases; and of the
progression by which its transmission is regulated on those
 occasons.  A  number of experiments  relative to  these
 questions are collected and compared in some of Dr. Mar-
 tin's essays ; and, in  my opinion,  a general account of
 what may be  inferred from  them may  be  given in three
 propositions, expressing general laws of the communication
 of  heat, and by explaining  a sort of deceptions which oc-
 cur in making the experiments.
   The first of these  propositions is, that for any thing we
 yet know to  the contrary, heat,  or the  matter of heat, is
 disposed to enter in to, or to leave  the  different kinds of
 matter with nearly equal  celerity; or, that among the dif-
 ferent kinds of matter, there is no difference, so far as we
 know, in the celerity with which the matter of heat is dis-
 posed to enter into them, or leave them.
    When the experiments which have been made to enquire
 into this point  are exaimned, they seem at first to lead to a
 iery different conclusion.   Some kinds of  matter, in the
 iame circumstances, are found  to be heated and cooled 
 82     COMMUNICAliOX  OF  HEAT.

 much faster than others.  The first opinion formed upon this
subject was, that the times necessary  for the heating or
 cooling of  equal masses  of different matter are directly in
proportion  to the density of the masses ;  or that the cele-
 rity with whKh bodies of equal size are heated or cooled,
is inversely as their density,  or directly as their rarity.
Lord Verulam first entertained this opinion, or rather of-
fered it as a  suspicion: but  Dr.  Boerhaave afterwards
adopted it, and was so much persuaded of it, that he gives
a list or a number of substances differing  greatly in densi-
ty, such as gold, quicksilver, water,  oil, spirit of  wine,
Kther, air,  &c. and he affirms, that  if equal bulks of these
different bodies be  reduced to the  same temperature, and
then exposed to the action of the same heating cause, capa-
ble of making them hotter by 100  degrees, for example,
each will require a different length  of  time from the rest
to receive that temperature ; the  denser bodies requiring a
longer time in porportion to their  density.
   But this opinion is entirely the  work of imagination,
without any  foundation  in fact;  and,  when we have re-
course to experiments, the result proves it  to  be  widely
 erroneous.  This appears from some  experiments of Dr.
 Martin's, one of which  is the experiment on  the heating
and  cooling of quicksilver and water,   already  described:
 ;n which the  quicksilver,  instead  of  requiring  a much
 longer time than water to its being heated or cooled, was
heated or  cooled in the  same  circumstances, much faster
 than the water.
   I am persuaded that when different  kinds of  matter
 are equally exposed to the  same heating cause, the mat-
 ter of heat enters into them all at  first with  nearly equal
 celerity;  at least we have no facts  to prove the contrary.
 But there  is  necessarily a deception  in  the appearances,
 some  of  them  becoming hot sooner,  and some  much
 slower than others,  and  for this  reason,  that those which
 have the greatest capacity for the matter of heat,  are not
 so soon saturated  with it as others.   They require  a
 greater quantity of it, and  a longer time for that quantity
 to be  communicated to  them; arid I  apprehend that the
 time n-ccssary for  the heating or  rooling  of different 
           COMMUNICATION OF  HEAT.       83

 bodies of the same size, and  in the same  circumstances
 and conditions,  will be  found nearly proportional to the
 capacity which each body has for the  matter of heat, or
 to the quantity of it necessary to raise  its temperature by
 a  given  number of degrees;  and that,  in  Dr.  Martin's
 experiment  with  quicksilver  and  water,   although  the
 quicksilver became hot,  and appeared to take in the heat
 faster than the water,  it did not in reality take it in faster,
 but, having less capacity'for heat, was sooner saturated to
 that  degree which accorded with  the circumstances in
 which it was placed.
    For any thing, therefore, that we yet know to the con-
 trary, there is no difference among the different kinds of
 matter, with respect  to  the  celerity  with  which heat is
 disposed to  enter into them, or leave them,  when the cir-
 cumstances are  exactly the  same.  (See Note 2. at the end
/ of the volume.)
    The second  proposition,  or general law,  which may
 be deduced from  the  experiments  on the  heating  and
 cooling of bodies  is  that the  celerity with which  heat
 is  communicated from hotter bodies to colder  ones, is,
 when other  conditions  are  equal,  nearly  proportional
 to the difference of their temperature,  or  to the  inten-
 sity of the heat  in the hot body abbve that of the cold.
    Of this general fact we  have examples  in the experi-
 ments that have  been made to learn  the  pi ogress  with
 which bodies cool, when exposed  to  a stream  of cold
 air.  The heat  is taken away fastest at first, and more and
 more slowly afterwards, in proportion  as the temperature
 of the mass  approaches nearly to that of the air which is
 moving against  it.
     Sir Isaac  Newton supposed, that, in this case, the  quan-
 tities of heat lost, in equal and small portions of the time,
 are exactly proportional to  the excess  of heat remaining
 in the hot body at the beginning of such small  portions,
 or to the difference between its  temperature and that of
 the air;  and, therefore,  that the quantities  of heat lost in
 equal divisions of time are a series of proportionals, or in
 a geometrical progression*.   This idea of Sir Isaac.New-
 ton's is supported or verified by some experiments which
               (  Phi' i av.b ab-idjred, IV. 2. p. 3. 
84       COMMUNICATION  OF HEAT.
 were made, in a great variety of circumstances,  by the
 ingenious  Professor  Richman,  and  published  in  the
 transactions  of the  Petersburgh academy for the year
 1750.
   Dr. Martin,  it is true,  also made some  experiments
 relative to this question, by which experiments he thought
 it appeared  that  the progression of  cooling  varied a
 little from this rule*.   But I am not  satisfied  with Dr.
 Martin's experiments on this subject.  I think they were
 attended with  a circumstance that  might deceive  him.
 There is room, however, for more inquiry into this point
 by further  experiments.
  It is proper, also, to remark,  that experiments made
with this view can have no  value, except when the heat is
 taken from the mass, or communicated to it, by a stream
 of air  playing constantly and uniformly  on it, and all the
 time at the same temperature; for in this case only can it
 act in  a regular manner.  If, on the contrary, the air be
 not in  motion, but in a stagnating state;  and still more, if
 the hot body  be inclosed with a small quantity of  it, the
 natural progress of cooling must be totally deranged, as in
 this case,  the  cooling  cause will not continue the same,
 the air becoming warmer  every moment,  while it  cools
 the hot body, and therefore  less fit to take away the rest
 of its heat.  This enables  us to understand why,  in cold
 weather, if the air is calm,  we are not so much  chilled as
 when there is wind along with the cold; for in calm wea-
 ther, our clothes, and the air entangled in them,  receive
 heat from our bodies ;  this heat is  accumulated in them
 to  a certain  degree, and  brings them nearer  than the
 surrounding air to  the temperature of  our  skin.   But
 in  windy  weather,  this heat is prevented,  in a  great
 measure, from  accumulating; the  cold  air,  by  its im-
 pulse.,  and rapid succession, both  cooling our clothes fas-
 ter, and carrying away the warm air that was entangled in
 them.
   The sensation  of  coldness,  therefore,  produced by
 wind,  or agitated air, is so much stronger than  that pro-
 duced by  equally  cold  air in a stagnating  state, that we
 are often persuaded the agitated air is actually  colder,
                 * Martin'* Essays, p. 55, &x. 
         COMMUNICATION OF HEAT.      jm

until we examine it by the thermometer ;  and Dr. Boer-
haave thought the deception so strong, that he contrived
an experiment to remove it completely (Boerhaave  Ele-
menta  Chemise).   He suspended a thermometer in the
air of  a large  room, for  some  time, and  noteing the
degree to which it pointed, he then directed against the
bulb  of it a stream of air, impelled by a large bellows, in
the same room.   That stream of air would certainly feel
to a  person who opposed any part of his body to it, con-
siderably colder than the rest of the air in the same room ;
but the thermometer is  not in  the  least  affected by it.
And it would be easy to exhibit  another experiment, to
shew,  that agitated  air is not made  colder by agitation.
A piece of ice,  for  example, being  suspended in the air
of a  warm  room, and blown upon by a bellows, instead
of being thereby kept  the  more  cool,  as  our  hand
would  be,  and preserved  the longer from  being totally
melted, would  certainly  be melted  so much the faster,
than when  the air is allowed to stagnate in some  measure
around it.
   The last of the three  propositions  which I said may be
deduced from the experiments on the communication of
heat is, that
   The  celerity with which heat is  communicated from
hotter bodies to  colder ones, when  all other conditions
are  equal,  is proportionable to  the extent of contact,
and closeness of communication, between the bodies.
   Every person is prepared to  admit  this general rule
by common  experience.   We  all  understand, that,  if
we had  two  balls,  and two  cubes  of gold, and one of
each was made very hot in an oven, and then laid  upon
the other,  the heat would be communicated milch  faster
from the hot cube to the cold one, than from the hot
ball  to  the cold one, which is touched by  it in one  point
only.
   Of the general fact expressed  by  this proposition, we
have examples in the experiments that  have been made
on the heating and cooling of masses of the  same matter,
but of different forms, and different sizes,  exposed at the
same time  to be heated by a constant and equal heating
   V< ..   T.                  T 
86       COMMUNICATION  OF HFAT.

cause, or to be cooled by a stream of cold air.  It being
at the surface that the body has contact and communica*
tion,  either with the cold air or with the  heating cause, ¥
the extent of  contact  and communication  of parts  is, in
this case, the whole  surface.   Accordingly, those  forms
which contain the same quantity of matter under the
greater surface,  are the more quickly heated or cooled,
in proportion  to the extent of that surface ;  an ounce of
gold in the form of  a cube, is  heated and cooled sooner
than the same quantity in  the form of a ball ;  and if it
be  beaten into the form of  a thin plate, it is heated and
cooled incomparably sooner, in  proportion  as the surface
is then  the more extended ;  and, for the same reason,
when the masses we  compare have the same form, and'
differ only in their size, the smaller are heated or cooled
faster than the larger, or in  the shorter time, in the direct
proportion of their dimensions.  If we have, for example,
two cubes of  gold, in one of which the diameter is only
the half of that of the other, and both of them are pl.unged
into boiling water to be heated ;  or after they are heated,
to be  exposed to a  stream of cold air to cool, the smaller
will be heated or cooled in  half the time  that is required
by the other;  for, in the smaller  body, the surface bears
twice the proportion to the  quantity of matter,  and con-
sequently to the quantity of heat, which is to be either
communicated or taken away, that it does in the larger,
the quantity of  matter being as the  cube, and the sur-
face only as the  square of  the  diameter.   And, accord-
ingly, some   experiments  made  by  Sir   Isaac  Newton,
and by Dr. Martin,  agree sufficiently with  this general
rule,  which applies equally  to all  masses of unequal size,
but of the*3ame matter, and having the same form,  what-
ever that be.
   This enables  us  to understand  how it happens, that in
those arts ia which much heat and fuel  are employed, as
in the extraction of metals from their ores,  foundaries, dis-
tilleries, and the like, experience has shewn, that the larger
they make their furnaces and stills, and  other such  uten-
sils,  they do their work with so  much  the less fuel;
'or, in the larger furnaces and stills, and other such uten-
sils,  the e>:uji:t of surface,  communicating with the  air, 
COMMUNICATION  OF HEAT.
 bears a smaller proportion to the contents of the  furnace or
 still', and to the quantity of fuel employed, or of heat produc-
 ed, than in smaller utensils of the same nature ; and the waste
 of heat is therefore less.
   We can also explain,, by the same general  principle, the
 slowness with which heat is communicated  from one part to
 another of those bodies, or assemblages of bodies, which are
 uncommonly rare or spongy, through which it passes much
 more slowly  than through the dense and  heavy.   A  thick
 rod of iron,  for example, or copper,  if one end of it be put
 into the fire, and  made red hot, transmits the heat so quickly,
 and in  such  quantity, to the other  end, that it  cannot be
 touched without burning the hand ;  while a stick of wood of
 the same size  can easily be held by the one end, while tho
 other is burning.   A piece of cork transmits heat through it
 still more slowly than the  wood, or is a worse conductor of
 heat;  and a mass of wool, or feathers, or furs, excel even the
 cork  in  the slowness with which heat is conducted through
 them.  Such  materials compose  a mass  so spongy and  rare,
 and between the parts of which there is so little closeness of
 contact, that the heat is communicated with difficulty through
 them; and, although the interstices or pores of such bodies
 are filled with air, this being a fluid excessively  rare, and
 which, in this case, is kept entangled, and is in a stagnating
 state,  it gives little assistance in the transmission of heat.
 These are the reasons why such materials are so effectual in
 keeping our bodies warm in very cold weather.  In very cold
 countries, the inhabitants  are not only wrapped in a thick
 clothing of furs by day, but cover themselves with feather
 beds by  night.  Frequent  experience of the effect of these
 materials disposes us to consider them as warm, or  as having
 the power of adding  heat to the bodies wrapped  in  them ;
 but this is a mistake, they only preserve the heat of such
 bodies from  being dissipated so fast  as it would otherwise
 be; and, by the  same quality, they may be, and are, employ-
 ed to keep things cool in some cases.  If, for example, we
 wish to preserve a lump of ice from melting fast in a warm
room,  wo cannot do better than to wrap it in plentv of flan- 
88        COMMUNICATION  OF HEAT.
nel, or furs, or the like materials, which produce the desired
effect,  by retarding  the  communication  of heat  to the ice
from the air, and other bodies contained in the room.
  These coverings, therefore, of wool,  furs, feathers, and
such like materials, keep  us warm, not by adding heat to our
bodies, but by preserving our natural heat  from being dissi.
pated too fast.
  In the same manner does snow preserve  the ground from
being deep frozen, and the vegetables from being destroyed,
in the  very cold  countries.   Nothing can be imagined more
spongy, soft, and downy, than snow that has not been com-
pressed; nothing is  more fit to serve this  purpose.   The
showers of  it that fall in this climate, may be considered as
accidental,  and  of little  importance; the  moderate tempe-
rature of  our winters rarely requires such  a  preservative.
But in those parts of the world that are exposed  to violent
cold,  it is well known to fall regularly in  the beginning  of
winter, until it cover the ground to the depth of several feet,
and, where  the cold is excessive, to  that of several yards, and
even of fathoms.  Thus nature,  which has clothed the ani-
mals of those regions with  the  richest and warmest  furs,
throws over the  ground this thick and downy covering, which
has a wonderful  effect in  preserving its heat.  It cannot, it is
true,  preserve it  at a higher degree than the freezing point;
for unless the surface be  cooled to this degree, the snow can-
not be on  it.  But this degree is a great degree of heat, when
compared with the terrible and deadly cold which is common
during the winter in the  air of those climates.  In  countries
exposed to severe winters, the thermometer commonly falls
to the beginning of Fahrenheit's'scale, or 32 of his degrees
below frost: a degree of  cold which  is just  as much colder
than  simple frost, as frost is colder than the warmth of our
summer weather.  And, in  the countries the most remarka-
ble for severe winters, a  cold has often been observed, which
was more than twice as far below frost.  In Siberia, at Kras-
nojark, east  of Siberia, and in some of our  settlements
in Hudson's Ray, the thermometer  has often fallen to 35 and
 « degrees below o of Fahrenheit:  a degree of cold in which 
          COMMUNICATION OF  HEAT.        89

quicksilver was congealed by simply exposing  it to the cold
 air.
  In such dreadful colds, a great depth of snow lying on the
ground has much effect in preserving the vegetables.
  It is  proper, however, in this place to remark, that this
effect of the rarity, or sponginess of bodies, this  retardation
 of communication, or transition of heat through them, does not
 take place in fluids equally as in solids.   Fluids generlly re-
 ceive heat,  and transmit it through them more qucikly, than
 the greater number of solids.  Of this we have an obvious
 example  in the air, which, though excessively  rare and light,
 cools the bodies exposed to it remarkably fast; much faster
 than we might expect from our knowledge of its rarity.
   This  depends,  partly, on  the  capacity which  such fluids
 may have for the matter of heat, but more on  their great ex-
 pansibility by heat, and on the easy mobility of their parts
 among one another; which two. last qualities, combined  in
 the same matter,  produce a particular effect, which  occa-
 sions the quick dispersion of heat through it.   This effect is a
 constant  motion, or change of the fluid in contact with the
 heated body.                                         '
    If, for example, we expose a hot body, as  a lump of red
 hot iron, to the air to cool, the air in contact with it becomes
 immediately  hot, while it  takes heat from the iron.  In be-
 coming hot it is also greatly expanded, and  rendered more
 light than the surrounding parts of the atmosphere.  It  is,
 therefore, pressed  upwards by these, in the same manner as a
 piece of Cork, a quantity of oil, or any lighter substance, is pres-
 sed upwards  when we attempt.to sink it in water.  It does not,
 therefore, remain in cor/tact with the iron, but is forced to rise
 in a stream, and leave its place to colder air, which comes from
 all around.  This again  is soon heated and expanded, and made
 to rise in the same manner, and thus fresh and cool air is ap-
 plied to  the hot iron in constant succession, by which its heat
 is carried off much faster than if the matter in contact with it
 were perfectly stagnant.
    This ascending motion of the air that is in contact with a hot
 body becomes evident when we hold our handperpendicularlv 
90            SOME EFFECTS  OF THE

over it, though at a considerable distance,  or  when we place
the hot body in the light of the sun, and so that the shadow of it
may be thrown on a white wall:  a waving stream, like a stream
of smoke rising from the shadow, is  seen on the wall.  This
appearance is caused by the rarefied air, which, having its
power to refract  or attract light considerably, changed by its
rarefaction, the light which passes through it is turned out of
its course to one side or the other,  in  a fluctuating manner,
and therefore does not fully and regularly illuminate that part
of the wall to which it was tending,  but produces an imper-
fect and unsteady illumination, which resembles the shadow
ofsmoke.
  Another effect produced by this motion of warm air, is the
tremulous appearance of objects seen at a*distance through the
smoke, or heated air, arising  from  a fir?,  or from chimney
tops, or from dry ground much heated by the sun ; for the
light which comes from these objects to the eye, is. disturbed
a little from its straight course, by passing through the streams
of warm air ; and coming, therefore, to the eye,, with succes-
sive changes of its direction, occasions an unsteadiness in the
apparent place of the object, or parts of the object, from which
it comes.
   We also learn from this effect of heat upon air, the reason
of the common opinion, that heat, or  fire, has a tendency up-
wards.  This opinion has been  founded  on the stream of
warm air which  may be felt ascending from any hot body
exposed to the atmosphere ; and the appearance and properties
of flame have also contributed to the  support of  such an opi-
nion.  Flame, in consequence  of its being composed  of a
luminous fluid, rarer than air, is driven upwards, in this last,
on every occasion.   To be convinced, however,  that heat has
not a prevalent tendency upwards, we need only  shut up a hot
body in a glass receiver, and  exhaust the air: we shall then
find the heat penetrating equally through  every part of the
receiver; or, if it affects some more than  others, it is those
 that are nearest the  hot body.  But, let the air  be admitted
 into the receiver, the uppermost part  of the glass will soon be
 the hottest,  although the most distant from the hot body. 
EXPANSION OF  AIR.             91
  Heat, therefore, by  rarefying air, and  thereby producing,
a motion in the parts of this fluid, is more quickly and equally
dispersed through it than through any solid quantity of matter,
and occasions the appearances above described.
  There is one other fact that depends on this principle, and
which deserves particular notice, on account of its importance.
This fact is the ventilation of mining works that are  properly
planned, andj|^ proper effect of fire-places and vents in houses
rightly constructed.
  In mines that are' properly planned, a stream of air passes
constantly through the mine, and with this remarkable circum-
stance, that it  has one direction in summer, and another in
winter.
  To  understand this, it is necessary  to know, that in be-
ginning to mine for metals or  minerals, the  mountain or
rising ground in which they are  contained, is first pierced in
an horizontal direction, that the water which  abounds in sub-
terraneous places may run out constantly,  while the workmen
proceed forwards to the vein of ore.  This horizontal  entrance
into the mountain, or mining ground, is called the level, drain,
or drift of the mine, or sometimes the adit.  But when they
have proceeded forward a certain length, they find themselves
distressed for want of  fresh air.  The long excavation they
have made contains a quantity of air in a stagnating, or nearly
stagnating state, which is  exhausted of its vital  principle by
their breathing it, and  by their candles.  Their candles, there-
fore, first burn dim, and  at last are  extinguished, and give
them warning that their own lives are in  imminent danger.
The best remedy for this is, to dig a wide pit perpendicularly
from the surface over the internal extremity of the level, so
deep as to communicate  with it.  No sooner  is this  done,
than there is plenty of fresh air in the mine, a current is im-
mediately established,  which in summer  enters through the
pit, and passes out through  the  level, and in winter as  in-
variably enters through the level, and rises "up through the
pit.
  The reason of this is, that the heats of summer, and the
cslds  of winter do not penetrate so quickly  into the interior 
    92           VENTILATION  OF MINES.

   parts of the earth, as to produce there the same  changes of
   temperature that happen in the atmosphere. The commumca.
   tion of heat through large masses of solid matter being slow,
   there is not sufficient time, during the summer, for the heat
   of the air, or that produced by the sun's light, to act so com.
   pletely on  the mass of this globe, or to give  to the internal
   parts of it the summer heat of the  atmosphere ;  and, for
   the same reason, the winter is too short to admj*^ such a loss
   of heat from  the interior parts of the earth, as^o bring down
   »he heat of these to a level with that of  the surface and the
   atmosphere.
      In the interior parts, therefore, the course of time has pro-
   duced  a temperature, between the heat of summer, and the
   cold of winter,  which undergoes  very little  variation.  Of
   this we have experience when we place thermometers in
   deep caves, or pits, that have little communication with the
   atmospherical air, or when we plunge them into the water of
   perennial springs, where they rise  out of the ground.  The
   thermometer points to the same  degree  in summer and in
   winter; which degree is the medium between the two seasons
   in  that particular part of the  earth's surface.  It is evident,
   therefore, that the internal parts of the earth, and along with
   them,  the  air contained  in the cavities  of  mines, must be
    warmer in winter, and colder in  summer than the external
    atmosphere.  Hence the column of air contained in the pit
    of the mine is heavier in summer, and lighter in winter, than
    an equal column  of the external air; and this occasions the
    motion which takes place,....for the internal column  gravita-
    ting on the internal extremity of the level, is counterpoised by
    an external column which gravitates on the external extremity
'^   of the same  level, and, whenever one exceeds the other in
 *'  density and weight, the heavier must force the lighter one to
    ascend. *The internal column, therefore, descends in summer,
    and is forced upward in winter,  its  place being supplied by
    external air, which enters during  the first season at the top of
    the pit, and during the latter at the external extremity of the
    level. And this air, when it enters, being very soon reduced
    to the temperature of the internal parts of the earth, the same 
       •    VENTILATION OF MINES.         93

conditions continue to take  place, and  the current of air
through the mine continues as long as the heat of the external
air is different from the temperature of the internal parts of
the earth.  The truth of this  explication is remarkably  con-
firmed, by a stagnation of the air in such mines, which now
and then happens in the spring and autumn,  the atmosphere
in those seasons sometimes assuming the middle temperament.
The cause of the motion is thus discontinued, and the workmen
must have, at such times, recourse to other means, to obtain
a renewal of  the air in the mine; such as,  kindling a fire at
the bottom of the pit, to give a little superiority of heat to the
column of air  which it contains,  or directing  into it the force
of the wind to set it in motion. But these expedients are em-
ployed only on particular occasions.  The ventilation of mines,
in general, depends on the  principle above explained.  If it
should appear difficult to imagine how the external air should
thus pass  constantly through the mine, without reducing it to
a degree of heat coinciding with its own, it  must be consider-
ed that the solid mass of matter,  through  which the mine is
dug,  is so enormously great,  when compared with the small
stream of air  which passes through the works, that this air
cannot have any sensible effect in changing the temperature of
the internal parts of the mine.  They, no doubt,  receive a.
little heat in this  way during  the  summer, and lose a small
part of their heat during the  winter; but this acquisition or
loss is too small to prevent the ordinary conditions  from con-
tinuing in force.  It remains only to be further observed, with
regard to  this subject; that many mines are so extensive, or
of such a nature, that it is necessary for drawing out the water
or the rubbish, or other materials cut out of the mine, to  dig
several pits,  at considerable  distances from one another, to
communicate with different parts of the subterraneous exca-
vations.   These different pits, however, produce  the same
effect as the  single  pit and level, above  supposed; for be-
ing different  from one another in depth, in consequence of
the unevenness of the earh's surface, the pits which are dug
in the higher parts of the ground contain an  higher column of
air than those dug in the lower; and, though the temperature
   VOL. I.                    U 
94          EQUABLE DISTRIBUTION

of heat is the same in them all, and  equally different irom
that of the atmosphere, the  difference of height produces a
prtponderancy in the one or the other;  and the same effect
is produced in ventilating the  mine.   The same  principles
explain the ventilation, or  draught of air,  as it  is called
in common  chimneys  and vents,....and  the  vents  of the
lower  rooms in  a  house carry up  the smoke better than
the vents  of the upper rooms that are nearer the  top of
the house ; but  the venting of chimneys  is liable to many
accidents and variations, which are  afterwards to be con-
sidered.
   Ail  these motions produced by heat in atmospherical air,
and the  quick dispersion of it through it,  depends  on  two
qualities, formerly mentioned as belonging to fluids in general;
I mean the easy mobility of their parts among one another,
and their  expansibility by heat; and as these two  qualities
are found  in air, in an eminent degree, it is,  therefore, the
more  remarkably affected in  the  manner I  have  describ-
ed.        4
   All other fluids,  however, are affected in a similar manner,
by heat applied to them ; but not being so expansible,  the
motion produced in them is not so conspicuous.  But a mo-
 tion is produced in them, as'well as in air, which occasions
 the heat applied to be more  quickly and equally dispersed
 through them than through most solid bodies.
   When water, for  example, is set in a vessel over the fire,
  the heat is necessarily communicated first to the lower parts
  of the fluid, and,  while these receive the heat, they are ex-
  panded and become lighter than the rest of the water; they
  are therefore driven upwards, and their place at the bottom is
  supplied  by the colder, heavier  parts of  the  fluid.  These,
  however, are  soon heated, and made to rise in their turn,
  and thus  there is a constant motion, or circulation of the parts
  of the water,  which occasions a quick and equal distribution
  of the heat through it.  For  this reason, water is  often em-
  ployed in chemistry,  as a medium through which heat is com-
  municated to vessels of glass, these being most secure from
  being broken  when  the heat is applied to them gradually 
                OF HEAT  IN FLUIDS.             95

 •and equally, over the whole of  their surface.   When water
 is applied to this purpose in chemistry, it is called the  Water
 Bath.
   A motion, ^depending on the same principle, is also the
 cause which hinders the water of deep lakes from being frozen
 during the winter, in this climate, and at times when the sur-
 face of pools, or other shallow collections of water, is totally
 frozen.   The cold air, while  it passes over the surface  of the
 water,  necessarily cools it.   The superficial water, by being
 thus  cooled, becomes denser  and heavier than the water that
 is below it.   It therefore sinks  downwards,  in  innumerable
 small streams, leaving its place at the surface to be occupied
 by the less cold water which rises from below.   This, in its
 turn, is deprived of a part of  its  heat, and  disposed to sink in
 the same manner; and thus a constant circulation of the parts
 of the water is established, a constant exchange between the
 surface  and the bottom ;  in  consequence of which there is
 such a quantity of water to be cooled,  when the lake is large
 and deep, that there is  not sufficient time, during our  frosty
 season,  for its being all cooled  to near the freezing  point.
 And it must be cooled to within a few degrees of that point,
 before that part which is at the surface will remain there long
 enough to be frozen*.
   But large  seas, which contain collections of water incom-
 parably greater than those of lakes, and which are agitated by
 winds and  currents, are,  in  consequence  of these circum-
 stances, so little  affected by the cold of winter, that their
 temperature  continues  nearly the same through the whole
 year, like that of the interior parts of the earth.  And this
 mild temperature of the ocean  affects the air which passes
 over it,  and the countries  that  are  entirely or mostly sur-
 rounded  with it;  when places surrounded  with extent of
 continent, or exposed to winds blowing over a northerly con-
 tinent, are affected in winter with violent cold.   As the heat
 of  summer penetrates but slowdy into the  solid mass of this
 globe, so, during the winter, it  comes out again but slowly.
  * When water is cooled to 6 or 8 degrees above freezing', it is not mads
sensibly denser by a diminution of heat.....De Luc. • 
96        UNEQUAL TEMPERATURE

The solid parts of the globe are at rest among one another,
and cannot be set in motion by moderate heat, like the parts
of fluids.  Heat,  therefore, is transmitted through them with
that slow progress  with which  it commonly  pervades large
masses  of solid matter.   The  cold winds, therefore, while
th* v blow over a great continent, very  soon cool the surface
of the earth.  The  heat of  the interior parts  of it does not
quicklv ascend to temper that coldness, as it  does from the
bottom  of large collections of water.  The surface, therefore,
is cooled  to a  great degree, and the cold winds afterwards,
while they pass over this surface, not meeting with any thing
that can  diminish their coldness, act with their full force on
the matter  exposed to them.   (See Note 3. at the end of the
volume.J
   When  we now reflect on those  motions which heat pro.
duces in  fluids, the circulation of  them which it occasions,
and the quick  and equal distribution of heat through  them,
 which  follows from thence, we, find one phenomenon  in na-
 ture, which, on  the first view of it,  may appear  inconsistent
 with these  general principles.  This phenomenon is the une.
 qual distribution of heat  in the atmosphere, in  which it  is
 well known that the lower parts are much warmer than the
 superior.   We have  satisfactory  proofs of  the  coldness of
 the superior parts of  the  atmosphere, when we ascend high
 mountains, and  other elevated parts of the earth's surface,  in
 whatever climate be their  situation.   1 he French academi-
 cians who  were sent to measure a degree of the meridian at
 the equator, when they left the sultry plains of South America,
 and ascended the high mountains  of that part of the world,
  passed through  a succession of  strata of the atmosphere, in.'
  u-easmg in coldness, and distinctly  marked  by their produc
  tions, as well as by the manner in which they affected their
  feelings and the thermometer, until they reached the regions
  of perpetual frost and the vast collections of snow, which are
  but little wasted there by the melting, but continue to accumu-
  late annually, untilthey overhang the precipices of the moun-
  tains, and fall at last with a dreadful ruin on the coontry that
li.ern. 
                 OF THE  ATMOSPHERE.            97

      The same perpetual frost, and accumulation of snows, with
   the consequences of it,  are well known to  take place on all
   other high mountains, such as the Alps and Pyrenees ;  and
   the coldness of the superior parts of the atmosphere has been
   further experienced by those who have ascended from the
   plains to a great  heiglnyby the use of baloons.  To all those
   proofs, we may add the showers of hail which sometimes fall
   in the heat of summer and the piercing cold with which these
,  are accompanied, in consequence of there mixing a part of the
   superior with the inferior air.
      It is, therefore,  plain  that only the lower region of the air
   is warm, the heat being found to diminish while we ascend in
   the atmosphere, and  that the  very high regions of it are ex-
   cessively cold.   This has long been known, and  attempts
   have been made  to explain how it happens.
      The explanation that has been commonly given is, that the
   light of the sun reflected from the  sides of the mountains, and
   other elevated parts, into  the valleys and plains,  is the cause
   of the  greater  heat of  the air in these.  But  it is easy  to
   shew that this opinion is a great  mistake.  If the declivities
   of the mountains,  and other elevated parts of the earth, had
   polished surfaces, the light would be reflected in the manner
   supposed.  But  the surface of the earth is every where com-
   posed of rough  uneven  materials, which absorb  the greater
   part of the light  that falls  on  them, and disperse  the rest  in
   every  possible direction; the consequence  of  which is the
   contrary  to what is here supposed;  for  the  mountains must
   receive more reflected or dispersed light from the plains than
   that which the plains can receive  from  the mountains.   The
    quantity  of  light dispersed from the surface of mountains
   and plains, is in general  proportional to the extent  of those
   surfaces;  and plains are generally far more extensive  than
   those  declivities of mountains, from  which  they can ra*eivc
   dispersed light";  and, moreover, the  greatest number of ex-
   tensive plains are at such a distance from mountains, that it
   cannot be supposed that  they  owe their superior heat to light
   reflected from these.  There are  parts of them, from which
   the mountains are at such a distance that they cannot be seen, 
 98         UNEQUAL TEMPERATURE
and  yet  these  parts  are as warm,  and generally warmer,
 than  those which are  in the neighborhood of  the  moun-
tains.
   The  principles from which a good explication of this phe.
nomonon may be deduced, are two well known qualaties of
the atmosphere,....its transparency, and its elasticity or com.
possibility.   A consequence of tire transparency of the at-
mosphere is, that the light of the sun, in passing through it to
the surface of the earth, does not  communicate heat to it, or
communicates only a very small quantity,  especially in  the
superior regions, which are much more clear and transparent
than the inferior.    It is  the  nature of light to excite heat,
chiefly in  opaque bodies,  which stop its progress and absorb
it;  but very little in those that are transparent, and allow it to
pass through  them.   To be satisfied of this, with respect to
air, we need only to reflect on our own*experience.   We have
all felt, that in a clear day in the spring season, or in  the be-
ginning of summer,  the air is often'very cool, although the
sun shines bright,  and has great power to warm the solid and
opaque- bodies which are expo'sed to rays.   Stones, slates
plates of metal, and other dry and  opaque bodies,'will become
warmer than  the human  body, though the  air at  the same
time feels remarkably cold.  We have another strong proof
of this, when we attend to the focus of a large burning glass,
or mirror, exposed to the sun.   Although any solid or opaque
body, introduced into the spot, is melted or dissipated, or con-
sumed  in an  instant,....before it is put there, we cannot, by
looking at the place,  perceive any signs  of that intense  and
terrible heat by which it is so quickly affected.  The truth is,
that there is no gr.-at heat in that spot, until  the opaque body
is placed in  it.   There is a great quantity of light  passing
through that  place, but the air being transparent, little heat is
produced. There is another experiment, by which the effect of
transparency, in a medium transmitting light, is further demon-
strated.   If  a large lens be  exposed to the sun, and a tub of
clear water be placed below it, so situated that the focus of the
lens shall be  formed below the surface of  the* water, and in
the centre of  its mass, there are no remarkable signs of great 
              OF THE  ATMOSPHERE.             99

heat produced in that focus ; but let a stick be thurst  down,
and introduced into that spot, the water will soon boil on the
surface of the stick, and the internal parts of the wood will be
scorched to a black coal, by the intense light which penetrates
into them, and acts so powerfully upon the surface.
  It is therefore  evident that the sun warms the atmosphere,
by first warming the surface of the earth, from which the heat
is communicated to the air immediately above it; and, no doubt,
it must act also  on  the vapors and effluvia  which diminish
the transparency of the lower region of the atmosphere.   But
its  greatest action is on the earth.  We can perceive this in
clear sunshine, and  when the sun is high, by looking at dis-
tant objects over the ridge of a rising ground.  Those objects,
or  their parts, appear to have a tremulous motion, the light
which comes from them to the  eye  being disturbed in its
course by the rising of warm'air, which is constantly receiv-
ing heat from the surface of the soil.
   Thus we have accounted for the h,eat being found chiefly in
the inferior region of the atmosphere : we must next explain
why it remains in a great measure there, and is not so quickly-
communicated to the superior regions as  we might expect,
when we consider the expansibility of the air, and the motions
which heat applied to it commonly produces.
   This partial distribution and confinement, in some measure,
of heat, to the lower region of the atmosphere, depends on the
elasticity and compressibility of the  air.  A consequence of
this quality is, that the lower region of it is condensed by the
pressure of the superior parts; and, although it be expanded
by the heat in it, and thus wiade rarer than it would otherwise
be, this expansion is not equal to the condensation it suffers
by the pressure of  the  superior parts*.   The lower air,
therefore,  although  heated and expanded, continues  still
denser than the strata  of the atmosphere that are considerably
above it, and is not  disposed to be pressed or driven far

  * When we descend from high mountains, the air is found warmer by one
degree for every 200 feet of perpendicular descent: but the pressure of 200
feet perpendicular of the atmosphere, has two and a-half times as much
effect in condensing the air, as one degree of heat has in rarefying ft 
ioo         UNEQUAL TEMPERATURE

upwards in them, but remains  in its place near the surface
of the earth, where the heat  is most useful for the production
and ffupport of vegetables and animals*.  Thus we are enabled
to understand the  excellent effects of  a proper degree of
shelter  in increasing the fertility of a country, and the comfort
of its inhabitant;.   Whatever preserves  the lower regions of
the air  from frequent and violent agitation, improves the cli-
mate  in our northern latitudes, by preventing more or less the
chilling effects of wind, and dispersion of the warmth accu-
mulated at the surface of the earth.  This shelter is procured,
to a certain degree, by a due proportion of plantations of tim-
ber trees, properly disposed  over  the face  of a country, so as
to break or diminish the force of the winds.
  If  it  should be here asked, what is  meant  by the  lower
region of the air, and what is the  limit between it and higher
regions  ? the answer must be, that there  is no precise limit;
but that, in general,  by the lower region, I mean the parts of .
the atmosphere which  do not reach  to an elevation equal to
that of high mountains.   Very  few of the highest mountains
rise to  the height of two miles above the  surface of the sea;
but we  know for certain^ by  the appearance of some meteors,
as well as by calculation, that our atmosphere exists,  and
is capable  of action at the  height of fifty miles,  though at
that height, by the want of pressure, it is in a highly rarefied
state.
   Even the lower  region,  as above pointed out, is very
different in different parts of it; those parts nearest the earth,
and in the lowest situations, being in general the warmest, as
well as  the most compressed ;  while the heat of those  above
them, and the pressure to which they are exposed,  gradually
diminishes.
   The  diminution of the heat is about one dpgree of Fahren-
heit for every two hundred  feet of elevation, and the multi-
  * The celebrated Dr. Franklin explained, in much the  same manner, the
unequal distribution of heat in the atmosphere;  and deduced from  these
principles the phenomena of whirlwinds, hurricanes, and water-spouts, which
never before were so ingeniously and satisfactorily  accounted for. See Phil.
Trans, vol. 55,....and Experiments and Observation- on F.lectricitv, he. bv
Benjamin r.->.-.k!in, p. 182 and 21 ^ 
              OF THE ATMOSPHERE.           101

plication of bulk is proportional (very nearly) to the elevation
of temperature.  The greatest difference of heat, therefore,
balances two-fifths of the effect of compression, and cannot,
alone, raise the air contiguous to the earth's surface to any
considerable height.   Were it not for the winds, the region
of perpetual snow, even under the equator, would  not be a
mile from the level of the sea.   Thus, there is  a gradual and
undefinable transition from the warmth of the lower air to the
coldness of the higher; and, in the higher regions, the same
gradual diminution of heat and compression takes place.
  This view of our atmosphere enables us to understand why
those horizontal  motions of the  air,  which we call winds,
and which have some effect in mixing the superior colder with
the inferior warmer air, produce this effect in a far less degree
than might be apprehended, although it often happens that
they have different directions  in the different strata; of the
atmosphere at the same  time; for the strata, which differ
greatly in their heat, may easily move horizontally in different
directions,  without mixing  together,  being separated by a
multitude of  intermediate strata,  in which there is a regular
and slow gradation of density and  heat.   We were led to
give the preceding views of the communication, distribution,
and progression of heat, as examples of some of the improve-
ments  we have made in our  knowledge of heat,  since the
invention of thermometers : and this invention arose  from the
discovery of the expanding power of heat.
  Having,  therefore, now  done  with  the consideration of
expansion, we proceed to consider* the  next of the general
-fleets of heat, formerly enumerated, which is fluidity.
VOL. i. 
102                 HEAT ALONE
              SECT. II....OF FLUIDITY.

   THE propriety of considering fluidity as one of the genera*
effects of  heat, is sufficiently obvious.  It is well known that
a  great number of different  substances, which commonly
appear in  a solid state, can be rendered fluid by heat, and will
continue,  so long as  they  continue heated to the  proper
degree.   But, besides this consideration, we find reason to be
persuaded, that fluidity,  wherever  we meet with  it in  the
various kinds of matter, is  always the effect or consequence
of heat;  experience, in  most  cases, having taught us, that
those bodies which commonly appear in a fluid state, and the
fluidity of which might be thought a natural  and essential
qu;.litv, do nevertheless derive this fluidity from the quantity
of heat they contain j^for, when we diminish their heat suffi-
ciently, they do not fail to assume a solid form ; from which
it is plain  that the fluid state in which we commonly see them
depends upon heat; nor  is  there any other circumstance, so
far as we  can observe, that is necessary and indispensable to
their fluidity.
   In general, therefore, we have the strongest reasons for con-
sidering heat as the efficient cause of fluidity, in  all bodies in
which this quality is found.
   But,  as there is a  great diversity among different bodies*,
with respect to the intensity  of heat necessary to produce this
effect on them, this shews that there  are certain peculiarities
in their constitution, which increase in some, and diminish in
others, the disposition to fluidity; and these peculiarities are
plainly certain differences in the nature, number, and composi-
tion of their constituent parts ;  a proof of which is, that we  can
by mixture, in  most cases, change a  body from its ordinary
state, in this respect.   Water, for example, by having salt or
spirit of wine mixed with it, acquires an  increase of its dis-
position to fluidity, and is enabled to endure, without freezing,
a more intense cold than it can bear in its  ordinary state. But
still its fluidity, however much it may resist the cold, depends 
             THE CAUSE  OF FLUIDITY.        103

 upon heat;  for, by continuing  to take heat from  it, it will
 freeze  at last.  Moreover, as long as it does  not undergo
 any new change with respect to mixturef no other  cause but
 a  certain quantity of heat alone can render and preserve it
 fluid.
   Our  experience of the freezing or consolidation  of fluids,
 when exposed to more or less powerful degrees of cold, is
 almost  universal.   The  exceptions  are  very few.   The
 strongest spirit of wine, and a few subtile and volatile oils, are
 the only substances which have not yet been congealed by any
 degree  of cold  hitherto  known.  As  these, however, are so
 few in number,  it  appears  unreasonable to believe  that they
 have a  nature and constitution so different from that  of all
 other bodies, that  fluidity  is in them an essenual quality, of
 which they  cannot be deprived by any diminution of  their
 heat.  As we have no certain knowledge of what is the lowest
 possible degree  of heat, or most extreme intensity of  cold,
 but, on  the contrary, shall have  reason hereafter to be per-
 suaded  that the most violent cold  which has yet  been ob-
 served,  is very far short of the  most  extreme degree, it is
 more reasonable to suppose that these  few bodies differ from
 others,  only in having a much greater disposition to fluidity,
 and therefore as requiring less  heat to preserve them fluitl;
 in consequence of which, we have never yet known  a degree
 of cold sufficient for congealing them ; but that they would
 undoubtedly congeal, like other fluids, were they exposed to
 a sufficient cold.
   Quicksilver was, not long since, one of this small number
 of bodies, which, having  never been seen in any other than a
fluid state, was considered as naturally and essentially  fluid,
and incapable of being reduced  alone  to  a solid  form,  until
experiments were made with it, first  in different parts of the
Russian empire, since the year 1760, and verified afterwards
in other places.   By these experiments, every person  must
be convinced  that  quicksilver is a metal, which  can become
solid and  malleable like the rest, but that it is melted with so
little heat, that  the lowest degree ever observed,  over the 
104.         ERRONEOUS SUPPOSITION
greater part of the surface of this earth, is more than sufficient
to preserve it fluid.
   In the same manner may we  consider all  other fluids  as
solids melted by heat.
   Some philosophers, however, have offered many objections
against admitting this general proposition concerning the na-
ture of fluids.  They thought  it necessary to suppose that
water is an exception.   They could not be persuaded that  in
it fluidity is the effect of heat, but supposed this quality to be
an essential quality of the water, depending on the spherical
form and  polished surface of its particles, and that the freez-
ing of it  depends on the introduction  of  some  extraneous
matter.  This opinion is defended by  Professor Muschen-
broeck, in his physics*, and he has collected all the reasons
and arguments which have been devised for supporting such
an opinion.
  But  when we consider these reasons and arguments  with
due attention, we do not find that any of them are valid.
  Many  of them are alleged  facts, adduced to  prove that
water is frozen on some occasions, and in some circumstances,
under which Mr. Muschenbroeck did not comprehend how
it s^uld  be cooled to the degree reckoned necessary to its
congelation ; and he therefore concludes that  the freezing of
it must proceed from some other cause than  the  diminution
of its heat to the proper degree.
  But had he  applied a  good thermometer  to it, he would
have found that it was actually cooled to the usual degree  in
every case in which it was frozen.   For the truth of this we
may depend on the testimony of Dr.  Martin, who, with the
assistance of his friends, took care to have experiments made,
in many  distant  parts of the world, and at different times,
with thermometers, on  the  goodness  of  which he  could
entirely rely.   There is no  doubt that  many mistakes   have
been committed by using bad ones, or by the  want of skill  to
use them properly. But the truth is, that in the facts adduced
by Muschenbroeck, no thermometer whatever was applied  to
the water itself, but only vague reasoning  is employed, to  make
                 * Muschenb. P"hys. de aqua-. 
             OF FRIGORIFIC MATTER.         105

 it appear probable that it was not cooled to the proper degree.
 We may, therefore, pass over the greater number of his rea-
 sons, and take soq^ notice of a few only of his facts, which
 are surprising in themselves, and could not be explained by
 any principles then known, and which besides  are stated by
 him in such a manner as to make them appear uncommonly
 perplexing.
   The fact which first induced him to think that ice is formed
 by the entrance of extraneous matter into water, is the en-
 largement of bulk; ice being always more bulky  than  the
 water from which it was produced,  in the proportion of about
 9 to 8.  This enlargement of bulk is the effect of a powerful
 cause acting  with great force, which has  been  shewn by
 many experiments.  " Now he thinks  it more probable  that
 u this enlargement of bulk,  and the force with which  it
 " is  performed, depend  on  the  entrance  of  some  subtile
 " matter,  than  on the simple diminution  of heat, the gen-
 " eral effect of which is to occasion  contraction in bodies."
   To the whole of this argument, however, it is easy to an-
 swer, that  the fact is better explained by the supposition of a
 particular modification of the attraction, which makes the parts
 of the water unite and cohere, <in one way rather than in any
 other.
   The attraction of cohesion, which, in this case, is the at-
 traction of the particles of the water for one another, is  un-
 doubtedly the cause of the concretion of water into ice,  as  it
 is the cause of the solidity and hardness of all  other bodies.
 No person doubts that the hardness of iron  is  owing to  the
 cohesive attraction of its particles for one another.  We  can
 make  these particles  change their^position among one ano-
 ther, by hammering the iron, or drawing it into wire, and
 they still cohere.  And yet,  when iron  that has been melted
 is allowed, by cooling,  to resume its solidity, which it does
 when it is yet in a strong red heat, the same effect is produc-
ed as in the case of water; the iron, when solid being a little
 more bulky than when it was fluid.  And  the reason is  the
same in both  cases,....the  attraction of  cohesion,  which
'.'• a?  counteracted and  overcome in  these bodies, by the  re- 
106       ERRONEOUS SUPPOSITION
 pulsive power of heat;  being, in consequence of the diminu.
 tion of the heat, allowed to operate, it acts in a peculiar man.
 ner, forcing the particles to assume an ar|jmgement different
 from that which they had while the body was fluid, and which
 requires a little more space.   This peculiar action of the  at-
 traction of  cohesion in  such bodies, I formerly compared t«
 that of a great number of small magnets set  afloat on the sur-
 face of water.  There  is no doubt that by  the  attraction of
 their north and south poles, they would arrange themselves
 in a particular manner,  different from that of the closest ar-
 rangement ; and  that something  like this does  actually hap-
 pen in freezing  water,  and congealing iron,  is sufficiently
 evident from a number of phenomena formerly described.
   Another remark of Professor Muschenbroeck is, that " in
 " moderate frosty weather, water,  exposed and preserved
 " from disturbance, remains fluid, but being  then disturbed, it
 " suddenly freezes."  To this it may be answered, that there
 is here no appearance which gives reason for concluding that
 the freezing of it depends on  the introduction of extraneous
 matter.  And when we consider the experiment more fully,
 which is to be done in the sequel,  we shall find that it can be
 better explained,  in a very different manner.
   Cdly, " A wet cloth hung in the  air, in cold and dry weather,
 ''■ will sometimes become stiff, by the freezing of the humid-
 " ity in it, when the thermometer shews the  cold of the air to
*" be insufficient for the freezing of water in  ordinary circum-
 " stances."   And Mr.  Muschenbroeck  supposed,  " that, in
 " this case, the ice  was formed in the  cloth by some other
 " cause than the diminution of heat, and the power of attrac-
 " tion."   But he formed this  judgment from the  indication'
 of the thermometer hung in the air.  Had he applied it to the
 cloth itself,  he would have found it cooled to the usual degree
 of 32 Fahrenheit; and  the  truth is, that a  wet cloth, hung
 up in these circumstances, becomes colder than any other
 body around it;  which fact shall be explained hereafter.
  4thly,  u Water, at 33  degrees of Fahrenheit, when the
 " strong nitrous acid is  added to it, becomes warmer to the
 " 41st degree.  But, if we pour the same strong acid op 
OF FRIGORIFIi MATTER.         lor
  u pounded ice, at 32 degrees of Fahrenheit, an enormous cold
  M is the consequence.  How can these different effects be
  " produced,  when the heat of the wat^r and  ice are  so
  " nearly the same, unless there  be  in the ice  some hetero-
  " geneous matter, the power of  which* is increased by the
  " acid r"
    To this we.can answer, that ice is certainly different from
  water, and produces different effects, when mixed with some
  other substances; and the Question is,  In what does this dif-
  ference consist?   Mr.  Muschenbroeck says that it consists
  in  the presence  of frigorific particles  in the  ice;  but we
  shall soon give another account of it, that shall explain  this
  experiment, without having recourse to such a supposition.
    Sthly, He concludes, " That in ice the  particles of water
,  " have corpuscles of  a different  kind mixed with them; for
  " this reasoi-0 that the water produced  by the melting of ice,
  " is unfit for boiling vegetables tender, or for drawing a good
  " infusion from tea,  coffee,  &c.  until  it  has been boiled for
  " some time."
    In this reason, it is stated as a fact, that the water of melted
  ice is different from good and soft water, but that it becomes
  soft and good by boiling it some time.   Now this fact appears
  exceedingly  improbable,' and before it be admitted, would
  need to be ascertained by accurate and  decisive experiments ;
  for, in whatever manner ice may be different from water, that
  difference should certainly be removed when the ice is chang-
  ed  again into water.   The  Professor does not mention  any
  accurate experiments  made  by himself or others, to  prove
  this alleged fact, and some I have" made disprove it entirely.
  I am persuaded  that the opinion of-its truth is vulgar and er-
  roneous, and has arisen from the coldness of the water which
  comes from ice ; for it is well known,  that very cold  water
  applied to soap, has little power to dissolve it, or  appears to
  be  hard  water,  when compared  with  the same  water less
  cold ; and it is a maxim in cookery, that water employed for
 the boiling of vegetables does not perform its office well,  when
  the vegetables are put into it in its cold state, although it had
  not been frozen for a long tim^efore.   14 mav therefore be 
I OS
EKRONEOWS  SUPPOSITION
•
 presumed, that the vulgar opinion of the unfitness of ice-water
 to dissolve soap,  or boil vegetables well, or to draw good in-
 fusions from  vegatable substances, has arisen from its  being
 applied to them colder than other water.
   Cthly, He states as fact, " That when a mixture is made of
 ,v snow and common salt in a proper vessel, and in the middle
 '• of this mixture  another is placed, containing pure water;  if
 " we now set  the whole apparatus upon a fire, while the salt is
 " dissolved, and the snow melts, the pure water is frozen, and
 " the fire*accelerates the freezing of the pure water ; for the
 " stronger the  fire is, the pure water is frozen so much the
 "■ sooner, which (he adds) could not happen, if the fire did
 " not chase the congealing particles  from the snow  into the
 "water; and he cannot believe  that  any person will assert,
 w that in this case, the fire chases  an absence or deficiency of
 ^ heat from the snow into the water."         9
   It must be confessed, that this  experiment, thus stated, ap-
 pears very perplexing;  but this  proceeds from the introduc-
 tion of a circumstance into it, which is no way necessary to its
 success,  and is introduced  for no other reason but to  make
 it appear more surprising.   The  circumstance I mean, is set-
 ting the whole  apparutus  upon a fire.  The plain truth is,
 that the salt and snow act on one another  in a way hereafter
 to be explained ; and, in consequence of this action, they unite
 together, and  are both liquefied, becoming at the  same time
 intensely colder than they were before.  This will be sure to
 happen, and with more effect, though we do not set the appa-
 ratus upon a fire.   But the power of this mixture, when made
 in proper quantity, is so great,  that  although  the heat re-
 ceived from the fire diminishes the cold produced, there is
 still  enough for freezing the water,  which, when frozen, is
 always as cold as in ordinary cases.  There is only one  case
 in which the application of heat can promote or quicken the ac-
 tion  of the mixture, and this happens when the snow and salt,
before they are mixed, are both as cold as the Zero in Fahren-
 heit's scale, or are near to this cold. When so cold as this, they
are little disposed to act upon one another; and if heat be applied
to them, it quickens their action.   But even in this case, the
heat  applied diminishes their power, upon the whole, to nr>. 
              OF FRIGORIFIC  MATTER.         109

 duce cold ;  for, if time be allowed, they will be sufficient for
 congealing a larger quantity of water, without the application
 of heat to them, than when it is applied.   And this experi-
 ment is to be explained in the sequel, without employing the
 supposition of congealing or frigorific particles*
   Upon the whole, there are many of Professor  Muschen-
 broeck's reasons for his opinion on the congelation of water,
 that are quite inconclusive.   And thoudj, some must appear,
 for the present, unaccountable facts, tnese are hereafter to be
 explained.   But, in the mean time, it is evident that none of*
 them give any satisfactory proof that the fluidity of water is
 an essential quality, or that any new matter is introduced into
 it when it is frozen.  The propensity which  many have to
 imagine water is a substance naturally and essentially fluid, is
 a prejudice contracted from the habit of seeing it often every
day  in a fluid state, or much oftener in this state than in the
 state of a solid.
  As we have sufficient reason, therefore, for beleiving that flui-
dity is in all cases the effect of heat,and not an essential quamty
 cf any of the palpable kinds of matter, so it appears to be one of
 those effects which are produced generally, and upon all kinds
 of bodies.   There are very few substances now known, which
 cannot, in some degree of heat or other, be brought  into a
state of fluidity by heat.   Those that are the least disposed to
 it,  are the earths and stones,  many of which were formerly
 reckoned incapable of being melted; but  later experiments
have  shewn this opinion to have been erroneous,  especially
some experiments with burning glasses, and some made in
consequence of improvements in the construction of furnaces,
 and in the management of fuel, by which means much more
violent heats have been produced than formerly, and which
have brought into perfect fusion many of those bodies which
 were reckoned incapable of being melted by heat.
  In considering this effect of heat, we may first remark one ob-
vious difference between it and expansion, which is, that where-
as expansion is, in general, an equable and regular effect, accom-
panying every increase of the heat of bodies, the change of a bo-
dy from the state of solidity, to that of fluidity does not take
                            Y 
110    MELTING  OR CONGEALING  POINT.

place, except when its heat is encreased, soas to rise above a par-
ticulardegree. This has been ascertained by innumerable expe-
riments made with thermometers.  If the heat of the body be
increased above this degree, it becomes fluid in all the higher
degrees.  If again its heat be diminished, so as to fall below
this  degree, it becomes solid, and remains  solid in all the
lower degrees. This at least may by stated as the general fact.
 ♦There are, however, many  substances in which the transi-
tion  is not  so sudden.  These, in a certain latitude of heat,
are reduced to an intermediate state, a state of softness, and
pass  through all the degrees of it, while they are  changing
from a solid to a fluid, by heat.   We have examples of this
in bees wax, resin, tallow, glass, and many other substances.
But, even in these, every degree of softness depends on a cor-
responding degree of heat, which has the power to produce _
it; and, in most of these bodies, there is also a remarkable
3tep  from the greatest degree of softness to  perfect fluidity,
which  always depends on a  certain intensity  of heat neces-
saijtyto the  perfect liquefaction of that particular body.
   We may therefore say in general, that each  different kind]
of matter requires to be heated to a particular degree, or above
it, to render it fluid, and that below this degree it is always
solid,  or'has some solidity.   This degree of heat is there-
fore called the congealing or the melting  point of such
body.   It is also called the  congealing point of such bodies
as appear commonly in  a state of fluidity,  and the melting
point of those that are solid, in ordinary circumstances.  This
point,  or intensity of heat, is invariably the same for each par-
ticular kind of matter, no other circumstance but heat appear-
ing  to have the  smallest influence in producing fluidity, so
long as the body itself continues the same,  unaltered in the
mixture or porpotion of its constituent parts.  But, when we
compare different kinds of matter, there is all the variety that
can  be imagined between those that require for their fusion
the  utmost violence of heat, and those for which so little is
sufficient, that they always appear, in ordinary circumstances,
 in a fluid form. 
VITRIFICATION.               Ill
   Among this variety there are a number, the liquefaction of
 which by  heat is distinguished by the term of vitrifica-
 tion.
   This term is applied to the fusion of those earthy bodies
 which undergo  a certain change  of their  appearance and
 texture, by being melted and allowed again to congeal. What-
 ever might  be their appearance before, in their natural state,
 they assume, after fusion, a resemblancg^of glass,  forming a
 hard brittle mass, which breaks wifn eqffin ease in every direc-
 tion, and into fragments, the surfaces of which are smooth,
 polished,  and undulated.   When an  earthly substance  as-
 sumes this form after fusion, it is said to be vitrified.
   Thus we have taken a general view of the subject, accord-
 ing to the notions which were formerly had of it.
   I must,  however, now add, that this account of fluidity, as
 an effect of heat, is not complete and satisfactory, if it be un-
 derstood in conformity with the common opinions which were.
 entertained of it. It is inconsistent with many remarkable phe-
 nomena; and these phenomena, when attentively considered,
 shew that  fluidity is  produced by heat, in a very different
 manner from that which was commonly imagined; a manner,
 however, which, when understood, enables us to explain ma-
 ny particulars relating to heat or cold, which appeared in the
 former  view of the subject,  quite perplexing and unaccount-
 able.
   Fluidity was universally considered as produced by a small
 addition to the quantity of heat which a body contains, when it
is once heated up to its melting point; and the return of such
 body to a solid state as depending on a very small diminution
 of the quantity of its  heat, after it is cooled to the  same de-
 gree ; that  a  solid body, when it is changed into a  fluid, re-
 ceives no greater addition to the heat within it than what is
 measured  by  the  elevation of temperature indicted after fu-
 sion by the thermometer ; and that, when the melted body is
again made to Congeal, by a diminution of its heat, jt suffers
no greater loss of heat than what is indicated also by the sim-
ple application to it of the same instrument. 
J12 ERRONEOUS NOTION OF LIQUEFACTION.

  This was the universal opinion on this subject, so far as I
know, when I began to read my lectures in the University ok
Glasgow, in the year 1757.  But I soon found reason to ob.
ject to it, as inconsistent with many remarkable facts when at-
tentively considered; and I endeavored to shew, that these facts
are convincing  proofs that fluidity  is produced by heat in a
very different manner.
  I  shall now describe the  manner  in which fluidity ap-
peared to me to be produced by heat, and we shall then com-
pare the former and my view of the subject with the pheno.
mena.
  The opinion I formed from attentive  observation  of the
facts and phenomena, is as follows.  When ice, for example,
or any other solid substance,  is changing into a fluid by heat,
I am  of opinion  that  it receives a  much greater quantity of
heat  than what is perceptible in it  immediately after by the
thermometer.  A great quantity of heat enters into it, on this
 occasion, without making it apparently warmer, when tried
by that  instrument.  This heat, however,  must be thrown
into it, in order to give it the form of  a fluid;  and  I  affirm,
that this great addition of heat is the principal, and most im-
 mediate cause of the fluidity  induced.
   And, on the other hand, when we deprive such a body of
 its fluidity again, by a diminution of  its  heat, a very great
quantity of heat comes out of it, while  it  is assuming a  solid
 form, the loss of which heat is not to be perceived by the com-
 mon manner of using the thermometer.   The  apparent heat
 of the body, as measured by  that instrument, is not diminish-
  ed, or not in  proportion to the loss of heat which the body
  actually gives out on this occasion; and it appears  from a
  number of facts, that the state of solidity cannot be induced
  without the abstraction of this great quantity of heat.  And
  this confirms the opinion, that this quantity of heat, absorb-
  ed,  and,  as it were, concealed in  the composition of fluids,
  is the most necessary and immediate cause of their fluidity.
    To perceive the  foundation of this  opinion,  and the incon-
  sistency of the former with many obvious facts, we must con-* 
 A NOTION MORE CONSISTENT WITH THE FACTS. IIS

 6*ider, in tRe first place, the appearances observable in the
 melting of ice, and the freezing of water.  ,
   If we attend to the manner in which ice  and snow  melt;
 when exposed to the air of a  warm room,  or  when a thaw
 succeeds to frost, we can easily perceive, that however cold
 they might be at the first, they are soon  heated  up to their
 melting point, or begin soon at their surface to be changed
 into water.  And if the common opinion had been well found-
 ed, if the complete change of them into water required  only
 the further addition of a very small quantity of heat, the mass,
 though of a  considerable size, ought all to be melted in a very
 few minutes or seconds more, the heat continuing incessantly
 to  be communicated from the air around.   Were this really
 the case, the consequences of it would be dreadful in many
 cases ; for, even as things are at present, the melting of great
 quantities of snow and ice occasions violent torrents, and great
 inundations  in the cold countries, or in the rivers that come
 from them.  But, were the ice and snow to melt as suddenly
 as they must necessarily do, were the  former opinion of the
 action of heat in melting them well founded, the torrents and
 inundations would be incomparably more irresistible and dread-
 ful.   They would tear up and sweep away every  thing, and
 that so suddenly, that mankind should  have great difficulty to
 escape from  their ravages.  This  sudden liquefaction does
 not actually  happen ; the masses of ice  or snow melt with a
 very slow progress, and require a long  time, especially if thev
 be of a large size, such as  are  the collections of ice,  and
 ivreaths of snow,  formed in some places during  the  winter.
 These, after  they  begin to melt, often require many weeks of
 warm weather, before they are totally  dissolved into water.
 This remarkable slowness with which ice is  melted,  enables
 us to preserve it easily during the summer, in the structures
 called Ice-houses.   It begins to melt in these, as soon as it is
 put into them; but, as the building exposes only a small surface
 to the air, and has  a very thick  covering of thatch,  and the
 access of the external air to the  inside of it is prevented as
 much as possible, the heat penetrates  the ice-house with a
dow progress, and this, added to the slowness with which the 
  114           USUAL PHENOMENA

  ice itself is disposed to melt, protracts the total liquefaction of
  it so long, that some of it remains to the end of summer.   In
 the same manner does snow continue on many mountains dur.
 ing the whole summer, in a melting state, but melting so slow-
 ly, that the whole of that season is not a sufficient time for it
 complete liquefaction.
   This remarkable  slowness with which ice and snow melt,
 struck me as quite inconsistent with the common opinion of
 the modification of heat, in the liquefaction of bodies.
   And this very phenomenon is partly the foundation of the
 opinion I have proposed; for if we examine  what happens,
 we may perceive that a great quantity of heat enters  the melt-
 ing ice, to form the water into which it is changed, and that
 the length of time necessary for the collection of so much heat
 from  the surrounding bodies, is the reason of the  slowness
 with which the ice is liquefied.  If any person entertain doubts
 of the entrance and absorption of heat  in the melting ice, he
 needs only  to touch it; he will instantly feel  that  it rapidly
 draws heat from his warm hand.   He may also examine the
 bodies that surround it, or are in contact with it, all  of which
.he will find deprived by it of a great part of their heat; or if
 he suspend it by a thread, in the air of a warm room, he may-
 perceive with his hand, or by a thermometer, a stream of cold
 air descending constantly from the ice ; for the air in contact
 is deprived of a part of its heat, and thereby condensed and
 made heavier.than the warmer air of the rest of the  room; it
 therefore falls downwards, and its place round the ice is  im-
 mediately supplied  by some of the warmer air; but this, in
 its turn, is soon deprived of some heat,  and prepared to  de-
 scend in like manner; and  thus there  is a constant flow of
 warm air from around, to the sides of  the ice, and  a descent
 of the same in a cold state, from the lower part of  the mass,
 during which operation the ice must necessarily receive a great
 quantity  of heat.
    It i», therefore, evident, that the melting ice receives heat
 very fast, but the  only effect of this  heat is to change it into
 water, which is not  in the least sensibly warmer than the ice
  was before.   A thermometer, applied  to the drops or small
*'" 
                OF LIQUEFACTION.             115

streams of water, immediately  as  it comes from'the melting
ice, will point to the same degree as when it is applied to the
ice itself, or if there is any difference, it is too small to deserve
notice.  A great quantity, therefore, of the heat, or of the
mafter of heat, which enters into the melting ice, produces no
other effect but to give it fluidity, without augmenting its sen-
sible heat; it appears to be absorbed and concealed within the
water, so  as not to be discoverable by the application of a ther-
mometer.
   In order to understand this absorption  of heat into the
melting ice, and concealment of it in the water, more distinct-
ly, I made the following experiments.
   The plan of the first  was, to take a mass of ice, and an
equal quantity of water, in separate vessels, of the same size
and shape, and as nearly as possible of the same heat, to sus-
pend them in the air of a warm room, and, by observing with
a thermometer the celerity with which  the water is heated,
or  receives heat, to learn the celerity with which it  enters
the ice ; and  the time necessary  for melting the ice  being
also attended  to,*to form an  estimate, from these two data,
of the  quantity of heat which enters the ice during its lique-
faction.
   In order to prepare for this experiment, I chose two thin
globular glasses, four inches diameter, and very nearly of the
same weight: I poured into one of them five ounces of pure
water, and then  set it in a mixture of snow and salt, that the
water might be frozen into a small mass of ice.   As soon as
frozen, it was carried into a  large empty  hall, in which  the
air was not disturbed nor varied in its temperature during the
progress of the experiment;  and in this room the  glass was
supported, as it were,  in  mid air, by being set on a ring of
strong wire, which had a tail issuing from the side of it five
inches  long,  and the end of this tail was fixed in the most
projecting  part of a reading  desk or pulpit:  And in this
situation  the glass  remained  until the  ice was  completely
melted.
  When the  ice was thus placed, I set up the other globular
glass precisely in the same situation, and at the distance of
&* 
 116       FORM .OF  AN  EXPERIMENT

 18 inches to' one side, and into this  I poured five ounces of^
 water, previously cooled, as near to the coldness of meltipg
 ice  as possible, viz. to 33 degrees, and suspended in it a very
 delicate thermometer, the bulb of which being in  the centre
 of the water, and the tube being so placed, that without touch-
 ing the thermometer, I could see the degree to which it point-
 ed.   I then began to observe the ascent of this thermometer,
 at proper intervals, in order to learn with what celerity the
 water received heat, stirring the water gently with the end of
 a feather about a minute before each observation.  The  heat
 of the air, examined at a little distance from  the  glasses, was
47 degrees of Fahrenheit's scale.
  The thermometer assumed the  temperature of the water in
less than half a minute, after which, the rise  of it was observ-
 ed every five or ten minutes, during half an hour.  At the
 end of that time, the water was grown warmer than at  first,
 by seven degrees ;  and the  temperature of it had risen to the
 40th degree of Fahrenheit's scale.
  The glass  with the  ice  was, when first taken out of the
 freezing mixture, four or  five  degrees  colder than melting
 snow, which 1 learned by applying the bulb of the thermome-
 ter  to the bottom of it; but after some minutes, it had  gain-
 ed from the air  those four or five degrees,  and was just be-
 ginning to melt, which point of time was then noted, and the
 glass left undisturbed ten hours  and a half.  At the end of
 this time, I found only a very small and spongy mass of the
 ice remaining unmelted, in the centre of the upper surface of
 the  water, but this also was totally^ melted in a few minutes
 more; and, introducing the bulb of the thermometer into the
 water, near the sides of the glass, I found the water there was
 warmed to the 40th degree of  Fahrenheit.  From this it
 appears, that when a considerable part of the ice was melted,
 and the  quantity of water around it  was increasing, the heat
 was not  transmitted through this water to the remaining  ice
 altogether so fast as it was received by the  water;  which is
 easily understood, if we consider that the distance 'between
 the remaining ice and  the external surface of  the  vessel
 through which  the heat entered, was gradually  increasing*,  j 
          FOR  DECIDING THE  QUESTION.    117

  and  that heat always requires time to pass through bo-
  dies, or to be communicated  from one part  of them  to
  another*.
    It appears, therefore, from this experiment, that it was
  necessary that the glass with the ice receive heat from the
  air of the room during 21 half-hours, in order to melt the
  ice into water, and to heat that water to the 40th degree
  of Fahrenheit.   During all this time, it was receiving the
  heat, or matter of heat, with the same celerity (very near-
  ly) with which the water-glass  received it, during the sin-
  gle half-hour in the first part of the  experiment.   For, as
  the water received it with a celerity which was diminish-
  ing gradually  during that half-ho^r, in consequence of the
  diminution of difference between its degrees of  heat and
  that of the air ;  so the glass  with the ice  also  received
 heat with a diminishing celerity, which corresponded ex-
 actly  with that of the water-glass, only that the  progres-
 sion of this diminution was  much more slow, and corres-
 ponded to the  whole time which the water surrounding  the
 ice required to become  warmed to  the  40th degree  of
 Fahrenheit.   The whole quantity of heat, therefore,  re-
 ceived by the ice-glass during the 21 half-hours, was 21
 times the quantity  received by the water-glass during the
 single half-hour.   It was,  therefore, a quantity of heat,
 which, had it been added to liquid water, would have made
 it warmer by 40-33X21,  or  7X21, or 147  degrees.   No
 part of this heat, however, appeared in the ice-water, ex-
 cept  8 degrees ; the remaining 139  or  140 degrees had
 been absorbed by the melting ice, and were concealed in
 the water into which it was  changed.
    The communication of heat to the melting ice was easi-
 ly  perceived, during the whole time of its exposition, by
 feeling the stream of cold air which  descended from the
 gh.HS*

  '  Wat-r, while ii■:, heat is increased from tbe 33d to the 40th degree of
['a'nrciihcit, suffers very little or no  expansion from the heat  entering  it
(Vide f) ■ T.vic, Rccberches,  &c. p. 418.) The water,  therefore, in this,
in .;-, would n >t he  made to move and circulate by the unequal heat of the
iniVruit part.-; of it, bnt './ulil remain  in a great measure at rest.
    !. OI.. i.                   7. 
118   ANALYSIS OF  iHE  PHENOMENA
  This experiment  was an  analysis of  the  manner  m
which ice is melted by the  heat of the air in ordinary cir
cumstances.
   But another obvious method of melting ice occurred to
me,  in  which it would be  still more easy  to perceive the
absorption and concealment of heat,  and this was by the
action of warm water.
   When hot and cold water are mixed together, the excess
of heat contained  in the hot water is equally  distributed
in an instant through the  whole mixture, and  raises the
temperature of it according to the greatnesfr of the excess
of temperature, and the proportion  which the hot water
bore to the cold.   If the quantities of hot and  cold water
are  equal, the temp'eraturejs the middle  degree between
that of the hot and that roT the cold.   No pjtVt  of thjfc heat
disappears on  this  occasion, so  far as we can perceive,
 but the intensity of it only is dv$fcnished,by its being dif-
fused through a larger quantity of milter.  It was, there-
 fore, obvious, that if a quantity of heat is absorbed, and
 disappears in the  melting  of ice, this would easily be per-
 ceived when the ice is melted with warm water.
    To  make this  experiment, I first froze a  quantity of
 water  in the neck of a broken retort, in order to have a
 mass of ice of an oblohg form.
    At  the same time I heated  a quantity of  water nearly
 equal in weight to the ice, in a very thin globular glass, the
 month of which  was sufficiently wide to take in the piece
 of ice.  The water was heated by a small  spirit of wine
 lamp applied to the bottom of the glass ;  it was also often
 stirred with the end of a feather, and a thermometer hung
 in it.
    While the water was heating, the mass of ice was taken
  out of  the mould  in which  it had been formed, and
  was exposed to the air of a temperate room,  until it was
  perceived to be  beginning to  melt over the whole of its
  surface,
     I then put a woollen glove  on my left hand, and taking
  hold of the ice, I  wiped  it quite dry with a linen  towel,
  laid it in the scale of a baiance on a piece of flannel, and 
OF LIQUEFACTION.
119
hastily counterpoised it with sand in the opposite sc&le,
that I might examine the weight of it afterwards ; and I
immediately plunged it into the hot water, and extinguished
the lamp  at the same time.  The lamp  being small, the
heat of the water had been increasing very slowly, and had
almost ceased  to  increase; and being examined imme-
diately before I put the ice into it, the temperature  was
found to be just 190 degrees.   The ice was all meked in
a few seconds, and produced a  mixture,  the temperature
of which was 53 degrees.
   The weight  of  the ice, when put into the hot water,
was seven ounces three drachms and a-half.  The weight
of  the glass, with  the whole mixture  in it, was sixteen
ounces,  seven  drachms, and seven grains.  The weight
of the glass alone was nearly one ounce.
   In considering this experiment, we may overlook quan-
tities less  than half a drachm, or thirty grains, and reckon
the quantities  of the  different articles, by the  number of
half-drachms in each.
   Thus the weight of the  ice was 119 half-drachms.
             ------Hot  water  135   -  - -  *-
             ------Mixture   254
             ------Glass alone   16
   The melting of the ice was affected,  not only by the
heat of the hot water, but also by that of the glass.  And,
by other experiments, I learned that sixteen parts of  hot
glass have no more power in heating cold bodies, than eight
parts of equally hot water ; we may therefore substitute,
in place of the sixteen half-drachms of warm glass, eight
half-drachms of warm water, which,  added to the above
quantity of warm water, make up 143 half-draehms.
   The heat of this warm  water  was  190 degrees,  that
is 158 hotter than the ice; and if this  heat had abated
in the mixture only in consequence of the quantity and
< oldness of the ice, and if nothing had happened  when the
ice was put in, but merely a communication of this heat,
and an equal distribution of it  through the mixture, the
temperature of the mixture should have been 158, viz. the
rxc.,oS 0f  'Jf>at in the warm water, multiplied by 143, the 
120   ANALYSIS OF  THE PHENOMENA

quantity of  the  warm matter,  and  divided by 262,  the
quantity of the whole, which gives 86.
   The mixture should have  been  86  degrees warmer than
melting ice  ; but it was found only 21 degrees  warmer.
Therefore a quantity of heat has disappeared, which, if it
had remained in a sensible  state, would  have made  the
whole mixture and glass warmer by  65 degrees than they
were  actually found to be.  But this quantity of heat would
be sufficient for increasing, by 143 degrees, the heat of a
quantity of  water, equal  in  weight  to the ice alone.  It
was,  however, absorbed by  the ice, without  in  the  least
increasing its sensible heat*.
   The result of this experiment coincides sufficiently with
that of the former ; the difference is not greater than what
may be expected in similar  experiments,  and might arise
from  the accident of the central  parts of the mass of ice
being colder than the  surface, by  one or two degrees.
   I have, in the same manner, put a lump of ice into an
equal quantity of water, heated  to  the temperature 176,
and the result was, that the  fluid was no hotter than water
just ready  to freeze.  Nay,  if a little sea salt be  added to
the water,  and it be heated only to 166  or 170, we shall
produce a fluid  sensibly colder  than  the ice  was in the
beginning, which has appeared a curious and puzzling thing
to those unacquainted with the general fact.
   It is, therefore, proved that the phenomena which attend
the melting of  ice in different circumstances, are  incon-
 sistent  with the common  opinion which  was  established
 upon this  subject, and that they  support the one which I
 have proposed.
   But  some persons  may, perhaps,  imagine that the heat
 which thus disappears does  not truly enter into the melt-
 ing ice, or  become combined with that into which it is
 changed.   This  heat is, perhaps,  entirely extinguished
 and  destroyed.   As heat has been  supposed  by some to
   * These two experiments, and the reasoning which accompanies them,
were  read by me in the Philosophical Club, or Socictv of Professors and
others in the University of Glasgow, in the year 1762^ and have been de-
gcribed and explained in my lectures, there, and in Edinburgh, eve:- vcar
gince.                                                •" ' • • 
OF LIQUEFACTION.
121
 consist in a rapid motion or tremor of the particles of bo-
 dies, or of  some subtile matter that is intermixed with
 them, those who choose  to adopt this opinion may imagine
 that this motion may meet with friction and  resistance in
 the ice, and that a part of it may thus be destroyed, or the
 moving parts brought to  rest*.
    This  supposition, again, as well as  the common opi-
 nion, will  be found inconsistent with many phenomena
 which attend the freezing of water,  while these phenomena
 strongly support the idea I have proposed.
    If, for example, we expose some pure water to the cold
 air, in a sharp frost, and  set beside it an equal quantity of
 equally warm water, with a little  salt  or  spirit of wine
 added to it, to prevent it from freezing ; we may perceive,
 by putting a delicate thermometer into each of these waters,
 that they are both cooled by  the air at first with the same
 celerity, until  they  are brought  down to   the  freezing
 point, at the  same,  or  very nearly the same moment of
 time.  -After this,  the  progress of the  salted  water  in
 cooling will be continued without interruption,  until it be
 as cold as  the air to  which it is exposed, which we may
 suppose  to  be ten  degrees  colder than freezing water.
 But while this  goes on, if we examine what happens in the
 pure water, when it has been cooled down to the freezing
 point,  we shall not find  in it the same uninterrupted pro-
 gress of cooling:  nor will it suddenly be  changed into
 ice, which it ought  to be, according to the common opi-
 nion.  It will begin to be changed into ice, but with a very
 slow  progress, which will  require  a long  time  to  com-
 plete it; and, during all this  time,  it will continue at the
 32d  degree  of Fahrenheit's scale,  or 10 degrees warmer
 than  the  air to  which it is  exposed, until  it be totally
  * Indeed this supposition seems inseparable  from this mechanical  theory
 of the formal cause of heat. It is amusing to observe how the mechanical
 explainers of the chemical phenomena produce whatever effects they have
 occasion for, by their invisible motions and actions.  Here heat is lost by
 friction and resistance, &c.  The heat is produced, in many chemical solu-
 tions, by the same frictions and violent knockings of the conflicting particles
in the act of solution....yet the same act of solution is accompanied with
intense cold, when salt and snow are mixed.
                                                 editor: 
 122    ANALYSIS  OF THE  PHENOMENA

 frozen ; after which,  it will again begin to become colder,
 and proceed without  interruption, until it  be as  cold as
 the surrounding air and the salted  water.
   But, from the beginning of its congelation to the com-
 pletion of it, although it does not become sensibly colder,
 it is constantly communicating heat to the surrounding air.
 A warmer  body cannot  be in contact with a colder one,
 without communicating heat to it.   We may easily satisfy
 ourselves, that the water, while  congealing, is continually
 imparting heat to the surrounding air ; for,  if we  suspend
 a delicate thermometer immediately above the water, we
 shall  find the instrument affected by an ascending stream
 of air, less  cold  than  the air around.  While the water is
 congealing, therefore, it is constantly  imparting  heat  to
 the air, without becoming colder  itself, and this heat must
 be that which had been absorbed and concealed in it, the
 last time it became water, since  now, in losing its fluidity
again,  this heat comes out of it, although it did not appear
 immediately before in a sensible  state*.
   In  the above  described common process  of  freezing
 •water, the  extrication  and emergence of the latent heat, if
 I may be allowed to use these terms, is performed  by such
 minute st-.ps, or rather with such a smooth  progress, that
 many  may  find difficulty in  apprehending it;  but I shall
 now mention another  txample, in which this extrication of
 the concealed heat becomes manifest and striking.
   This example is an experiment,  first made by Fahren-
 heit, but since repeated and confirmed by many others.
   He wished to freeze water from which the air had been
 carefully extracted.   This water was  contained in small
 glass  globes, about one-third filled  and accurately closed,
 to prevent the return of the air in to them.   These globes
 were exposed to the air in frosty  weather, and remained so
  * Dr. Black made this experiment also, and many registers of it are to be
found among his memorandums : but, in our temperate climate, it is sel-
dom that a freezing cold continues with sufficient steadiness for admitting
 a fcimple calculation.  He made many experiments on the congelatl n of
 other substances, in more  usual and steady  temperatures : b*t, in this
 elementarv course,  intended for the instruction of the most  uninformed
 hearer, he chose to confine his proofs to the most familial- observations.
                                                 EDITOR. 
                OF CONGELATION.            123

long exposed, that he had reason to be satisfied that they
were cooled down to the degree of the air, which was six
or seven degrees  below the freezing point.   The water,
however, still remained fluid, so long as the glasses were
left undisturbed, but, on being taken up and shaken a little,
a sudden congelation was instantly seen.
   It has since been found, by the  trials of others, that the
experiment will succeed, although the water be  not de-
prived of its air, and that the circumstances the  most es-
sentially necessary are, that it be contained in vessels of a
small size, and preserved carefully from the least  disturb-
ance.  The vessels, therefore, ought  to be covered  with
paper, or otherwise, to prevent slight motions of the air
from affecting the surface of the  water*.   In these cir-
cumstances, it may be cooled to  six, or seven,  or eight
degrees  below the freezing point, without being frozen ;
but, if it be then disturbed, there  is a sudden congelation,
not of the whole, but of a small part only, which is formed
into feathers of ice, traversing the water, in every direction,
and forming a spongy  contexture of ice, which  contains
the water in its vacuities, so as to give to the whole the
appearance  of being frozen.  But  the most  remarkable
fact is, that while this happens, (and it happens in a moment'
of time) this  mixture  of ice and water suddenly becomes
warmer, and makes a thermometer, immersed in it, rise to
the freezing point.
   Nothing can be more inconsistent with the old opinion
concerning the cause of congelation  than the phenomena
of this experiment.  It shews that  the loss of a little more
heat, after the water is cooled down to the  freezing point,
is not the most necessary and inseparable cause of its con-
gelation, since the  water is cooled 6, 7, or 8 degrees below
that point, without being congealed.
   Some may, perhaps, imagine  that the  water  remains
fluid in consequence  only of some state  of arrangement
and connection between its  particles, which  is favorable
or necessary to its fluidity, and that this state of  arrange-
ment and  connection continues in  this over-cooled water,
                   * Atari.in siir la Glace 
 121;    ANALYSIS  OF THE  PHENOMENA

 in consequence of its "being carefully preserved from every
 disturbance.                                          **
    Although this be only a supposition, let us admit it for-
 a  moment, and try how it will agree with the facts.
    If this supposition were true, then,  by disturbing such
 over-cooled  water, we  should  suddenly bring on  a total
 congelation of it, and it should continue as cold as it was
 immediately before ; but neither the one nor the other of
 these events  takes  place.   A  small portron  only of  the
 water is thus  suddenly frozen,  and it  does not continue
 equally cold ;  a quantity of' heat  suddenly appears in it,
 which was not perceptible immediately before : And this
 heat,  being diffused in a moment through the whole mix-
 ture of water and newly  formed ice, raises the temperature
so high, that no more of the  water can be frozen, until
 more of its heat  be taken away.
   But these pK^Tiomena, though inconsistent with such a
 supposition,  aria with the old  opinion of  the manner in
which  heat produces  fluidity,  strongly support  the one
which I have proposed, and can all  be explained by it.with-
 out difficulty.  They actually shew the extrication from the
freezing water of that quantity of heat which is concealed
 in its composition so long as it  retains the.form of a fluid.
 This  experiment shews,  that when water  is  cooled in a
 state of perfect rest, in a small vessel,  it is disposed to re-
tain this concealed heat, which I have been used to call it's
latent heat, a little more  strongly than in ordinary circum-
 stances.   In common circumstances, the water retains the
 whole  of this heat, until  it be cooled to the 32d degree of
 Fahrenheit, or  a very.little lower.   If, in ordinary circum-
stances,  we attempt to make it colder, we may perhaps
 succeed  in mahing it still colder by  one degree or two,
but no more, for then the latent heat begins  to be extri-
cated from a small part of the water, and to appear in the
form of sensible heat, that small portion of the water which
loses it,  assuming consequently the form of  ice.  Thus,
 he latent heat  of  the rest of the  water continues  after-
wards to be extricated slowly and imperceptibly, in propor-
 tion as  we abstract  the sensible   heat  into  which  it  is
 changed, until  the whole  of the water is frozen, or has lost
tlv.: whole of its latent heat- 
                OF  CONGELATION.            125

   But, when the  water is cooled in the circumstances of
 the above mentioned experiment, it retains its latent heat
 and fluidity longer, or until its sensible heat be diminished
 to seven or eight  degrees  below frost ;  and, when it is in
this state, if it be  agitated, or suddenly disturbed by the
 impulse  of  the air, or the falling into it of a \ittjfe bit of
 ice, or other such matter, this occasions the extrication of
 a part of the latent heat, which now becomes sensible heat,
 and that part of the water which thus loses its latent heat
is at the  same time changed into ice.   But the heat thus
 extricated at once being in  greater  quantity than what is
extricated in any  one moment in the common process of
 congelation, it is more conspicuous, by suddenly increasing
very remarkably the  sensible  heat of the materials, and
limiting  the  quantity of the  ice that is thus suddenly
formed.
  This surprising experiment, therefore, which formerly
appeared so  strange  and unaccountable, can thus be ex-
plained.   When  this experiment is made with great care,
in a vessel inaccessible to the external air, and in a place
where it is not disturbed by the tremulous motion occa-
 sioned by walking on  the floor, or the rattling of heavy
 carriages, it  is possible to cool the water even ten degrees
 below 32°.   If the water be now touched, ever so gently,
with a slender spicula of ice, formed by crystallization, or
 a flake of dry snow, it  instantly shoots into beautiful spiculae,
 which rapidly form and branch out in all directions, and
the thermometer, which had been left in it, rises slowly to
 the 32d ^degree.
   But farther, by the  same principles I also explained some
 other remarkable facts relating to cold, of which no ex-
planation had before been attempted.  The facts I r jean,
 are those experiments in  which intense degrees  of cold
 are produced by mixture.
   The  mixtures most effectual for this purpose are those
of ice and snow with different salts; and it always happens,
that while the cold is  produced, the salt and snow dissolve
together into a liquid.  This liquefaction of the salt and
 snow is absolutely necessary to the production of the cold.
 In  order to explain  this phenomenon, we need  only  to
   vol.  i.                  \ a 
126   EXPLANATION  OF THE  EFFECTS

recollect,  that  the  liquefaction, which  suddenly happens,
requires, as on other occasions, that a quantity of heat be
absorbed, and combined, in a latent state, with the lique-
fied  matter ; that the mixture cannot assume the form of
a fluid without absorbing into it a quantity of heat, or of
the matter of heat  ; and that this heat becomes  latent in
the  composition  of the  fluid that  is  produced,  being
combined with  the  matter of  the  fluid more closely  and
intimately  than heat that is in its  ordinary sensible  and
moveable state.   This requisite heat can be had only from
the materials.   A  part, therefore,  of  the sensible  heat,
which was in the materials before they were mixed, sud-
denly assumes that form,  and disappears, so as  to  be no
longer perceptible by the feeling of heat, or by means of
the thermometer.   The mixture, therefore, appears much
colder than the materials were immediately before. More-
over, we see  in this example the converse of what happens
in Fahrenheit's experiments with  over-cooled water.   In
these cold  mixtures we make  a quantity of ice melt sud-
denly, without adding  heat to it;  and this liquefaction
occasions a great degree of cold to be instantly produced,
by the convulsion of a quantity of sensible hea%into latent
heat.  In  Fahrenheit's experiment, we occasion a quan-
tity of water to freeze suddenly, by  disturbing it only, and
without taking heat from it. A quantity of heat suddenly
appears  in it, or is suddenly changed from  a latent to a
perceptible state.
   To understand better  the power of *hese mixtures in
producing cold, and to what  extent they  can produce it,
we must recollect,  that  salt added to water increases  con-
siderably its disposition to fluidity but increases it only to a
ce-^&in degree.   In a strong brine of common salt, for ex-
ample, the disposition of  the water to fluidity is so much
greater than in pure water, that the brine will  bear to be
cooled down to the  beginning of Fahrenheit's scale, or even
a few degrees  lower,  without shewing any beginning of
congelation.   But if, by exposing to the action  of a suffi-
ciently powerful cooling cause, we cool it a little more, for
example, t  rive or si:: degrees below Zero, then the chemi-
cal attraction of the salt and water for one another  begins 
OF  FRIGORIFIC MIXTURES.       12?
 to be overcome by the cohesive attraction of each of these
 substances :  a small part of the salt concretes into grains
 like sand, and falls  to the bottom, and a small part of the
 water freezes.   This is accompanied by the extrication
 and  emergence  of  a proportional quantity of latent heat,
 or the conversion of it into sensible heat, which increases
 the sensible heat of the brine a little, and puts a stop to the
 farther separation of the salt and congelation of the water,
 until this small addition of sensible heat be taken  away
 again by the cooling cause.   But as soon  as it is  taken
 away, a little more of the salt concretes, and a little more
 of the water freezes.   All which repeatedly happens, until
 at  last,  the  whole  of the latent having been  gradually
 extricated, the salt will be found at the bottom, all in small
 grains, and the water over it all congealed.
   From  this we can explain the peculiar phenomena of
 some of  these experiments.   For example, it is remark-
 able of the mixture  of  ice and common salt, that it never
 produces  a cold greater than Zero, or a few degrees below
 Zero, ^ut that it continues cold  to  this degree for a  con-
 siderate  length of time, during which  the salt and. snow
 are gradually melted. Now it must necessarily happen, that
 as soon as they begin to melt, so much of their heat will
 be converted into latent heat, by the melting of  a  small
 part of these materials, that their remaining sensible heat
 will be  greatly diminished.   It  will be only-efficient for
 heating them to  the degree Zero on Fahrenheit's scale ;
 but, when the remaining solid ice and salt are cooled to this
 degree, they cannot act further on one another by chemical
 attraction.   The cohesive attraction of each is become too
 strong to admit  of  this; they cannot act further, until a
 little more heat be communicated to them  from the sur-
 rounding bodies.  As soon as this happens, they again act,
and a small part of them melts,  which again brings down
their sensible heat  to the low  level; therefore they  must
necessarily unite together, and  melt, with a very slow pro-
gress, regulated  by  the gradual  communication of heat
 from the  surrounding bodies,  which heat, as fast as it is
communicated, is i hanged into latent heat, and the sensi-
ble he,<r of the mixture must continue at the same low  de* 
128   EXPLANATION Ol  THE  EFl'EC
giee,  so long as any considerable part of the ice and salt
remains unmeited.   Some other  salts  increase,  to a still
greater degree,  the disposition of water to fluidity,  and
will therefore produce  a much greater degree of  cold in
this wav.
   It  will not be unacceptable,  I presume, on this occa-
sion,  to mention the most successful and convenient pro-
cesses for producing great diminutions of heat, or, as it is
more   commohly expressed, for producing intense colds,
This  is not  a  matter of mere  curiosity, but is frequently
the means of  philosophical investigation ;  and some very
interesting facts have been discovered in this way.
   I think you are now sufficiently aware, that, since fluidi-
ty is the consequence of a certain combination of calorific
matter with the  substance of solid bodies, it must follow,
that if any substances  are induced, by their chemical pro-
pcrties, to act on each other, so  as mutually  to  dissolve
and become fluid ; and if there  be no external source, from
which the necessary latent or fluid-making heat can be ob-
tained, it must be furnished by the materials themodves;
that is, some of their sensible heat must disappearpMj^heir
temperature, indicated by the thermometer,  must be lower-
ed; and this must happen in a degree so much the more re-
markable, as  there is more of the solid  matters  rendered
fluid, and also, in proportion as the substances so liquefied,
require a greater quantity of latent heat for this purpose.  An
ounce of one substance may thus absorb as much as two
ounces of another substance.
   You can easily see further, and I have already mention-
ed some examples of  it, that,  whatever are the substances
employed, there  must be a limit to the diminution of tem-
perature.  If the liquid mixture congeals at any particular
:emperature at 40° for example, it is impossible to pro-
d ice  a greater cold by  mixing those substances.
   These observations, therefore, suggest certain maxims
or rules for the powerful operation of chemical mixtures
lor producing intense colds. We  must choose substances,
whose chemical action induces them strongly to dissolve
 fach other in a liquid form. They should also be such as
 require a great  quantity of latent heat;  and they must be
 such whose mixture will bear a very  great diminution of 
           OF FRIGORIFIC  MIXTURES.      129

 temperature  before it freezes.   I cannot say that these
 circumstances have been made the subject of particular in-
 quiry, A as to enable us, a priori, to make the most effective
 seleeton of materials.   It  has generally been  by random
 and numerous trials.
   There is another maxim which may be deduced from
 the knowledge we have acquired.  When a certain  sub-
 stance requires a determined quantity of latent heat for its
 appearance in a liquid form, it  is plain, that  when other
 circumstances are  the same it will produce, not a certain
 determinate  temperature, but a certain determinate dimi-
 nution of temperature.
  Of this consequence we can make a most effective use.
 Suppose that a certain mixture  produces  a diminution of
 20 degrees, and that it is made in the temperature of 50,
 it will acquire the temperature 30 degrees, and no lower.
 But we can employ this temperature to cool another quanti-
 ty of the same  ingredients separately to the tempearture
 of 30 degrees.   Let these ingredients be mixed: we shall
 then produce  the temperature 10 degrees; and if  this  be
 employed in the same manner,  to cool another parcel of
 the same ingredients, their mixture will produce the tem-
 perature 10°.   By the repetition of this process, we shall
at last reach the utmost cold that is producible by those in-
 gredients.
   I have just to observe further, that as, during this  pro-
 cess, the external air, must be continually imparting  heqA
 to our mixtures, and this with rapidity, when the cold al-
 ready produced  is very intense,  it is necessary to cut off
this supply as much as possible.    The experiments should
be made when the air is very cold.   The vessels  should
be set in other  vessels, made of bad conductors of heat;
and the interval should be  occupied  by furs, or down,  or
 other spongy matters.
   I  thought it best to give you these general principles
and maxims, deduced from the  knowledge we  have ac-
quired,  that  you  may  perhaps  fall  upon  more effective
methods than any I can relate or describe. 
 130         FRIGORIFIC  MIXTURES.

   The  substances generally  employed have been  salts
 dissolved  in water.   None  has been found more effica-
 cious than sal ammoniac.  If three parts of water jjf added
 to one part of this salt, the  liquefaction lowers the tempe-
 rature fully 20 degrees of Fahrenheit.   We can therefore
 freeze a little cold water when the air is at 48°, the tempera-
 ture of ordinary fountains in this country.  Two such opera-
 tions would freeze a little water even in our  ordinary sum-
 mer heat.
   A  much shorter  and effective process  was discovered
 by  Mr.  Walker,  apothecary, in  Oxford.   He mixed 11
 ounces of sal ammoniac,  10 of saltpetre, and 16 of Glau-
 ber's salt.    He then poured on this mixture of dry salts
 32 ounces of water, stirring the mixture, to accelerate their
 solution.   Thus he produced at once a diminution of 48
 degrees of Fahrenheit.  The experiment being made when
 the temperature of the materials was 65°, it  was changed
 to 17°, which*i*15 degrees below frost.
   He produced still  greater colds,  by mixing some salts
 with acids.  Three  parts of aquafortis mixed with. 12
 of Glauber's  salt, sunk the thermometer 52  degrees : and  ,
then, adding six parts of sal  ammoniac, he sunk  it  eight
 degrees futther.   He froze quicksilver, by  repeating this
 process.  The vitriolic  acid  also, diluted with an equal
 weight of water, produced intense cold, by dissolving crys-
 tallized Glauber's  salt.   It sunk the  thermometer (then
 very low) 46 degrees.
   Dr.  Guthrie writes me from Petersburgh,  that Mr.
 Lowitz, in December 17g2, when the thermometer was at
 60 degrees of Fahrenheit, froze completely four pounds of
 quicksilver, by the mixture of  dry snow with crystallized
 caustic potash.  As the preparation of this salt  is very
 troublesome, and as  it corrodes every thing it touches, and
 is dangerous  for the operator, he tried another salt, called
 axed-ammoniac, which is the res.iduum in the manufacture
 of ctustic volatile  alkali,  and  is very cheap.   When this
 is evaporated, till a drop of it on a plate fixes immediatelv,
 it is then allowed  to  cool and grow solid.   It attracts wa-
 ter with greediness, and must be kept from deliquidating, in
 '.veil •. or!.ed bottles.    Triis salt, mixed with dry snow, i = 
       POWERFUL FRIGORIFIC  JKTURES.  131

    very nearly as powerful as the potash,  and also froze the
    quicksilver.   It is, perhaps of all ofhers, the most generally
    convenient: it is cheap, and easily preserved fit for this use:
    and when used, it will serve  again as well as ever, by eva-
    porating the water produced in the experiment.  Pounded
    ice may be used instead of snow in summer ; but it must
    be inferior in  efficacy, because there adheres to it some wa-
    ter already provided in latent heat.  Mr. Lowitz  found that
    both of these salts were twice as  powerful in producing cold
    as smoking nitrous acid, which produces the depression 41  #
    degrees.   He found that these salts produced about 80 de-
    grees.  If these numbers have been accurately reported to
    me, this method of  Mr.  Lowitz's is the most  efficacious
    that I know*.
      As soon as the solution ceases, the cold begins to  abate,
    by the influence of external warmth.   The process must
    therefore   be conducted  expeditiously,  and with the de-   •
    fences mentioned  above.   Ice and common salt, melting
    slowly, produce a  cold of considerable continuance and in-  .
    tensity.   But I do not imagine that it  can be lower than
t    about----5° of  Fahrenheit, because, about that tempera-
    ture, the salt,  being no longer attracted by the water, be-
    gins to crystallize  and  give out its latent heat.   I  am un-
    certain what may be the limits of those other mixtures.
    Thus I have given the facts relating to the melting of
    ice and freezing of water, connecting them together, so as
   to  lead to the same conclusion ;  and  I have communicated
   the notions and reasonings which  I founded on those facts.
   I must now add, that the same notions and reasonings may
   be  applied to the liquefaction of many and, probably, of all
   other  bodies by heat:  at least, a great number exhibit
   appearances when they  are liquefied, and when they return
   from the fluid to the solid state,  very similar to  those ex-
   hibited by ice or by water, when undergoing these changes.
     The  late ingenious  Dr.  Irvin, who was my pupil at
   Glasgow,  and afterwards my successor in the  chemical
   chair, making some  experiments, at my desire, on the

    * I find the" same  numbers in the account  given by Creil in his Chem. An-
  vils, 1793. 1  3x?.                                EDITOR. 
    132 SEVERAL**|AMPLES OF LATENT HEAT.

   melting and  congealing of spermaceti, and bees-wax, and
   some other substances, found  that the spermaceti in mel-
   ting, absorbed a quantity of heat, which, though it did not
   make it sensibly warmer, but only fluid, would have been
   sufficient for  making melted spermaceti warmer by 141 or
   148 degrees.
      Bees wax absorbed a quantity equal to what is sufficient
   for making  melted bees  wax warmer by  175 degrees.
   And from  experiments with tin, he found reason to con-
 * elude, that this metal, while melting, absorbs a quantity of
   heat equal to what might add 500 degrees ©f heat to it in
   its solid state.
     It appears also from Dr. Guthrie's experiments on the
   congelation of quicksilver*, that it absorbs a great quanti-
   ty of heat, while it is changed by heat from a  solid  to a
   fluid.   A cylinder of congealed quicksilver, adhering to
■  the bulb of a thermometer, being carried into a very warm
   room, very soon began to melt; but it required a consider-
   able time  to melt it completely,  the thermometer involv-
   ed in it pointing, all the while to the----40th degree of
   Fahrenheit.  As the  quicksilver all this time was so very
   much colder than the air around, it must have received
   heat in  a great quantity from that air.
      Appearances also occur  in the refining  of copper and of
   silver, and the  melting and moulding of phosphorus, which
   not only shew  that those substances,  when melted,  con-
   tain a quantity of latent heat, but that, in certain circum-
   stances, they can, like water, retain this  latent heat until
   they are cooled considerably below their congealing point.
   Another opportunity will be taken of  mentioning some of
   these facts, which are very curious, but cannot be fully ex-
   plained without a previous knowledge of the peculiar pro-
   perties of those substances.  I am  persuaded also, that
   the great and sudden contractions of the quicksilver in Pro-
   fessor Braun's thermometers, depended on  the same cir-
   cumstances.    The quicksilver was cooled beyond the de-
   gree sufficient for its  congelation, yet still retained its la-
   tent heat  and fluidity, but being then disturbed, was made
* Phil. Trans. 1786 
HEAT  ABSORBED.              133
 to congeal, at least in part, and the congelation of it is at-
 tended with extraordinary contraction*.
    There is reason to think, that not only  the fluidity of
 bodies, but that even the  softness of such as are softened
 and rendered plastic by heat, depends on a quantity of heat
 combined with them in the  form of  latent heat.   In the
 experiments  made  by  Dr.  Irvin on the  congelation  of
 bees wax, it was quite  plain,  that after the wax had lost
 its fluidy, and while, from its soft state, it slowly became
 harder,  till it acquired its greatest hardness, a quantity of
 latent heat was gradually extricated,  and,  assuming the
 form of sensible heat, kept up the heat of the cooling wax
 much longer, and occasioned its abating much more slow-
 ly than it would otherwise have  done.   The  doctor was
 surprised at it, and was at a  loss to  account for it, until I
 pointed  out to  h,im the cause that appeared the most  pro-
 bable, which was, that the soft state of wax is produced by
 a certain quantity of latent heat in it, through a much less
 quantity than that necessary to  give  it fluidity.
    On account of this,  and  some other facts,  I began to
 suspect,  that the malleability and ductility of metals depend
 on the same cause ; and, after mature reflection, I am  per-
 suaded that this is the case.  While metals are  hammered,
 and extended under the hammer, they become warm, and
 in some cases very hot; but at the same time they become
 Very  rigid, and are no longer malleable.   If  we attempt

  t Dr. black, in several courses of his lectures,  extended this  doctrine
 much fa-ther, namely, to the cases of solution of salts in water, and several
 others, when solid bodies  are rendered fluid by the action of proper fluids
 upon them; as also to some cases where fluid bodies become solid by che-
 rriical action, as in the churning' of butter, the formation of cinnabar &c.
 Some of these are exceedingly curious, and greatly puzzled the philosophers.
 Thus, if a saturated solution of Glauher's salt, in hot water, be allowed to
 cool in a place  where it suiFers no agitation or tremor, it will frequently
 grow cold without crystallizing.   If it  be now shaken, it crystallizes sudden-
 ly, and the vessel becomes very warm, and even  hot.  In the combination
 of mercury with  sulphur,  by the assistance of heat to melt the  sulphur,
when the tr; e chemical union takes place in a considerable quantity at once,
it becomes stiff on a sudden, and heat emerge* sufficient for kindling the su-
perfluous sulphur-
                                                  EDITOR.
      VOL. I.                »b 
134 IN PRODUCING SOFTNESS DUCTILITY.
to beat  them out farther,  they are sure to crack at their
edges, not having the same toughness and softness as be-
fore.   The  only  way now to restore that toughness is by
annealing thtm, that is, by making them hot in the fire,
and  allowing  them to cool.   This being done, they are
found malleable again,  but liable to lose this malleability
bv a second hammering, and they need the introduction
of heat  into them  again, if  we desire to  hammer them
more.
   The following experiment gave me a proof of that. In
order to anneal them, the single circumstance of their be-
ing heated  is not  sufficient,  but it is  necessary that the
heat be communicated to them from other matter.   I de-
sired a smith  to make iron red hot by hammering it.   He
very  soon did it by hammering the extremity of a rod of  ,
iron properly prepared.   It assumed a moderate, or dull
red heat under the action of the hammer.  I desired him
to let it cool,  and then hammer it again, to make it red hot
a second time.   He told me this could not be done  with-
out first annealing it, or  softening it in the fire; and that
if he should attempt to hammer it again without this pre-
paration, it would  not bear  the  strokes of the hammer
 without splitting and breaking into pieces, of which he
 satisfied me a little while  after.   But when the iron was
 in this brittle state, it needed only to be heated in the fire
 until it had a dull red heat,  similar in appearance to that
 which had  been produced in it by the strokes of the ham-
 mer ;  being then allowed to cool, it was found malleable.
 It is plain, that  if nothing were requisite for annealing the
 iron in  this experiment, but making it red hot, and allow-
 ing it to cool slowly, this is already done ;   for the iron is
 left red hot by the hammer.
   I  therefore consider the  metals as substances,  which
 have the power  to retain strongly a certain  quantity of la-
 tent heat, which gives  them their toughness  and mallea-
 bility, but I imagine that this heat is driven out  of them
  »y the violent agitation, compression,  and friction of their
  parts,  in hammering  them strongly into  another  shape.
  i hose called the more perfect metals retainthis heatwiththe
  rn> :u,=t force, and retain it in some cases, though extended 
             ANNEALING OF  METALS-       135

 by skilful hammering to an amazing degree.  Tough iron,
 which is a purer metal than  steel, contains more of it than
 steel does, and shews a little more power to retain it;  from
 iron it cannot be expelled but by the  strokes of the ham-
 mer, or violent compression ; from steel it can be seperated
 not only by hammering, but also by sudden and violent refri-
 geration of the steel from a red hot state. This happens in
 the operation called the hardening of steel;  The steel is
 made red hot in the fire,  and then suddenly plunged into
 cold water ;  thus it is made  excessively hard,  but at the
 same time perfectly inflexible and brittle. We must there-
 fore conclude,  that this sudden and violent refrigeration
 prevents  its retaining a due portion of latent heat, which it
 would have retained, had it been allowed to cool slowly
 and quietly.  Iron, when treated in the same manner, loses
 but very little of its latent heat.
  And now I have given, what appears to me a just account
 of the nature of fluidity and  softness,  considered as gene-
 ral effects of heat, and of the manner  in which these quali-
 ties  are produced by heat; together with a number of facts
 belonging to this subject, from which  it  may be perceived,
 that when properly understood, they  are connected toge-
 ther, and explain one another, as  well as some other facts
 of which no explication had been offered before.
   But since  I have been accustomed to communicate this
 view of the subject,  attempts have  been made to explain
 some of the facts a little differently.  It has been imagined,
 that in order to  account for the phenomena, I suppose a
 cause which is not at all necessary, namely, that it is a spe-
 cial combination of the substance of bodies with the materi-
 al cause of heat, which, while it continues in  this state of
 combination, I call latent heat. This supposition has been
 said to be unnecessary, because the appearances can be ex-
 plained by another principle, which, it is  said, is an uncon-
trovertible fact,  which must  necessarily produce the  very
 appearances that  we observe.
 The alledged fact or  principle is this :  A body, while in
 its fluid state,  has a greater capacity  for heat, or absorbs
 more heat in order to rise one degree in its temperature, 
 136  THE  PHENOMENA OTHERWISE EXP.

than when it is  in  the solid form.   Thus, for  example,
ice, it is said,  has less  capacity  for heat  than water
has. When ice, therefore is changed into water, a quantity
of  heat must go into this water, without making it sensi-
bly warmer; and when water is  changed into ice,  a quan-
tity of heat must come out of the freezing water  as fast
as the ice is formed, and this without leaving it sensibly
colder. The absorption of heat, therefore, into the melting
ice, is not the cause of its liquefaction, but rather the con.
sequence of it;  while on  the other hand, tfje extrication of
what I call latent heat from the  freezing water,  is not the
cause of  its becoming solid, but the  consequence of it.
  This way of conceiving  the subject occurred first to Dr.
Irvin.   I never had an  opportunity  to learn the terms in
which he stated this opinion. I  have  here stated it in the
manner in which I understood  it from report,  and" from
what Dr, Cawford afterwards published**.
   But to this statement  I  answer, that the alleged fact of
disparity between the  capacity of ice and water for  heat,
may indeed be  supposed  to account for the thermometrical
phenomena just now recited ;   but  the principal fact or
phenomenon ia not accounted for by it, I mean the change
 of the solid into the fluid.   Solidity, we all know,  depends
 on cohesive attraction ; but on what cause does fluidity de-
 pend ?   Will it be said  to depend on  the absence or ces-
 sation of cohesive attraction ? I  cannot imagine  how
 cohesive attraction  can  entirely cease, or be suspended,
  * It has already been mentioned, when treating of the unequal distribution
of heat, how Dr. Irvin was led to attend very minutely to the different capa-
cities of bodies for heat; and his notion  of the heat, which appeared when
we  nyix vitriolic acid, in its most concentrated  state, with water, has also
been  noticed. This heat is frequently such as to be  insupportable by the
hand, and splits the glasses in which the mixtures are made.  It is not won-
 derful, that Dr. Irvin  should think this sufficient to account for the slow
 extrication of 140 degrees of heat.  It must be added, that the few experi-
 ments which this temperate climate allowed him  to  make, on the compara-
 tive heats  of ice and water, and the low temperature which he thought was
 the commencement of  absolute heat, authorised him to say, that the sum
 total of the excesses of the specific heat of water, above those of ice, was
 more than sufficient to  account for all the heat that emerges in  freezing, of"
 li> absorbed in melting.
editor. 
ABSORPTION OF  HEAT.
 or,  if  this  should  happen,  how  it should  easily be
 again restored : Nay, we have evident proof, from the
 round form of the drops of liquids,  that it  is not  in  them
 entirely suspended, but only weakened to such a great de-
 gree, that the solid hard body is become  a liquid.   Now
 this is  such a violent change, that I  cannot help  thinking
 it must depend on some powerful cause.   It may be said,
 perhaps, that no  other cause is necessary but the  increase
 of sensible heat above a certain degree, together with some
 singularity in jhe nature of cohesive attraction;  such as
 that this attraction, though very strong at certain small and
 imperceptible  distances,  becomes  suddenly very  weak.
 when we increase, beyond a certain limit, the distance of
 the  particles of matter from one another, and this increase
 of distance is produced  by sensible  heat.
   If this were true, the increase and diminution of distance
 by the  action of sensible heat, ought  always invariably to
 produce, each its  appropriated effect,  on the power of the
 cohesive attraction ;  as  water, when its degree of heat is
 above 32°, is always liquid, so when its sensible heat is
 below 32°, it should always be solid.   But this we know is
 not  always the fact.   Water,  in some circumstances, can
 be cooled to 7 or 8 degrees below 32° without being con-
 gealed ; and many other substances exhibit the same phe-
 nomenon, by retaining their fluidity in some circumstances,
 after their sensible heat is diminished considerably below
 their ordinary congealing point.   And I now ask, what is
 the cause which hinders  the cohesive  attraction  from pro-
 ducing  its effect,  and changing  the liquid into a solid i
 When I find  by experience, that, upon  disturbing  such
 over-cooled liquids, a quantity of heat is extricated from
 them, which did not  appear  immediately before, and that,
 while this heat is extricated, a proportional part of the
 liquid congeals ; I cannot help considering this latent heat
 which was in it,  as having been the cause of its protracted
 fluidity.
   We have also, in the experiment on  iron above describ-
 ed, a striking example of the difference between sensible
 and latent heat, and of the want of power in the one to pro-
duce the effects of the other.   When the latent heat of the 
138     THE REAL CAUSE  OF FLUIDITY,

 soft iron was e'xtricated by hammering, and changed into
 sensible heat, it had no longer the effect to give toughness
 and malleability to the iron, but the greatest part of it was
 communicated to the surrounding bodies,  as sensible heat
 always is; thus  leaving the iron deprived of the circum.
 stance necessary to its toughness.   Moreover, by no other
 means can this quality be again "restored  to the iron, but
 by giving it an opportunity to recover in the  fire, and re-
 tain,  in  a state  which  does not affect the thermometer,
 this quantity which had been  expelled from it.  In which
 process of annealing the iron, it is not necessary to heat
 it to a greater degree of redness than that which was pro-
 duced in it by the eruption of its latent heat.
   I am, therefore, still  of the  same opinion  that I men
 tioned to you a little ago upon  this subject.   I consider
 fluidity as depending, immediately and  inseparably,  on a
 certain quantity of the matter of heat which is combined
 with the fluid body, in a particular manner,  so as not to
 be communicable to a thermometer,  or  to other  bodies,
 but capable  of being extricated again  by other methods,
 and of re-assuming the form  of moveable or communica-
 ble heat.
   We now see the reason for the sixth  caution given for
 conducting the experiments  for determining  the  specific
 capacities of bodies for heat.  If they  act on each other,*
 so as to produce  the solution, or even  the softening, of
 one of the bodies, heat will.be absorbed;  or, if tfie mere
 heat of the mixture melts or softens either of them.  For
 the same reason,  we cannot  examine the temperature of
 intense heats in  Newton's method, when  the body exam-
 ined freezes during its  exposure to the  stream of cold
 air; nor can we  discover  it by pouring the melted body
 into water,  if the temperature produced be  so low that
 the body freezes.   (See  Note 4. at the  end  of  the  Vo-
 lume. J 
      OF VAPOUR AND  VAPORISATION.   139



                    SECT.  III.

      OF VAPOUR AND VAPORISATION.

    HAVING considered, in sufficient detail, the appear-
ances which are exhibited when solid  bodies are  melted
by heat, and having shewn the connection which the difn
ferent circumstances accompanying this change have  with
the general operation of heat, we now proceed to treat of
another, and more remarkable  change, produced by this
great agent  in nature; I mean the conversion  of bodies
into an  elastic vapour, that is, into a fluid extremely rare,
light, and expansive, like air, capable, like it,  of being
easily reduced into less  space, by external pressure, and
resisting, like it,  the force  which  thus compresses  it.
That you may conceive this more distinctly, I shall direct
your attention chiefly to this operation of heat as it affects
fluid bodies, and,  taking water as the most familiar in-
stance,  I shall describe the manner in which it suffers this
change.   Suppose a  single tea-spoonful of water  to  be
put into a globe of glass, or  metal,  capable of holding
some gallons, and suppose this vessel exhausted of air.
If we apply heat to the globe, we shall perceive the water
gradually to  disappear, so that presently the globe  shall
seem empty.   But the fact is, that it is  completely  filled
with the water, now existing  in the  form of a  perfectly
transparent  vapour;  for, if the heat be still farther in-
creased, the expansive force of what  now fills the  globe
will also increase to such a degree, as even to'burst it.
   Upon this change of water from  its ordinary  state into
that of  a rare expansive fluid, depend the effects of  what
are  called candle bombs.   These are  hollow spherules of
glass, formed in the lamp,  on the end of slender tubes.
A little  bit of the tube  is left adhering to the ball.   By
holding the  ball  in the flame of a candle, part of the air
is driven out by expansion.  The end of the tube is then
dipped  into water, and, as the air  in the ball  collapses 
140         GENERAL PHENOMENA.
again by cold,  the water is forced into it by  the pressure
of the atmosphere, till it is perhaps half  full.    The tube
is then sealed  hermetically.   If this  little ball be stuck
into the wick of a burning candle, the water is converted
into elastic  vapour,  which  bursts  the  glass  with a loud
crack.  As  the vapour expands in all directions, the candle
is blown out, and the wick is commonly beaten down, as if
by a blow with  a hammer.
   In  like  manner,  the  aeolipile  affords  a very  plain
proof of the great rarity and elasticity  of the vapour into
which water is  converted by heat.   The aeolipile is a hol-
low ball of copper, having no opening but a narrow pipe,
which is bent at the top, nearly at right angles.  It is partly
filled with water, in the same manner as the candle-bomb.
If it be now set on burning coals, with its pipe pointing into
the midst of the fuel, we shall soon perceive a violent blast
proceed from it, which, when properly managed, will blow
up the fire like a pair of  bellows.   This will continue till
the water is all expended.   We shall afterwards learn that
it blows up the fire, chiefly, because  the vapour erf  the
water drags along with it the surrounding air,  and forces *
it through the  interstices of the  fuel.
   It is partly owing to this conversion of water into elas-
tic vapours  that it is so dangerous  to allow it to  come near
hot oils.   The boiling of lintseed oil, for example, is al-
ways done without doors, to prevent accidents;   and we are
obliged to choose a dry day, because a drop of  rain falling
among the  oil would sink down through it, and would be
suddenly converted, by the excessive heat of the oil, into a
great bulk of expansive vapour,  which would dash the oil
out of the vessel into the fire.   I  said that this vaporisa-
tion of the  water is partly the cause of such accidents, be-
cause it is  known, of late years, that the water is by  this
means converted  into another expansive fluid, much more
bulky and elastic than the mere  hot vapour of water.  Of
this  you will  learn more  afterwards.   It  is  the  water
 which  all oils  contain, in a small quantity, that is the cause
 of their first frothy boiling, after which they remain quiet,
 till raised to a  vastly higher temperature.  In like manner, it 
                  OF  VAPORISATION.             141

   is  dangerous to admit water, or even dampness, into the
   moulds in which metals are cast.   In the casting of iron,
   they form the moulds of sand, which being very porous,
   allow the humidity to escape without any violence.  When*
   copper is melted in very large masses, it contains a prodi-
   gious quantity, both of sensible and latent heat, which gives
   it a great  power of converting water  into  expansive  va-
   pour.   It has sometimes happened that a person, by care-
   lessly spitting in a copper foundry, has occasioned an  ex-
   plosion that destro)ed the'whole building.
     The bulk into  which water expands, by the ordinary
   boiling heat,  producing a steam equally elastic with com-
   mon air, is very great.   It has been, however, much over-
   rated by some very respectable authors.  Dr. Desaguiliers,
   on the authority of Mr. Nieuwentyt, and some  of  his v
   own  experiments,  says that it expands  into 14000 times
   its natural bulk.    But, by my.own  experiments, and those
  of my friend Mr. Watt, whose interest in this matter made
   him  uncommonly  accurate,  its  greatest   expansion   in
   these circumstances, does not exceed 1800 times its former
■: bulk.
    This change is produced, like fluidity, in consequence of
  the  heat of the body being increased  beyond  a  certain
  temperature.   This is found to be  different for almost
   every different kind of matter.*   Like  fluidity,  also, this
  temperature .must be continued, in order that the vaporous
  form may remain.  If the heat be again diminished the va-
  porous form is destroyed, the vapour loses its plasticity, and
  again collapses into that form from which it was produced.
  You must all have  observed  this i,n  the case of water, the
  steam of which is condensed into water again, whenever it
  strikes any mass of cold  matter, such as a lump  of iron.
     In general, therefore,  the conversion of anv particular
  kind  of matter into vapour,  depends on its  being heated
  above a certain degree, and  this vapour collapses again, as

   * The earliest notice that I can meet with of this observation, is that by
 Dr. Hooke, who proposed the temperature of boiling water, as a l,x \ point
 in the  scale of the thermometer.  This was in 1634.  See Birch's Hut. of
 tbe Royal Society, vol. iv.                               editor,
     VOL.  I.                   « C 
142
EFFECT  OF PRESSURE.
soon as its heat is diminished below that degree.  Thi*
degree, orvaporific point of heat, is found (with respect
to most bodies) to be higher than that which is necessary
for giving them fluidity.  Almost  every  particular  kind
of matter requires a different temperature for this pur-
pose.
   Hence the  chemical terms of  volatile and fixed.
Those bodies  which are most easily converted  into vapour
are generally called volatile, and this disposition  to vapo-
risation is  called  Volatility.   Those  bodies, on  the
other hand, are called tixed, which are with greater diffi-
culty converted into vapour, or require a higher tempera-
ture for  this conversion.   Sometimes,  indeed, the  term
fixed, or fixedness, has been understood  to mean a total
want of volatility,  or capacity of being converted into va-
pour by any degree of heat.  But this is not the true mean-
ing, and it is doubtful whether any body in nature  is unsus-
ceptible of vaporisation by heat.   These terms, therefore,
are only terms of comparison, and a fixed salt  is not a salt
incapable of vaporisation by heat, but one that requires a
high temperature for its conversion.
   I have represented the degree of heat at which any par-
ticular kind of matter begins to be converted  into elastic
vapour, as steady or determinate, in the same manner as the
degree which  gives it fluidity.   I must now restrict this
notion, so  far  as to observe that it is steady,  only  when
bodies are  heated  under the  same  circumstances.   But
there are some circumstances, which, when they are vari-
ed, occasion very great differences in the vaporific  tempera-
 ture of every kind of matter.   .This point, therefore, is
 not so unchangeable as the  melting point.   So far as ex-
 perience goes, the melting  point of each kind of matter is
 always the same;  nothing but a certain invariable intensity
 of heat can give fluidity to  a particular solid substance, nor
can any variation  of circumstances hinder that solid from
 becoming fluid,- when raised  to the  melting point, and  a
 proper quantity of  heat is  communicated to  it.  But the
 change of a body into vapour is affected by pressure,  as
 well  as by heat.   In proportion  as the  external pressure 
PAPIN'S  DIGESTER.           US
 upon its surface is greater, it endures a greater heat, with-
 out assuming the vaporous form, than it would otherwise
 do ; and thus we may say, that vaporisation is retarded or
 opposed by external pressure.
   This was first observed,  I  think, by Boyle, when ma-
 king experiments with his air  pump.  Some time after
 this, Fahrenheit, when graduating his thermometers,  oh
 served that the degree of heat which  produces elastic va-
 pour from water, and is called its Boiling point, was lia-
 ble to a^ittle variation at different times, even when  ex-
 amined with the same thermometer. Attending to all the
 circumstances that accompanied these  variations, he found
 that they always corresponded  with  the changes  of  the
 weight, or pressure of the atmosphere.   In future, there-
 fore, he always attended to the height of the barometer,
 when he marked the boiling point upon his thermometers.
 The thing was  soon very well known, and the variations
 of the  boiling point, corresponding to  the heights  of  the
 mercury in the barometer,  were noted with  precision  by
 those who were interested in this subject.   As the chan-
 ges in the barometer are  not very great, so the variations
 in the temperature of boiling water are but inconsiderable,
 never exceeding two or three degrees in these  countries.
 (See Dr. Martin's Essays on  Heat----Sir Geo.  Shuck-
 burgh  on  the  Variations of the  Boiling Point____Phil.
 Trans, vol. 69.)
   The  variations of  the  boiling point of fluids, by still
 greater changes of external pressure, have been more par-
 ticularly considered  by different  writers,  and  machines
 have beencontrived where  their effectshave been muchmore
 conspicuous.   The most remarkable of these is the vessel
 called  a  digester,  the  invention of  Mr. Papin,  a very
ingenious French physician, residing in London.  Papin's
 digester is a copper vessel, generally cylindrical, having a
 lid nicely fitted to it, and  kept fast by screws.   It is usual
to interpose a slip of soft  paper between the digester and
its lid, to prevent the elastic vapour frOm fovcing its way
out.   If this vessel be half filled with  water, and the lid,
 rrewe4 tight down,  and if it be tHen set upon burning 
144           PAPIVS  DIGESTER.
coals, a portion of the water is soon converted into steam
(this is toe  name given  to  tne  clasiu vapour of  boiling
vat.r,)   This conversion begins in  the boiling tempera-
tup of the water ;  but the elastic vapour being confined, it
presses on the surface of the water, and thus prevents the
conversion of any more of it into  steam,   But the conti-
nued  application of heat  raises the temperature of  the wa-
ter above its ordinary boiling point, and occasions the con-
version of a  small  portion more of it into vapour, whose
elasticity  is at least equal to that of the steam already  ex-
is:,ug.    And thus the production of steam is again stop-
ped rill the heat be raised still higher, and the elasticity of
the steam be still farther increased.    We are not very
vvtl; informed concerning this internal procedure, nor very
cc'i tain whether i.ny second, or  subsequent, portion of wa-
ter is thus converted into steam,  by the increase  of heat.
1 his  must depend upon  the rate at which the increase of
h  i augments the elasticity of the steam  already produ-
ce. .
   We know,  however,  that by  the confinement  of  the
stream,  and the consequent increase of the pressure upon
t)  e surface of the water, it may  be made  to endure very
 considerable degrtea of heat.   The  ingenious  inventor
employed it  chief!v for increasing the dissolving power and
 general activitv of water, and water) fluids, in various oper-
 ations of cookery,  pharmacy,  and  other chemical arts.
 He  thus enabled watei completely to dissolve horn, tor-
 toise-shell,  cartilage, and  even bone,  into  a jellv.  He
 made many  extracts, which could not be obtained in  any-
 other manner.   These  eiiccts  induced him to call his ap-
 paratus  a digester.  Mr.  Muschenbroeck found,  that a
 heat might be produced in the water contained in the di-
 gester, sufficient for  melting bits of tin and lead, that were
 hanging  by  wires in  the midst of it.   These  are very haz-
 ardous experiments,  and terrible accidents have sometimes
 happened, bv the bursting of the digester.
   This  variation in the  temperature, necessary for the con-
 version  of borlies into elastic vapour, is not peculiar to wa-
 ter, but   obtains  universally.   Seeing, however,  that the 
    BOILING  TEMPERATURE IN VACUO. 145

pressure of the atmosphere does nbt vary very considera-
bly in any one place, where observations are made, the va-
porific temperature of  water and other bodies, when not
subjected  to  any  artificial pressure,  is  pretty  steady.
These are the temperatures which are generally considered
and mentioned as their  natural vaporific points.  No doubt,
in one sense, they  are so, being the temperatures at which
such  bodies are converted into vapour,  in  the  ordinary
course  of observation.   But they are by no means the
heats at  which  such  bodies  would begin to assume the
vaporous  form,  if left to their own natural disposition
alone, and exposed to no  other action but that of  heat.
But,  under the pressure  of the atmosphere, every body is
forced to receive much more heat than it would otherwise
endure, without assuming the form of vapour.  Some ex-
periments, made by Mr. Robison,  upon water,  spirit of
wine, mixtures of spirits, and water, and sulphuric acid,
shew.that the difference between the temperatures at which
they  boil, in vacuo, and in the open air, is nearly the same
in all, namely, 120 degrees of Fahrenheit's scale*.
    As the pressure of  the atmosphere does not commonly
affect our senses, it does not engage our attention, and we
bear it without inconvenience, or rather our constitution is
 such, that we cannot exist without it.   But we  must not
overlook this  when treating the subject  of  vaporization.
 We  must remember  that we  live under an ocean of air,
 which covers  the  whole globe, to the  height of about 50
miles, and which  has  a  very  sensible weight.   Thus, we
 find  that  a large bottle  is heavier,  when filled with air,
 than after it  has been extracted from it, a cubic foot of
 air weighs about 1* ounce troy.  Therefore, when we boil
 a fluid at the  top  of a high mountain, we do it under a
 smaller pressure than upon the sea-shore. We must attend

   * These experiments were made by me in 1764, by the help of a very in-
different air-pump.  I have repeated them since, in air still more rarefied,
and still have reason to think that the difference of temperature is very nearly
the same in all of them,  but 22 or pcrliaps 25 degrees more than what I then
found.
                                                 EDITOR, 
 146,    PHENOMENA OF  EBULLITION.

 to this in trying the boiling heats of different fluids, at dif-
 ferent elevations  above the sea.
   Let us now attend to some other differences of circum.
 stances and conditions, under which vapour may be pro-
 duced.   When a small quantity of  any fluid, a drop  of
 water, for instance, is heated  very quickly, as when we
 throw it upon a piece of  red hot iron,  it is soon  heated
 up to its vaporific point, hecause  every part of this quantity
 of matter receives the heat nearly in an equal manner, and,
 almost at the  same  instant,  arrives  at that temperature
which it  cannot pass without being converted into vapour.
Hence it happens that the drops run about upon the iron
plate like quicksilver,  for the  iron communicates heat so
very fast to the water, that the adjoining parts are  imme-
diately  converted into vapour,  which prevents  the water
from coming into contact with the iron.   Hence  also it
happens, than when a large lump of red hot iron is thrown
into water contained in a pewter or wooden vessel, it will
melt,  or  burn a hole in the bottom, because  the constant
atmosphere of vapour around it keeps off the water all the
while, and the  iron is but slowly robbed of its heat.
  But when we heat a large quantity of a fluid in a vessel,
in the ordinary manner, by setting it on the fire, we have an
opportunity of observing some other phenomena which are
very instructive.   The fluid  is gradually heated,  and at
least attains that temperature which it cannot pass without
putting on the form of vapour.   In these circumstances,
we always observe that it is thrown into a violent agitation,
which we call boiling.   This  agitation continues as long
as we throw in more heat, or any of the fluid remains, and
 its violence is proportional to  the celerity with which the
 the heat  is  supplied.
   Another peculiarity attends  this boiling of fluids,  which,
 when first observed, was thought very surprising.   How-
 ever long and violently we boil a fluid, we cannot make it
 in the least hotter than when it began to boil.   The ther-
 mometer always  points at the same  degree,  namely, the
 vaporific point of that fluid.    Hence the vaporific point of
 fluids is often called their boiling point. 
              ERRONEOUS  OPINIONS.          147

  When these facts  and appearances  were first observed,
they seemed surprising, and  different opinions were formed
with respect to  the  causes  on which  they  depend.   Some
thought that this agitation was occasioned by that part of the '
heat, which was more than the water was capable of receiving,
and which forced its way through, so as to occasion the agita-
tion of boiling; others again  imagined, that the agitation pro-
ceeded  from air,  which water is known to contain, and which
is now  expelled by the heat.
  Neither of these accounts  however, is just or satisfactory;
the first is repugnant to all our experience  in regard to heat;
We have never observed it in the form of an expansive fluid
like air. It pervades all bodies,  and cannot be  confined by
any vessel,  or any sort of matter, whereas the elastic matter
of boiling water  can be confined  by external pressure, as is
evident in the experiments made with Papin's digester.
  Yet, notwithstanding the incompatibility of this explanation
with our most familiar experience, it had a very general cur-
rency.  No wonder that so ill-founded an opinion was accom-
panied  by  consequences  and  deductions  equally  fanciful.
Even philosophers gave  credit to the vulgar notion, that the
vessel in which water boiled  with violence has very little heat
in it, because, say they, the fire runs up through it as through
a sieve, without heating it; therefore, when the finger is applied
to the bottom of  a boiling  tea-kettle, it is felt no more than
moderately warm.  But this is a silly mistake.  The trial is
commonly  made  with a sort  of fearful hurry, the fire all the
while affecting the back of the hand, and obliging  us to with-
draw it before we can form any distinct opinion of the matter.
But the great cause of this vulgar opinion is, that the bottom
of a tea-kettle is  usually  covered with a spongy crust of soot,
which is almost the worst conductor possible of heat.   If it be
scraped clean, and especially if the wet finger be applied to it
in that  state, it will be scalded in an instant.
   The  second explanation  is equally unsatisfactory,  nor is it
intelligible;  for, if we continue to apply heat to the water, it
will continue to boil to the last drop, and we should therefore
conclude that it is all air. But to be satisfied that this opinion 
 148 INFERENCES FROM THE PHENOMENA.

 is not true, we need only apply cold, or a cooling cause, to this
 supposed air, and we shall immediately find it condensable into
 the same quantity  of water from which it was produced.
  A morejus: explanation will occur to any  person, who will
 take the trouble to consider this subject with patience and  at-
 tention.  In the ordinary manner of heating water, the heat-
 ing cause is applied to the lower parts of the fluid.    It  the
 pressure on the surface be not increased,  the water soon ac-
 quires the greatest heat which  it can bear, without assuming
 the  form of vapour.  Subsequent additions of heat, therefore,
 in the  same instant in  which they enter the water, must con-
 vert into vapour that  part  which they thus  affect.  As these
 additions of heat all enter at the bottom of the fluid, there is
 a constant production of elastic vapour there, which, on account
 of its weighing  almost nothing, must rise through the sur-
 rounding water, and appear to be thrown up to  the surface
 with violence, and from  thence it is diffused through the air.
 The water  is thus  gradually wasted, as  the boiling continues,
 but its  temperature is never increased, at least in that part
whfch remains after long continued and violent boiling.   The
parts, indeed, in contact with the bottom of the vessel may be
supposed to have  received a  little mure heat, but this is in-1 j
stantly communicated to the surrounding water through which
the  elastic vapour* rises.
  This has the appearance of being a simple, plain, and com-  "
 plete account of the production of vapour, and of the boiling
 of fluids ;  and it  is the only account  that was given of this
 subject before I began to deliver these lectures  : but I am
 persuaded that it is by no means  a full account of  the matter.
 According to this  account, and the notion that was conceived
 of the formation  of vapour, it was taken for  granted that,
 after a body is heated up to its vaporific point, nothing further
 is necessary but the addition of a little more heat to change it
 into vapour.  It was also supposed,  on the  other hand, that
 when the" vapour of water is so far cooled as to be ready for
 condensation,  this condensation, or return into the  state of
 water, will  happen at once, or in consequence of its losing only
 a very small quantity of heat. 
        HEAT IS ABSORBED AND COMBINED.  U'-j

      But I can easily shew, in the same manner as in the case of
   fluidity, that a very great quantity of heat is necessary to the
   production of vapour, although the body be already heated to
   that temperature which it cannot pass, by the smallest possible
   degree, without being so converted.   The undeniable conse-
   quence of this should be,  an explosion of the whole  water,
   with a violence equal to that of gunpowder.  But  I can shew,
   that this great quantity of heat enters into the vapour gradually'
   while it is forming, without making it perceptibly hotter to the
   thermometer.  The vapour, if examined with a thermometer,
   is found to be exactlyjof the same temperature as the boiling
   water from which  it arose.- The water must be raised to a
V certain temperature, because, at that temperature only,  is it
   disposed to absorb heat; and it is not instantly exploded, be-
   cause, in that instant, there cannot be had a sufficient supply
   of heat through the whole  mass.  On the other hand, I can
   shew that when the vapour of water is condensed into a liquid,
   the very same great quantity of heat comes out of it into the
   colder matter by which it is condensed ; and the matter of the
   vapour, or the water into which it is changed, does  not become
   sensibly colder by the loss of this great quantity of heat.  It
   does not become colder in proportion to the quantity of heat
   obtainable from it during its condensation.
     All this will become evident, when we consider with atten-
   tion  the gradual formation  of vapour, in consequence of the
   continued application of a heating cause, and the like gradual
   condensation of d^iis vapour, when we continue to apply to it a
   body that is colder^  Thus, if we apply a heating  cause,  to a
   quantity of water,....suppose by setting^ tea-kettle with water
   upon the fire, the heat flows into it very fast, from the begin-
   ning of the  experiment until the water is heated to its boiling
   point.  Perhaps, during the last five minutes, we find that the
   water has increased its temperature 20 degrees.  It is a mat-
   ter of general observation, that the passage of heat from one
   body to another is  nearly in proportion to their difference of
   temperature, other circumstances remaining the same.   In the
   present case, therefore, since  the  water  does  not  sensibly
   change its temperature during the boiling,  we may conclude
      vol. i.               d d 
150 INFERENCES  FR(5M THE  PHENOMENA,

that the heat continues to flow into it nearly at the same rate,
and that it receives four degrees of heat in every minute. This
supposition leadsto no sensible error;  for I have repeatedly
found that,  when water rose 20 degrees  during  the last five
minutes, it took 40 to gain 162 degrees of  heat; and when I
observed it  to rise  38 degrees  in the same time, it had risen
162 degrees in 20 minutes*.   Now, if  the common opinion
were just, it is plain, that, in a few minutes  more, the whole
water would assume the form of vapour, and  would produce
a most violont explosion, even sufficient to blow up the houst^
Nothing like  this happens;  for,  after several minutes have
elapsed, we shall find that only  a very'small quantity of water
has become vapour, and that a great while indeed must elapse,
and a continual application of the fire must be made, before
the whole of it can be made to assume that form.  In the ordina-
ry course of such experiments in this climate, it requires about
six times the number of minutes to boil off a small quantity of
water, that it takes to bring it to boil.  I  can scarcely remem-
ber the time in which I had not some confused idea of this
disagreement  of the fact  with the common opinion: and I
presume that it has come across the mind of almost every per-
son who has attended to the boiling of a pot or pan.  But the i.
importance of the surmise never struck me with due force, till
after I had made my experiments on the melting of ice.   The
regular procedure in that  case, and  its similarity to what ap-
pears here,  encouraged me  to expect a similar regularity
in the boiling  of  water, if my conjecture  was well founded.
But almost  every time  I thought of it, it appeared to me so
difficult, if  at all possible,  to procure a supply  of heat that
should be tolerably uniform, or to ascertain its  irregularities,
that I was  discouraged from  making an  experiment which
would in all probability be so anomalous.  But I was  one day-
told by a practical distiller, that, when his furnace was in good
order, he could tell to a pint, the quantity of  liquor that  he
would  get  in  an hour.  I immediately set about boiling  off
\
  * In both cf these cases, the heat had entered somewhat faster in the be-
ginning of the experiment, while the temperature of the water was lower.  - 1J
                                                   EDITOR- 
       HEAT CONGEALED IN THE  VAPOUR.   151

    small quantities of water, and I found that it was accomplished
    in times very nearly proportional to the quantities, even al-
    though the fire was sensibly irregular.
      I, therefore, set seriously about making experiments, con-
    formable to the  suspicion that I  entertained concerning the
    boiling of fluids.   My conjecture, when put into form, was to
    this purpose.   I imagined that, during the boiling, heat is ab-
    sorbed by the water, and  enters into  the composition of the
    vapour produced from it, in the same manner as it is absor-
    bed by ice in  melting, and enters into the  composition of the
    produced water.  And,  as the ostensible  effect of the  heat,
    in this last case, consists, not in warming the surrounding bo-
'•  dies, but in rendering the ice fluid ; so in  the case of boiling,
    the heat absorbed does not warm surrounding bodies, but con-
    verts the water into  vapour.   In both cases, considered as
    the cause of warmth, we do not  perceive its presence: it is
    concealed, or latent, and I gave it the name of latent heat.
      I shall now  describe a few of the experiments, which, I ap-
    prehend, will fully establish the justice of  my conjecture.
      Experiment  1st.   I procured  some cylindrical  tin-plate
    vessels,  about 4 or  5 inches diameter,  and flat-bottomed.
•'•■  Putting a small quantity of water into them, of the tempera-
    ture 50 degrees, I set them upon a red hot kitchen table, that
    is, a cast-iron plate, having a furnace of burning fuel below it,
    taking  care that the fire should be pretty regular.  After
    four minutes, the water began sensibly to boil, and in twenty
    minutes more,  it was all boiled  off.   This experiment was
    made 4th October 1762.
    Experiment 2d  Two flat-bottomed vessels, like the former,
   were set on the iron plate, with eight ounces of water in each
   of the temperature 50°.   They both began to boil at the end
   of three and a half minutes, and in eighteen minutes more, all
   the water was boiled off.
     Experiment  3d.   The same  vessels were  again supplied
   with twelve ounces of water in each, also of the temperature
   50".  Both began to boil, at the end of six  and a quarter min-
   utes, and the water was all boiled off, from  the one in twenty-
   eight  minutes,  and from the other  in something  more  than
   twenty-nine. 
152
EXPERIMENTS WHICH PROVE
     I reasoned from these experiments in the following manner:
   the'vessels  in the first  experiment received  162 degrees of
   heat  in four minutes, or 40| degrees each minute.  If we,
   therefore, suppose that the heat enters equally fast during the
   whole ebullition,  we must suppose that  810 degrees of heat
   have  been absorbed by the water, and are contained in its va-
   pour.  Since this vapour is no hotter than boiling water, the
   heat is contained in it in a latent state, if we  consider it only
   as the cause of warmth.   Its presence is  sufficiently indicated,
„  however, by the vaporous or expansive form which the water
   has now  acquired.
     In  experiment second, the heat absorbed, and  rendered  j
   latent, seems to be about 830.                              V
     In the third experiment, the heat absorbed seems to be some-
   what  less, viz. about 750.  The time of rising to the boiling
   heat,  in experiment third, has nearly the same  proportion to
   that in experiment first, that  the quantities  of water have,
   The deficiency, therefore, in the heat absorbed, has been pro-
   bably only apparent, and arising from irregularity in the fire.
   Upon the whole, the conformity of their results with my con-
   jecture was  sufficient to confirm me in my opinion of its jus-
   tice.   In the course of further experiments made both by my-
   self and  by  some friends, and in which the utmost care was
   taken to  procure a perfect uniformity in the heat appliett, the
   absorption was found extremely regular, and amounted at an
   average to about 810 degrees.
     There are  other cases where this absorption appears in a
   much more  singular manner.  I put into a very strong phial,
   about as  much water as half filled it, and I corked it close.  The
   phial was placed in a sand-pot, which was gradually heated,
   until  the sand and phial were several degrees above the com-
   mon  vaporific point of water.   I was curious to know what
   would be the  effect of suddenly  removing the pressure of the
   air, which is well known to prevent water from boiling.  The
   water boiled  a very short while, but the ebullition  gradually
   decreased,  till it was almost insensible.   Here the formation
   of more vapour was opposed  by a very strong pressure pro-
   ceeding  from the quantity of vapour already accumulated, and 
          THE ABSORPTION  OF HEAT.       153

confined in the upper part of the phial, and from the increased
elasticity of this vapour, by the increase of its heat.   When
matters were in this state, I drew out the cork. Now, accord-
ing to  the  common  opinion of the  formation of vapour by
heat, it was to be expected that the whole of the water would
suddenly assume the vaporous form, because it was all heated
above the vaporific point. But I was beginning by this time to
expect a different event, because I could not see whence the
heat was to be supplied, which the water must contain when
in the  form of vapour.  Accordingly, it happened  as I ex-
pected ; a portion only of the water was converted into vapour,
which  rushed out of the phial with a considerable explosion,
carrying along with it some drops of water.   But^ what was
most interesting to me in this experiment was, that the heat of
what remained was reduced in an instant to the ordinaty boil-
ing point.  Here, therefore, it was evident that all that excess
of heat which the  water had  contained  above the boiling
point, was spent in converting only a portion of it into vapour*
This is plainly inconsistent with the common  opinion,  that
nothing more is necessary for  water's existing in  a vaporous
form under the pressure of the atmosphere, than its being
raised  to a certain  temperature.  The  experiment  makes it
more probable, that if the influx of heat could at that instant
hs&e been prevented, it would have remained in the form of
water,  although raised, in a very sensible degree, above the
boiling temperature.
   I was anxious to learn whether the heat which disappeared
in this experiment was in an  acurate proportion to the quan-
tity of  vapour produced, or the quantity of water that had dis-
appeared.  But the drops of water that were hurried along by
the explosion, without being converted into vapour, made it
impossible for me to  ascertain this with any tolerable accu-
racy, although I repeated the experiment several times.
   This experiment  was afterwards  made by my friend Mr.
Watt,  in a very satisfactory manner.  His studies for the im-
provement of his steam-engine, gave him a great interest in
every  thing  relating to the  production of steam*  He put
three inches of water into a small copper digester, and, screw- 
154  EXPERIMENT WITH THE  DIGESTER.

ing on the lid, he left the safety-valve open.  He then set it
on a clear fire of coaks, and after  it began to boil and pro- 'J
duce steam, he allowed it to remain on the fire half an hour,
with the valve open.  Then taking it off the fire, he found
that an inch of water had boiled away.  In the next place, he
restored that inch of water, screwed on the lid, and set it on
the fire; and as soon as it  began to boil, he shut the safety-
valve,  and allowed it to remain on the fire half an hour as
before.   The temperature of the  whole was  many degrees
above the boiling point.   He took it off the fire, and set it  up.
on ashes, and opened the valve  a  very small  matter.  The
steam  rushed out  with great  violence,  making a shrieking
noise for about two minutes.  When this had ceased, he shut
the valve, and allowed all to cool.   When he opened it, he
found that an inch of water  was consumed.
  It is reasonable  to  conclude  from this  experiment,  that
nearly  as much heat was expended during the blowing of the
Steam-pipe, as had been formerly expended in boiling off  the
inch of water.  For, before opening the valve,  the tempera-
ture was many degrees above  the boiling point, and  all this
disappeared with the  vapour.   The same inference may be
drawn from the time that the digester continued upon the fire
with the valve shut,  because we may conclude that the heat
was entering nearly at the  same rate during the whole titrie.
It is plain, however, that  the experiment is  not of  such a
kind as to admit of nice  calculation;  but it is  abundantly
sufficient to shew that a prodigious quantity of heat had es-
caped along  with the particles of vapour  produced from an
inch of water.   The water that remained  could not be hot-
ter than the boiling point, nor could the vessel  be  hotter,
otherwise  it would have heated the water,  %nd converted it
into vapour.   The heat, therefore, did not escape along with
the vapour, but in it, probably united to every particle, as one
of the ingredients of its  vaporous constitution.  And as  ice,
united  with  a certain quantity  of  heat, is water, so water
united with another quantity of heat, is steam or vapour. 
          WATER BOILING IN VACUO.        155

  There are some other instances of the same kind, which
may perhaps appear still more striking, I mean the experi-
ment made upon the boiling of fluids in  vacuo,  which are
easily explained upon these principles, and serve at the same
time to support them.  The first person who made experi-
ments of this kind, was the Honorable Mr. Boyle.  He pla-
ced phials with hot water, which had been well boiled to ex-
pel its  air, under the receiver of his new invented air-pump,
and then exhausted the air.  Long before the vacuum  was
nearly perfect,  the water began to boil, and continued to do
so with great voilence while he worked the pump.   He ex-
presses his surprise at the great heat which the receiver ac-
quired, and at the sudden diminution of the heat of the wa-
ter, which he expected  to  find »till scalding hot when he ad-
mitted the air and took off the receiver, whereas it was only
lukewarm.  This experiment has been repeated by others
many hundreds of times,  and with more particular attention
to the different circumstances.  They all agree in this,  that
the water, from being hot,  becomes lukewarm  immediately,
or very fast, and plainly in consequence of its boiling or pro-
ducing vapour.     '*
   At the time that I was engaged in these experiments, pro-
fessor Robinson, then one of my pupils, told me, that in some
experiments which he had made with the air-pump belonging
to the university of  Glasgow, however hot the water was at
first, the heat of it was rapidly  diminished, till it  fell to the
90th or 91st degree of Fahrenheit.  Mr.  Watt also assured
me, that, in a very good vacuum,  the water will continue to
boil or produce copious vapour, till its heat be quickly redu-
ced to the 70th degree of Fahrenheit.   This  sudden disap-
pearance,  therefore, of a great part of the heat of  the water,
while a very small part only assumes the vaporous form, is an
additional proof  of  what I  have advanced  concerning the
manner in which heat produces vapour.
  Similar to these examples, are also,  in my opinion,  some
surprising experiments made by  Dr. Cullen,  with  some very
volatile fluids, and published by him in the Physical and Lit-
erary Essays of Edinburgh.  The Doctor employed the late 
 156     DR. CULLEN'S  EXPERIMENTS

Dr. Dobson, at that time his pupil, to make experiments on
the heat or cold produced by mixing different fluids and solid*
with  one another.   Dr. Dobson, in  making these experi-
ments, observed  that the thermometer, when lifted out of
many of  the  fluids,  and suspended a short time in the  air
beside them, fell down to a lower degree than that indicated
by another thermometer which hung constantly in the same
air.   And, after varying his observations on this phenomenon,
he  found reason  to conclude, that it was occasioned by the
evaporation  of the last drop of fluid  which  adhered to the
bulb of  the  thermometer;  the sinking of the thermometer
being always greatest, and most quickly produced, when this
instrument  was taken out of the most volatile fluids.   Dr.
Cullen had then a curiosity to try whether the same phenom-
enon would  appear on repeating these experiments under Ihe
exhausted receiver of the air pump; and to satisfy himself,
he placed on the plate of the air pump,  a glass goblet con.
taining water, and in the water he placed a wide mouthed phial,
containing vitriolic aether. The whole was covered with an air-
pump receiver, having, at the upper end, a collar of leathers,
in a brass socket, through which a thick smooth wire could be
moved, and  from the lower end of this wire, projecting into
the receiver,  was suspended  a thermometer.   By pushing
down the wire,  the  thermometer could be dipped  into the
cether; by drawing it tip, it could be taken out and suspended
over the phial.
  The apparatus being thus adjusted, the air-pump was work-
ed to  extract the air.   An unexpected phenomenon immedi-
ately  appeared, which prevented the experiment being made
in the manner intended.   The aether was thrown into a vio-
lent agitation, which Dr. Cullen imputed to the extrication of
a great quantity of air.  It is evident,  however, from several
particulars, that the aether was not emitting air principally, but
a great quantity of ethereal vapour, and that it was truly in a
boiling state : for the elastic fluid, extricated by working the
pump, had a very strong odour of  sether, and the quantity of
the sether in the phial, was considerably and suddenly dimin-
ished, while this boiling continued.  And, what  is especially

                                                     0 
         ON VAPORISATION  IN VACUO.      137

to be attended io at present,.the sether became suddenly so cold,
that it froze the water in the goblet around it, although the
temperature of the air, and all the materials, were at the 54th
degree of Fahrenheit, in the beginning of the experiment.
   You will now easily understand how the phenomenon may
be explained, by the principles which I have already establish-
ed.   It is evident, that the vitriolic aether is a fluid of such
great volatility,  that, were it not compressed by the weight of
the atmosphere, which forces the parts of it to remain together,
it would for ever be in the form of vapour, in all the variations
of heat or  cold, which we commonly  experience;  that the
proper vaporific heat of this  fluid,  or the lowest degree at
which it would boil, were it totally free from the pressure of
the atmosphere, is far below the freezing point of water.  It
is therefore always more than hot enough to boil and produce
vapour,  whenever we take off the pressure'of the atmosphere
from its surface.  Accordingly,  when we take off this pres~
sure from it, vapour begins immediately to be produced, and
such a great quantity of the sensible heat that was in the ae'ehen
is suddenly changed  into latent heat, or absorbed in the va-
pour, that the  remaining aether  must necessarily become ex-
cessively cold.   It may become so cold at last, as to be no
longer in a condition to produce more vapour even in vacuo.
   This experiment, therefore,  has  a good deal of analogy
with  the production of cold, by mixing snow with  different
salts.  In those experiments,  the snow is made suddenly to
assume the form of a fluid,  without adding heat  to it.  In
the above experiment, the aether is made suddenly to assume
the form of vapour,  without  its receiving any  addition of
heat.
   We may further remark, that the above supposition, with
regard to the lowness of the proper vaporific point of the vi-
triolic aether, is not assumed without good reason:  it is sup-
ported by the analogy of the proper  vaporific heats of  other
fluids.  All of these which  have hitherto been tried begin in
a vacuum to boil, and emit vapour, when their heat is lower,
by 120 degrees at least, than their vaporific point under the
pressure  of  the  atmosphere.   But the vitriolic aether is well
   vol.  t.               e e 
158     COPIOUS ABSORPTION OF HEAT

known to boil under the pressure of  the atmosphere at  about
100 degrees of heat.  We have reason therefore to conclude
that it will boil in vacuo so  long as its heat  is higher than
the 20th degree below  the  beginning of  Fahrenheit's  scale, <
or the 52d degree below the cold sufficient for the freezing of
water.
   The more carefully we consider the production of expan-
sive vapour, in all that variety of circumstances that have been
already narrated and many others of  the same nature, the more
satisfactory evidence have we, that a great quantity of sensible
heat is somehow absorbed by the water, in passing from the
liquid to the  vaporous form, or disappears in some more un-
accountable manner  during its  formation.   For it does not
immediately appear in the steam.  This is evident by placing
the bulb of a thermometer in the way of the steam,  so as to be
completely surrounded by it.   It raises the liquor in the stew
of the thermometer  no higher  than it would be raised  by
plunging it in the boiling water.  As a farther proof that it is no
hotter, it is a matter of daily experience, that the steam of boil-
ing water does not scorch the most delicate flowers, nor spoil, in
the smallest degree, the fragrance or delicacy of their odorous
principle.  On the contrary, we  obtain this  principle  in  its
greatest  perfection, by placing those flowers in the upper part
of a common still, in which water is made to boil.   The heat
'of the passing steam volatilizes, and carries off along with it
the precious aroma; and it is found in the water  condensed
by the worm plunged in a refrigeratory filled with cold water.
This process is found to be vastly preferable (for such plants
 as are uncommonly delicate)  to the immersing them  in the
boiling water.   It  is plain, therefore, that the great quantity
of the heating principle, which we suppose to be flowing con-
tinually  from the fuel in the furnace into the water contained
in the still, is not to be found in the  steam, in the same active
form, or as the cause of heat.  But it is not lost, and is really
contained in the  steam, in such a manner as to manifest itself
by its former effects, by heating any body  when the vapour is
again converted into water.   As a particle of water, in the in-
stant of  its becoming a particle of vapour, attracts and unites 
SEPARABLE BY CONDENSATION.     l y,(
  with itself one or more atoms of this cause of heat,  if only-
  raised to the requisite temperature, and retains them as parts
  or ingredients of its vaporous form, so we may reasonbly con-
 jecture, that when a particle of vapour again becomes water,
  these atoms  of heat are set  at  liberty  by the fixed  laws of
  chemical affinity, and are then at liberty to enter into any other
  body which is in a fit condition for receiving them.  Such a
  body should be warmed by them, and this should be sensible
  by means of the thermometer.  If indeed the body be on the
  verge of liquefaction or ebullition, it may be as much disposed
 to absorption of heat as the actually  collapsing  vapour of
 water is disposed to  its  emission.   Yet,  in this case also, we
  should obtain equally incontrovertible indications  of  the ex-
' trication of heat from the elastic steam.   The temperature of
 the body may not be raised, but it will melt or boil.      •
   When this opinion got hold of my'mind,  I was  peculiarly
 pleased with the explanation which  it gave me of many curi-
 ous  facts  observed in distillations, and other chemical pro-
 cesses, which greatly perplexed the chemists, who being igno*
 rant of their  cause, could neither  explain them,  nor avoid
 them when hurtful.  The multitude of important consequences
 which seemed to result from this combination of heat with the
 volatilized body, made me anxious to obtain accurate informa-
 tion ; and the road that I was to follow seemed to be very ob-
 vious and easy.  The object was to obtain, not only  certain
 proof of the extrication of a great quantity of heat  contained
 in the vapour, and not perceivable in it by means of the ther-
 mometer, but also to discover whether all that heat was to be
 obtained from vapour while condensing, which had disappear,
 ed during its formation.   The proposed analysis was perfec'1"  ,
 similar to  what I had accomplished, so much to  my satK' .
                           .                     '    being
 tion, in the congelation  of  liquids  compared with  the 1
 faction of the frozen mass:  and the  mode of proceedin. r  .
 equally obvious and  familiar:  I had only to attend to
                                                     vUlllC*
 happens in the working of a common still.              ,  .
   All of you know well enough how the operation of      r
 raon distilling  is conducted.  The water, or  other  liqi come
bo distilled, is put into a boiler, which is set on the fire.  **•   '' 
    160    ANALYSIS  OF* THE' PROCEDURE,

    vapour produced by its ebullition is not allowed to  rise and
    mix with the air, but  is confined by the head of the  boiler,
    called the still, and it is conducted laterally by a  pipe.  But if
    nothing more were done, the vapour would only blow  out
    through  this pipe, as we observe it to do from the spout of a
    tea-kettle, when it boils with violence.  This pipe is, there-
    fore, inserted into another, which is made to pass through a
    large vessel, called the Refrigeratory, or Cooler, but without
    communicating with it.  The refrigeratory is filled with cold
    water.   This, being in contact with every part of the pipe,
    keeps it  cold, and the pipe being  made of  metal,  which is
    known to transmit heat very fast,  the inside  of  it is quickly
    robbed of any heat which the vapour can communicate to it;
    and this heat is  transmitted to the water in the  refrigeratory,
    and must raise its temperature.  That this may  be more copi-
    ously done, the pipe is generally made of a spiral form, like a
    cork-screw, its upper end communicating with  the pipe from
    the still-head, and its lower end being  turned  sidewise, and
  *  led through the side of  the  refrigeratory, and  a vessel is set
-^ ^jjj&ow its -mouth, to receive the liquor formed of the condens-
    *A vapour.
       From  this brief  description of the  apparatus  (of which
    more particular  notice will be taken in its proper place)  you
    will easily comprehend the whole of the process, and the cir-
    cumstance to which our particular attention must be directed-
    The heat of the fuel penetrates the body of the still, raises the
    temperature of  the water  (which  I shall  suppose tq be the
    substance under examination) till it can no longer remain in
        liquid form,  but boils,  that is, it  is converted into elastic
      bo'pur j  and, if my opinion be just, it absorbs, in the instant
      of trfnversion,  or is  converted by absorbing,  a prodigious
      tinuaity of heat, namely, as much as would raise its tempera-
      in dugout  800  degrees, if it acted on it in the same manner
      form>re its boiling.  I imagine that this heat is contained in
      conta^duced vapour, and goes off with it  through the £ipe.
      by itsj its passage  along the convolutions of the spiral pipe,
      againm, as it is called, the hot vapour comes in contact with
      srannrcumference of  the  cold  pipe.   The fact is, that it is 
          IN  ORDINARY DISTILLATION.      161

completely condensed by this contact.   For, when things
are properly conducted, by giving a very extensive surface
of contact, and keeping a plentiful supply of cold water in the
refrigeratory, nothing issues from the mouth of the pipe but cold
water, and the air which occupied the upper part of  the appa-
ratus, in the beginning of the distillation, and a small quantity
which is contained in almost all waters, and is extricated from
them by boiling.  In the mean time,  it is a fact, that the
water in the refrigeratory grows warmer and  warmer as the
distillation goes on, and indeed soon grows extremely hot,  if
it be not allowed to run off, and its place be not supplied by a
continued stream of  cold water.  When this  is not carefully
secured, the liquor which drops from the worm is warmer and
warmer,  in the same  proportion as the water in the  refrigera-
tory (for  it  evidently cannot  be colder, but  must  rather be
somewhat warmer), and at last it is almost boiling hot.   Now
the condensation ceases, and steam  blows through  the worm
in abundance.   This  has sometimes happened in the^distilla-
tion of spirituous liquors, and  has occasioned  fatal accidents,
because these vapours are inflammable.
   Such are the facts.  My notion of  the internal procedure
was this:  the particles of vapour which come  into immediate
contact with  the  circumference of the spiral pipe,  having
a smaller attraction for the heat  combined with them, than
those of the cold pipe have, allow them to unite with the sub-
stance of  the pipe,  whence they are  soon transmitted to the sur-
rounding  water, so that the pipe is still in a condition to rob the
subsequent particledrof vapour of their natural  heat also. The
particles of vapour, deprived of their latent heat, are  no longer
vapour, but water, which falls to the  lower sideof the worm, and
trickles down along it.  The exterior particles of vapour being
thus removed  out of the way, those which were nearer to the
middle, or axis of the  pipe,  being pushed forward by the
steam behind  them, and, at the same time expanding, come,
in their turn,  to be applied to  the sides of the pipe, and are
also condensed into water;  and thus, the different strata of
vaporous  particles,  from the circumference to the axis, come?
jn turn, to be deprived of  their heat, and, by the^deprivatiqn, 
 I02         INTERNAL PROCEDURE.

 become water again, cooled down to the temperature of that
 part of  the pipe by which they are condensed.   These strata
 of  vapour may be conceived  as like  the sliding tubes of a
 pocket spy-glass ;  those next the axis always getting beyond
 the stratum which surrounded them,  before they  expand and
 are condensed by  touching the sides.   By this process, the
 upper parts of the worm, and the surrounding water, become
 soonest hot, and are always hotter than those farther down the
 refrigeratory  (if the water be not stirred).  It is also reason-
 able to suppose that those parts of  the pipe which are coldest
 condense the  passing vapours most copiously, and that, when
heated very near to the temperature of boiling water, they con-
dense it very slowly indeed.   Therefore  more uncondensed
vapour  passes forward, to a part not yet so warm, where it is
 rondensed, and by giving out its heat, that part of the pipe is
more quickly  heated;  and thus the  heating of the pipe and
water proceeds to the bottom  of the refrigeratory.
  If this conception of  the process be just,  it should follow
that no heat  should  disappear in the experiment, except
 ,vhat is unavoidably  lost by warming  the vessels,  and,  by
 their intervention, warming the surrounding air, and being
 carried off by it.  All  the heat transmitted from  the fuel
 should be found in  the distilled  water,  and in the  water
 of the refrigeratory, except the loss by waste now mentioned.
  Thus were the steps of my examination  plainly pointed
 out to  me.   I must first measure the heat expended in boil-
 ing off a certain quantity of water.  I must, in  the second
 place,   measure  the  heat communicafed to  the   distilled
 water, and to the water of the refrigeratory;  and I must
 compare those quantities together, and find them equal, or un-
 equal.
  When, for example, we set a still to work, the refrigeratory
 of  which contains 100 gallons of water, of the temperature
 50°, and  we  distil off one gallon ;  if  we now examine the
 water of the  refrigeratory, and find that it has gained 11 de-
 grees of heat, we must reason  in  this way:  had the steam,
 while it heated this water, produced no more effect than in
 proportion to its perceptible heat, and  the quantity of water 
                MANY  INDICATIONS            163

 which composed it,  which is one gallon, it  should not have
 increased the heat of the boiling water more than 1 \ degree.
 For it is little more than 150 degrees  perceptibly hotter (the
 distilled water  having  the temperature  61°  nearly).   But it
 has actually raised the temperature of 100 gallons 11 degrees.
 Here, therefore, are 9\ degrees of heat added to each of 100
 gallons of cold water, without our being able to account for it
 according  to  what has been the common opinion.  But a
 quantity of heat which is sufficient for  increasing the sensible
 heat of 100 gallons of water  by 9^ degrees, could it be all put
 into one gallon, without converting it  into  vapour,  would be
 sufficient for increasing the  sensible heat of that gallon, by
 950 degrees.  And all this heat was actually contained in the
 one gallon, whilst in the form of vapour, and without shew-
 ing itself  by  affecting the thermometer more  than boiling
 water does.
   I made several experiments, in conformity to those already
 mentioned, on  the  conversion of water into steam, which
 fully satisfied my own mind that my opinion concerning the
 nature of elastic vapour was just.  Indeed, when my mind
 was occupied by this thought, conviction flashed on me  from
 every  quarter,  that the quantity  af heat in  all  vapours was
 prodigiously greater than what was indicated merely by their
 sensible heats or temperatures. Every person knows the scald-
 ing power of steam,  and that a momentary  puff of it from the
 spout  of a tea-kettle,  which will scarcely render  the  hand
 damp, and does not contain the fourth part of one drop of
 water, will in an instant raise the whole hand into  a blister,
 which  a thousand drops of boiling water could not do.  There
 is hardly a person who has not been struck by the great heal
 produced in the  refrigeratory  of a common still;  and  those
 who manufacture spirituous liquors for  the market have often
 found as great difficulty and eXpence attending the  supply of
 cold water for their  refrigeratory, as they had in supplying
 their furnace with fuel.   This has prevented the erection of
such manufactures in great cities,  where it would  otherwise
be most convenient  and profitable; because  they cannot be
supplied  with cold water.  The more I reflected on these 
164    OF HEAT COMBINED IN  VAPOUR.

things, I was the more astonished that a thing so obvious, and
so interesting to a number of active people,  attentive to their
interest, and accustomed to estimate every thing by measure-
ment of some kind, had  never attracted their attention,  and
engaged them,  or speculative men, who are always  ready to
assist them, in some comparison between the fuel expended
and the remarkable heats  produced.   It  now appeared won-
derful to me, that a competent knowledge of this subject was
not acquired almost as soon as  the scale of the thermometer
was  determined by the fixed  points of boiling and  freezing
water.   And, when that noble machine the steam engine, was
invented,  and was continually in the hands of most ingenious
men, such as Dr. Hooke, Dr. Papin, Dr. Desaguilliers, and
others, and almost prescribed this examination as indispen-
sably necessary for its very practicability, it is still more sur-
prising, that so obvious a  thing never occurred.  But no such
examination is to be met with in the writings of either  the
mechanicians or the chemists.
   The latent heat of steam seemed to me to be so completely
established, that I was little solicitous of more experiments
for my own conviction,  or for making the doctrine clearly
comprehensible to others.  I, therefore, taught it in my  an-
nual courses, and have reason to believe that it was perfecly
understood by my students.  But it was a desirable thing to
obtain exact measures of  the heat contained in steam, because
this  appeared to me  the most  accurate  way for  obtaining
measures of  the heat produced  by fuel,  and might be of
great service in making trials of  the most  economical meth-
ods of applying  fuel, a  point of immense consequence in
ninetenths of  all our manufactures.  It was some  consider-
able time, however, before I could execute this to my satis-
faction, owing to the occupations of a double academical duty,
and the attention due to my patients.  And, at last, indisposi-
tion obliged me to commit the conduct of the  experiments to
 one of my pupils,  the late  Dr.  Irvin of Glasgow, a young
 gentleman of an inquisitive and philosophical mind, of great
 ingenuity, and peculiarly qualified for this task, by  the habits
 of mathematical study,  and scrupulous attention to all kinds 
                 FIRST MENSURATION            165

   of measurement.   The following was the first of the experi-
   ments for this purpose, which I  made in Glasgow,  with the
   assistance of Mr.  Irvin,  on the 9th of  October 1764.
     Five measures (each containing 4 lb. 5 oz. and 6 dr. avoir-
   dupois) of water, of the temperature  52°, were poured into a
   small still in the laboratory.  The fire  had been kindled about
   40 minutes before, and was come  to a  clear and uniform state,
   The still was set into the furnace, and, in an hour and twenty
   minutes, the first  drop came from  the worm ;  and  in three
   hours and forty-five minutes more, three measures of water
   were distilled, and the experiment ended.   The refrigeratory
  'contained 38 measures of  water, of which the temperature, at
 "the beginniug of  the experiment, was 5&0.  When one mea-
   sure had come over, the water in  the refrigeratory was at 76°.
   When two had come  over, it was at  100° ; and when three
   had come over, it was at 123°.
     In  this experiment, the heat,  which  emerged from three
i   measures of water in the refrigeratory from 52° to 123°, or 71*.
   Now 3 is to 38  as 71* to 899^, and the heat would have raised
   the three  measures 8994, degrees in  its temperature, if this
 •  had  been possible without converting it into vapour.  The
   heat of the vapour from which this emerged, was 212°, or 160°
  more than that of the water.  Taking this from 899°, there
  remains 736°, the  heat contained in  the  vapour in  a latent
  state.
    But this must be  sensibly less than the truth.   During the
  experiment, the  vessels were very warm....the head of the still
  as hot as boiling water, and the refrigeratory gradually rising
  from 52°, which was within a degree or two of the tempera-
  ture of the air of the laboratory, to  123°.  Avery consider-
  able  portion of the latent  of the steam must have been car-
  ried off by the air in contact with a considerable surface, some
  of which was exceedingly hot.   A great deal must also have
  been carried of in  the  steam which arose very sensibly from
  the water in the refrigeratory, towards  the end of the experi-
  ment.  Mr. Irvin also observed, that,  during the distillation,
  the temperature of  the water which  ran from the worm, was
  about 11° hotter than the water in the refrigeratory.  The.
     vol. i.               f  f 
166  OF  THE HEAT LATEN'l  IN STEAM.

steam, therefore, at a medium, was not  160°  hotter than the
water which ran from the worm, bu* 125*»,  its heat tempera-
ture  being about 87".  This consideration  alone  will make
the latent heat of the steam not less than 774 degrees, with-
out any allowance for waste.
   Some comparison may also be made between the heat ex.
pended in the production of the steam, and  that which emer-
ges during its condensation.  The time which elapsed during
the raising of the temperature of the five measures of water
from 52° to 212°, that is 160°, was one hour and twenty mi-
nutes, or 80,....and 225' elapsed during the boiling off of three
measures.  Therefore,  since 80 is to 225 as 160 to 450, as
much heat was expended as would  have raised the five mea-
sures 450°  in temperature.  This would have raised three
measures  750°  above the boiling heat already  produced.
This gives 750  for the latent heat of the steam, besides what
was unavoidably lost by communication to the ambient air, and
what was expended in heating the vessels.
   This is an outline of the experiments, which must be made
for ascertaining this fundamental point.   A few weeks after,
Mr.  Watt made similar experiments with a smaller still,  bet-'
ter fitted for a trial of this question, and much more manageable.
The  medium result of these trials gave 825° for the heat con-
tained in the steam.  I have narrated the procedure, that you
may  see the  circumstances  which must be  attended to, in
order to draw a conclusion from them.  What I have related
is sufficient for establishing the main principle, namely, that
the heat which disappears,  in the conversion of water into
vapour, is not lost, but is retained by the vapour, and is indi-
cated  by  its expansive  form,  although it does not effect the
thermometer.   This heat emerges again from this vapour
when it becomes water, and recovers  its former quality of af-
fecting the thermometer; in short, it appears again as the
cause  of  heat  and  expansion.    But much  remains to be
done, before the experiment can be made use of for giving us
a precise measure, either of  the heat  imparted from the  fuel,
or of that  which emerges from the steam.  A train of experi-
ments must be  made  for obtaining the precise rate  at which 
      OTHER MEASURES OF LATENT HEAT. 167

 the heat of the fuel passes into the water during the rising of
 its temperature, in order that from thence we may conclude
 the rate at which it continues to enter while the water is dis-
 tilling.   Another train of experiments is necessary for ascer-
 taining the whole loss of heat by the vessels, and by commu-
 nication to the air. To relate these would consume much time,
 and they will  readily  occur to  any person much conversant
 with chemical processes.
   I think it sufficient to inform you,  that Mr.  Watt, in the
 course of his studies on the steam-engine,  has  made, all the
 necessary experiments with the most scrupulous care, know-
 ing that the improvement of that noble engine must  depend
 entirely on an exact knowledge of the procedure of nature in
 the formation and condensation of steam.  Mr. Watt informs
 me, that he has observed as exact coincidence  between the
 heat rendered latent in the vapour, and that which emerges
 from it, as can be desired; and that the heat obtainable from
 steam, capable of sustaining the ordinary pressure of  the at-
 mosphere, is not less than 900  degrees of Fahrenheit's scale,
 and that it does not exceed 950.
   But this is not the only way  in which it may be measured;
 there being other indications of the presence of heat, that are
 equally susceptible of measure.   I have already observed,
 that if the body employed for condensing the steam be in an
 absorbing state, its temperature will not be increased by what
 is thus absorbed.  But, as this  absorption and new combina-
 tion of heat, is accompanied by a change in the constitution of
 the body, which is sufficiently remarkable, viz. the body being
 either melted  or volatilized; ani^, as  the quantity of  matter
 thus changed in its form, must be'Jjteoportional to the heat com-
 bined with it,  it  follows, that the  heat which emerges from a
 given quantity of vapour, of the temperature  212°, will melt a
 precise quantity of ice, or any othqr body; and therefore, the
 quantity of heat latent in steam, may be compared with that con-
tained in the boiling hot water fron\which it is produced, name-
ly, by comparing the quantities office melted by both.  This,
accordingly, was one of the first methods which occurred to me
 for the measurement,  and  it came naturally into my thoughts, 
168  THE THEORY OF LATENT  HEAT ESTAB.
as I had, not long before this, been employed in the analysis of
liquefaction.   I projected some experiments for this purpose
to be tried in the ensuing  frosts.   But my friend Mr. Watt,
being now warmly engaged in  the  same  enquiry, which had
become so interesting  to him, I was soon supplied by  him
with measures abundantly accurate for my views,  The cele-
brated philosopher, Lavoisier, took  this  other method of
measurement,  and  invented a most ingenious apparatus for
the purpose of  measuring  all productions of heat,  by the
quantity of ice melted by those operations of nature in which
it appears.   The  calculations deduced from such  of his ex-.
periments as have come to my knowledge, gave a somewhat,
higher value of the  latent heat of steam, making it 1000 de-
grees, or perhaps a little more.
   It is unnecessary to add any thing more to the preceding facts,
to prove  that the  interruption in the calorific  process, in the
conversion of fluids into elastic vapour in the act of ebullition,
is perfectlyr similar to that in the  process of liquefaction ; and
I flatter myself that we may now take it as a point fully es-
tablished, that, when a  fluid body is raised to its boiling tem-
perature, by the continual and copious application of heat, its
particles suddenly attract to themselves a great quantity of heat
and, by  this combination,  their mutual relation is so changed,
that they no longer attract each other, gathering into drops and
forming a liquid, but avoid each other, separating to at least ten
 times their former distance,  (for a cubic inch of water forms
 much more than a thousand cubic inches of vapour J, and would
 separate much farther, were they not compressed by the weight
 of the atmosphere; and in short, they now compose a fiuid,
 elastic, and expansive, like air.
   This new form of aggregation is the effect of a new combi-
 nation of heat with the primary particles of  water,  and is a
 sufficient indication of this union, in the same manner as
 fluidity was a sufficient mark of a sudden and copious combi-
 nation of heat with the  particles of ice.   We cannot, perhaps,
 form a  very distinct notion of the nature of this combination,
  so as to understand its distinction from that which takes place
  in all lower temperatures, and is accompanied only by expan« 
             SEVERAL  OBSERVATIONS          169

 sion of the ice, without any change  of form.  In the same
 manner, we do not know the difference between the mode of
 union, which takes place between heat and water in all tem-
 peratures, from 32° to 212°, and that which obtains whenever
 we  attempt to increase this last  degree of  heat by  a more
 copious supply.  We must content ourselves with  observing
 the  fact, and remembering that the form of  elastic vapour is
 the  characteristic of a very peculiar combination of heat with
 the  elements of other bodies.
   We had some difficulty  in conceiving how 130 or  140 de-
 grees  of heat could combine with the particles of  ice, with-
 out  increasing its bulk in the same proportion as it had done
 in lower temperatures, and as it continues afterwards to do in
 water;  but,  in the present case,  we have  no such difficulty,
 because the bulk of the compound being much more increased,
 than in  the proportiop of the preceding  expansion*, there
 seems to hie more room than before, where it may  be lodged
 without  redundancy.  We are  not surprised,  therefore, at
 finding that the temperature is not increased; for,  instead of
 thinking that heat should  flow out, and enter a thermometer
 already expanded 212 degrees, we should  rather expect, that
 when  the,bulk has increased 1800 times, while only 900  de-
 grees  ot heat are thrown into it, there would rather be a dis-
 position in the heat to quit the thermometer, and enter into
 this empty space.  I do not pretend to say  that heat is con-
 tained in vapour, or in water,  as water is contained in the
 pores  of a spongef, but only that there does not appear  any
  * Mr. Watt has made a great number of experiments,  for ascertaining
 the bulk into which a cubic inch of water is expanded when it composes the
 steam,  which, at the temperature 212°, sustains the pressure of the atmos-
 phere ;  some of these M'ere direct, by evaporating a known quantity of
 water....others were more  circuitous, deduced from the performance of his
 engines.  The medium result gave about 1800 cubic  inches. We may say,
that a cubic inch of water forms a cubic foot of such steam.      editor.
  f There are phenomena, indeed, which have induced many naturalists of
reputation to think so.  Air, steam, and other elastic fluids, raise the ther-
mometer when suddenly and greatly compressed;  but this inference is too
Ijasty, and a proper examination of the union between these substances and
heat,  renders it extremely doubtful, whether this emission of heat should ne-
cessarily follow the compression.                          editor. 
 170   ON THIS COMBINATION  OF HEAT.

 thing unaccountable in the fact, that this great accumulation
 of heat does not  affect the thermometer.   We now  see,  in
 some degree,  how it cannot.  Ihe thermometer cannot rise
 without being hotter;  but, if hotter, it cannot condense steam;
 and if the steam be not condensed, its  heat cannot emerge,
 because whatever caused the absorption of heat maintains it
 absorbed.  But, whatever be the nature  of the equilibi ium of
 temperature among bodies,  it  remains  unaffected  by the
 change which the water has undergone in  its form.  When-
 ever a body hotter than 212° comes in contact with the vapour
 it raises its temperature and expands it:  and whenever a body  ,
 colder than 212° touches it, the vaporific union is immediately
dissolved;  heat quits the vapour, and it becomes water.  The
heat emerges in all  its intensity and abundance, if there be
enough of colder matter to receive it, but not else.  Therefore,
although the  steam from the mouth of  a  tea-kettle will not
raise  a  thermometer above 212°,  a single puff of it, which
does not contain one grain of water, will scald a considerable
extent of the skin, as much as if it were plunged for a moment
in boiling water.  The rapidity with which this transition of
a great quantity of heat is made, is wonderful, when the sur-
face of cold matter  is sufficiently extensive.   Two* English
gallons of ice-cold water, dashed in small  drops throagh the
 capacity of a  steam cylinder holding three hogsheads, will
condense the vapour which fills it, in less than one-fourthfof a
second.
  There are other^ways, however, in which vapour may be
 condensed, without drawing off its latent  heat by  means of
cold bodies.   It may be condensed by  mechanical compres-
sion, and in this case also we see its latent  heat emerge.
  This is a nice and troublesome experiment, and requires a
 very nice apparatus.  A metal cylinder,  nicely bored, is fitted
 with a piston ; and  the cylinder is  made to terminate below,
 in a small glass tube, close at the end.   A  thermometer is so
 fixed to the cylinder, that its bulb is in the  axis, and the tube
 projects ontwards through the  cylinder.  A  little water  is
 poured into it, so  as to fill the glass tube at  the bottom.  The
 piston is put into the top, and a small hole in the piston is left 
DISENGAGEMENT OF HEAT BY PRESSURE. 17

open.  A large glass cylinder is filled with water, kept boil-
ing hot.   The water in the glass tube is made to boil briskly,
the steam blowing through the hole in the piston.   In this
state it is immersed in the  boiling water, the hole in the pis-
ton speedily stopped, and the piston thrust down with great
force,  at least half way.  The thermometer  rises  imme-
diately, and the water in the glass tube is seen to increase in
bulk.  The whole must be done with great despatch.
   We now see clearly why the boiling point of a fluid is  so
constant, and why no violence of fire can make water in an
open vessel hotter than  212 degrees.   The instant that any
portion of it attains that temperature, it absorbs the heat as
fast as it can be transmitted through the vessel, and becomes
elastic vapour, which rises through the rest, and escapes. This
rapid absorption of heat  is most distinctly perceived by means
of the little philosophical toy, called a pulse-glass, the ingeni-
ous contrivance of Dr.  Franklin.  This consists of two thin
glass balls, A and B,  (fig. 2.) connected by a  slender pipe.
One of the balls,  and a small part of  the pipe,  is filled with
water; and this being made to boil violently, the steam blows
all the air out of the other ball, and follows it.  By stick-
ing the end of the pipe C into a bit of tallow, the hole is stop-
ped  up,  and the steam  in the  ball B soon collapses,  and
becomes water, so there is now a fine vacuum in this ball. The
flame of  a blow-pipe being now directed on the neck of the
pipe C,  it is soon hermetically sealed.  Thus we have  re-
moved the pressure of the atmosphere from the surface of the
water, and it will now boil  by th^ heat of the hand, if the ball
B  be kept a little colder, that it may  condense the  steam as
fast as it comes into it.  It will be a long time, 'however, be-
fore the warmth of the hand heats all the water in A sufficiently
and it will not boil till this  be done.  But if we hold the end
B  a little down, so that  some water may run into it from A,  -
and thus leave a part of A empty, and then, holding the instru-
ment level, if we grasp A in the hand, the heat is sufficient
for immediately converting into vapour the thin film of water
which now lines the empty  part of A.   The vapour expands,
pushes all the water out of Ainto B, and then rushes up through 
   172               STEAM-tJOOKERY.

   it, throwing it into the agitation called boiling, and it is a real
   boiling, although with a warmth that is imperceptible.   On the
   contrary, the ball A feels even cool to the hand. If we allow all
   the water to come back again, except so as to have a very small
   bubble in the upper part of A, and now apply the tongue to that
   part, we shall immediately throw it into a violent ebullition in
   the ball B, and A  will feel very cold to the tongue. The ebulli-
   tion and the cdjd will continue till all the film of water which
   lined  the inside of A is completely evaporated, and it  has be-
   come perfectly dry. The boiling in B now ceases, and,  with it,
   the coldness of A.   Nothing can shew, in a  more convincing
   manner, the prodigious quantity of heat absorbed by water, in
   the act of its conversion into vapour, than the coldness pro-
   duced,  during a very considerable space of time, in order* to
   evaporate this trifling film of water.
     It is  this great quantity of heat  contained in steam that
   makes it so powerful and effectual in  the business of cookery.
   The knowledge now  acquired of the latent heat of steam, has,
   of late years, been applied by ingenious artists to this purpose,
   with very great effect, giving to all the operations of the .kitch-
   en, where boiling or  stewing is concerned, a neatness and easi-
   ness of management that is very remarkable; and this is at-
   tended  by  a great saving of. fuel.  Whoever attends to the
   manner in which heat is applied, in  our kitchens, to all cook-
   ing vessels, will be convinced that not less than three-fourths^
   of our fuel is wasted, in the  production of a heat which is ex-
   tremely troublesome to the  cook,  and, after all,  goes up the
   chimney, without doing any  service.   A great surface of fuel,
   in a state ofmcandescence, heats a great quantity of air, both
   by  radiation and by material communication.   We ^pt a
   boiler on  this open fire, in the midst of a  copious  stream of
   heated air, which is incessantly rushing up the chimney. Only
/  a  very small  quantity  of this comes  in  contact  with our
   boiler.  The bottom of it is indeed,  heated by the fuel  also on
   which it rests;  but the greatest part of all the heafe product
   is running to waste.   If, instead of setting the veSfel on the
   open fire, we set  it on a well constructed furnace," where  the
   heated air shall all be forced to come in contact with  its hot- 
STEAM-COOKERY.               lri
  torn and sides,  we are certain that more  than nine-tenths of
  the fuel may be employed in communicating heat to  water
  boiling in the vessel, the remaining tenth being exrAided in  IL
  maintaining the heat of the furnace, or escaping with the air by
  the flue.  By this operation, the water is converted into elastic
  vapour,  furnished  with its  latent heat,  which is ready  to
  emerge and affect any body that is colder than 212°. It would
  therefore, very quickly heat the water in another similar ves-
  sel  up to the same  temperature, and would keep it  in that
  state,  although it cannot cause it to boil.  And it will do this
  in a manner far superior to the  hot air of a  fire ;  for this last
  acts only  by the film,  which is in perfect  contact with the
  vessel; and  the  hot air,  which is half an  inch  farther off, fy
  passes by, without doing the least service.   But, if the steam
  from  our  boiler be admitted  into a box,  already  heated  to
  212°, and  containing a piece of meat, or a vessel  which con-
  tains any thing to be  dressed by a boiling heat, not a particle
  of vapour  will pass by it,  but will all come  and deposit on it
  the whole of its latent heat.   And this will  continue without
  interruption, till the  vessel, with its  contents, have attained
 the temperature 212°.  Then, and not till then, will the steam
 fill the box,  and  blow through any hole made in its  top  or
 sides.  This hole may communicate with another box.   The
 steam, having  nothing left it to do in the first, will go into
 the second, where it will be equally effective.  Steam is the
 most faithful carrier of  heat that can be  conceived, and will,
 deposit it only on  such bodies  as are  colder than  boiling
 water.
   In constructing an  apparatus  for this purpose, the  most
 adviseable  method seems to be,  to  convey the vapour from
 the boiler into a long wooden box, divided by partitions  into
 portions fit for holding the different vessels of  cookery,  and
 always to employ those  which are nearest to the boiler.  If
 any thing be dressing in a remote  division, and we put a colcf"
 vessel into an intermediate one, the operation in,all the divi-
 sions beyond it will be suspended, till the  new vessel be boil-
ing hot; and the others will now be cooling, and we must ex-
pend  steam in order to restore their proper heat.  Wooden
   vol. i.               o g 
174 THE M. OF WORCESTER'S STEAM ENGINE.

boxes are recommended, because they will waste less heat by
communication to the air;  but  they must be lined with thin
metal, otherwise they will soon be destroyed by the steam.
   Employed as we are just now, in the consideration of the
production of elastic vapour, and its condensation into water
again, it would be unpardonable to omit taking notice of the
application which has been made of these properties to the
arts of life, by  the invention of the Steam  Engine;  an  in-
vention,  which, I may truly say, is, in its present state, the
master-piece of human skill*. Nor was it, like the pump, the
mariner's compass, the telescope, the production of a chance
observation, but the result of deep  thought and reflection, and
really a present by philosophy to the arts.
   The power of heat to increase the elasticity of steam to the
most enormous degree, had been obse*"- ?d by many, and Dr.
Papin had made an ingenious use of the heat accompanying
its great compression in his digester.   The Marquis of Wor-
cester, a gentleman of extensive natural knowledge, and most
inventive mind, observed farther, that steam, when exerting
this enormous  force, could  be, in an instant, deprived of it, by-
dashing a little cold water through it, and  that a vacuum
would thus be  produced  in the  vessel,  which the moment be-
fore was ready to burst.   He therefore, about the year 1660,
contrived, most ingeniously, to make this vessel V (fig. 4.)  A'
 communicate alternately with a boiler B, from which it should
 receive, through a cock S, this highly elastic steam, and with
 a cistern C,  from which it was required to raise or  force the
 water.  The vessel being supposed to be  filled at first with
 common air, the steam was admitted,  aqd it quickly blew out
 all the  air.   The steam-cock being now shut, a  jet  of cold
 water was admitted by another cock I, which, in  a moment,
 condensed the steam, and made a vacuum,  and water rushed
 in from the cistern and filled  the vessel.   The  condensing
 cock  bein^now shut,  the steam-cock was  again  opened;
 strong steam was admitted, which by its violent pressure on
 the surface of the water,  drove it out by another pipe  EF,
 which conveyed it to where it was wanted.
               * See Note 35 at the end of the volume.            / 
         WORCESTER'S  AND NEWCOMEN'S    175

    This most  curious  contrivance was immediately  put in
 practice, and excited the wonder of every person in that most
 ardent and inquisitive age.   The engine was found, however,
 so limited in its practicable powers, and so difficult, and even
 hazardous to  manage, that  it never  came  into very general
 use.  But, about the beginning of this century, its principle
 and form underwent a great  revolution.  The great elasticity,
 which made it so hazardous, was found  unnecessary ;  and
 the steam, no stronger than common air, was employed merely
 as a mean of rapidly making a vacuum in a vessel by its con-*
 densation.   The contrivance was as follows:          >
   The steam-vessel V,  (fig.  5.) is  now a metal 'cylinder,
 smoothly bored, communicating  with the boiler B, through
 the steam-cock S, and with the cold water cistern A, through
 the injection-cock I.  A  great piston, P,  exactly fitting  the
 cylinder, hangs by one end of  the working beam DH, which
 turns  round an axis or gudgeon, G.   From the other end of
 this beam, hangs the piston Q of the pump, which draws the
 water  from the pit.  This end of the beam is made to pre-
 ponderate, so that,  when the great piston P is at the bottom
 of  the cylinder, the weight  hanging on H shall be able to
 overcome all the frictions, and to drag the great piston to the
 top of the cylinder.  The sketch, therefore  represents  the
 engine in its natural inactive  position.
   The water being made to boil strongly in the  boiler, the
 cock S is opened, and the steam rushes into the cylinder, and
 drives., all the air out of  it by the  pipe  E near the bottom.
 When  the  attendant observes the steam following in abun-
 dance,  he shuts the steam-cock, and opens the injection-cock
 I.   Cold  water now spouts up into the cylinu^MgAjd^s
 against the piston P, and, scattering  over  the  w^M   ■url,
 immediately condenses the steam.  The pressure ofj Fat-
 mosphere shuts the valve lying on the mouth of th^MMfc^
 pipe E  ; and a vacuum is now produced in the cylinder.^. The
atmosphere presses on the upper surface of the great pist&n,
with a force of nearly 15 pounds on every inch, or a ton on
every square foot.   This forces down  the great piston P, and
 draws  up the piston Q  with  the column of pit-water corre*. 
176             STEAM ENGINES.
 ponding to it.  When the piston has reached the bottom of
 the cylinder, the attendant shuts the injection-cock, and opens
 the steam-cock.  The steam, being as elastic as common air,
 performs the same office that the opening of the valve E would
 do, that is, it allows the preponderance at H to drag the great
 piston to the top of the cylinder, where it is again  ready to re.
 ceive  the  pressure of the atmosphere, and make a second
 working stroke.
   It is plain that  this construction  has totally changed  the
'engine, both in form  and in principle.   We no longer need a
 highly  elastic  steam, in  order to  raise  a  great column of
 water.   We need only to enlarge the diameter of  the cylinder,
 till the pressure of the atmosphere  supplies a force equal to
' the  task.   A cylinder of 30 inches diameter supplies a force
 of five tons.  It is  a vulgar mistake, that the prodigious
 power of this  engine is owing to a  great  force in the steam.
 Its  only use is the rapid production of a vacuum, by the con-
 densation  produced by the injection of cold water.
   Such is Newcomen's  engine.   By its assistance, many
 mines have been continued, and many have been opened and
 worked, with  great  advantage to  the  proprietors and to the.
 public, which  must have otherwise been abandoned, notwith-
 standing the earth was full of treasure ; and the abundance of
 pit-coal procured, by these means, to these kingdoms, gave il a
 superiority  in many manufactures, which  has  been of un-
 speakable advantage  to this nation.
    I have  the pleasure of thinking, that the knowledge which
 we have acquired concerning the nature of elastic vapour, in
 consequence of  my fortunate observation of what happens in
 its  fopnyuion and condensation, has contributed, in no incon-
 siderable  degree,  to the  public good, by suggesting to my
 iriend Mr.  Watt of Birmingham,  then of Glasgow, his im-
 provements on this  powerful engine.  The great  hindrance
 to  its employment, for every purpose, not only of mining, but
 even  of manufactures, by giving us a moving power of un-
 limited force, has been the prodigious expence of fuel.  An
 engine, having a cylinder  of three feet diameter, and working
 continually, consumes about 2000 chaldrons (London measure) 
IMPROVEMENTS OF THE STEAM ENGINE. 177

of coals in a year, or almost 3000 tons*.  Many attempts
have been made by engineers to diminish this enormous
expence ; but no way occurred to them but  improving the
construction of their furnaces, and by preventing all waste
■'of  heat that  could be avoided.  The "flame was  made to
circulate round the boilers ;  the  fire was placed in the
middle of the boiler ; the machine was carefully defended
from the air, in all places where its heat was great.  Mr.
Watt also directed his  attention to these things ; but he
 soon found that all these were but trifling improvements,
 and that the  copious production of steam was not enough,
 unless  it was afterwards husbanded with the most rigid
 economy.   He saw that  more heat was wasted  after the
 steam was produced, in consequence of its being misman-
 aged, than by the most injudicious construction of furnaces.
 For, as steam contains  such a great  quantity of heat,  a
 waste of steam is a sure waste of heat.   As much heat
 is  contained in one gallon of water in  the form of steam,
 as would bring 5\ gallons of ice-cold water  to  a boiling
 heat.
    In  Newcomen's engine, a  vast quantity of steam  is
 wasted by unavoidable  condensation, the moment it enters
 the cylinder ;  for the  cylinder has been cooled  down  to
 100°, or 110°,  by the injection ; and there is  lying at the
 bottom a quantity of water, perhaps  even colder than this.
 It is also cooled by the descent of the piston, covered with
 several inches of cold  water.  All this must be heated
 again to the temperature 212°, before the  piston begin to
 rise ;  and, as it goes up,  more of the cooled surface of the
  cylinder is laid open to the  steam, and condenses  it ; and
  the piston cannot  reach  the top till the whole  cylinder be
  again  raised to 212°.  Then comes  the injection, which
  again  cools every thing.   It  is plain that this waste Will
  be increased by injecting too  much cold water ;  therefore V
  the engineers  were careful not to do this.  But they blun-
  dered as much by injecting too little ; for this quantity of
   * 100 pounds weight of the best Newcastle coal, when applied by the
  most judiciously constructed furnace, will convert about l^wine hogsheads
 pf water into steam, that supports the pi'ossure of the atmosphere. 
    178 MR.  WATT'S  REFLECTIONS  THEREON.

   water, not being sufficient to absorb all the latent heat of
   the steam, was raised  to a considerable temperature, and
   therefore allowed a steam to remain, whose elasticity bore
   a very  sensible proportion  to  the  pressure of  the atmos-
   phere ;  an^this balanced that portion of the pressure, and*
   rendered the piston  unable  for the full load of 15 pounds
   on every square inch.   To preserve this power,  requires
   the injection of an ale pint  of cold water, for every  cubic
   inch of water  that has been converted into steam,  or for
   every cubic foot of steam condensed.   Mr. Watt imagined
   that half of the  steam produced in the  boiler was thus
   wasted.
     But  how was this to be remedied ?  The waste is una-
   voidable, while the  steam-vessel is thus cooled by  every
   injection ; therefore it must be wasted  as long as the con-
   densation is performed in the cylinder, or steam-vessel.
   He therefore contrived to perform  it in another vessel,  in
   the same way as it is done in the common process of dis-
   tillation.  His condenser is  a separate vessel, occasionally
   communicating  with  the  great cylinder.  He knew that
   steam, being an elastic  fluid,  will go  into any communi-
   cating  vessel,  if emptied of its air,  or  other contents.
      And if this condensing vessel be kept extremely cold, by
   being immersed  in cold water, or  any other method, he
   presumed that  it would all be  condensed there.  But all
   this js  not enough.   The cylinder  will still be cooled by
   the descent of the piston, and the cold water above it. Mr.
   Watt, therefore,  did not allow the external  air to  press
   down the piston, but  employed the hot steam from the
   boiler, which is equally powerful, its elasticity being already
   a balance for the pressure of  the atmosphere : it  will,
   therefore, press  as strongly  on the upper surface of the
   piston.   Thus  is the cylinder kept continually hot,....as
7?  hot as boiling water ; and it may be inclosed in a case, to
   prevent loss of heaif from so hot a>surface of great extent.
     Mr. Watt's construction is,.therefore, as follows : the
   steam-vessel D (fig. 6.) communicates  with the boiler F,
 • by the pipe A A leading from its top, and having a  stop-
   cock A.  It communicates at the bottom with the conden-
   ser E, which is immersed in a cistern of cold  water.  The 
              GENERAL £CCOUNT.           179 '

pipe of communication has a stop-cock B.   A communi-
cation is  also  established  between the upper and  under
parts of the steam vijpsel, by means of the pipe C C, also
furnished with a stop-cock C.   Both D and E are  neatly
bored, and  have pistons,  which  move  smoothly up and
down, without allowing any air, or other fluid, to pass  by
their sides.  Both pistons are furnished with shanks, which
move air-tight, in holes made  in  the covers of the  cylin-
ders.   The quiescent position of the engine is such, that
the pistons are at the top of the  cylinders D and E, be-
cause they  hang from the inner  end of  the  great  beam,
and the outer end of it is made to preponderate a little, so
as to drag both pistons to the top  of the cylinders.
    In setting the engine to work,  all the cocks are opened.
The steam, passing through the cock A, gets into the top
 of  the cylinder, and, from thence, along the pipe C, into
the bottom of  the cylinder, and fills it, driving out the  air
 through the cock B into  the condenser, and then through
 the valve of the condenser-piston.   This being done, the
 cock C is shut, and now, as the steam is condensing rapidly
 in the condenser,  it is rarefied in the  cylinder below the
 piston, while it presses on the upper surface of it with its
 full  force.   It therefore presses  down the piston,  almost
 to the bottom, but not altogether, by reason of some air
 remaining  in it.  When the piston is at the bottom of the
 cylinder, the  cocks  A and B are shut, and  C is opened.
 Thus, by establishing the communication between the top
 and bottom of the cylinder,  the  steam presses equally on
 the upper  and under surfaces of the piston, and now the
 preponderancy of the outer end of the beam is permitted to
 drag the piston to the  top again ; and the steam, which
 was formerly  all above it, is now  all  below it, having come
 round by the cock C.  This cock is now shut, and A and
 B are  opened.  There  is, at present, a vacuum  in  the re-
 frigeratory E, by the rise of its piston." The steam, there-
 fore, in  the cylinder expands into this empty space, and,
 as fast as  it enters, it is condensed  by its told sides, and
 falls to  the bottom, in the  form of  water.   A vacuum
 being thus made below the piston, it is pressed down by
 the steam, even although this steam were-no stronger than 
 ISO     OF WATT'S  STEAM ENGINE.

common air.   It presses  down with a force  equal to the
sum of  the pressure of the atmosphere,  and the  excess of
its elasticity above that pressure.   The water produced in
the refrigeratory, by  the  condensation of the steam, and
the small quantity of  air which is extricated from it, or
has entered  by crevices, or remained after the first descent
of the piston,'  is extracted at the next upward movement
of the piston of the condenser.                           /
  By this construction, the cylinder is kept boiling ho^ and
wastes no steam.   The vacuum also, made by the conden-
sation, may be as perfect as we please, because there is no
occasion to be sparing of  cold water for the condensation.
This circumstance gives a great mechanical superiority to  J
this  construction.   For it has  been found by long expe- |
rience, that  in the old engine, it is adviseable to work
against no greater  load than six or seven pounds on every
square inch of the piston, because the quantity of cold water
that must be employed, for producing a greater unbalanced  «
pressure on  the piston, cools the cylinder so much, that
more loss is sustained by the expence of fuel to replace the
wasted steam, that is gained by the increase of power.  In
Mr. Watt's construction, the engine will  work very tolera-
bly against a load of 13* pounds, and is never loaded, with
less than 11^ ;  but its most  conspicuous  advantage is, that
an engine of this  construction consumes  not more,  and  j
generally a good deal  less, than one-third of the  fuel used
in the very best common engine of the same size.
  - Mr. Watt still farther improved this engine, by making
it exert an active stroke in both directions. This was very
obvious and  easy.  He had only to make both the  upper
and under parts of the cylinder communicate with the boiler
and condenser.    When the  upper  part communicates
with the boiler,  and  the lower with the condenser, the
piston is strongly  pressed down ;  and when these commu-
nications are reversed, it  is as strongly pressed up. More-
 over, by an artful cutting off, or greatly diminishing the
 supply of steam, before the piston has completed  its move-
 ment, he has enabled  a given quantity of steam to produce
 a much greater effect, and  at the same  time has brought
 the engine so much,  and so  instantaneously, under com- 
                MR.  WITT'S STEAM  ENGINE,     181

      mand, that it is really more manageable than the exertion
      of any animal.   But the explanation of these particulars
      is not in  our department, but that of the mechanician.
        I must not, however, omit an important chemical obser-
      vation which occurred to Mr. Watt in the course of those
      improvements, and particularly in the prosecution of a pro-
      ject which he  had formed for distilling  in vacuo.   This
      is a project which I have thought of ever since I saw the
      curious experiments of Dr.  Cullen, already related to you.*
      In these  it was  plain, that  when the pressure of the air is
      removed, volatile liquors will boil,  and therefore distil, at
' 1, '";, vei7 l°w beats.  Mr. Robison's experiments, already men-
 ,*    tioned, also shew that water will certainly distil at the  tem-
 |(f  perature  90°, and  vinous  spirits at the temperature 55°.
      I see that Fontana has had the same project;  and indeed
  <    it is very obvious.   Mr. Watt, having carefully examined
      the elasticity of watery  vapours, in a  variety of tempera-
      tures, between C2° and 212°, concluded that water would
      pass  copiously  by distillation, when  of  the temperature
      70°, if the refrigeratory be  kept in melting snow ; or,  that
'      it will distil equally well of the temperature 90° or 95°, in
      the ordinary temperature of this climate.
        He accordingly made the experiment, and it succeeded
}&L   Per^ect^  we^» A verY small still was half filled with water,
-|jjj   and then  closely united with the  vessel which was to re-
      ceive the  distilled water.   A very small hole was made in
      the bottom of the receiver,  and a plug was fitted to it. The
:      water being made to boil  violently, sent the steam through
      the whole apparatus, (there being no water in the refrige-
      ratory to  condense  it) forcing the  air out  by the hole.
      While the whole was boiling hot, and the steam blowing
      through the hole, this was Suddenly stopped up by the plug,
      and the bottom of the still was  set on ice.   This soon
      cooled its contents ;  and  the steam, which  occupied  the
      rest of the apparatus, collapsed into a  few drops of water,
      A lamp was now set under the still, and in a few  minutes
      the whole apparatus  grew warm :  a  proof  that steam  was
     now produced from  the  water, and  passed over into  the
            * I see this project? in -A i-.ote.book written before 1757.
                                                    editor.
        'OL. I.                 H  h 
1>>.          AND SPECULATIONS.

deceiver.   Cold water was put into the refrigeratory, and
the distillation  went on ;  slowly indeed, but very well;
and the  ebullition  was very distinctly heard  and felt in
the still, although the head of it was scarcely warm to the
hand.   But the  result of the process was, that the latent
heat of the steam was greatly increased by this diminution
of its sensible heat.  The temperature of the steam in the
experiment was found to be 100.   The water in the refri-
geratory was raised by the condensation of the steam^ from
57° to 77°,  and  the vessel had acquired as much heat as
could have raised the water one degree.  Therefore," 21
degrees of heat  had been acquired from the steam.   The.
quantity of water distijjpd was 7"T of that in the refrigera-
tory ;  therefore, the increase of temperature, namely 21°, 'h'
multiplied by 54,  will give  the heat extricated from the
steam.   This is 1071. From this take 100-77, or 2J, the  A '
sensible heat lost by the steam, and there remains 1048 for
the latent heat of this steam.
   Mr. Watt has made other experiments, with much  more .
care,   and in various states of heat and elasticity of the
steam.  He finds that water distils perfectly well, when of
the temperature 70°,  and that,  in this state, the lategg heat
of the steam approaches to  1300, and certainly exceeds   I
1200.   The unexpected result of these  experiments is,  ^M
that there is no advantage to be expected,, in the manu-  tit"
facture of ardent spirits by distilling  in vacuo.   Fqjc/vc  mM
find that the latent  heat of the steam is at least as much
increased as the sensible heat  is diminished.   This will
undoubtedly be  attended with  an increased expenditure of
fuel.   For the increase of 100 degrees of sensible heat oc-
casions an increase of  fuel, only while we are raising the
temperature of  the still to the  ordinary heat of boiling
water; in the beginning of the distillation.  If the  furnace
be judiciously constructed, and due  precautions taken to
prevent dissipation, it requires very little fuel  to maintain
this temperature.  But 100 degrees of latent heat is an ex-
pence that is continual,. and which no contrivance whatever
can prevafK.
   This train of experiments suggested to Mr. Watt  many    \
very curious and important reflections concerning the com-   ** 
            DISTILLATION  IN  VACUO.        183

 binations of bodies with heat ;  and his opinions on  this
 subject have been farther established, by the whole course
 of his experience, in the construction and management of
 his double-stroke steam engine.
    We are now much more#able to understand the nature of
 vapour than formerly, and to trace same of the steps by
 which it is produced.   It is surely an approach to the ex-
 planation of any phenomenon in  nature, when we obtain
 certain information of circumstances indispensably neces-
 sary for  its  production ;  and the clearness or competency
 of this explanation is greatly augmented, when thosq, cir-
 cumstances  are of a  measurable  kind, and invariably'con-
 nected with certain  gradations in the  phenomenon itself.
 We  seem to  have attained this  desirable  point in  the
 present case, and in that of liquefaction and congelation.
 There is a something to which  I have given the familiar
 name of heat, which is susceptible of degrees.   It not onlv
 gives us  the sensation of greater and less warmth, or heat,
 but it also produces other effects, which are  equally varia-
 ble and distinguishable in quantity.   We might take any
 of these variations for the measures of its degrees, if \*e
 find them always consistent.  We have not been able to
 measure  our feelings of heat with accuracy,  and we even
 observe,  that the most accurate  measures of them Wjould
 not be accurate measures of  their cause i because, in cases
 where  we know that the general cause is  the  same,  the
 degree of the sensation may be different.   The expansion
 of certain bodies has been  found a very good  measure,
 merely because, in cases in which we know, by other proofs,
 that the cause is the same, or double, or triple, the expan-
 sion is also the same, or double,  or triple.   But, in  the
 application of this measure to our purpljfcesy we met with
 two very remarkable  irregularities, which,  had they  oc-
 curred earlier,  might have hindered us from adopting  ex-
 pansion as a measure  of heat, and deprived us of  the
 thermometer,.... I mean the passage of a body from solidity
 to a liquid form,  and from  this  to the  form of vapour-
 Had we only examined bodies which  change their  state
 gradually, such as sealing  wax,  wc should perhaps have
farm-.'d very different  notions of the scale of heat, afteV 
184  OUR  KNOWLEDGE OF  HEAT IS  INC.

which  we should have been  in continual  mistakes,  while
considering the heats or temperatures of  such  bodies as
water, lead, mercury, &c   But the observations I  have
laid before you have removed all those difficulties, and have
enabled us to  include even those great anomalies in our
general scale ;  so that we continue to have the same confi-
dence as before, in the scale of the thermometer, as a scale
of the differences of heat.  We have learned,  that when
ice melts, or when water boils, the  cause of the  phenome-
non is the same with that of expansion ;  because we ob-
serve, that what produces a double or triple expansion, also
produces a double or triple liquefaction, or vaporisation;
and  that  congelation  and condensation accompany con-
traction, which is  always  produced  by the  abstraction of
what^was the cause of expansion.   But we see reason to
think,  that the manner  in which the general cause of all
these  phenomena acts, in producing its effects, is somehow
different.   We find, that while the  action  of heat expands
a body, it does  not melt it ;  and when  it melts it, it does
not expand it.   If we conceive heat to  be a substance sui
generis, susceptible of union  or combination with bodies,
similar or analogous to those  combinations which the che-
mist observes between numberless varieties of substances,
we must conclude that the union  which produces expan-
sion is somehow different from that which  produces lique-
faction, Sec.
   When we perceive that what we call heat disappears in
the liquefaction of ice, and reappears in the congelation of
water, and  a  number  of analogous phenomena,  we  can
hardly avoid thinking it a substance, which may be united
with the particles of water, in the same manner as the par-
ticles of  Glauber';;  salt are  united with them in  solu-
tion, and may be separated as these are.   But,  since heat
has never been observed by us in a separate state, all our
notions of this  union must be hypothetical.   Moreover,
this hypothesis must be combined with some other hypo-
thesis, which we may have adopted  concerning the unions
of those  other substances ;  for it must be acknowledged,
that our notions of those more palpable and familiar com-
binations are all hypothetical.  We think that we can con- 
        REFLECTIONS  ON  THE  UNION     185

ceive how a particle of common salt  can draw around it,
and attach to itself, particles of water, and how they may
adhere, and compose a saline  crystal.....We  transfer  this
notion to the  relation  between  bodies and  heat,  and
(without much reflection, or distinct  confeeption) we sup-
pose, that, in like manner, a particle of a body attaches itself
to a number of particles of heat.  Heat is, therefore, sup-
posed to be somehow contained or lodged in the pores of *
bodies, and we endeavor to account for the changes of
sensible appearance, such as increase of bulk,  or fusion, or
vaporisation, &c. by  shewing some resemblance between
those  appearances,  and those which occur  in chemical
unions or mixtures.
  Many have  been  the speculations  and views of inge-
nious men about this union  of bodies  with heat.   But, as
they are all hypothetical, and as the hypothesis is of the
most complicated nature, being in fact a hypothetical appli-
cation of another hypothesis,  I cannot hope  for much
useful information by attending to it.   A nice adaption of
conditions will make  almost any hypothesis agree with the
phenomena.   This will please the imagination, but does
not advance our  knowledge.  I therefore avoid such  spe-
culations, as taking up time which may be better employed
in learning more  of  the  general laws  of chemical  opera-
tions.   I content  myself with saying, that in liquefaction
and vaporisation,  water absorbs a great quantity of heat,
because this expression immediately' raises the notion of a
sudden,  and somewhat  copious,  accumulation of heat.
And I say, that this great quantity of  accumulated heat is
latent in the water, or in the vapour,—merely because the
thermometer,  our usual test, does not give us any indica-
tion of its presence;  of which presence we are not allowed
to doubt, when we see the same quantity emerge again, in
its usual thermometrical  activity, when the water free2es,
or the vapour  collapses.   Without saying that I have any
clear conception of  the union between bodies and heat,  I
am well entitled,  by the phenomena, to say, that this vapo-
rous  combination differs  in some peculiar manner from
that which merely produces expansion. And, because I  find
that vapour cannot exist without this latent heat, any more 
186         OF BODIES WITH HEAT.

than aquafortis  can  exist without nitrous acid,  I consider
latertfheat as constituting a part of aqueous vapour.
   Some chemists think that the phenomenon of the produc-
tion of elastic vapour is  brought about in a different way.
Almost immmediately after my discovery of this remark-
able absorption and concealment of heat, in the phenomena
of  fusion and ebullition, Dr.  Irvin,   whose speculations.
were always ingenious, conceived that there was no occa-
sion for the distinction between sensible and latent heat,
in the peculiar  circumstances of these  phenomena.   He
imagined,  that in every case there is  a portion  of heat
which does not affect the thermometer ; and that the rea-
son why there is a seeming disappearance  in liquefaction
and ebullition is, that when the  element changes its form
fromlhat of water to that  of vapour,  its capacity for heat
is suddenly increased; and, consequently, it requires much
more heat to be thrown into it, in order to make it appear
equally hot by the thermometer. If it were in our power,
by a wish, to convert an ounce of water, of the tempera-
ture 50°, for example,  into vapour,  without adding heat
to it; this vapour, if examined immediately by a thermome-
ter, would appear excessively cold, because the heatalready
in this ounce of  water'is not  sufficient to heat an ounce
of vapour to this temperature';....its heat is that of an ounce
of vapour of an incomparably lower temperature ;   and it
would therefore affect the thermometer as such,  that is,
would rob it of its heat, or cause the liquor to contract. The
vapour has a much greater capacity for heat than the water
from which it was produced.   Heat will,  therefore, flow
 into it from the surrounding bodies,  till they are reduced
 to an equilibrium of temperature.  And, on the contrary,
 when the capacity of the vapour is diminished, for example,
 by violent compression, and it is changed  into  water, this
 heat  is  redundant;   the substance has not capacity  for
 retaining it; it must be thrown out,  as water is squeezed
 out of a sponge.
    This is an ingenious thought, and gives a very simple
 and intelligible view of some of the appearances: but it does
 not assign a cause for the formation  of vapour.   The ab-
 sorption or disappearance of the heat is here considered Is 
   THEORY  OF DR.  IRVIN INSUFFICIENT. 187

the consequence  of the formation.   The vaporoos form
must precede the absorption, if a greater capacity for re-
ception of heat be the cause of this absorption*
   Although this attempt to explanation, by an increase of
capacity  se£ms  to have given very general  satisfaction,
I  acknowledge that it does  not appear to me to be any
thing more than a different expression for what I have
called absorption.    There  was surely something  in the
water, which made it unsusceptible of more heat,.without
rising in  its temperature, and that something is now re-
moved, and the  water is now capable  of receiving more
heat without sharing it with a thermometer.   But,  if by
capacity we are to understand more  space in which heat
may be lodged, I apprehend  that we are speaking without
sufficient authority.  The same explanation is given of li-
quefaction,  although it  be well known  that many sub-
stances occupy less room in their fluid than in their solid
state*.
   Next to  Dr. Irvin, the ingenious  Dr. William Cleghora
applied his peculiar theory to the same subject, and shewed
that it is  remark#bly adapted  to it.  This theory is pretty
much the same with the opinion gene*|ally entertained by
the  modern dynamical  philosophers, as they are called, on
the continent,  who consider heat as consisting of particles
mutually repellent, and strongly attracted by the particles
of all bodies.  The different kinds of matter will  attach to
them different proportions of this substance, in consequence
of the different strength with which a particle  of each sub-
stance attracts a particle of heat.   The chief limit to the
quantity attracted is  this repellent power between the par-
ticles, by which it becomes a sort of plastic fluid.   The
more that its particles are  constipated by the attraction of
the particle of other matter which accumulates them round
it, the more active must their mutual repulsion be.   And
thus, when the attractive force of any substance for heat
is given,  the quantity  that can  be  constipated by it in a
given space i3 limited.   Each  body,  while  it  receives

  * 1 would  ask the partisans of this doctrine of capacity how it will explain
the hunting- of vessels by steam *
                                                EDITOF 
 188     DR. WM.  CLEGHORN'S  THEORY.

 heat, is supposed to attract and condense more or less of
 thesejnutually repelling particles according to its strength,
 and when this is in equilibrio with the  whole of their mu-
 tual repulsion, no more will be collected from the surround-
 ing bodies, which have also  their respective attraction for
 them.   But when this equilibrium is attained, in the case
 of water, for example, if we suppose it  suddenly converted
 into vapour,  the  particles of heat in it, having now much
 more room, are much less constipated round any one parti-
 cle of the vapour; therefore, the repulsion of one of the par-
ticles, for the others surrounding that  particle of  vapour,
 must be greatly diminished, by their removal to a greater
distance.  But the watery particle, having still the same, or
nearly  the same  attraction  for heat as  before, can now
accufliulate many more around itself, before  their mutual
repcllency balances its attraction.  They will,  therefore, be
drawn from the neighbouring bodies, and  while this goes
on, the vapour must appear colder than them.   Dr.  Cleg-
horn adduces, in proof of this theory, the well known fact,*
that a thermometer,  in the  receiver of an   air-pump,
descends when the air is rarefied, and rises when it is con-
 densed.            ,                 ft
   There is no doubt but that this supposition  agreesupretty
well with this single phenomenon.  We can form a pretty
 distinct notion of it; but, like the former, it gives no expla-
nation of it.  It does not shew us how the gathering a cer-
 tain quantity of heat round  the particles of water should
be followed by an immediate and great  separation of those
particles from each other, each  carrying  with  it  its due
 proportion of calorific matter, so  as to leave  room  for the
 reception of a great deal more : far less will it shew us how
 the abstraction.of  th* smallest  portion of  this  additional
 heat by a colder body, instantly brings all the particles of
 the vapour together, again expelling the heat.   An active
 cause for the change of form is wanted, before we can speak
 either of enlarged capacity, or less repulsion.
   Many other strong objections may be made to  this ex-
 planation.   Some of these will  occur  when  we consider
 the nature of chemical union ; and it is needless to dwell
 more on this speculation, because it gives very little ad'! 
   CLEGHORN'S EXPLANATION....UNSATIS. 189

 tion to our chemical knowledge.  I content myself with say-
 ing, that it is much  more reasonable to conclude,  that*'the
 absorption and entrance of this great quantity of heat into the
 composition of vapour is the cause of its formation, than that
 it is its consequence; because we have the greatest reason
 to be satisfied that heat, in one shape or other,  is the. princi-
 ple or only cause of the formation of vapour, as it is of fluid-
 ity*.   (See Note  5. at the end of the volume.J
   I have already observed, that the heats which produce va-
 pour from  bodies under the pressure of the atmosphere, are,
 with regard to the greatest  number of them, higher  than
 those which produce simple fluidity.  This is, however, not
 the case with respect to. all kinds of bodies.  There are some
 solid substances, which, when we heat them,  evaporate en-
 tirely, or are totally changed into vapour, without previously
 melting,  or shewing any tendency  to become simply fluid;
 and the vapour into which  they  are changed,  when cooled
 again  condenses into'solid matter, not into a fluid.   Such are
 camphor, the volatile alkali, sal ammoniac, arsenic, corrosive
 sublimate of mercury-, and some others.  These bodies, how-
 ever, are capable of being brought into a melted state,  by in-
 creasing the pressure on their surface above that which they
 commonly sustain from the weight of the atmosphere, which
 must be  done by  confining  them and their steams in  close
 vessels.  The consequence of this is, that they are forced to
 bear more heat, and several of them can  thus be made so hot
 as to become perfectly fluid.   This shews, therefore, that the
 principal difference between  them and other bodies is, that
 whereas other bodies are more easily melted  than  changed
 into vapour by heat,  under the  ordinary pressure of the air,
 these,  on the contrary, are sooner changed into vapour than
 melted, and some  of them so much  sooner, or with so much
 less heat, that  we  dare not venture^ to confine  them so long
 as is necessary to learn what degree of heat is requisite for
 their  fusion.  They  are so  volatile that  they  would  burst
  * See  Lavoisier's Mem. 1777, in which he explains his notion of the man
ner in which all elastic fluids are formed of some solid matter, and the mat>
ter of heat.
    VOL. I.                 I » 
PARTICULAR OBSERVATIONS.
rmv vessels in which we could attempt to make the experiment'.
  These differences of bodies from one  another will enable
you to understand the nature of several chemical operations,
which depend upon the production of vapour; such as evapo^jh
ration, distillation, and sublimation.  These  are  operations
occasionally performed with mixtures of bodies, or compound
substances,  the different ingredients of  which we  desire to
separate from one another;  and they can only be performed
with those mixtures or compounds which contain ingredients
differing in volatility,  and which are not united together by a
strong attraction.  When this is the case, all that is necessary
is to expose the whole to heat,  very gradually  increased, so
that  the  more volatile may be gradually converted into va-
pour, and wholly separated, before the more fixed.  It  gene-
rallvihappens  that the  separation effected in this  way  is not
complete.  The action of heat in the conversion of a  body
into vapour, seems not to destroy altogether the former com.
bination,   A very scanty supply of heat promotes  the separa-
tion, seeming to allow  the difference of volatility to produce
its effect with greater certainty.   This however seems  a dif-
ficulty.   An operation  of this  kind is  called  evaporation,
when performed with a view to obtain the more fixed  parts,
and when we are careless about the more volatile, and there-
fore expose the matter to heat in  open vessels, and allow the
volatile matter to fly away in the air, as in the manufacture of
common  salt: but in  distillation  and sublimation, we pre.
serve the  volatile parts, which is done by performing the ope-
ration in  vessels and  instruments, by which the vapours are.
not allowed to escape, but are directed into a part of the ap-
paratus in which they are cooled and condensed  again.  In
this,  distillation  and, sublimation agree, and the  distinction
between them depends on the state and appearance of the con-
densed matter producedjrom the vapour.   In distillation,
this condensed matter is a fluid; in sublimation, it is a solid
substance.  In one of these operations, therefore, we work
upon subjects that are more fusible than volatile, or  more
easily melted than volatilized; in the other we work  upon
those that are more volatile  than fusible. 
 EVAPORATION, DISTILLATION, SUBLIMA. 191

   There are several other terms which might be taken no-
tice of here, but it is  not to  be supposed that you would
remember them,  though  I should enumerate  and  explain
them at present.  I shall have occasion to notice them here>
after.
   Beside thus  explaining  these  chemical  operations,  the
doctrine of vapour  will enable you to understand many par-
ticular effects which heat produces on  certain bodies of a
compound kind ;  such as entirely changing their appearance
and properties,....converting them,  as it were, into new sub-
stances.   This happens, because such compound bodies con-
sist  of ingredients  which  differ  in  volatility,  and cohere
weakly.   Heat dissipates some in the  form of vapour; and
both the volatile and.more fixed ingredients;~combine in a
new manner.  Hence wood contracts, instead  of expinding;
and instead  of melting afterwards,  is converted into water,
oil, salt,  charcoal,  earth.   And clay contracts  always the
the more it is heated/and remains ever after in the contracted
6tate.
   Nothing  general can be delivered on this  head,  because
these are all instances of peculiar properties.  We can  only
see clearly, that substances, which are brought into particular
states of combination by the operation of nature or art, may con-
tinue in that state unchanged, while their temperature  remains
the same.   But when we raise  this to a degree  which  vola-
tilizes several of their ingredients, and when these are such as
will unite, either in  this high temperature, or in a lower,
when each has collapsed, it must happeirhhat we shall pro-
duce compounds which  did not exist in the body from which
they have been raised.  And it may happen, that the ingre-
dients that are left  behind, are such as will unite, but had
taken another combination, by the intervention of the ingre-
dients  now  volatilized.  We shall thus obtain other  com-
pounds which did not exist in the body.
   These results appeared very wonderful, while the  science
of chemistry was in its infancy, and led into many mistakes
as to the compositions  of bodies.   The  detection of those
mistakes, constitutes the chief  part of the  advances which 
192  HEAT  PRODUCES NEW COMBINATIONS.

have been made in the science to this day, and will occupy the
greatest part of the following course of lectures.
  It remains now to  be considered,  whether  this effect of
heat may be supposed universal with regard to all bodies, or
whether there  are any exempted from this  power of heat.
Many philosophers  are inclined  to suppose that it would be
found universal, were it in our power, to employ heat of the
utmost intensity; j»nd we  have, no doubt, very extensive ex.
perience of the possibility of converting into vapour  a great'
many of the different kinds of matter, many of them  such as
might not at the first  view seem  much disposed to undergo •
this change; such as all the different kinds of salts,  and the
inflammable  substances, and  the metals.  Bdt still we  find
much difficulty with respect to the, earthy bodies.  The
greater number of these shew no dispositfon to volatility, but
on the contrary,  resist the action of  our most violent  fires,
without anv signs of their being changed into vapour.  There
are, however, one or two of them that, in very violent heats,
are more or less dissipated;  and from these examples, some
may perhaps imagine that the rest might likewise be volatiliz-  *
ed, were it in our power to command and employ more, vio-
lent heats ; but this is only a supposition.
   Some philosophers have taken such an enlarged  view of
this  power of heat,  that they have  not only  supposed the  ' \
 most fixed bodies  might be volatilized, had  we a sufficient v^jj
 heat in our power,  but that every palpable, elastic, • fluid in
 nature, is produced and preserved in that form by the action
 of heat.   Mr. Amontons, an ingenious member of the late
 Royal  academy of sciences  at Paris, was the first who pro-
 posed  this idea with respect to the  atmosphere.   He  sup-
 posed  that it might be deprived of the whole of its elasticity,
 and condensed,  and even frozen into a solid matter, were it
 in our power to apply to  it a sufficient cold;  that  it is a sub-
 stance  which differs from others, by being incomparably  more
 volatile; and which is therefore converted into vapour,  and
 preserved in that form, by a weaker heat than any that ever
 happens or can obtain in  this globe, and which therefore can-
 not  appear under any other form than the one it now wears, 
         IS VAPORISATION UNIVERSAL?     103

 so long as the constitution of  the world remains the same as
 at present.
   This opinion, though it may appear, at the first view of it,
 to be an extravagant flight  of the imagination, is, however,
 supported both by analogv, and,  in some measure, by direct
 experience.  We know that  water is easily convertible by
 heat into a vapour, which, so long as it is kept sufficiently hot,
 has many of the qualities of air.   It is  very light and rare,
 even more so than air, and it is equally transparent.  It is also
 very compressible, and reacts with an high degree of elasticity
 against the force compressing it,  in the same manner as air;
 and  its elasticity is greatly increased by  additions  of .heat, as
 well as diminished by the abstraction of it.
   As heat can thus produce  from water an elastic vapour,
 which resembles air in so many of its qualities, it is not im-
 probable that air itself is produced in the same  manner from
 some other matter. The presumption is very strong; and, when
 we consider the effects of heat on air more attentively, we find
 reason to be the more presuaded that its  rarity and elasticity
 depend upon heat;  for the  more its heat is increased, it be-
 comes always the more elastic, and, if it  has room  to expand,
 the more rare: and although  we  are not able  to  cool  it so
 much as by that way alone  to reduce it  to a dense unelastic
 body, we find, however, that  the more it is cooled, its  elas-
 ticity is the more diminished ; and it is the more condensed,
 without  any bounds to this  condensation and diminution of
 elasticity, except our inability  to make it  colder.   x
   In the modern doctrines of chemistry, therefore it is now
 considered as an indubitable truth, that  the air has its rare
 and elastic form from the heat it contains;  and it is assumed
 as an established principle, that all other permanently elastic
 fluids derive their elastic vaporous form from the same cause;
 that each of them is  a compound of a peculiar matter, and of
 the matter of  h«at, united in the state of latent heat, (or ca-
 loric, as it is named by the French chemists,) with the other
 matter, so as to constitute with it an elastic fluid or vapour.  I
observe,  that Fourcroy,  in his Elemens de  Chymie, says ex-
pressly, that by Calorique, his countrymen  mean the same 
194      REASONS  FOR THIS OPINION.

thing that I named latent heat, and that they reserved the
common word Chaleur to express moveable or sensible
heat.   I am not altogether certain of Fourcroy's being suffi.
ciently warranted in this use of the words, nor do I think it
the most proper.  Were I to write French, I would use the
term chaleur latente for those combinations  from which heat
is abstracted  by the touch of a colder body, and I would re-
strict the term cahrique to the combinations in which it is in-
deed equally concealed, but from which it cannot be extracted,'
but by presenting a  proper third body,  at the  same, or even
at a higher temperature, namely, to the gaseous combinations*
whence it is generally disengaged, accompanied  with light,
exhibiting the  phenomenon called combustion.  I am  the
more disposed to make this distinction, because I think that,
this is  a general mode of combination, containing several dif-
ferent steps, analogous  to those observed in the combinations
of latent heat.  But of this we shall occasionally hear more
afterwards.

                 OF  EVAPORATION.

  Thus, we have come near the end of this subject;  but to
make it complete, it is  necessary to observe, that, besides the
elastic steam produced  from volatile bodies, when a sufficient
quantity of heat is thrown into them, there is a different kind
of vapour produced from many of them, in much lower de-
grees of heat than what is necessary to the production of the
former.  But this second sort of vapour is always produced
slowly and imperceptibly without the smallest  commotion or
agitation, and only occasions a very slow and gradual diminu-
tion of the mass, until  it  be entirely changed, and disappear.
Thus, camphor, or volatile salts, even though wrapped in pa-
per, or other such containing matters, are continually emitting
vapours, which are  perceptible to the smell, though invisible
to the  eye, and are  thus gradually wasted, until they are en-
tirely lost.
   We have a familiar instance of this sort of vaporisation, as
well as of the other, in water.  If water be exposed in open
vessels, it is sure to exhale away imperceptibly, in this man- 
 HEAT THE CAUSE OF EXPANSIVE FLUID. 195

 ner, until it totally disappears, whatever "be  the temperature
 of the  air to  which  it is exposed; faster,  indeed, as the
 weather is warmer; but it has been  observed, that even in
 frost, and when the water itself is in the form af ice, it always
 suffers some loss this way.
   This, to distinguish it from the former, may be called Na-
 tural Evaporation.   It  happens through a considerable
 latitude of heat, but becomes more remarkable, as the heat'is
 increased.  The vapour thus produced, differs, by an obvious
 distinction,  from the one I have hitherto described, and which
 may be called the elastic vaporisation, which is produced only
 by increasing  the heat to the boiling degree.  The vapour,
 produced by the boiling heat, and which may be called steam,
 has great elasticity and force, and requires violence to confine
 it; whereas that produced in natural evaporation, unless it
 be made as  hot as the  other,  has no elasticity that is percepti-
 ble in ordinary circumstances, but may be  detained in the
 weakest vessels,  with  ease, and does not exert the smallest
 sensible force upon the vessel which confines it.  Every body
 knows that water can be preserved for any length of time in
 close vessels of glass, however thin.  Provided they be such
 as that  the water cannot penetrate  or  dissolve them, it never
 will be diminished in quantity, nor will it produce any vapour
 that will shew the smallest disposition to burst them.  As this
 sort of evaporation of water exposed to the air goes on always
 in a quiet and imperceptible manner, without any commotion
 or agitation of the fluid, it is plain that it must proceed en-
 tirely from the surface: and  accordingly, it appears by expe-
 riments that it is always more copious, as the surface is grea-
 ter.
   Another observation, with  regard to this kind of evapora-
 tion, must not be omitted ; that if this sort of vapour be allow-
 ed to stagnate over the body,  or be confined upon  its surfaqe,
 it proves an obstacle to the formation of any more, or to the
 progress of evaporation.  In  order to have a continued  pro-
 duction of this vapour, it is necessary that it be carried off, or
 removed from the surface as fast as it is  formed.
   Hence wind  greatly promotes natural evaporation.  I  like-
wise observed  before,  that it is greatly promoted by  heat. 
 196     OF NATURAL EVAPORATION.

The production of this vapour does, indeed, so much depend
upon heat, that when, ifi consequence of more heat than^ordi-
nary applied to bodies, a great quantity has been produced, it
is condensed again,  like the other, when this heat is dimin-
ished.   Thus, when a bottle or glass of cold water is brought
into the room,  where many persons  have  been sitting for
some time, its  outer surface is immediately  dimmed  by
the vapour contained in  the air  of the  room, now condens*
ed  upon  it.   Hence, too, arise  the  moisture and  hoar-
frosts,  on the inside of our window's, in very cold weather;
as also on  our walls, whey a hard  frost is  followed  by
a sudden thaw, brought  on by a wind coming f'ora the
south,  and filled with  vapour.   It is a vulgar  error to as.
cribe this moisture  on our stone walls  to the weather pene-
trating through them.   It is the vapour contained within tile
house, condensed by the cold walls.  We observe none when
the walls are covered  with laths and plaster, and think that
this prevents the penetration.   But the true reason is, that this
thin, detached covering, soon acquires the warmth  of the
house,  and condenses no  more vapour.  The glass of our
windows condenses it copiously, because very cold,  yet it is
impenetrable by  the weather.
  From the same cause arise the mists,  which hover over low
marshy grounds in the cold mornings and evenings,  especi-
ally after a warm day, in which the evaporation has been very
copious.  In this case, the smallest diminution of heat pro-
duces a condensation, and the vapour becomes visible, and is
precipitated.   The mist seems to rise,  and we see a stratum
of clear air below the stratum of dew, because the precipita-
tion begins in what is next the ground, and proceeds gradually
upwards, as the strata cool in succession.   For the same rea-
son, we frequently see a smoke apparently  rising from  holes
made in the ice which covers deep lakes  and rivers.   (See
Note 6. at the end of the volume.J
   This sort of vaporisation, and particularly the natural eva-
poration of water, has been much attended to, as being closely
connected with  some of the more important an.d extensive
operations of nature. 
                 OPINIONS OF  PHILOSOPHERS       197

         By this sojt of vaporisation, water arises into the atmos-
       phere, from the surface of tfie sea, and other large collections,
       in the form of transparent vapour, which, at a certain height,
       is condensed by the cold of the superior regions, into mist or
       clouds.   These clouds are suspended at different heights, pro-
       bably in consequence of the particular temperature and rare-
       faction of that portion of a'ir which is contained in them, and
       they ;>.r» liable to become  electrified, so as sometimes to oc-
       casioa lightning and thunder.  Often also, in consequence of
       a more quick and perfect  condensation of  the vapour, or by
       mean* of an electrical attraction of the little globules  of water
 '      for one another, they form drops of water, which descend in
^F     rain, which is  indispensably requisite for the nourishment of
       vegetables and animals, and gives origin to springs, rivers, and
     ♦kt,.
H        As this circulation of water, by which we all live,  depends
'      entirely on  natural vaporisation,  it has  greatly  attracted the
       attention  of philosophers,  and Has occasioned many attempts
       to discover and explain in what manner  it happens.
         1.  The  first remarkable attempt was made  by Dr. Der-
       ham, in his Physico-Theology, B. I. ch. 2. and B. il. ch. 5f
       He says that fire, being the lightest of all natural things, rises
       up  through the water,  and carries off with it a quantity of
       water.  The fire, bursting out of the water,  is enveloped with
       a thin film, in the form of a small bladder, or vesicula, which
       rises  in the air b\ its extreme lightness, till it attains a height
       corresponding  to its specific gravity.   Myriads  of such vesi-
       cula? form clouds,  which float  about  in  the atmosphere, till
       they are condensed by cold, or coalesce by being driven against
       each other by  the winds.   Then they descend in dews, fogs,
       rains, snow, or hail.
        This was a very careless attempt to explain so curious and
      important a phenomenon.  Vesicles of such a constitution
       could not remain a minute  in the air, but must collapse as soon
      as they  are formed^  Nay, we cannot conceive their forma-
      tion.  Fire has never been met with in the form of a palpable
      expansive fluid, like air,  fit  for  distending these vesicles.
      They must instantly collapse,  acquiring *the  temperature of
         vol. r.              k k 
  198 CONCERNING THE ASCENT OF VAPOURS.
                                                      '■*» (.'
  the air ; and now, being much heavier, must immediately fall
  down again.
    2. The theory of Desaguiliers is founded on calculations
  of the space which he supposed water to occupy, when chang:
  ed into vapour, with different  degrees of heat.  He first en-
  deavors to calculate how much, or to how many times its
  own bulk, water  is  expanded, when  changed into steam by
  the boiling heat, or  elastic vaporisation, under the pressure
  of the atmosphere ;  and, from some  premises, he concludes
  tha--, when changed  into steam, under  that pressure, it  is ex-
  panded to 14000 times the bulk of the  water.  In the next
  place, he  supposes that the different degrees of heat, between
  frost and  the ordinary  boiling  point of water,  must produce
  from water vapours  less rarefied, but still  very rare, and ap-
  proaching in raritv to the steam  of boiling water, inproportiOT
  as the heats by which they are produced approach to that heat,
  and are distant frotn frost, or the cold of melting snow.  And,
  from these principles, he concludes, that the heat of the weather
  in summer, spring, and autumn, and even  in the greater part
  of winter, is still sufficient to form vapours  from water, which
©shall be lighter than  air  ; and therefore disposed to ascend, or
  to be driven upwards in the atmosphere.
    But his very first  proposition is totally false, and the falsity
  of it overcrows the whole of  his reasoning.  Water,  when
  changed into steam, is not expanded into 14000 times its ordi-
  nary bulk.  I examined, with great attention, that part of his
  book in which he had stated this proposition, to see if I could
  learn on  what it  was founded.  I could not obtain the least
  satisfaction: and  I can assure you, that, by a simple and deci-
  sive experiment, which I lately described to you, the space
  which water occupies,  when changed  into steam,  under the
  ordinary  pressure of the atmosphere, is only about 16G4 times
  its bulk, when in the form of water. More  recent experiments
  bv Mr. Watt, make 1720, or rather more ; but he is not per-
  fectly precise in his opinion.
    3. A muchrmore ingenious explanation was proposed  by the
  c.-U-brated astronomer and philosopher Dr. Halley.   In some
  oi his writings, indeed, fce gives Derham's explanation  of the 
           OPINIONS OF  PHILOSOPHERS      199

 rise of vapours by vesicles, distended, not by fire, bqt by heated
 air.  (See Phil. Trans. No. 192.)   This is as untenable as the
 other.  But Dr. Halley, afterwards, in a dissertation on the
 origin of springs and rivers.  (Sjee Phil. Trans, abridged, \ oh
 2, No. 127,) expresses himself as follows :
   " I take it that the air itself will imbibe a certain quantity of
 " aqueqqs vapours, and retain  them, like  salts dissolved in
 " water. The sun warming the air, and raising a more plentiful
 " vapour from the water in the  day-time, the  air will sustain
 " a greater portion of vapour, just as warm water will hold
 " more salts dissolved, which, upon the absence of the sun in
 " the nights, will all be again discharged in dews, analogous to
 " the precipitation of salts on  the cooling of the liquors.'1
   It is surprising that an explanation, suggested by so eminent
 a philosopher, and supported by so extensive analogy, did not
 meet with more general acceptance.  Although this essay of
 Dr. Halley's, and another much  connected with it, are fre-
 quently quoted with the greatest respect, especially on the
 Continent, this  passage is never mentioned.  And  hi 1743,
 when the  Academy of Sciences of Paris published this  as a
 prize question, the many performances of the competitors, and
 even those of Kratzenstein and Hamberger, which shared the
 prize, take no notice of it But the explanations given by these
 authors are very inadequate indeed ; yet the theory of Krat-
 zenstein, who adopts the vesicular form of the matter which
 composes cloud, fog, or dew, is accompanied  by  many  very
 curious observations and  ingenious reflections on  these vesi-
 cles.   He produces the vesicles, by forcing a  quantity of air
 into a clear glass globe from the lungs; the globe has a leather
 valve, which keeps in the air ; when this is pierced with a pin,
 the air comes out slowly, and the vesicles appear in the glass
 in  strata,  exhibiting beautiful colours.  In  1750, however,
 the same Hamberger,  in his Elementa Physices, published at
 Halle, adopts Dr. Halley's solution-system completely,  but
without seeming to have taken it from him, so unskilfully does
 he manage it.
  In the following year, Mr. Le  Roy, physician at Mont-
 pellier, (I think)  and  correspondent of  the Academy of 
200  CONCERNING THE ASCENT OF VAPOURS.

Sciences, gives this system in full detail, "supported by a nume-
rous train of resemblances  to the solution of substances in
their solvents, and applied to the chief phenomena with great
distinctness and ingenuity.   The celebrated Dr. Franklin, the
late Lord Kaims, and Dr. Hamilton of Trinity college Dublin,
have also published dissertations  to the same purpose, sup-
porttd by the same arguments, and applied nearly in the same
manner.
  The circumstances adduced by these authors, in which the
rise of vapour, and the principd phenomena of the atmosphere,
resemble chemical solutions, are indeed very many and very
specious.
  There is an actual solution, for the  dampest air is perfectly
transparent: bodies dry7 very fast in warm air, but slowly in
cold, and in the cool of evening, the dews are more copious
after a warm day.  Every temperature of the air appears to
have an appropriate power of taking  up vapour,  and part is
precipitated as it cools.  This perfectly resembles the satura-
tion which obtains in all solutions.  Extent ot surface greatly
promotes evaporation.  More water  will evaporate  from a
moist cloth in  an hour,  than from a bowl of  water  in a day.
Wind also greatly promotes evaporation, j.3t as a renewal of
water or agitation promotes solution.  Stagnation, on the con-
trary, puts a stop to both.
  To these  arguments  may be added,  that evaporation pro-
duces cold,  in proportion to its extent and rapidity, just as it
is produced in the solution of salt.
  This theory of the ascent of vapours, when thus proposed,
soon obtained a general concurrence, and was freely employed
by the  meteorologists.   But an experiment related by Walle-
rius, in which he observed  a copious evaporation in  the ex-
hausted receiver of an air-pump, raised some  doubts about the
validity of this explanation. The experiment has been repeated
with great  care  by many philosophers, and it is  now well
known, that evaporation goes on much faster in vacuo than in
the  open air, and is more  and more retarded, in  similar cir-
cumstances of temperature, &c. in proportion  as the  evaporat-
ing body is surrounded  by denser air.  It is found that heat 
     HEAT THE SOLE CAUSE  OF EVAPO.   201

expands water into elastic vapour in all temperatures.   In an
experiment by Mr. Watt, water of the temperature 57° pro-
duced a vapour  which supported half an inch of mercury.
This experiment was  a very curious  one.  A  single drop of
water was let into a  very fine  barometer ; as soon as it got
above the mercury in  the tube, the mercury sunk half an inch.
Mercury was now poured into the cistern, and it rose also in
the tube; but the mercury in the tube still wanted half an inch
of its first elevation above that in the cistern.   This depres-
sion continued while mercury was still poured into the cistern^
till at last the vapour of the water was all condensed again.
This is analogous  to  an experiment which I mentioned some
time ago, in which boiling hot steam was condensed by pres-
sure.                            *
   Now, in this experiment of Mr. Watt's, there was no air to
dissolve the water.   Mr. Nairne produced, in like  manner,
vapour of the same strength in  the air-pump receiver, from a
bit of moist paper.  Mr. Saussure set a  barometer into a tall
glass, the air of which was rendered perfectly  dry by means
of quick-lime, which absorbed all moisture.  He then threw
into it a small piece of moist linen, and instantly shut the ap-
paratus.  In a short time, the vapour produced from the cloth
raised the barometer half an inch.   Comparing these three
experiments, we see  that  the presence of the air, in the last
experiment, had not contributed to the production of vapour.
   It is therefore considered as very doubtful, at least, whether
the active dissolving  power of  the air is  the cause of tiie rise
of vapour in the  atmosphere ;  and it seems to be  the most
prevailing opinion  that heat is the sole  agent.  Those who
continue to  maintain  Le Roy's system, (Mr. de la Place and
Mr.  Monge) say that there is less cold  produced in  sponta-
neous evaporation in  the air, than in the  artificial evaporation
in vacuo: but another experiment by Mr. Walt shews the con-
trary.  Two vessels, perfectly similar,  were exposed, filled
with water of the same temperature ; the evaporation from
one of  them was  prevented by  a covering of oil;  the heats
lost by both were  compared, and the grains of water evapo-
rated were  measured ;  the difference of  the heats lost greatly  . 
 202     EVAPORATION PRODUCES COLD.

exceeded the heat lost by evaporating the  same  quantity of
 water in the air with a boiling heat.   Thus, the solution-sys-
tem seems to lose ground. Mr. Saussure thinks that heatalme
 expands the water into vapour, an*  that the incumbent air
dissolves this  vapour.  Mr. De Luc  has another theory, but
 jt is too completed to be considered  here.  See his Meteoro-
Iggie.   (See Note 7, at the end of the  volume. J
   I mentioned the cold produced by spontaneous evaporation.
Dr. Cullen's experiments, which  I adduced on another occa-
sion, are the most remarkable examples.   Mr. Mairan and
 Mr. Franklin give many examples of the cold produced by the
 evaporation of water and  other fluids from the skin.   It is
from this cause that a wet cloth exposed to the air, often freezes
stiff.  Although the thermometer stands some degrees above
32°, water evaporates, and this cools the remainder so as to
freeze it.
   When these phenomena were first  taken notice of by our
philosophers, they were thought very wonderful; but they have
been long familiarly known in India and in China.   In those
sultry regions,  water is kept  cool by putting it  into very
porous earthern jars.   These are ever covered with moisture,
 in a state of  copious evaporation.    The traveller  carries
 water in an earthern mug of the same kind, or covers his can
 with  woollen cloth ;  he has this slung on his shoulders,  and
 dips  it into every pool or puddle.   Ihe land-winds being
 excessively dry, cause the water to  evaporate very fast, and a
 great deal  of heat is thus absorbed.   The wealthy have their
 dining  halls open on  all sides, the  roof being supported on
 pillars, and the intervals  are hung with curtains.   Servants
 without doors scatter water continually7 on these curtains, and
 its evaporation absorbs a vast deal of heat, or makes the hall
 cool  and refreshing.
   Nay, such is  the effect of  evaporation  in absorbing heat,
 that the rich inhabitants of those countries can lay up stores of
 artificial ice,  in  a climate where the thermometer is never
 under 40°, and frequently rises to 110®.   I shall explain the
 curious process  for this purpose, as  I have it from a  gentle-
 man who  resided nine years at Benares,  one  of  the ancient 
                  INDIAN PROCESS              203

 seats of arts and sciences.  He saw the process performed
 several tifoes, and his letter on the subject is to be seen in the
 philosophical transactions for 1792 or 1793.
   Benares is between the 2oth and 26th degree of north lati-
 tude, a great way up the Ganges and has an immense continent
 extending from  it in every direction.  The process is per-
 formed in the  middle of their winter only, and in clear dry
 weather, when the thermometer falls to within 8 degrees of the
 freezing  point;  and although no ice is ever formed on the
 pools and1 other small collections of water, they succeed in pro-
 curing it  by this process.
   Shallow pits or beds are made four or  five  feet wide, and
 about four inches deep, separated from one another by narrow
.borders, and so numerous as to cover an extent of about four
 acres.  These pits are  filled  with dry  straw  in the proper
 season, and on the ptraw are placed rows, close togethtr, of
 shallow earthern  pans, into which a few inches depth of water
 are put every evening of the winter season, when the weather
 is clear, and the nights  very cool.  In the'morning they find
 more or less ice in the pans, which is taken out before sun-rise,
 or about five o'clock ; and that it may seperate the more easily
 from the earthern vessels, the inside of these is rubbed with a
 little butter before the water is put into them.   The number
 of earthern, vessels or pans in one place is about 100,000; and
 about 300 men,  women, and children, are employed in the
 morning in  gathering the ice, that  it may be done quickly,
 before the heat of the day begins.   They wrap it  in  thick
 coarse woollen stuff, and carry it to the ice-house.  The clear
 and cold weather, which is necessary to the production of the
 ice, by this process, is undoubtedly attended with a  northerly
 wind.  And in  such  a country, the northerly winds,  or
 breezes, are excessively dry, in consequence of their coming
 over a great extent of  dry northern continent.  They have,
 therefore, great power to promote evaporation.   The shallow
 earthern vessels, exposed in this maner, and the water in them,
 must become colder, by the quicker  than ordinary evapora-
 tion of a part of the water} than any other  matter around
 them.  Care also is taken, by the contrivance of the process, 
 204              FOR  MAKING  ICE.

to prevent the coldness of the vessel and water from-being too
much diminished  by communication  with the warm soil that
is below.   The thick bed of straw, on which the vessels are
laid, is evidently intended to serve this purpose.
  In another part of India, farther south, or nearer to .Calcutta,
they find more  difficulty  in preparing  ice by a process of this
kind.   But experience has taught them, that  boiling the water,
before it is exposed, promotes their  success.  This fact was
communicated to me in the year 1774, or 1775, by the late Sir
John Pringl., president of the Rcyal  Society, who tlesireu to
have my c pinion on it.   Before I gave any opinion, I thought
it was proper to enquire  into the certainty of the fact, and I
accordingly  made  some  experiments relating to it.  But I
cannot say thai they terminate.; in any very confident opinion.
I" found very little difference, if any, in tlu quantity of ice pro-
duced from boiled and unbr iled water. »  I think  that, in most
of my trials, the boiled water first begfln  to freeze, and, when
the  heat of the air was' but very little bAo  ' 3^°, suppose not
lower than 2\)°,  the difference was certain, and more conside-
rable.   I found always, that when the one had frozen a little,
and the other not, the smallest touch or disturbance,  with a
tooth-pick, made it freeze  instantly.  *The  difference, there-
fore, which I think maybe  stated between them, is that boiled
water begins  to freeze as soon  as the  air has cooled it to 32°,
but that unboiled water will continue fluid a little longer, if not
disturbed. I recollect that the experiment  in which this curious
fact was first  observed, was made by  Fahrenheit,  with wa; r
which had been purged of air by boiling. He had sealed the
vessel  containing  it.   Wishing to apply  a  thermometer, he
broke off the end of the neck, and the water froze as soon as
the  air touched it.  Perhaps  this was effected by the  water's
beginning immediately to absorb air  again.   Perhaps air, as
well as salt, retards the congelation of water.
  On the whole, this operation is an example of a curious art,
which in its present state of perfection and  simplicity, seems
the  product of  great knowledge and ingenuity, requiring un-
common talents in any  individual to invent and direct it.
But it probably  even began in ancient times, in a cooler cli- 
     HEAT MORE TOLERABLE  BY EVAPO.  205

 rnate, and'extended slowly southward, in consequence of re-
 peated improvements,  derived from experience, the source
 from which all our useful knowledge has proceeded.
   I am persuaded that the heat absorbed in spontaneous eva.
 poration greatly contributes to enable animals to bear the heat
 of the tropical climates, where  the thermometer frequently
 continues to shew the temperature of the human body.  Such
 heats, indeed, are barely supportable and enervate the animal,
 making it lazy and indolent, indulging in the most relaxed
 postures, and avoiding every exertion of body or mind.  The
 inhabitants are induced to drink large draughts of  diluting
 liquors, which transude through their pores most copiously,
 carrying off with them a vast deal of this troublesome and ex-
 hausting  heat.  Still however,  these heats are most oppres-
 sive to the poor,  who must work, and  often produce great
 mortality, when they continue long unabated.  There is in
 the body itself a continual laboratory, or manufacture of heat,
 and,  were the surrounding air of such a temperature as not
 to carry it off, it would soon ace umulate so as to destroy life.
 The  excessive perspiration, supplied  by diluting draughts,
 performs the same office as the cold air without the tropics, in
 guarding us from this fatal accumulation.  It is not  unlikely
 that that constitution of the vessels of the lungs, and the pores
 of  the skin, which unfits  them from bringing forward  the
 lymph, in sufficient quantity for carrying off, by evaporation,
 the heat  generated by the vital functions,  is the immediate
 cause of the heat, in ordinary fevers. •
   It is really wonderful what heats may be carried off in this
 Way. Dr. Fordyce staid for a considerable time,  and  without
 great inconvenience, in a room heated  to  260° of  Fahren-
 height's scale.   The lock of the  door, his watch  and keys
lying on the table, could not be touched  without burning his
hand.  An egg became hard ; his pulse  was at 139°; yet a
thermometer held in his mouth was only 2° or 3° hotter than
ordinary.   He perspired most profusely.  (See many  curious
and instructive  observations and reflections on these  experi-
 ments in Phil. Trans, vol. 64. and 65.)
   VOL.  !•                  J- 1 
 *06 CURIOUS  OBSERVATION  OF PROF*.  WIL.

   There is  only one phenomenon remaining that may be
 thought worthy of notice, and it was discovered by the wor.
 thy and ingenious Professor of Astronomy at Glasgow, Mr.
 Wilson, and described by him in a paper of  the first volume
 of the transactions of the royal Society of Edinburgh : and he"
 also communicated some of his observations on this subject to
 the  Roval Society at London*                               (
   During a winter,  when the cold was uncommonly severe
 and intense  in this country, and  when the thermometer fell
 some nights  to the beginning of Fahrenheit's scale, and, even   j
 in the day time,  stood for many hours at 10 or  12 degrees   \
 only above it,  the Professor,  who passed  some  part of the
 night in the  Observatory,  took with  him several thermome
 ters, and placed them in different situations.   Some were laid
 on  the surface of the  snow,  others hung up in the air, and
 otherwise ; and he very soon perceived, that those which were *
 laid on the surface of the snow,JMmbhe vjwy soon pe»eehist^iWt
^jbose whujjfcwere laid ojubesurfitceof thesnow fell considerably \"-
 lower than the rest;  sometimes they were 14 degrees lower. ~*f-
 He then sunk some of  them below the surface of the snow,
 which was- uncommonly deep, and found, as he expected, that  v
 they rose and stood higher, than when  they were hungup in
 the air;  they rose the higher the more deeply they were sunk
 in the snow,  and  the nearer they were placed to the surface of
 the  ground.  This was easily^mdcrstood, but the surprising
 and unaccountable fact  was, that they should indicate a grea-
 ter degree of cold when laid on the upper surface of the snow,
 than vhen hung up in the air above it.   3This fact excited his
 curiosity,  and he made a great number of judicious experi-
 ments to discover the circumstances with which it  was con-
 nected.   He learned by these experiments,  that it was con-
 nected  with  a disposition of hoar-frost, and that it was only
 when the thermometer was placed on  a surface of snowor
 other matter, on which a disposition or condensation of hoar-
 frost was going on, that it sunk so low.   This he ascertained
 by a number  o." decisive experiments :  but he observed that it
 happened only  when the cold of the air was at the same time 
IGNITION.                   207
 violent, so as to make the thermometer fall 20 degrees, or
 more, below frost.   Still, however, it was a most unexpected
 and surprising phenomenon.   From the  knowledge we have
 of latent heat,  we might rather expect to see an increase of
 sensible heat, on this Occasion, thfan a diminution of it.   (See
 Note 8, at the end of the Volume. J

           SECT. IV.....OF  IGNITION.

    WE are now to attend to the fourth general effect of heat
' commonly called Ignition, or Incandescence.
   By this term, is  meant that glow, or emission of light from
 bodies, which appear when their heat is increased  to an high
 degree of intensity, about the 650° of Fahrenheit's scale; and
 which, when it  first begins, is weak and obscure, and of a red
 colour,  but increases in brightness as the. heat becomes
 Stronger until, in the most violent intensity of heat which we
 can produce, it  becomes so bright and dazzling  a  whiteness,
 that the eye cannot bear it.
   A difficult question here occurs, concerning the connection
 between heat and light: how happens it, that each is naturally
 disposed to produce the other?  We know by experience, that
 whenever a body is heated intensely, it always becomes lumi-
 nous, or erupts light;  and, on the other hand,  it appears as
 plain, from the effect of the light of the sun, the most luminous
 body in nature,  and, especially,  when this light is  condensed
 by convex glasses, or concave mirrors, that a strong light, di-
 rected upon bodies, is sure to produce in them very quickly
 the most violent heat.
   The opinion of some of those who have ventured to form
 conjectures on this subject, is, that light, as well as heat, is pro-
 duced by that subtile, elastic, self-repellent matter,  which has
 been  imagined to be the cause of the sensation heat;   That
 when the matter, in its ordinary and most communicable state,
 abounds in a body,  the body appears hot, or is in a  hot state;
 but that,  when it abounds to a high degree,, some particles of
 this subtile matter are thrown off with inconceivable velocity,
corresponding to its high degree  of elasticity, or the powerful 
208   CONNECTION  OF HEAT  AND  LIGHT.

 repulsion of its parts for one another ; and that these projected^
 atoms constitute light.  (See  Note 9,  at the end of the  Vo*
 lume.J
    Dr. Boerhaave considers the connection between heat and
 light at great length in  his treatise on fire ;  but he perplexes
 himself, and gets into a confusion of  ideas that is surprising.
 He takes much pains to  find the reason,—why the light of the
 moon, though collected  by mirrors, or burning glasses, should
 not sensiblv affect the most delicate thermometer;  while that
 of the sun, collected by the same means, produces very quickly
 the  most vehement heat of which we  have any experience,
 and has great affect on  bodies even when it is not condensed.
 At last, he thinks he has found  the reason of  the  difference,
 which is, that the rays  of the sun are more parallel than those
 of the moon.
    B_ut this is the most unfounded notion that can be imagined.
  Neither the rays that come from the sun, nor those that come
  from the moon, are parallel to one another.  Those, indeed,
 that come from any single point of either of thr se luminaries,
  are very nearly parallel when they reach us.   On account of
  their coming from a  point at such a very great distance, they
  are so nearly parallel, that they atopear, by experiments, to be
  exactly  so;  and there is no sensible difference between the
  ravs of the  sun,  and those  of  the moon, in  this respect.
  Though the sun is more distant/ than the moon, the moon is at
  too great distance to admit of our perceiving the smallest sen.
  sible divergency in the rays that come  from the same point of
  her surface.  But if we consider the  rays  which come from
  different parts of the surface, either of the sun  or of the moon,
  it is plain that they are far  from being parallel.  The sun's
  disk subtends an angle of  half a degree ;  and his  rays, if we
  consider  the whole of them together, are not so parallel as
  those of the moon, whose diameter subtends an angle not quite
  so large.   It  is,  therefore,  surprising that  Dr. Boerhaave
  should have taken up this idea,  which, at any rate, does not
  in the least make  the  fact more intelligible.  If the effect of
  the sun's rays, in heating the  bodies upon which they  fall,
  depend upon then  parallelism, the light which comes from 
    FAINTNESS OF THE MOON'S  LIGHT.   209

Jupiter and Saturn, in  some of their positions,  should be
more powerftd than that of the sun.
   The true cause of the great difference between the effects
of the ravs of he sun and those of the moon is the excessive
feebleness  of the moon's light.
   The moon is plainly illuminated by the sun, and the light
which comes from her to us, is a part of the sunrs light re-
flected from her surface.  But it is plain that, even supposing
it to be all  reflected,(which is very improbable) this reflection
must happen in such a manner, that the light is dispersed
equally in all directions ; and therefore, a very small part of it
only can come toward the earth; such a  small part, that it is
rather difficult to understand why she appears so bright.  Ac-
cordingly, some philosophers  think that the surface of the
moon must be of the very brightest kind,  as if it were  covered
with  snow.  But I do not think it necessary to form such a
supposition, in order to account for her apparent brightness at
night.  I  was surprised at the appearance which a baloon of
oiled paper, filled with inflammable air, made in a serene blue
sky, and bright sunshine,  though the reflection of light from
it, when near the earth, was not remarkable.  It appeared like
a star, or like Venus, when it was so high that its shape #as
not seen.
   That the light reflected by  the  moon to  our  earth is,  in
reality, next to nothing, when compared with that of the sun, is
plain from.some obvious and easy calculations.      JflV*
   Bouguer's experiments were made by causing a beanfof the
sun's light  to diverge by means of a concave glass.   Thus the
light was weakened as it receded from the glass, the strength
of it being, at different distances from the focus of glass, In-
verselv  as the squares of these distances.  Then he  investi-
gated, bv experiment, the distance at which it was equal in
strength to the light of a candle thrown upon paper, placed at
proper distances from the  candle.  And he  afterwards com-
pared  the  light given by  the  same candle  with that of the
moon.  In making this last comparison, it was not necessary
to weaken the light of the moon, in order to make it equal to
that of the candle. It was, on the contrary, necessary to  weaken
that of the  candle.  The final result of Mr. Bouguer's  calcula- 
210   HEAT  MAKES  ALL BODIES SHINE.   0

tions is, that the sun's light is more than 300,000 times denser
than that of the moon ; which last is, therefore, quite undis-
coverable by our most delicate thermometers.
   Ignition  may be considered as one of those effects of heat
that is produced in the most similar  manner, upon different
bodies, of any kind whatever.  It is also a very general effect
of heat.  We  do not know of any bodies that are exempted
from it, unless it be those whose volatility is so great, that they
cannot be made to endure the degree of heat necessary for this
effect.   But even  these, when in the form of vapour, may
perhaps be  heated red hot, though many  of them must be so
very greatly expanded before it acquire the necessary degree,
that itlirould not be easy to perceive whether or not they are
in the state of ignition. (See note 10, at the end of the volume. J
  Upon thepwhole, we have reason to consider this as an effect
of heat, to which all bodies, without exception, are liable, and
all at the same, or nearly the same, degree of heat.  But it is
difficult to  know exactly the place of this degree in the scale
of heat.   We depend on the acuteness of our sight to inform
us of the beginning of ignition, and we are not sure that it per-
ceives the very weakest light.  The presumption is rather the
contrary, as we  know that other animals see objects in such
weak light, as appears to us to be utter darkness.   And even
our eyes will,  in one condition, be affected by a weak light, to
which they are quite insensible in another.
  S^Mtaaac Newton, however, attempted to estimate the de-
gree or  heat, which is attended with the weakest  perceptible
light, by the method I formerly described, viz. exposing a
lump of very hot iron to a stream of  cold air, and observing
exactly the  time when it ceased to emit light; and having also
observed the celerity and progression with which it cooled, or
lost its heat, from the beginning of the experiment until it was
reduced to the temperature^of the air, he was enabled to esti-
mate the heat  which remained in it at the last moment of its
ignition.
   By a calculation of this kind,  he reckons that iron,  in the
lowest state of ignition, or when it just ceases to shine per-
ceptibly in  the dark, is  heated to the 635th degree, in a pro-
longation of Fahrenheit's scale ; that it just ceased to shine in 
                EXTENSIVE SCALE              -if

the twilight, when it was heated to the 884th degree ;  and
that, when it was as red hot as a common fire could make it,
the degree of its heat was 1049 or 1050.
   To conceive, with  some  distinctness, the  relaticms which
these intense heats have to the temperatures  with which we
are more familiarly acquainted, it is by no means enough to
attend to the numbers which express them.  This gives but  a
very imperfect notion of the comparative magnitude of their
intervals.  We must  have recourse to a scale, or straight line,
divided into a great number of equal parts, or units, and place
the physical character, or effect of each temperature, at the
corresponding units of this scale.  By looking at such a figure,
we perceive at once,  that the  difference between the heats of
boiling oil of turpentine, and boiling water, is nearly twice as
great as that between the heats of boiling and of freezing water;
and that this last difference is nearly the same with that of
boiling mercury and melting tin, &c.   The inspection of such
a scale  shews us how very small a part of the scale of mea-
surable heats comprehends all the  temperatures of the habit-
able climates, and those of  our most familiar and intexesting
•experiments.                                    •••'sfft*'
   But the temperature of glowing iron is far from being the
 highest of those of many chemical operations, with which  it
 is necessary to have  a sort of practical acquaintance.   Some
 metals  are observed  by those  who manufacture them, to melt
 when red hot, some  before, and some after they are red hot.
 The  cutlers have familiar marks when steel  and iron are fit
 for certain operations, and  have terms which express those
 States, such as  cherry  red, worm red, a white heat, a blue
white, Sec,
   You will readily perceive that many of the  temperatures in
 the upper part of the scale are such as cannot be determined
 by the expansion of a thermometer.   Thjey have been deter-
mined in Newton's way, (indeed by Newton himself) already
described.   Some  of them have  been determined by  Mr.
Irvin, at my desire,  by means of the heat communicated to
water, by the lump of iron,  immediately after the freezing of
the metal.  Some of Newton's determinations were  by his
lintseed  oil thermometer.  With respect to the temperatures 
OF MEASUREABLE HEATS.
far below Irost, I have paid no regard to those low stations of
the  mercurial thermometer described  by  Professor Braun.
These were undoubtedly owing to the  irregular contractions
of portions of the mercury, as they froze.   But as the spirit
thermometer pointed at the same time to....l48°, I have con.
sidered this as the temperature of the experiment.   For the
same reason,  I have admitted....l20° as a natural cold, once
observed in Siberia.

            TABLE  OF SENSIBLE  HEATS.

     1050  Iron,  as hot as a common fire could make it.
      884  Iron red, in the twilight.
      752  Iron bright red, in the dark.
      635  Lowest ignition of iron, in the dark.
      600  Mercury, and also ol. lini, boil (644 according to Mr. De Se-
          condat.)
      560  Ol. Terebinth boils.
      546  Sulphuric acid boils.
      540  Lead melts (Newton) 585 (Secondat) 594 (Irvin).
      460  Bismuth melts. 476 (Irvin.) also tin 1 pt. and lead 4 pts.
      408  Tin melts. 413 Irvin.
      334  Tin, 3 pts. lead, 2 pts. melt.  Also tin, 2 pts. bism. 1 pft  «
      283  Tin, 1 pt. bism. 1 pt.                                    U
      242  Nitric acid boils.
      212 Water boils.  Bism. 5 pts. tin 3 pts. and lead 2 pts. melt
      174 Alcohol boils.
      156 Serum and Albumen ovi coagulate.
      142 Bees-wax melts, Irvin.
      107 Feverish heat.
      100 to 96, Animal heat, or blood-warm.T
       64 Summer-heat in this climate.
       32 Frost.       ^
       30 Milk freezes.
       28 Vinegar freezes.
       25 Blood (human) freezes.
       20 Strong wines freeze.
        7 Spirit of wine 1 pt. mixed with 3 pts. water, freezes-.
        0 to °, or from o to—4, salt and snow melt.
      —7 Brandy, or equal pts. of alcohol and water, freezes.
      —11 Spirit of wine, 2 pts. mixed with 1 pt. water, (Reaumur).
      —14 Cold, observed in the air at Glasgow observatory, anno 1780<
      —-34 Reaumur's spirit of wine thermometers at Torneao.
      —40 Fahrenheit's  experiment,  and congelation of  mercury; alsf1
               nitrous acid crystallizes 
       Scale  of measureable  heats.   2hj.

     —69 Cold produced by Mr. M'Nab, at Albany Fort in  Hudson's
          Bay, in December 1784, by means of sulphuric acid and snow.
    —121 Siberian cold  7   ,     , .„,    . ... c  .   »
    —148 Braun's expt.  \  observed with a spirit of wine thermometer.

   1049  or 1050, therefore, is the greatest heat which Sir
 Isaac Newton attempted to measure, so as  to learn how far
 it exceeds other heats,  the strength of which is more easily
 ascertained.
   But the late Mr. Wedgewood, who published two papers
 in the philosophical transactions, on the measurement of vio-
Tent heats,  has gofce much farther, and has shewn great  in-
 genuity  in the contrivance of  a  method for comparing and
 distinguishing violent heats.
   The principal mSan which he  employed is the contraction
 of the purest kind of clay in the fire.
   The properties of the more pure and perfect kinds of clay,
 with respect to heat, are well known to be, 1**, a power to re-
 sist the most intense heat, without melting.   2d,  A disposi.
 tion  to  contract, in all its dimensions, when it  is heated
 strongly; which contraction is always  greater, the  greater
 the heat is to which the clay is exposed.
   This singular property of fine clay, at first, makes it appear
 an exception to the general law of expansion of bodies by heat,
 and contraction by  cold.  In  reality, however, it is not an
 exception ; for the general rule of expansion by heat, aid con-
 traction  by cold, is  only applicable to bodies which suffer no
 permanent change in their composition or texture, by the ac-
 tion of heat.  But a mass of dry clay  dftes suffer a change in
 its texture  by the  action of a strong heat ; it becomes mure
 compact and hard.   I once supposed that it suffered a change
 in its composition, by  evaporation of its volatile  parts ; but
 Mr. Wedgewood has shewn that this only happens in the be-
 ginning  of ignition.
   The contraction which happens afterwards  must be only
 from a change of texture, a closer arrangement and contactof
 parts, which is permanent, and gives great hardness.  Query,
 how does heat produce this  change of texture ?.  Heat may be
 supposed to produce it, by softening the clay, and thus dispo*
    vol. i.              Mm 
214     WEDGEWOOD'S THERMOMETER.

sing the particles of it to enter, by their attraction, intd-'iclosei
contact.   A mass of chy,  simply dried, or made  simply red
hot, is exceedingly porous,  and imbibes  water remarkably.
The particles of it have not a close contact or connection with
one another.  It may be compared to dry clay in loose powder,
which we know becomes far more compact and dense, by being
softened  with water, and then allowing it to dry again.   (See
note 11, at the end of the volume.)
  In whatever mann< r heat increases the density and hardness
of clay7, this effect of the heat is permanent, and remains after
the  heat  is gone again ; and a mass  of clay, thus changed,', is
never more affected bv any inferior heats in a different manner
from  other bodies.  In these inferior heats, it is expanded^
and by cold contracted, like other bodies.  It is only by ex-
posing it to a  more violent heat than it  has suffered before,
that any further permanent contraction can be produced.
  Of this property, therefore, Mr. Wedgewood takes advan-
tage, in his method of measuring violent heats.
  He forms a chosen, and  very pure clay, into small, short
cylinders, which, by means of instruments properly contrived,
are  made exactly of the  same size.   The cylinders are then
leaked with a low red heat, to expel the air and water, which
are  contained  in raw clay.   Being thus  prepared,  they are
ready for the measurement of strong heats, of which they will
give information in a few minutes.   That time is sufficient for
penetrating them thoroughly with the heat to which they are
exposed ; and if they be then taken out and cooled, they are
always found contracted, more or less, in their dimensions,
according to the intensity of  the heat to which they were ex-
popped.   This contraction being but a very small quantity, the
contrivance of  a micrometer is necessary for subdividing it.
To accomplish this, the clay piece is introduced between two
brass rulers, firmly screwed to  a plate, and  set,  with a very
small inclination, to each other.   Suppose them -r^of an inch
farther asunder at one end than at the other, and that they just
admit the clay cylinder, after its due preparation,  at th« wide 
        WEDGEWOOD'S THERMOMETER.    215

 end:  and  suppose that the clay-piece  has been  in  a
 pottery furnace, and that it is now admitted to the 75th
 division of the rulers (the whole length being divided into
 100 equal parts) :  we learn, by this experiment, that the
cylinder has contracted TV? of ^ of an inch, or has con-
 tracted ^^ of an inch, or ^ of an inch.
   Mr. Wedgewood's peculiar views made him less solici-
 tous about the inferior heats, and he resolved to begin this
 scale at the temperature called a full red heat.   In order
to give numbers to  his scale, which should convey soiree
 familiar ideas, he took  the melting heat of fine gold for
 a fixed temperature, and by continual bisection,  divided it
 into 32 parts, which he made the degrees of his scale.
   By this scale he ascertained the temperature  of several
 chemical processes of  great importance  in the arts and
 manufactures.
   The highest of these  extended to 240°  of his scale of
heat.
   But this  gives  us little information as to the relation
Which those  increments of  temperature  bear to  those
which we have ascertained by means of the common ther-
mometer,  even supposing the temperature  of full red
heat, ( the o of Mr. Wedgewood's scale) to have been
well determined,  which  is a matter of some uncertainty.
We do not know,  from the process now mentioned,  how^
many degrees of  Fahrenheit's scale correspond to one of
Wedgewood's. 
116  CONNECTING  WITH FAHRENHEIT'S,
   Mr.  Wedgewood  connected his      t   s   w
scale  with  iahrenheit's in the follow-
ing manner:  the  heat  which  raised
Fahrenheit's thermometer from 50° to  1913 ---92   64
212°,  expanded a piece  of silver from
0° to 8°,  of a certain scale.   A heat,
which expandedthis piece of silverfrom
0° to c6°of the silver scale, correspond-
ed to 2-1: of the clay or  Wedgewood's
stale, and  92° of the diver scale cor-
responded  to 6^of Wedgewood's si ale.  1386---
1 herefore, let the  lines  F f, S s, W w,
parallel to  each other, represent Fah-
renheit's the silver, and Wedgewood's
scale.   Fahrenheit's scale being divide 0190
ed into its  degrees,  draw through  50
and 212 the horizontal lines,  cutting
S s in 0°, and in 8°.  Dividing 0-8 into
eight equal parts, and carrying forward
those divisions, draw lines through 66
and 92, cutting  the two lines F f  and
W*w.  In Fahrenheit's  scale, these
lines will fall on 1386 and  1913, and on
Wedgewook's scale they  give  21 and   212
6j from which we readily find the place
Oto correspond to 1090 of Fahrenheit.    50
Mr. Wedgewood  means  his scale to
commence with the heat given  to iron     0
by a strong fire, and reckons  this 1077      F    S   W
of Fahrenheit, differing from this  construction about T\ of
his degree, which is nearly 13° of Fahrenheit.   (See  Phil.
Trans, vol. 72.)   But  I am  by no means  certain that the
contractions of clav are proportional to the  heats;  but
although they are not,  this  ingenious contrivance is still
fitted  for  giving  us  very useful  information  concerning
those intense heats.  Such heats certainly induce chemical
changes;  such as tendencies to vitrification,  or even this
contraction observed in the purest clay, which may  effect
the  heat communicated by them to water, which was the 
SCALE OF THERMOMETERS.      217
only other method which promised much accuracy in de-
termining their temperatures.
       SCALE  OF WEDGEWOOD'S  THERMOMETER.
                                     FAHRENHEIT'S.
Extremity of the scale, or highest tempera.
   ture observed,       -        -           240
Greatest heat in an air-furnace eight inches
   square,         -      -        -      -     160
Cast-iron melts,       -        -       »     130
Smith's forge hottest,         -              125
Welding heat  of iron,        -       -        95
                   to                         90
Fine gold  melts,        -                      32
Fine silver,                                   28
Swedish copper,       * ^    -        .       27
Brass,        -       -        .              21
Enamel colours,       -        -               6
Red heat fully visible in day-light,     •>       O
Red heat fully visible in the dark,     -       O
Mercury boils,
Water boils,      -     -                       0
 To these may be added some other temperatures, which
     are interesting to many classes of inquirers :
Hessian crucible melted in an iron furnace at   150
Soft iron nails melted with the bottom of the
   crucible,     -              -        -     154
 (The parts untouched by the nails remained unaltered)
Flint-glass furnace strongest heat,     -     114
Another do. do.        -        -      -       70
Plate-glass strongest heat,         -     -     124
Settling heat,  28 (4717) or                    29
Working do.                                  57
Delft-ware fired, 40 (6277) or          -      41
.Cream-coloured,       ...       86
Stone-ware, or Pots de Gres,        -        102
Worcester china vitrified, at         -         94
Mr. Sprimont's  Chelsea china,       -        105
Derby do.         -        -         -        112
Bow do.      -       -        -        -     121
Bristol china, no  vitrification, at      -        135 
218
INFLAMMATION^.
  Common Chinese do. no vitrification with any heat, but
softened and sunk down at 130°.
  True stone nankeen continues bibulous to the last.
  Dresden is more refractory than common china. Cream-
colored or queen's-ware bears heat  as well as the Dres-
den.
  To melt equal parts of chalk and clay, which Pott says
is a master-piece of art, requires  123.  -
  From a general comparison of all these observations, we
see that the greatest range of observations which have been
made  by means of Fahrenheit's thermometer, does not
exceed ■& of that of the observations made by Mr. Wedge-
wood, and which daily occur in the practice of  the arts.
It is also evident, that by a prosecution of such observa-
tions, much valuable instruction may be obtained.

     SECT.  V.....OF INFLAMMATION.
   THIS is the phenomenon familiarly known by the name
of  Burning,  or Combustion.   It obtains in such bodies
as take fire, or are kindled in a part, by the application of
a very hot body, or otherwise,  and continue to give out
light and heat to other bodies, till  they are burnt all over,
or  their faculty of burning is exhausted, which we express
in common language by saying that they are consumed.
    Seeing that all bodies will not  kindle  and burn, as is
 well known, it may be thought improper to include this
 among the general effects of heat.   But it is very general,
 and happens in many cases where we do not attend to it}
 such as lead, iron, and other metals ;  nay, even in the dia-
 mond.  But my chief reason is, that an acquaintance with
 the distinguishing phenomena of combustion, is indispen-
 sably necessary for our farther progress.
    With these views, I observe, in the first place,  that no
  natural  operations, nor  any experiments, shew  us that
  those substances which do not burn,  either suffer any per-
  manent changes in their principal qualities, as subjects of
  chemical examination, by being thus heated, or afterwards
  give out more heat than they had received.  They readily
  receive heat, and transmit it to others ; but without either 
INFLAMMABLE  SUBSTANCES.      219
 increasing or diminishing the quantity of that heat, so far as
 we can perceive,  or being themselves altered in their princi-
 pal qualities, in consequence of having received it and lost it
 again.   Thus,  water, or any pure  saline or earthy substance,
 readily receives any quantity of heat to which it is exposed.
 It will be expanded, perhaps  may suffer a change from the"*
 state of a solid to that of a fluid, or from the state of a fluid or
 solid to that of vapour.   Or, if it be a fixed substance, it may
 be ignited, and will preserve its new form and appearance as
 long as the heat continues ;  but, if it be placed beside a colder
 body, it will quickly lose its superiority of heat, and, at the
 same  time,  return to its former condition in other respects.
 ""The vapour of water or salt,  returns again to  water  or salt.
 And a salt or earth, after having received so much heat as to
 be ignited, or perhaps melted, returns, in cooling, to the same
 state as before; at least it is not altered in its principal properties.
 It is still a salt,or an earthy substance, while, at the same time,
. the heat which it had received, is transmitted to other  bodies,
 without increase or diminition, that can be distinctly observed.
   But such is the nature of inflammable substances, that they
 are  different from others in these two particulars.  For, if
 they be heated to a proper  degree in  the open air, they not
 only continue hot, but become  hotter; and this, in  some cir-
 cumstances, to  a violent degree of  intensity.   And they con-
 tinue in this state, called the state of Inflammation, for a
 longer or shorter time, during  which they lend out of them,
 into the surrounding matter, a  continued, and often very co-
 pious, stream of  heat and  light, the  whole of which,  it is
 plain, they have not received in the usual manner, or by com-
 munication from other bodies.
   Further, this  flow of heat comes to an end at last, and then
 we find that  the inflammable body  has undergone a most re-
 markable change.   It has lost the qualities by which it  was
 distinguished as an inflammable body.  It cannot now be
 again made a scource of heat, nor is it qualified any7 longer to
 exhibit the phenomena I have  described, but is  on the same
footing with  other uninflammable matter, with respect to
heat. 
220     PRINCIPLE  OF INFLAMMABILITY*.

  Such is, therefore, the general nature of the inflammable
substances,  and  the  distinction  between  them  and  other
bodies.   There  are  some of them which,  to  a careless ob-
server, may seem, in some respects,  exceptions to this gene-
ral account;  which,  during inflammation,  instead of  being
changed  into a new sort of matter, appear to be totally spent
and consumed, so that nothing remains in their place.  Such
is spirit of wine.  Dr. Boerhaave considered as a pure pabu-
lum of fire.   Such also is sulphur.  But the appearances here
are deceitful.   Spirit of wine is changed by  inflammation
into a great quantity  of water....Sulphur into a great quantity
of acid.
  The matters  produced from the other inflammable sub-
stances are very  various.   From phosphorus  we obtain, by
inflammation, a fixed acid;  from others  we obtain water,
earths, salts of different kinds, and aerial matter; and some
afford several of  these substances at  the same time.  This
is the case of the common animal and vegetable inflammables
substances.                                                .
  All the inflammable  substances,  therefore,  are changed,  «
during their inflammation, into one  or more principles, or
kinds of matter, which are no longer inflammable.        ■
  Another particular, which must be attended to in  consi-
dering the general nature of inflammation, and of the inflam-
mable bodies, is;the necessity of air to mflammation.
  I began this subject with saying that the  inflammable sub-
stances  enter into a state of inflammation,  and suffer the
change described, when heated to a certain degree in the open
air.   The being heajted to a certain degree, in the first place,
is a circumstance undoubtedly necessary to the beginning of
inflammation, though the degree or intensity of heat necessary
is  very different, in the different kinds of them.   More or
less heat, however, is necessary in all cases ; and I have cho-
sen, therefore, to introduce this subject of inflammation among
the effects of heat.
   But it is equally necessary that the body be exposed to the
action of air, and even of fresh air, constantly renewed at its
surface.   If we attempt to inflame a body in a space exhaust 
  GENERAL PHENOMENA OF COMBUSTION. 221

  ed of air, we never succeed.   If we even attempt it, in air
  that is confined, the inflammation  soon stops,  although the
 . inflammability of the burning body be not exhausted.  ^The
  air with which it was inclosed undergoes  a change,  which
  renders it unfit any longer to continue the inflammation which
  it promoted at first*.                                      '"
    These are the chief facts relating to diis subject, and which
  have been remarJSed for a long time.  It will be proper, now,
  to attend a little to the opinions which have been formed con-
  cerning the nature and causes of these.phenomena.
    !The opinion which was first formed among  the chemists,
  and was received as just and adequate for a considerable time,
  was this:
*   It was imagined that the quality7 of inflammability depend-
  ed on a principle, or material substance, which they supposed
  to be contained in all the inflammable substance^ and to be
  the same in alfcthe different kinds of them ;  and that the di-
versity among them, in external appearance, and other quali-
  ties, proceeded from the other  principle, or number of princi-
  ples, with which this common principle is combined;  and the
  chemists considered inflammation,  with the several  phe-
  nomena that attend  it, as depeiiBing on a gradual separation  .'
  and dissipation of this common principle, the <p*X«y«rr«v, which
  being once separated, what  remained of the  body ftould no
 longer be an inflammable substance, but must  be similar to
 the other kinds of matter.                       ■>.
    This opinion was  founded chiefly on some facts and expe-
 riments, of which it is proper to give you a  general know-
 ledge.  There are numerous experiments and operations in
 chemistry, whereby  the  uninflammable* matter,, into  which
 some of these  bodies are changed during their inflam nation,
 can be restored again to its former state, and rendered again

  * There are, indeed, a few cases where bodies will bum, and be consumed,
in the closest vessels. This you all know  to happen with gunpowde:-, and
all substances containing saltpetre, and any ordinary combustible.  But
chemistry teaches us that this is no exception, and that the saltpetre is de-
composed by the heat and the combustible body, and yields a continual sup,
ply of the purest air,                                   :jdjtoiw
    vor.. j.                   N n
MA 
 ■222    PRINCIPLE OF INFLAMMABILITY.

 capable of being inflamed.  We have noted examples ol this  *k
 in sulphur and in phosphorus.   Both of these mflammable
 substances are changed,  by inflammation, into acids, the one.
 of which  is called the sulphuric acid, or vitriolic acid, and
 other  the phosphoric acid.  But each of these acids can be
w again  restored to the state of an  inflammable body;  the one
 to the state of sulphur,  the other  to that of phosphorus,  as
  before: and this restoration is effected by mixing them vf'itfy
  charcoal dust, and exposing  this mixture  to  a strong heat.  *
  The charcoal employed, or a part of it, is consumed, and dis-
 appears,  while the acid resumes' the  form of an inflammable
 body.  The  disappearance or diminution of the charcoal  in
 these  operations,  made the chemists imagine that a part of it
 was combined with the acid matter, and brought it back togthe
  state of an inflammable substan^s, by restoring to it some prin-
  ci| le  whic,h .'St had lost  during its own  production from the
  inflammablCjSubstance, by inflammation.    %
     This supposition having been formed, certain variations of
  these experiments were thought to prove that this supposed jM
  priaciple^is the jsame in all the inflammable  bodies; for, in
  restoring the acid of sulphur to  the state of atilphur again, or
  the acid of phosphorus, to til? state of phosphorus, we are not
  limited to the use of any pne kind of charcoal, or indeed any  '
  one kind of combustible matter, if it  be sufficiently  fixed
  to bear the necessary heat, without escaping in volatile fumes.
     Upon these facts, therefore, was founded a persuasion that
  the  inflammable substances  contain a  principle which gives
  them the quality of inflammability,  which they, have in com-
  mon, and which principle the chemists called phlogiston, as-
   serting thal^it is precisely the same in all the species belong-
   ing to this class, and  that it is dissipated from them during
   their inflammation, but  can  be again  restored by  processes
   like those I  have just now described.
      But when we inquire further, and endeavor to learn what
   notion was formed of the nature of this principle, and what
   qualities it was supposed to have in its separate state, we  find
   this  part of the  subject very obscure and unsatisfactory, and
   the opinions very unsettled. 
TERRA  SECUNDA,  OR  PHLOGISTON.  228
     The elder chemists, and the alchemists, considered sul-
  phur as the universal inflammable principle, or at least they
  chose to call  the inflammable part of all bodies, that are
  more or  less inflammable, by the name  of their sulphur,
  which they appear to have considered as a general prhui-
  ple of inflammability.    The  famous  German  chemist,
  Becher, was, 1 believe, the first who' rejected the notion of
  sulphur being the  principle  of inflammability  in bodies.
  He considered sulphur as one of the inflammable substan-
  ces, which contained the principle of inflammabilitv like the
  rest, but which was  not the pure principle of inflammability
  itself.
     Hjs notion of the nature of the pure principle of inflam-
  mability was afterwards more fully explained and supported
  by Professor  Stahl, who, agreeably to  the cfbctrine of
  Becher, represented the  principle of inflammabilitv  as a
  dry7 substance,  or  of an   earthy  nature, the panicles of
  which were exquisitely subtde, and  were much disposed
**to  be agitated, and  set in  motion  with inconceivable velo-
  city.   You have a full account of this explanation in his
  Theoria Chcmi* Dogmatic*, a work of great merit and in-
  genuity, and which was long regarded as the infallible  code
  of chemical science.         *
    The opinion of Becher and Stahl concerning their Terra
  Secunda, or Terra, Inflammables, or Phlogiston,  was, that
  the atoms of it are, more  than all others, disposed to  be
  affected with  an excessively7  swift whirling motion, motus
  vorticillaris.   2The particles of other elementary substances
  are likewise  liable  to be  affected with the  same sort of
  motion,  but not so  liable as,  these o^the  Terra Secunda:
  and, when the particles of any  body are agitated with this
  sort of motion, the  body  exhibits  the phenomena of heat,
  or ignition, or inflammation, according to the violence and
  rapidity of the motion.  This notion of heat, as depending
  on a motion or agitation induced in the particles of bodies,
  was founded on the common observations  and conclusions,
  first inferred by Lord Verulam, from the consideration  of
 the  different  means  by which heat may  be produced or
 excited in bodies.   Becher and Stahl,  therefore, did not
 suppose that heat depended on the abundance of a peculiar 

224   MORE PLAUSIBLE  EXPLANATIONS.

matter, such as the matter of heat or fire is now supposed
to bi, but on a peculiar motion of the particles of matter ;
and that their phlogiston, which was supposed to be con.
tained in all the inflammable  substances, was, most of all
other  matter, disposed  to  assume  this  sort of motion.
V'de Juncker, vol. l. Tab. de Principiis, et Tab. de Instrument     i
tis Chevu*essentialibus.
   This very  crude opinion of the earthy nature of the
principle of  inflammabilitv  appears  to have been deduced  ,   j
from a quality of many of  the inflammable substances, by
which they resist the action of water as a solvent..'•< The
greater number of  the earthy substances are little, or not
at  all, soluble in water ;  and  some time ago they were
reckoned,  when in their pure state,  perfectly insoluble.
And,  as Becher and Stahl found those compounds, which,
they supposed contained phlogiston in the largest quantity,
to  be  insoluble in water, although the  other  matter, with
which the phlogiston was supposed to be united, was, in its
seperate state, exceedingly soluble in that  fluid, they con-*
eluded that a dry nature, an incapability to be combined with  ^^
water, was an eminent quality of their phlogiston ; and this  $r^
was what they meant by calling it an earth, or an earthy
 substance.               -                                .$
    They thought it was most abundant, and  least clogged   *
 with^natter in charcoal, and in soot, both of  which are
 ver\  inflammable,  and insoluble in  water,  and, in some
 measure, shew even a repulsion for that fluid.   The finest
 kind  of  soot  (lamp black)  is  very difficultly wetted with
 water.
    But these authors supposed, at the same time, that the
 particles of this dry and earthy phlogiston were much  dis-
 posed to be excessive-ly agitated with a whirling motion;
 which whirling motion, exerted  in all directions from  the
 bodies in which phlogiston is contained, produced the phe-
 nomena of inflammation.
    This appears to have been the notion formed by Becher
 and by Stahl,  conceding the nature of  the  principle of
  inflammability,  or  the  phlogiston ;  a notion which seems
  the least  entitled to the name of explanation, of any thing
  we  can think of.  I  presume that few persons can form
  any clear conception of this whirling motion, or,  if  they 
OF  COMBUSTION.            225
can, are able to explain to themselves, how it produces, or
can produce,  any  thing like  the phenomena of  htat or
fire.
   Succeeding chemists, however, refined afterwards upon
this notion, and formed an opinion which had much more
appearance of probability.
   This opinion was founded in the phenomena of inflam-
mation, and,  and on some new facts relating to this subject,
which Becher and Stahl had not attended to, or had not the
opportunity of observing.
   When a combustible body  is set on fire, and is  under-
going the change which I have described what appears the
most  obviously, and evidently,  to be separated from it,
and to be thrown off, is heat and  light.   But, from  a great
number of considerations, there is reason to conclude, that
heat and light depend on the motion and action of an incon-
ceivably subtile, penetrating,  and elastic matter, the par-
ticles of which  are  minute  beyond what the utmost efforts
of our  imagination can conceive, and  have a strong re-
pulsion for  one another ;  and thus are  capable  of being
thrown off from bodies, or set in motion, with a velocity
that is astonishing.   It has been proved*, by astronomical
observations, that light comes from the sun to us in 8'. 13".
A cannon  ball, flying with the velocity of 600 feet per
second,  would require more tthan 25 years to go  to the
sa-me distance.  The velocity of light, therefore,  is per-
fectly astonishing, and would enable it to beat into powder,
and disperse  in dust, every object on which it shines, were
not the particles of it so inconceivable  fine and  minute, that
they are thereby deprived of this destructive power. Their
minuteness is  such,  that  some  have demonstrated,  that
200,000,000,000 of them, at least, can pass through a pin-
hole  made in  a card,  crossing one-* another  in every di-
rection, and  in the same moment of time, or at least in a
time so short,  that it appears indivisible.
   This  exquisitely  subtile and active  matter, therefore,
was,  by  the later chemists,  supposed  to be capable  of
being fixed and combined with other matter, so as to-fenter
into the composition of certain bodies ;  and,  in this'com-
bined  and fixed state, gave to such bodies the qualitv  of 
226     MORE  PLAUSABLE  THEORY.

inflammability, and was separated from them during their
inflammation.
   This is the  opinion I  had of  it formerly,  before the
existence  of  a phlogiston, or principle of  inflammability,
began to be doubted.  Mr. Macquer, and other chemists
also formed the same opinion.
   One principal foundation of this opinion was a number
of experiments and observations, from which it appears that
certain bodies, not  inflammable, are  changed into inflam-
mable bodies, by simply exposing  them to  the light of the
sun ;  or, if they be not actually changed into inflammable
bodies, they  suffer the same changes  as those they suffer,
when we apply to them other inflammable substances, with
the intention  to communicate to them  the quality of inflanw
inability, or other qualities which  are connected with it.
   We have an example of this, in the production of the
green matter of the leaves  of  plants.  It  has  long been
known, that  the green colour of the  leaves of  vegetables
is produced by the  lighjt of the sun.   The  most direct ex.
periments  to ascertjjn this fact,  were made  first by Mr.  .
Dufay,  and  others of the French academicians ; and the
investigation  has been since carried farther by Mr. Senne-
bier of Geneva, and others.  When seeds are sown in a
dark place, the plant grows luxuriantly, but with a foliage
extremely different  from what is common to it, and entirely
without  the  green  colour.   This is the  case, although Jt
plenty of fresh air be admitted to  them.   When the plant ^
is in this state, if  the light be admitted, it immediately
begins to acquire a green  colour, which in time arrives at
its ordinary  fulness.   What is  farther remarkable in this
experiment is, that the plant directs its growth to the light,
although  the . jjiole by which this  is admitted be closed up
by a piece of glass. *-
   Professor Robinson informed  me, that he  saw tansy
growing  in a coal-pit, with a very abundant foliage,  but
white, and without smell ; and that, when the clod was
brought  above ground,  this  subterranean growth died
down, and was succeeded by fresh leaves of  a full  green,
and strong aromatic smell.   Hence he concluded, that the
sun's  rays not only produce the green faecula,  but  also 
          STRONG OBJECTION TO IT.        22r

 the essential oil of plants.  Hence we may account for the
 deep green, and the great abundance of  aromatic plants,
 shrubs, and trees, in the warm climates.   (See note 12
 at the end of the volume. J  Upon this depends the art of
 blanching endive, celery, and manyother garden plants, for
 theuseof the table, in order to have them tender and sweet,
 and free from any strong biting taste.   We bind up their
 leaves as they sprout, keeping them from the light.  Some
 esculent vegetables produce such a tuft of leaves, and these
 so broad and compacted, that the  external leaves alone get
 the benefit of the light, and are colored green by it.  Such
 vegetables, therefore,  scarcely require the aid of  art to
 blanch them.   Such are some kinds of lettuce and cabbage.
 It is  quite  plain, therefore, that the green colour of their
 leaves is  produced by light.  A long time ago, I had the
 curiosity  to examine  this green matter, to learn the nature
 oi it.    It separates from the rest of the matter of the plant,
 when that is mashed and macerated with water.  It may
 be completely taken up by spirit of wine.  This fluid dis
 solves the green faecula completely, and may afterwards be
 expelled from it by heat. *It is highly inflammable, and
 burns completely,  without leaving any sensible quantity of
 ashes.                                      ^      }
   Thus have I briefly stated the opinions concerning  the
 nature of inflammation, and of the  inflammable  bodies
 which had  been  formed  among the chemists until about
 fifteen or twenty years ago.   Before that time, however, a
 fact had been observed with surprise that was very incon-
 sistent  with this opinion, viz.   That some of the inflam-
 mable bodies, when  changed  by inflammation  into unin-
 flammable matter, afford a quantity of this matter of greater
 weight than the inflammable body itself.  About the period
 I just now  mentioned, or since that period,  thisTact has
 been  better ascertained,  and discovered to  be true with
 respect  to all the inflammable  bodies, and in all cases of
 inflammation.  The reason why  the chemists wefe so late
 in discovering this fact, with respect to a great number of
these  cases, was, that the uninflammable matter produced
is, in  many  of the experiments,  a volatile  substance, and
reaped during the examination.    But, ingenious methods 
228   SPECULATIONS  OF PHILOSOPHERS.

having been devised for collecting and weighing the whole
of this volatile matter, as well as the rest of the uninflam-
mable substances into which bodies are changed during the
inflammation, it has been found,  that in general, there is
more of this uninflammable matter than there was  of  the
body before  it was  inflated.  From an ounce of  phos-
phorus, for example, we may perhaps get an ounce and a
a half, or more, of the solid acid  salt,  into which it is
changed by being inflamed.
   This appeared to many  an insuperable objection to  the
theorv of the chemists ;  and, though some  attempts were
made to obviate  this objection, they rested on suppositions
so very subtile and difficulty comprehended, perhaps absurd,
that they were quite unsuccessful.
   Another theory, therefore, which I may say is the oppo-
site or contrary to this, has lately  been formed, and is fast
gaining ground,  especially on the continent.   I cannot  ex-
plain to you at present the whole of this theory, or trace
the steps by7 which it has been brought to its present state.
We  shall have better opportunities  for doing this in other
parts of the  course, as you will have by that time become
well acquainted  with several  substances, whose properties
have a great influence in producing the phenomena which
furnish  the  chief arguments  in support of the theory.   I
can  only remark just now, that  it is  founded  on  a very
great number of experiments and discoveries, which have
been made within these twenty vears (1789) on the  nature
and qualities of atmospherical air, and of a number of other
elastic fluids.
   Ingenious men in different parts of  Europe,  incited by
the  remarkable properties which I had discovered  in  one
of those fluids in 1756, began to employ themselves in such
experiments, and they have  made  many astonishing and
most interesting discoveries.  The fundamental  experi-
ments were first made,  and the  leading inferences were
first drawn in this  country, by Dr. Priestley,  the Honor-
able Mr. Cavendish, and my  friend Mr. Watt.   But it was
chiefly  in France  that they were repeated, with  proper
attention to all the  circumstances that would affect  the
result, and this result was  maue the foundation of  a new 
OF MR. LAVOISIER.          229
   theory  of combustion. '  These experiments have been
   made  there with extraordinary ingenuity  and accuracy,
   and on  so large  a scale, that the effecis'"produced were
   the  more clearly and fully  perceived.   The unfortunate
   Lavoi-d r, who fella sacrifice to the ambition of his phil-
   osophical associates, and whosjs unjust and cruel fate, and
   the  loss which science has suffered  by it, cannot be too
  "much deplored, was the most active in this inquiry, spar-
   ing neither  labour nor expence.   His own exertions  dis-
   play the greatest ingenuity.   He also employed the talents
   of other eminent philosophers in ascertaining some of the
   most important and interesting facts ;  but Lavoisier him* v
   self was the principal author of the new theory.
      It had  long  before been known, as I have already in-
   formed  you, that air, and even  a constant supply of fresh
   air,  is necessary to inflammation; that if a burning body
   be  shut up  with a certain (moderate quantity only of air,
   the  inflammation stops after some time,  although the in-
   flamjnability of the body be not  exhausted.   The inclosed
   air has  undergone a change which makes it totally unfit
   for continuing the inflammation.  In some  of the experi-
   ments made by Dr. Hales, and other  persons, it had  also
   appeared, that the air is generally diminished on this oc-
   casion,  or, in the language of Dr. Hales, a part of it was
   absorbed.
     But this has been ascertained with much more-preci-'
   sion, and  in a much greater number of cases, by the later
   experiments.   It has become clear and evident, by many
'   of these,  that a considerable quantity of  the air is really
   absorbed,  and combined with the matter of the burning
   body, so  as to  form, in many  cases, a dense compound,
   in which the air so absorbed is totally deprived of its usual
   for'm of  an elastic fluid: and the additional  weight which
   the  matter of the burning body  acquires, has been found
   to correspond exactly to the weight of the air which has
   been absorbed by it.
     Farther,  the  two  very different combinations of' heat,
   which I  had discovered, in which it produces fluidity and
   vapour,  encouraged Mr. Lavoisier to presume that there
      vol.  i.                 o o 
2J0       MR. LAVOISIER'S THEORY.

was  another combination,  which  produced a permanently
elastic fluid, not decompoundable, like a liquid, or a va-
pour, by  the touch of a colder body*;  and the different
capacities for heat, having been already discovered, it ap-
peared  no difficulty to account for the vast quantities set
loose in  combustion.  This  new  theory,  therefore,  is
founded on the doctrine of latent heat, and is, indeed, an
extension of it.
   For, in the  remarks I lately made to  you on the pro-
duction of  vapour from bodies  by the action of heat, I
made it evident, that vapour contains a very great quantity
of heat, beside that which we  perceive in it by the simple
application of the thermometer ; and that this heat, which
appears to be a necessary ingredient in the composition of
the vapour, comes out of it  again when  it is condensed.
In Mr. Lavoisier's theory, the same thing is  supposed to
happen during the action of air and'burning bodies on one
another.   The matter of those bodies attracts the matter,
or basis of the  air, or rather of a part of the air, which is
different from the rest, and with which I shall make you
hereafter better acquainted.   The burning body collects,
and  condenses this sort of air, depriving it at once of its
vaporous and elastic  aerial form; or, if it does not abso-
lutely  condense it to a solid  matter,  it diminishes, to a
great degree, its capacity for heat.   All, or  the greater
part, of that great quantity of  heat, therefore, which was
combined with the matter of this air, and thermometrically
latent in it,  is suddenly let loose, to diffuse itself among
the surrounding bodies: and there is a continued emission
of it, in this manner, so long as the air and burning  body
continue to act on one another, or until that portion of
the air, which is capable of being thus attracted, and  com-
pounded with the burning body, is totally expended.   This
theory, therefore,  is directly  contrary to the  theory the
chemists had formed before.   (See note  13, at the end of
the volume. J

  * This is not strictly true,  for the oxygenated muriatic acid gas is con-
densed by a cold....32o of Fahrenheit.
EDITOR. 
            ELAME  INFLAMMATION.        231

  According to this theory, the inflammable bodies, sulphur,
for example, or phosphorus,  are simple  substances.   The
acid into which they are changed by inflammation is a com-
pound.   The  chemists, on the contrary, considered the  in-
flammable bodies as compounds, and the uninflammable mat-
ter as more simple.   In the  common theory, the heat and
light are supposed to emanate from, or to be furnished  by,
the burning body7.  But, in Mr.  Lavoisier's theory, both  are
held to be furnished by the air, of which they are held to be
constituent parts, or ingredients, while in its state of fire-sup-
porting air.
   Such are the outlines of this celebrated theory of combus-
tion.   I  cannot enter on it in greater detail,  in this part of
the course, but this brief sketch was necessary for the under-
standing what follows concerning the management of fuel in
our furnaces, and for some other purposes.   The full conside-
ration  of it will be given, when those fuels are considered as
subjects  of chemical  examination, by  the help of doctrines
depending  on the chemical  nature  of other substances, of
which the treatment is more elementary.
   After  these observations on the general  process and phe-
nomena of combustion I must explain to you some appearances
which, though not universal, yet obtain to a very great extent,
and occasion important differences in the phenomena, and in
the employment of combustion, as  an instrument of chenu
istry.
   Flame,  and that  particular form of  combustion which is
properly  called  Inflammation7,  requires some  particular
consideration.  With this view, therefore, I  must first  re-
mark that different inflammable substances have very different
degrees of  volatility.  Some cannot be converted into vapour
with any degree of heat which we can command.   These do
not suffer any loss from  heat,  if we preserve  them from  the
action of the air; and when the air is admitted, their combus-
tion  is attended  with  a  gradual consumption of their surface
only.   But many others are more or less volatile, and many
so volatile,  that they begin to  be converted into vapour before
we can heat them to the degree necessary for their inflamma-
tion.  It is evident that we cannot  inflame these,  except in 
.232   FLAME  PERFECTLY TRANSPARENT.

the form of vapour;.....they are more volatile than inflamma-
ble.   The  usual practice, therefore, is to heat them, or a
small part of them, until they emit vapour, and then,  at the
same time, to set that  vapour on fire.  Thus we produce
Flame, the heat of which occasions more vapour to rise, and
to be inflamed, in a constant succession, until the whole mass
has undergone inflammation.   This is illustrated by examin-
ing attentively the flame of spirits of wine, burning in a small
vessel or di-h, l£ or 2 inches diameter.   (See note 14, at the
end of the volume.)
  Thafthe flame, even of oily,  substances, as oil, or tallow, is
truly a transparent vapour, which appears otherwise in a dark
place, in consequence of its emitting  heat and light, becomes
evident, when we place  a candle in the  sunshine.  The light
of it being too weak to be perceived in the strong light of the
sun, the flame is not seen.   'We see objects placed beyond it,
as well as if it were not interposed ; and when we observe the
shadow of it on a wall, we see no shadow but that of the wick;
the ravs of the sun pass through the flame, and  reach the wall
without interruption.  They are only a little refracted, bent
from their  straight course, while they pass through  the rare
vapour of which the flame is composed, or through  the air
heated bv the flame ;' and therefore the  shadow of the candle,
when" thrown -on a white wall, shews an appearance of thin
smoke arising from the  wick.

             GENERAL REFLECTIONS.

   I  have now finished the account I proposed to give of the
more general effects of heat; in communicating which, I have
followed the plan which  appears to me the most proper to lead
to improvements in  our knowledge  of this subject, and by
following  which,  many improvements  have  actually been
made.
   One part of  the subject, however, may appear to you to
have been omitted;  and it is a part upon  which Dr. Boer-
haave has  bes'owed  a great deal of  labour and study.  The
part I now refer to is the different means  by which heat may 
             GENERAL REFLECTIONS.        233

be produced, or increased, and even diminished also;  or the
different means for producing heat, or cold.                ,
  But although  I  have not attempted, as Dr. Boerhaave has
done,  to bring together all the facts belonging  to this part  of
the subject,  I have taken occasion to mention  the  greater
number, and the  principal of  those facts.   To have treated
them as minutely as  that authcr, would have taken  up too
much of our time  at present;  and we shall  have a better op-
portunity hereafter to notice those parts of it  which remain
untouched.   But,  from  the  progress we have  already  made
in studying the gt mral effects of heat, I am persuaded you
will view the subject as one of the most curious and  interest-
ing parts of the study of nature.
  There is, perhaps, no inquiry more worthy of the attention
of the philosopher than the nature of heat, and  the manner in
which  matter in general, and the different kinds of it, are af-
fected by this wonderful agent.  Its influence lis manifest   so
universal,  and its action so important and necessary io the
progress of all the operations of nature, that>  tcPthose Ijftp
consider it with some attention, it will appear to be'the gene-
ral material principle of all motion, activity, and life, in this
globe.  Heat is inseparably necessary to the very existence of
vegetables  and animals.  Without heat, plants cannot live*
Without it, they want the power to attract their nourishment,
or to set it in motion through their system,  or to refine and
ripen it in their different parts.   Their vigour and life depend
on its influence.   It is only when enlivened by7  heat, that they
"make it  assume the various forms and qualities which  we
find in the  wood, the root, the leaves, the juices, the fruit,
the seeds ;  and the beautiful  forms and colours displayed  in
"the flowers.   They decay and  die  when heat departs.  Nor
is animal life  less  immediately dependent on heat, for sup-
port, than vegetable.   Heat is the main spring in the corporeal
part of an animal, without which  all motion and life would
instantly stop.  There are few  facts more unaccountable than
the effect of heat on an egg, though there are few to which we
pay so  little attention.   We see a lump of apparently dead
matter,  which, left to  itself,  would  continue  dead and
inactive for ever.  By  the  application of a  gentle degree 
284          GENERAL  REFLECTIONS
 of heat, it soon has an animal formed within it, which quick-
 ly increases in size and perfection, until it breaks open its in*
 closure, perfect in all its parts, and ready to perform its prou
 per functions.
   The effect of the heat, upon this  occasion, does not depend
 upon the particular manner in which it is produced or applied.
 In most cases,  it is  supplied  by the mother;  but  it is  well
 known, that in  a very great number, no heat is necessary but
 that of the sun : and art has succeeded in employing the heat
 of fuel, and that produced by masses of fermenting or putrefy-
ing matter.
   In the incubation" of an egg, therefore, we see an example
of an animal brought into existence by heat, operating as a
circumstance necessary or  indispensable  to its production.
 But, after the animal is thus brought into existence, heat is
still  necessary7 for its support.   If heat be diminished to a
 certain degree, although no visible damage be produced, all
motion  and life  are  quickly  extinguished.  The  animal  is
seized with a sleep and insensibility, under which  it expires.
 This is well known in the countries where the colds of winter
are violent; and the inhabitants are upon their guard to shake
off the first appearance of drowsiness, produced by excessive
 cold. There are, it is true, some kinds of animals, as swallows,
bats, frogs, &c to which this sort of sleep is not fatal; that is,
it  does not destroy the seeds of life so effectually «s to take
away the  possibility of its returning.   But even  this sleep, f"^**
 when it continues, is attended with  all the symptoms of death,
 except putrefaction,  and nothing but the return of heat can
 restore to the animal its life and activity.
   Thus, therefore, we see that the whole of the vegetable
 creation, and all the varieties of  animal life, depend on heat
 for production and support, or cannot be produced and sup-
 ported without  it.  How wonderful and extensive,  therefore,
 in its operation and influence, as  confined within limits set to
 it  by the Author of  nature, and  as subject to  the gentle and
 regular vicissitudes of day and night, summer and winter !
   It is plain, that not only all animal and vegetable, life, but
that the whole face and appearance of nature, the very form and 
ON THE IMPORTANCE  OF HEAT.     *35
 powers of the elements  themselves,  depend on this limited
 action of heat.  There are none of the elementary bodies with
 which we are better acquainted than water.   Let us attend a
 little to the powers and qualities  by which it acts its part in
 this system of beings.  We all admire its pure  transparency
 in a spring;  the  level and  polished surface with which  it
 reflects objects that are on the banks of a lake ;  the mobility
 with which it runs along the channel of a brook, and the inces-
 sant motion of its  waves in a  stormy sea.  But, when viewed
 with a philosophical eye, it appears much more an object of
 admiration.  Th J same  water, which, under its usual form,
 is such a principal beauty in the scene of nature,  is employed
 in her most extensive operations, and is necessary to the for-
 mation of all her productions.   It penetrates the interior part*
 of the earth, and appears to assist in the production of various
 minerals,  stones,  and earths, found there, by bringing their
»different ingredients  together, and applying them to  one
 another properly, that they may concrete.  We know it arises
 in vapours from  the surface of the ocean, to  form the clouds,
 and to descend again in rain upon the dry land, and give origin
 to springs, rivers,  and lakes; or, upon proper occasions, to
 form deep snow, which  protects  the  ground and vegetables
 from the intense and mortal cold to which some parts of the
 world are exposed ;  and, after it has performed this useful
 office, it readily yields to the heat of summer, and returns to a
state in which it serves the same purposes  as rain.  By its
 fluidity and tenuity,  it penetrates the soil, and  the seeds of
 plants which that soil contains.  These it causes  to swell and
 germinate into plants, which depend on water for  support.  It
 passes with freedom and ease  through all their minutest tubes
 and vessels, and carries with it materials necessary for nour-
 ishment and  growth, or changes its appearance  so as to be-
 come part of  the  plant.   There is no plant or vegetable sub-
stance, that does not contain in  its composition a large quan-
tity of water, easily separable from  it.  The hardest woods
contain a great deal.   The softer and more succulent parts
of vegetables are  almost totally composed of it.  Even the oils
and resinous substances can be resolved in parti into water. It 
 236  GENERAL REFLECTIONS  ON  THE, &c

 is plainly  as necesaarv to the  animals,  and is found to  be as
 copious an ingredient in the composition of their bodies, and
 of all the different parts of them.         ,
   These  are the numerous and extensive uses of this beautiful
 element.   But, in this succession of  forms  and  operations
 whivh it undergoes, you will perceive that it is set  in motion,
 and  adapted to these ends, by the nice adjustment  and gentle
 vicissitudes of heat and cold,  which attend the returns of day
 and  uight, and summer and winter;  and that even the form,
 under which it and the other elements play their parts, depends
 on the limited action of heat.   Were our heat to  be  dimin-
 ished, and to  continue diminished, to  a degree not very far
below the ordinary temperature, the  water  would  lose its
fluidity, and  assume the form  of a  solid'hard body, totally
unfit for  the numerous purposes which it serves at  present.
And, if the diminution of heat were to go still farther, the air
itself would lose its elasticity, and would be frozen to a solid
useless matter like the water; and thus all nature would become
a lifeless,  silent, and dismal ruin.   Such being the  important
part  allotted to water, in the magnificent serjes of natural ope-
rations, in consequence of the qualities communicated to it by
heat, all its properties become interesting objects of contem-
plation to  a sensible heart.  That  peculiarity, by  which the
expansion and contraction of water by heat is distinguished
from the same effect on other substances, I mean its irregu-
larity between  32° and 40° of Fahrenheit, naturally attracts
attention.. Even  this  seemingly  trifling distinction   has
been shewn by Count  Rumford to have  a mighty effect in
rendering our habitation more comfortable.
   On the  other hand, were the heat which at present cherishes
and enlivens this globe, allowed to increase beyond the bounds
at present prescribed to it; beside the destruction of all animal
and vegetable life, which would  be the immediate and inevit-
able  consequence, the water would  lose its present  form, and
assume that of an elastic vapour like air; the solid parts of the
globe would  be melted and confounded together,  or mixed
with the air and water in smoke and vapour ; and nature would
return to the original chaos. 
MIXTURE.                     23*
  Such are the objects and reflections which present them-
selves to our minds, when we contemplate the general effects
of heat in the universe : and with these I now finish the first
section of the first part of our course.
                 PART II.

         -     ^                  ' ■

GENERAL EFFECTS OF MIXTURE.
      INFORMED as you now are of the general effects of
heat, your minds will be greatly assisted in your  endeavors
to form distinct conceptions of the results of the mixture of
different substances.  This, you know, is the other great mean
of accomplishing the chemical changes in nature or art.
   Upon this subject we have not so many general facts to
communicate as on the subject of heat. The effects of mixture
are so extremefy diversified,  that it is more  difficult to find
such as occur in a great number  of  cases, or which can be
considered as general effects.   Yet there  are some of this
kind ;  and it is proper to take notice of them here, that we
may have an opportunity to become acquainted with the bond
of chemical  union, which is the  characterestic of our science,
and from our notions of which are principally derived all our
attempts to explain the chemical phenomena of nature, or to
reason concerning them.
   It is therefore chiefly with a view to prepare you for under-
standing the principles by  which many of  the chemical phe,
nomena are considered as explained, that I am now to lay
before you the more general facts relating to mixture ; and
they are these:
   tol. i.               ?P 
238 EXAMPLES OF MORE GENERAL EFFECTS.

  1. In the first place, we have learned, by  innumerable ex-
periments with the various kinds of bodies, that, though some,
when mixed together,  or applied to one another with a view
to mixture, cannot be made to unite ; as oil with water, water
with mercury, sand with  water ;....others readily unite in the
most intimate manner, and form compounds, which have, how-
ever, all the appearance of being simple and homogeneous, or
in which the eye cannot discover, even with a microscope, the
smallest signs of any diversity in the constituent parts.
  2.  In some of these cases again,  this intimate union  is ef-
fected slowly and quietly, and is not  accompanied by any other
phenomena that are very remarkable ;  as, when pure common
salt  is dissolved  in water into a clear brine ; when sugar is
dissolved into a syrup, or when vinous spirits are diluted in
water.  But, in other cases, the subjects unite with an appear-
ance of violence,  or with rapidity, commotion, and the produc-
tion of heat.  Of these cases you will have more  distinct
knowledge by some examples.
   First Example.  If equal measures of vitriolic acid and
water (as  eight  ounces) be poured into a Florence flask, we
observe that the  intimate union is  performed with rapidity,
commotion, and  the production of heat.   The heat produced
from the sudden mixture of this quantity of strong acid and
watery is so great, that it will split the glass vessel, unless ex-
tremely thin.
   Second* Example.  Another  instance may be taken, where
 the appearance of violence and commotion is still  greater,
 though the heat produced is not so considerable.  Put into a
 tall glass a small quantity (suppose four ounces) of the com-
 mon spirit of hartshorn, called by the chemists mild volatile
 alkali, united with wate*, and  pour into it moderately strong
 vitriolic acid,  by small quantities  at a time: the liquor in-
 stantly boils up, foaming and frothing, and sputtering up many
 small drops. This turbulent motion ceases after a litde while,
 but is renewed by the next addition of acid.   After a few such
 additions,  the foaming  becomes  less remarkable, but will
 increase by agitating the vessel, or by stirring the mixture.
 After some time, no more of it appears.  We have now pro- 
     EFFERVESCENCE....INTIMATE UNION.  239

 duced a mixture perfectly transparent and uniform in all its
 parts.
   •The appearances of turbulent union, and most violent action
 of the substances on each other, are still more remarkable in
 some cases ; and even smoke and flame sometimes burst out
 with violent explosion.
   I may take  this opportunity to remark,  that this sort of
 violent and  turbulent intestine motion, and foaming, which
 accompany theunion of different bodies, is called EffeH-Ves-
 cence, among the chemists, and that the bodies are said to
 effervesce with one another.
   3. These examples are cases of the union of fluids  with one
 another.   There  are many others, which, at the  first view,
 may perhaps appear more surprising, in which even solid, and
 sometimes very hard bodies, when thrown into certain liquids,
 begin immediately to  be  torn  asunder, with  more or less
 rapidity and violence,  into atoms so  minute as to  become
 entirely imperceptible to our senses ; in which state of ex-
 quisitely  subtile division, they  are dispersed through evtry
 part of the fluid, and compose, with it, an apparently homoge-
 neous liquor.   They form, with the fluid, a liquor which is
 not  only7 quite  transparent, and, in  appearance, pure and
 simple at first,  but which,  though kept for ages, will remain
 so, without depositing the smallest sediment, or shewing any
 other signs of its being compounded.  This happens, too, in
 many cases, although the solid be composed  of much  heavier
 matter than the fluid, and  sink  in it, at first,  as  a stone in
 water.  Notwidistanding its superior weight, it is soon com-
 bined  with the fluid,  in the manner I just  now mentioned,
 and the particles of it never afterwards separate  or  subside
 to the bottom, but will remain  for**ever equally dispersed
 through every part of the fluid,  and  closely and intimately
 united with it.
   This happens to different solid bodies, with more  or  less
 rapidity, and appearance of violence.   As examples, there-
 fore, of such combinations, I shall state one case, in which the
 solid is divided and dispersed quietly, and  one in which its
division is accompanied with appearances of violent action. 
240   SOLIDS  DISPOSED TO  UNITE WITH  FL.

We shall  afterwards, as we go on,  have occasion to see ex-
amples of this kind in great abundance.  The first example is
the solution of camphor in spirit of wine.  In^ this mixture,
nothing can be observed but the slow and gradual diminution
of the camphor, and the sensible diffusion of its pungent and
hot taste in every part of the liquor.  The second is the solu-
tion of  a bit of marble in muriatic acid,  or spirit of salt.  In
this experiment, we find that the bit of marble  is no sooner
dropped into the liquor, than the particles of it begin to be se-
parated one from another, and  to  be dispersed in invisible
atoms through the fluid  so that in a  short time  the marble
totally disappears ;  and, while this  goes on, there  is an effer-
vescence, or appearance of  constant agitation,  and turbulent
commotion, all round the bit of stone, and innumerable bubbles
arise from it,  as if the fluid  were boiling.  The  same effer-
vescence is seen here, though less violent, as that which ap-
peared  in the mixture of the vitriolic acid and volatile alkali.
   Besides these symptoms of violence,  with which certain
bodies unite together, it appears,  from other particulars, that
some of them have a very strong propensity to unite. They will
unite under circumstances very unfavorable or when the union
might be  thought  to be attended with considerable difficulty.
Thus,  some substances will collect  the water that  is in the air
in the form of invisible vapour,  and will be dissolved by it,
although  the air appear very dry, and contain but little of such
humidity,  so that other bodies  containing humidity,  would
 quickly be  dried in such air.  The vitriolic acid which we
 made use of is one example of  this.  There are other salts
 which  can be reduced to a dry state,  and are usually kept in
 that form, but which it is necessary to keep in close vessels,
 shut up from communication with  the air ;  as salt of  tartar,
 he.   This phenomenon is called Deliquescence, or running
 per DELiquiUM ;  and  the salts which have this disposition
 are called Deliquescent salts.
    To  return again to our experiments.   We may take notice,
  that when a solid is thus attacked by a fluid, thus divided and
  dispersed through  it  so  completely and  intimately  as to
  become  perfectly  fluid along  with it,  the chemical  term for
  this change of condition is Solution.   A  solid bodv, te 
DELIQUESCENCE, SOLUTION, MENST. &c. 241

which this happened, is said to  be dissolved, and the fluid
which  has this  effect  upon  it is  called the  Solvent,  or
Menstruum.  This last term is said to have  been invented
by some of the alchemists as the name of some very powerful
solvent,  which  they made a secret, and which required a
month to produce its effect.
  I wish you here to attend carefully to this sort of combina-
tion of solids with fluids, and to the characters by which it is
distinguished; that vou mav not be misled by inaccuracies in
common language, in which the term solution, or dissolution,
is often misapplied.
  It is not uncommon to say,  " That clay dissolves, or can be
" dissolved,  in  water;" because, when put into water, it U
first softened, and afterwards reduced to a mud so fine, that,
by stirring it well,  it can be equally dispersed through  the
whole'of the water.  But, in  this case,  the clay is not intim-
ately combined with the water, or reduced along with it to  the
form of a fluid.   The particles of  it are only separated from
one another, and dispersed.   There is no strong and intimate
combination, or  change of the clay to the form  of a fluid, and
incorporation with the  water.    The mixture has not transpa-
rancy;  it is quite muddy, and,  if we allow it to stand at rest
for some time,  the whole of the clay subsides again to  the
bottom.  The proper name for this kind of mixture,  in che-
mical  language, is Diffusion.   We say, that the  clay, is
Diffused through the water.
  But,  in the case of solution,  the solid is incorporated with
the fluid, and the mixture is always transparent.  The par-
ticles of the solid body become invisible, and provided we do
not afterwards make a change  in the mixture, by adding some-
thing else to it, or abstracting some of its parts, the soluble mat-
ter  which it  contains will never afterwards subside from the
fluid, but will remain for ever intimately combined with it.
  From these examples, you may form some idea of  the  in-
numerable cases of union of  bodies, with various  degrees of
the phenomena I have  described and shewn you.
  But farther: there are some  other particulars remarkable
in the greatest number of such cases, which may therefore  be 
 242 SOLUTIONS  ARE TRANSPARENT....SATU.

 also enumerated among the more general phenomena of mix.
 ture.   Thus.
  4. We find that in most cases we cannot combine such bodiqg
togetherin every proportion that we might choose. There isalimit
to the quantity of the one that can be combined with the other.
 Thus, waterwill dissolve only a certain quantity of salt; spirit of
 wine will dissolve only a certain quantity of camphor; and in.a
great number of the cases there is even a limit upon both sides to
the quantity of each that can be joined with the others and the
bodies can  be  properly united in one proportion only.  This
limitation to the quantity of a substance which can  be joined
chemically  with  a certain quantity of another, is denoted, in
chemical language, by the  term saturation?  Substances
which are added to one another, or mixed together exactly7, in
the  largest  proportion in which they will unite are said to be
mutually saturated with one  another.  Of such subs>ance*
we shall have examples in the salts.
  Saturation again is of two kinds,  First,  simple, as in the
case of common salt dissolved in water.  Second, reciprocal,
where neither of the  bodies can be combined with  the other
except in one proportion, as in the case of a saline crystal.
  5.  A distinguishing mark of chemical union is, that the
substances cannot be separated by filtration, or other  mechani-
cal means.   It is even difficult, and, in some cases, impossible
to separate them by7 the action of heat.
  A volatile substance, when thus united to another which is
more fixed, cannot be separated again so easily by  heat ap-
plied, as we might be led to expect from our knowledge of
their difference in point of volatility7.   When we apply heat
to the compound, the  more volatile ingredient is not changed
into vapour by that degree of heat which we  know to be suffi-
cient to produce this  effect in its separate state.  In some of
the  cases in which the volatile body is combined with a very
fixed one,  no-heat that we can easily command   is strong
enough to  change  it  into vapour.   In others, though it may
be forced to assume the form of vapour, it always requires a
heat much stronger  than that  which is sufficient to produce
 this change in its separate state; and in many such cases,
 when the heat is raised so high, the volatile body does not rise 
     IMPERFECT SEPARATION BY  HEAT.   243

 alone; the more fixed body is raised along with  it,  and the
 eompound therefore is converted into vapour,  without being
 separated into its constituent parts, though the  more fixed in-
 gredient would not have been converted into vapour by7 itself
 in the same heat.  Several of the experiments which have just
 been exhibited  are  examples  of what we are now  saying.
 Thus, in the case of water and vitriolic acid, we shall be dis-
 appointed if we  should expect that by heating this  mixture to
 212°, the water  will quit the acid.   In  the first place, it will
 not boil  till considerably hotter,  that is, the volatility of the
 water is diminished.  But farther, the  water carries off with
 it a great part of the acid.  Again,  in the example of volatile
 alkali, and vitriolic acid;  this mixture will not separate at all
 by  heat.  It has nothing of that pungent smell which indicated
 the volatility of the salt of hartshorn.  And, when we increase
 the heat a good deal, the whole mixture rises undecomposed,
 and the vapour condenses  into a  solid salt.  Lastly, muriatic
"acid, and crude calcareous earth: this acid has such a great
 degree of volatility, that we cannot obtain it in a coherent, or
 a fluid form ; and, when pure, it is  always  a vapour:  Yet,
 when combined with the calcareous earth, it stands a red heat,
 long continued, without appearance of separation.
   It is, therefore, impossible, in the greatest number of these
 cases, to separate from one another, by heat alone,  the dif-
 ferent substances which have been thus combined.
   6. But, by multiplying our experiments in the way  of mix-
 ture, a discovery has been made, which has  been of infinite
 use to us in  chemistry, and has  greatly enlarged  our power
 over a great number of different compounds.   The discovery
 I mean is, that the addition of some suitable third body to a
 compound of two ingredients, which are united strongly to-
 gether by chemical combination, will, in many cases,  dispose
 them to  separate from one another.  The examples of  this
 are very numerous; but as our  purpose at  present  is only
 to form a general idea of these facts, I shall only call your at-
 tention to a few  examples, which will be sufficient to serve this
 purpose. 
244   SEPARATION BY  A THIRD SUBSTANCE.

  Example 1st. If we pour oleum tartari (which is the liquor
into which pearl ashes melt when long exposed to the  air)
into a quantity of vitriolic acid, mixed with water, .we shall
have a violent effervescence,  and we  shall see a quantity of
crystalline salt fall to the bottom.  If we now taste the liquor,
we shall find its  acidity greatly diminished,  if not  entirely
gone.   Thus have we procured a separation of the vitriolic
acid from the water.
   2d. The salt which is produced by the mixture of vitriolic
acid and the salt of hartshoJfO, is called vitriolic  ammoniac.
It has no smell.  If this be dissolved in  water, and we pour
into  it an equal quantity  of  soap-maker's ley, called by the
chemists caustic alkali, we shall instantly be struck by the most
pungent smell of the salt of hartshorn, greatly exceeding the
usual pungency of that salt.   At the same time, a quantity
of crystalline salt will fall to the bottom, without smell, and
having  very little taste, in comparison of the vitriolic ammo-
niac.
   3d. If, to the solution of camphor  in spirit of wine, which
I described a little ago, we add a quantity of water, we shall
instantly produce snow-white clouds in the liquor;  and, al«
lowing  the mixture to settle, we shall find all this white matter
collect together at  the surface, if we have added enough of
water, and that it is pure  camphor which thus floats at top.
   4th.  If we pour oleum  tartari on the solution of marble in
the marine acid, we shall  observe a copious subsidence of the
cloudy  matter, which is immediately produced;:  This falls to
the bottom; and, when examined, is found to be the same
in all its  chemical properties with the marble which we dis-
solved.  This is an evident separation of it from the acid.
   This form of separation, when the detached substance falls
to the bottom, is called a  Precipitation, and the substance
bo separated is called the  Precipitate.
   We  have here an opportunity of observing further, in all
 such cases,  that the substance which is added to the  com-
 pound, in order to effect  a separation of its constituent parts,
 unites  strongly with one  of the ingredients of the compound,


                                                      f 
          PRECIPITATION....PRECIPITATE.     244

 which are thus  separated from  one another; and we have a
 new compound.
    The  four separations just  now exhibited are, every one,
 examples*of this.   Thus, in the first experiment, the salt at
 the bottom is found to contain the vitriolic acid united  with
 the salt of the oleum tartari; that is, with pearl ashes*  In th»
 second experiment, we find  the very same salt at the bottom.
 In  the third experiment, the liquor is spirit of wine,  diluted
 with water, but void of camphor.  And in the fourth  experi-
 ment, the  liquor is found to be> the solution  of digestive salt
 which consists of marine acid, and pearl ashes.
   These examples are a sufficient illustration of the new com-
 bination which generally results from the separation by a third
 body.  There is often a gradation in these separations.  Thus,
 silver dissolves in aquafortis, just as marble did in that liquor,
 but after  it is  dissolved, it can be separated  by mercury.
 Aquafortis can also dissolve mercury, and the mercury  may
 be separated by copper, lead, or  iron.  And copper, dissolv-
 ed in the same manner, can be separated by iron or lead ;  and
 lead, so dissolved, may be separated by iron.
   7th.   There is one other effect of mixture, which is as ge-
 neral as these already enumerated ;  but tl\e knowledge of  it
 has not yet been found of so  much importance to the chemist,
 and has therefore been less  attended to,  and is  but seldom
 mentioned in chemical books, though it is a remarkable fact,
 and worthy of more notice.  It is this: that in most cases of
 the union of bodies with one  another,  if their nature  and
 manner of  union be such as  to admit of the proper examina-
 tion, we find that the bulk of the two when combined, is  differ-
 ent from the sum of their bulks in their separate state.  It is
 generally rather less, but in some cases greater.   Thus, it is
 found  that  100  cubic  inches of  water, mixed  with  100 of
 spirit of wine, do  not  make 200 cubic inches of mixture:
 they rarely make 198, and often considerably less.   Experi-
ments  have shewn this  also in  mixtures of metals with one
another,  as in the  mixture of mercury and  silver and still
more remarkably, in the mixture of copper and tin. Although
tin is the lightest of all the metals, yet this mixture, in  a due'
   vol.  i.                   ^9 
246 MIXTURE CHANCES THE SPECIFIC GRAV.

propnraon, produce's a mixture which is considerably heavier
than the copper; that is,....a pound weight of tin being mixed
win a pound weight of copper, the mixture will be kss bulky
than two pounds of copper,  and far less than two pounds of
tin.   The same thing happens inlbnixtures of salts and water.
The difference is commonly not great;  but it is sensible, and
does not proceed from  the heat or cold produced.  Hence it
appears that we cannot form an exact judgement of the pro.
portion of the differ, nt ingredients in a compound, by exam-
ining its specific gravity7,  trdfbgh we know what these ingre*
dients are, and what their specific gravities are in a separate
state.   Therefore  Archimedes was in  an error  when he
thought  that he had  solved the  problem  about the  golden
crown.  I think too that this phenomenon, when attentively
considered, will be found to be inconsistent with some com-
mon opinions,....as that of the nature of water, and the cause
of  the difficulty  found in compressing  it.   For it  is not
enough to say that the Other ingredient occupies less  room,
by  lodging in the interstices between the spherules of  water,
which only touch each other in single points.   For,  when a
certain quantity of" some  substances are  mixed with  water,
the bulk of the mixture  is  even less than  that of  the water
alone.  (See Hahn de effcacid misturae in mutandisvolumini-
bus corporum.)   The particles of water must be nearer than
they were before ;  that is, they were not in contact at the first.
 Mr.  Achard of Berlin, and Mr. Brissonof Paris, have made
 a vast number of experiments on this subject. *
   But this general fact, of the change of density in conse-
 quence of the union of bodies into compounds, is not so in-
 teresting to  the chemist as  those which I before  enumera-
 ted.

    OF  THE  THEORIES OF CHEMICAL  MIX-
            TURE AND  COMBINATION.
   In stating the  theories of  the  effects of mixture,  I may
 begin by remarking, that during the first ara of chemistry in
 Europe,  from  its first  introduction to the time of Lord
 Verulam, the chemists had no intelligible theory.  This was
* 
THEORIES                  247
     occasioned, partly by their being too little acquainted with the
     principles of reasoning that  are most familiar to the rest of
     the observers of nature, and partly by the  nature of their sci-
     ence.
       When we give an explication of any extraordinary appear-
     ance or property of bodies, we always do this by shewing that
     it is not at bottom so extraordinary, or so very much uncon-
     nected with every thing else, but that there is a connection
     between it and  other things, which we know very well, either
     by a resemblance which it bearq^othemm certain particulars,
     or by its deriving its origin from the  same causes with more
     familiar events.
       But the chemists, having confined their attention solely to
     chemistry, were strangers, in a great measure, to the rest of
     the world.   They could not, therefore, explain the chemical
     facts, by shewing a resemblance between them and other things
     better known. They were more disposed to explain other things
     by chemistry, than chemistry by any thing else.  Hence arose
     their attempts to set up a system of medicine, upon chemical
     principles ; and a number of other attempts of a similar kind.
     And, in tfieir  reasonings concerning the phenomena of che-
     mistry itself,- they never  went  further than  explaining one
   '  chemical  fact by another.    Thus, after observing that the
     salts called acids effervesce  with the salts called alkalis;  if
     they found any new substance not an alkali, which  effervesced
     with acids, they said thatdt did'this because it was of an alka-
     line nature, or contained hidden alkali in its composition.
       But, beside the ignorance of the chemists in other things,
     it must be  allowed that fhe nature of  their science may have
9   been one  cause of  difficulty to  those who  would  have at-
     tempted to improve it in the way of reasoning and explication.
     Most of  the chemical phenomena are  so extraordinary, and
     it is so difficult to find any thing else to which they bear a '
     resemblance, any  thing to which they  can be compared, and
     by which they may be explained, that on this  account alone,
     many of the first attempts could not miss to  be unsuccessful.
     It is much more  easy  to give a Satisfactory  explication of
    surprising facts in  some other sciences. When we  are  told 
248      OF CHEMICAL COMBINATION.

that an ingenious mechanic has contrived to  move  an im-
mense mass of rock, of enormous size and weight, and this
by the force of  one man only: if we are at all acquainted
with the powers of mechanism, we shall  not be greatly sur-
prised at hearing this.  We know the thing to be possible, and
when we see the machine by means of which it was done, we
shall understand his contrivance, and the effect which it pro-
duced, completely.   We shall perceive that his machine is a
system of levers or  wheels, and perhaps of screv s, or wedges,,
or pulleys,  properly combined  and that the whole is a con-
trivance for increasing force, at the expence of velocity. We
shall, therefore,  understand how the force of  one man might
be  sufficient to  produce the  effect; but that he could only
move the mass with a very  slow, and almost imperceptible
motion.  But, if the same rock were moved by a chemist, we
shall be disappointed if we expect to be as readily satisfied, or
to understand as quickly, the nature of his operation.   If we
are told that he  mixed a small quantity of saltpetre, sulphur,
and charcoal together,....laid this mixture under the rock, and
applied a spark of fire,...'.and that the rock was instantly thrown
up with great violence and velocity odt of its place ; we find
nothing here that is reducible to the common laws of mecha-
nism—no contrivance for increasing the force at the expence
of the velocity :....we wonder.
   Of this uncommon and unaccountable nature are the effects
which generally present themselves in chemical experiments.
At the same time, there is so wide a field  of inquiry, and it is
so easy for  a person who applies  himself  to  it, to make new
discoveries of curious  and useful facts,  that  those who for-
merly employed themselves in improving  chemistry, took
chiefly this way.  They made experiments, and enlarged the
 collection  of  facts, finding this much easier than to explain
them.  Or, if they attempted explications, it was in theman-
ner already7 mentioned, and which none else understood.
   Such was the state  of  our  science until the time  of Lord
 Bacon ;  when it became more commonly known to those who
were masters  of other branches of knowledge.   From this
 period, attempts were made to speak more intelligibly than be- 
            CHEMICAL PHENOMENA      -    249

fore; by endeavoring to explain the chemical phenomena from
mechanical principles.  I must here observe, that as the me-
chanical powers of bodies are commonly the most familiar of
anyrand most distinctly understood, the mind of man is always
prone to have recourse to these for explaining the phenomena
of nature.  We have always a propensity to suppose that any
extraordinary effect or poweitof a body depends on some me-
chanism in it, something particular in the form, size arrange-
ment, or motion of its parts.  And when any remarkable fact
or appearance  is shewn to depend on principles of this kind, it
is generally thought to be* well explained.   The account thus
given of it is plain and intelligible.
   Lord Bacon, therefore, recommended this sort of reasoning
in preference to  the language which was common in his time
among the chemists ;  and Mr.  Boyle  afterwards attempted
to introduce and support it more fully.  But the consequence
was, that for some time after, this sort of reasoning was applied
to chemical phenomena with so  much boldness,  and so little
judgment or consideration, as quite disgusted those who had
real discernment; and they formed an opinion, that chemical
facts could not be explained in a satisfactory manner by reason-
ing from mechanical principles.
   There is a noted example of a bold reasoner in this way, in
a French author, Lemery, who in other respects was a chemist
of great  merit;  but he was led into this manner of reasoning
bv the fashion of the times.  According to this author, a fluid
which has the power of dissolving a solid body, abounds with
sharp and pointed particles, having  the forms of needles  or
wedges, which are agitated in the fluid  with a rapid and con-
fused intestine  motion.   The solid  again has pores of  such
sizes and shapes, as  are fitted to the  pointed particles of the
fluid, in  consequence of which they are penetrated and torn
asunder.   In like manner, he explained the precipitation by a
third body, as when potash causes marble to be precipitated
from its solution in aquafortis, by say ing that the particles of
the potash were porous and spongy, and, by this configuration
and confused motion, they took hold of, and broke  the spiculae
of the acid, &c. &c.   This supposition appeared at first to ex- 
250       EXPLAINED BY MECHANISM.

plain some particular facts very conveniently. Thus aquafortis
dissolves iron or copper, but not gold ; the reason was plain,
because iron and copper have wider and more numerous pores
than gold.  But aqua regia dissolves gold, and does not act on
silver.  In like manner, mercury penetrates  and dissolves
gold,  and will not touch  iron^ and very difficultly  copper.
Many other examples of  the  same kind might be adduced.
Indeed, this sort of theory, in the way in which it was manag-
ed, was quite inconsistent, with sound mechanical principles,
upon which alone, however, it was meant to be founded.  The
motion of the  particlqp of fluids, necessary for producing the
agitafion, could not continue;  they must necessarily stop one
another.   Yet spirit of salt, though kept for ages, will act on
marble ;  and we have proof by cxperim«att that it is not ne-
cessary  that the particles of a body  be  fluid or  moving, to
make it  capable of acting upon  another.  There are some
examples of solid bodies acting on one another, and mutually
dissolving, and we cannot  imagme that the particles of solid6
are moving among themselves.
   The experiment which  I have often mentioned to you, in
which an intense cold  was produced by mixing dry salt and
dry snow, is of this kind:  they melt into a brine, that is, they
dissolve  one another.  In like manner, corrosive sublimate,
and regulus of antimony,  lime, and crystallized mild volatile
alkali, act on each  other, and unite in a different manner from
their former state, exhibiting a true chemical solution.
   It is needless to' insist longer on these whimsical theories,
so gratuitous and unsupported by evidence, and  so deficient
even in the plausibility of explanation.   We can have no dis-
tinct conception of such action, and without this no theory7 can
have any value.
   These, and  some other attempts of the same kind, to ex-
plain  the chemical  phenomena, appeared therefore quite crude
and unsatisfactory to those who had more discernment and
penetration than ordinary.  But we  had no chemical theory
that connected this science properly with other parts of our
knowledge of nature, until Sir Isaac Newton  published that
edition of his optics, at the-end of which he has given a num-
ber of queries,  containing his speculations and  conjectures 
             BY  SIR  ISAAC NEWTON.          251

eoncerning many of the curious and difficult ficts which occur
in the study of nature.  There are a number of those queries
that relate to chemistry.  In these he lays open a view of the
more remarkable phenomena  which occur.in chemical mix-
tures, which is altogether his own, and which is  much more
satisfactory, and makes them appear much more conformable
to the rest of nature, than any that had ever been offered be-
fore.
  This great author had already given to the world his much
admired system of the universe, in which he explained the
connection  of the  heavenly bodies with one another, and the
regulation of their motions, by shewing that they are retained
in their orbits by the same power, or principle of motion which
determines a stone to fall to the ground ; a power commonly
called gravitation, or the attraction of gravitation.  He con-
ceived that there  are other forces or principles of motion  in
nature, more  or less similar^to this, by which certain bodies
act, or ippear to act,  at a sensible distance, on one another.
This is  evident in the case of the magnet and iron, and in
electrified bodies.   He suspected that  there were still other
similar forces, whose sphere of action was still smaller, even
so small as to escape our senses.   He imagined that  these are
manifest in the attraction and cohesion of polished  planes, and
of metals, in  the  attraction of small vacuities  for  different
fluids, commonly called capillary^ attraction, and in  the in-
flection and deflection of light, as it passes near the edges  of
solid bodies.  It  is  worthy  of particular remark,  that the
force which acts  in these last instances is amazingly great.
Notwithstanding the  astonishing velocity7 of light, about 200,
000 miles in a second, and the almost  unmeasurably minute
space through which the deflecting force  acts on the light, it
turns it  greatly out of its former direction.   From the quan-
tity of this deflection, we could calculate the proportion of this
force to that of gravity: for Newton, with wonderful sagacity,
has been able to give us the exact measure of the distance  at
which this deflection is performed.  This force  is  not less 
  252  SIR ISAAC NEWTON'S EXPLANATION

  than twenty millions of millions of times greater than gravity*.
    Sir Isaac, reflecting ott these examples of the action of bodies
  on one another, and being struck with the effects he had ob-
  served in making chemical mixtures, suspected that these last
  might depend on powers, or principles of motion, somewhat
  resembling the others.
    You cannot have a more striking view of his ideas on these
 subjects, by any account of them, than that which you will get
 by reading those passages at the end of his Optics, (Quest
 22, &c.) in which  he first communicated these conjectures of
 his to the world.   His style  is so  simple, and at the same
 time so nervous, that it cannot fail to make an impression.
   By  these Queries, in which Sir Isaac Newton first proposed
 the doctrine of attraction in chemistry, an intelligent chemist
 cannot miss to perceive that he had acquired  extensive and
 accurate knowledge in  the different  parts of this science : and,
 when  we consider the  time when they were  published,  and
 how little had been done  to explain the chemical phenomena
 in  a satisfactory manner, we  see in these queries a  proof of
 that capacity of mind, and that acuteness and originality of
 genius, by which Sir Isaac distinguished himself on every oc-
 casion.  Not only is the  path pointed out,  in  which great
 reputation has been acquired by some of the most eminent
 chemists, but the chief steps are actually made, and the manner
 in  which farther progress may  be made, is  distinctly de-
 scribed.                           $
   His explanation of the chemical phenomena  was received,
 by the unprejudiced, with the highest admiration and pleasure:
 and there is no doubt but that it opened a view of some prin-
 ciples of action in  nature, which certainly exist,  and which,
 with some few modifications,  explain many of the most re-
 markable chemical facts.   It  is evident that  there  are such
principles of motion in nature, as Sir Isaac pointed out;  for

  *  Dr. Herschell's discoveiy of the refrangibility of heat, in precisely the
same manner and degree as light, shews the  immense force which connects
it with other qptter; a force fully adequate to the most remarkable effects
which we see it produce.
                                                   EMTOP 
OF CHEMICAL PHENOMENA.
  there is  no denying that there  are such  motions.   The
  reality of the  powers which .connect the particles of matter
  is clearly proved by a multitude of facts and experiments:
  and, that attractions  somewhat resembling these,  act in
  ehemical mixtures, and that with great force, is equally
  plain from the phenomena.
    For, when marble, for example, or iron, are dissolved in
  the muriatic acid with rapidity, and appearances of vio-
  lence, a very- great force  must act  to produce this effect,
  overcoming the strong cohesion of  the particles of  those
  fcodies.   Were we to attempt to divide them mechanically
  into very minute  parts,  it would cost us as much labour
  and toil  to effect  this ; and our utmost exertions, though
  ever so  long  continued,  would never divide them  into
 atoms so minute as those  into  which they are divided by
  chemical solvents.  And, ■ even after they are divided by
  these, we  have the  plainest  evidence  of  the  continued
  action of an attractive  force,  which prevents  the heavy
 matter of the solid from being ever deposited again by the
 less ponderous fluid, and which, binds to the particles of
 the iron  or marble, the particles of the volatile solvent,
 thereby greatly diminishing,  or repressing, the volatility
 which it had in  its  separate state.
    Sir Isaac's theory, therefore,  explains all the  most diffi-
 cult parts of the  subject.   It supplies  a force  by which
 solid bodies are divided,  and remain for ever suspended.
 It explains why some salts are deliquescent, and why vo-
 latile and fixed substances,  when united in this way,  are
 afterwards so strongly connected together, and so difficulty
 separated from one another by heat.
   I must not, however, omit to  observe, that some of the
 effects which were  at first imputed to the violent action of
 these forces, have been since discovered to  proceed from
 other causes.   Such is the effervescence which  occurs in
 making many chemical mixtures.   Sir Isaac supposed it to
 proceed from the violent friction and collission of the par-
 ticles of the diffc rent bodies, while they rushed together to
 unite.  It was very natural for him to form this supposi-
tion.   It was  even, I  may say, a  beautiful part of  his
   vol. i.               R r 
254   CHEMICAL ATTRACTION....AFFINITY.

theorv, as it presented to the imagination, an explanation
which appeared at that time very probable, and was easily
understood.   (See note  15, at the  end of the volume.)  It
is known however now, by late experiments and discoveries
to proceed,  in most cases, from  the.liberation of  some
elastic aerial matter, which was contained in those bodies
in a condensed state.  And the heat  is now explained on
other  principles,  viz. by a diminution of capacity in the
compound, to contain  the heat which was contained, in its
ingredients, or (to express it more  cautiously) by the libe-
ration of a quantity of heat not necessary  for that tempera-
ture of the  compound.   But all the  other phenomena of
mixture  are  still considered as depending entirely on these
specific attractions.
   Notwithstanding the  very great merit  of this theory in
comparison with  others, it was not very7 well received at
first bv the chemists abroad.  They objected to the word
attraction,  as implying either  an active quality  in matter,
which we cannot conceive to be possessed of activity,  or
some connecting intermediate substance, by which the par-
ticles  of bo lies were drawn together, and for the existence
of which no proof  is offered.  They therefore chose to
substitute the term  Affinity instead of  Attraction.
   But their objections to the word  attraction were certainly
unreasonable.   Sir, Isaac  in the beginning of these que-
ries,  expressly declares,  that when he uses this word, he
does not pretend to assign the causes, natures, or manners
 of acting, of those forces by which bodies are disposed to
rush toge her into union.  He only means the  forces them-
selves, or the  disposition to joinv manifested by the fact
itself, leaving it to others  to discover the cause, and ob-
serving that it is improper to attempt it,  until we become
beuer acquainted with its  manner of  acting, by studying
tlic tacts.   And when the word is used with  this precau-
tion,  merely as a term expressing  a fact,  a phenomenon,
I do not see any advantage in preferring affinity to it.   It
would sound ve;-y ill,  to speak of the affinity of gravitation,'
 of electricity,  or of magnetism.   Attraction is more  ex-
 pr«. s.!\t; and affinity  implies, or suggests, some similarity, 
    ITS  DISTINGUISHING  CHARACTERS.   255

which, in most cases, is not agreeable to fact, seeing that
we  generally  observe  the  greatest dissimilarity in  those
bodies  which  are  eminently  prone to unite.  (See  note
16, at the end of the volume.)
  As  Sir Isaac's theory, however is  now well established
wherever the science  of chemistry has made any progress,
and, as we shall find it most extensively useful to  us in
assisting us to understand a great variety of the ch; mic'al
phenomena, I shall consider it with  more attention,  and
endeavor to give you as clear and correct an idea of it as
possible, as well as of the facts which it expTains.   With
this intention, I shall make  a few remarks on it.
   1.   The first 1  shall offer is, that it differs  from  some
other kinds of attraction in this respect, that, so far as we
can  perceive,  it cannot set  in motion large and visible
masses of"  matter.   It only acts in  such a manner,  as to
dispose the minute  and invisible particles of one substance,
to separate from one another, and unite themselves severally
to the  minute and  invisible particles of another substance.
An obvious example of which we have  in a lump of salt or
sugar, and a quantity of water.
   It is a necessary consequence, therefore, of the nature
of  these attractions,  that the attraction of cohesion is an
antagonist to them, or counteracts them more or less ir. all
cases.   By the attraction of cohesion, or the cohesive attrac-
tion, is meant that attraction which similar particles, or the
particles of the  same  kind of matter, manifest for  one
another.   (See note 17, at the end of the volume.)
   We have an obvious example of the action of  this sort
of attraction in two bits of  lead, or of elastic gum, which
being made quite clean, and closely  applied to one another,
will join and cohere  as  strongly  as if they  had  been
melted cr soldered together, in those parts of their surface
that have been  brought into very  close  contact.   Iron
affords another example ; different pieces of it  uniting per-
fectly together,  when they  are softened by a strong heat,
and very  closely applied.    Clay is an  example still  more
familiar, and, in my  opinion, perfectly similar to all  the
other examples.   Although, in these  cases, we do  not 
256
OF  CHEMICAL ATTRACTION.
perceive the action of cohesive attraction,  in  making the
masses cohere,  except when  they are brought into what
appears to our senses the closest contact; yet other experi-
ments have shewn that it  can act at small  distances, only
that its power  is diminished so fast  as  the  distance is
increased,  that it soon becomes  imperceptible.   The ex-
periment that I have chieflyjn view here, is that first made
by  Mr. Huyghens, and mentioned in the Phil.  Trans. .No.
86.   He found  that two finely polished  plates  of glass
attracted each other, and that the  one lifted  the other, even
when a singleMibre of  silk was interposed, but not when
two fibres crossed one another.
   Without this cohesive  attraction,  no body would have
solidity or hardness, but would be a powder,  or a liquid,
or  in the form of vapour, if  heat were present,  or in the
state of inconceivably subtile unconnected particles.   This
attraction is in general strongest  in the most solid, hard,
and dense bodies ;  less strong in those which are soft or
brittle ; particularly modified  in those  which are tough and
ductile.    Its effects  are  reduced to a  great  degree of
weakness in those which  are  fluid,  and almost to nothing
 in those which are  in the form of vapour*.    It is  in all
 these cases counteracted, less or  more,  by the repulsive
force of the matter of heat.
   This sort of attraction (viz: cohesive), as you will perceive,
does not in the least counteract some other kinds of attraction,
 by which certain bodies  act  on  one  another, such as the
 attraction of the  magnet and iron, and the  attraction of
 electricity and of gravitation.   But the  chemical attrac-
 tions are directly counteracted and opposed by it.   When
 a lump of iron, for example,  is thrown into nitrous or mu-
 riatic acid, there is a powerful chemical attraction, which
 disposes the particles of the iron, and those of the acid, to
 unite together.   But it is evident that the cohesion of the

   * The immediate, or what we may call the formal cause  of these different
modes of aggregation, are distinctly, but concisely, explained in Newton's
 questions, at the end of his optics.  They are considered at great length by
Mr. Boscovich, in his Theory of Natural Philosophy, a work of the greatest
 ingenuity, and ricli'y deserving the serious perusal of" a philosophical chemist.
                                                  EDITOR; 
            CHEMICAL COMBUSTION.         257

parts of the iron with one another is  in opposition to their
union with those of the acid;  and were not their attraction for
the acid  particles stronger than their cohesion with one ano-
ther, the iron could not be dissolved.   It is stronger, however
and the consequence is the dissolution of the iron, notwithstand-
ing the strong cohesion of its particles. (See Note 18,  at the
end of the Volume.)  But although this cohesion is not strong
enough to preserve  the iron from being dissolved, it must
necessarily resist or counteract, to a certain degree, the pow-
erful attraction of the acid :  and the iron would be dissolved
with much greater rapidity, were it in our power to deprive it
of its cohesive attraction before it is put into the acid.
   There are  even examples,  in which,  by  increasing the
power and action of  cohesive  attraction, we can enable it to
overcome a chemical attraction which  it was not able to over-
come before.   In  the hammering of iron, -and other metals,
the latent heat is forced out, the chemical attraction between
it and the metal being overcome, by increasing the power of
the cohesive attraction, in consequence of the condensation of
the parts of  the  metal, which  the hammering  produces*.
The metal, after being hammered, is  both denser and harder
than before.  And the cohesive  attraction, be;ng in all cases
an  antagonist  to  the chemical  attraction,   by which latent
heat is retained in bodies, when the one of these attractions is
increased the other must give way or be overcome more or less.
Even the chemical attraction of acids for iron is courteracted,
and reduced  in  its  power  and  efficacy, by  hardening the
iron.
   In confirmation of these remarks,   I observe that we have
learned, by the most extensive experience in chemistry, that
bodies which  have chemical attraction for one another, act and
unite together much more readily, when we take them in a
state in which their cohesion is weak,  than when it is strong.
Of this you will see numerous examples as we proceed.  We
have a  good example of this counteraction of cohesive and
chemical attraction in the phosphorates of silver and of nico-

  * $ee what was lelivered on this subject in page 140.  The diminution
of the chemical attnr-7 - * ' ■■••' hardening the iron, is very doubtful,  editor. 
 25S       RULES  FOR PROCURING IT.

 Ium.   These  compounds  retain the phosphorus while  they
 are fluid by heat,  but when the cohesive attraction increases
 by the diminution of  the heat, the phosphorus  is thrown out,
and burns on the  surface of the metals,  while they  congeal .
 (Annates de Chymie, T. 13.)
   And this proceeds from that peculiarity which I mentioned
just now in the  nature of  chemical attractions, that  they do
 not act so as to dispose large, or any way perceptible masses of
 matter, to move towards  one another, and join, together  in
 their massy state.   They only act so as to dispose the minute
 and invisible atoms of one species of matter to  unite severally
 with   the  minute  and   invisible atoms  of  'another,   or
 of some  others.  And these  attractive  forces between  the
 atoms of  such different kinds of matter, act  so powerfully
 as to  tear asunder and totally  destroy the  masses which these
 different kinds of matter formed separately before.
   2.   We may in the next place remark, with  regard to this
 attraction, that when  two bodies unite in this manner,  they are
 each of them divided into particles of such extreme minute-
 ness,  that the utmost efforts of  imagination cannot form  an
 idea of it.  I had one clay a desire to  see into how many sen-
 sible  parts a body may be actually  divided by solution.   I
 found that one grain of silver  gave a very  sensible tinge to
 about 38 troy ounces of water, or 18060 grains.   I found by
 repeated trials, that  a drop of  water weighs very  nearly a
 grain, for 50 drops weighed  from 49  to 55 grains.  An ordi-
 nary microscope exhibits  100,000 sensible parts in a drop when
 spread out.   Suppose that it exhibits only 1000 such parts: in
 every one of these there are many particles of the silver, for the
 visible particles are all littleflocculi,of differentraggedforms,and
 dierefore consisting  of many atoms.  From this observation,
 I was well entitled to say  that a grain of  silver could be divi-
 ded  into much more than 18 million  of  sensible parts.   You
 may form some notion of this  number by this circumstance,
 that this number of common pease,  laid close to each other,
 would occupy 81,43 miles.   This is  net a singular or  extra-
 ordinary example;  we have  reason to think that a similar di-
 vision takes place in all case3. 
   FLUIDITY PROMOTES COMBINATION.  259

  3. I observe  that the influence or energy of chemical at-
traction reaches only to  an exceedingly small distance round
the particles of  bodies, a distance so extremely small, that it
is  not perceptible ; and, if we trust our  senses, no action
seems to take place until the particles come  into the  closest
contact.                                       *-
  By attending to these particulars in  the nature of chemical
attraction, we are enabled to understand the propriety of some
canons or rules relating to the practice of chemistry, or to the
most proper manner of performing many7 chemical operations,
or of making experiments.
  First rule.  When it is desired to make two bodies act che-
mically on one another, to produce any change, or to obtain
any  product in chemistry, it  is in most cases necessary that
one  or both be fluid when they are mixed, or that they be ren-
dered fluid, or disposed to fluidity, immediately after.  The
cases are very few indeed, where any considerable action can
be  induced without previous fluidity.   You will get a very
clear conviction of its utility, by taking a small quantity of sal
ammoniac, and of salt of tartar in dry powder, and mixing
them.   No sensible change  is produced;  but pour in a little
water, and instantly you have a strong smell of spirits of
hartshorn.  This is  instantly  formed by the assistance of the
water.
   In like manner, dry acid of tartar, and dry salt of tartar, re-
main inactive when mixed; but a little water being poured in,
you have the violent  action of the ingredients, and obtain a
compound salt.
   The dry salt composed of copper and  aquafortis, being
wrapped up in tinfoil,  also remains inactive ; but if, before
wrapping up the salt,  you very slightly wet the tinfoil with a
pencil dipped in water, and quickly roll it up after sprinkling
the salt on it,  you have a  most violent action, accompanied
with  explosions, and  even a bright flame and burning heat.
Ihe tin leaf is all corroded or eaten into holes.
   The propriety of this rule arises from many considerations.
In the first place,  the fluidity produces an  extent of contact
unattainable  in  any other way.   The  finest powders  touch 
  260        CHEMICAL  COMBINATION.

 each other but in few points, and the greatest part of each
 particle of the powder lies from another far beyond the reach
 of chemical attraction.  But the very atoms, when floating in
 the fluid, are applied to the other substance  all round, at a
 di-.tance fully within the sphere of action.
    But,  secondly, it appears that some chemical actions evi-
 dently assist  one another, not perhaps directly, but by coun-
 teracting the  cohesive attraction in the particles of solid mat-
 ter.   The  cohesive attraction is  unquestionably7 an antagonist
 to all  chemical  actions whatever; it must be overcome in the
 first instance.  This  is one of those chemical facts or laws,
 which the chemists on the continent call examples of predis-
posing affinity.  Some curious examples will occur afterwards.
 Perhaps we may not understand the cause or nature of this
 fitness or predisposition in every case; but we may use the
 term as the expression of a phenomenon,  an undoubted and
 important fact,....important,  as it very often enables us to ac-
 complish what would otherwise be attempted in vain.
   For  similar reasons,  it  is not unfrequently  the practice to
 convert substances  into vapour, in order to  make them unite.
 Here the cohesive  attraction is completely destroyed.  The
 vapours are most intimately mixed, and they unite when con-
 densed.   There are several instances, however, where two
 gases, that is,  permanently elastic fluids,  shew no disposition
to unite when  completely formed, vet will unite, if one of them
be presented to the other in  its nascent state, that  is, in the
very act of formation.  This happens in the  case of inflamma-
ble and vital airs.
   Second rule.  That the solution of  solids in fluids is pro-
moted  by previously  dividing the solid  mechanically,  or
otherwise enlarging its surface, as by  pulverisation, granula-
tion, filing,  plating.  The reasons for this are t6o plain to re-
quire illustration.   These preparations increase the surface of
action.
   Third ride.    Frequent agitation of the  solvent also pro-
motes solution, by removing the saturated parts of  it.  This
is a curious subject, and of  most extensive influence.  (See
vote 19, at the  end of the vohmr*.) 
           OF ELECTIVE ATTRACTIONS,      36*

    A Fourth Pule is, That the action of bodies in general upon
 one another, and their union together, is promoted by mode-
 rate heat.   This rule is founded upon a very extensive expe-
 rience.  I do not remember any exception from it, any exam-
 ple of bodies disposed  to  act on one another,  in which the
 action is not promoted and increased by  a moderate increase
 of their heat.   Thus salts and water act in general more pow-
 erfully as the mixture is hotter.  There are even examples by
 which we might be led to conclude that chemical activity would
 not exist without more or less heat.  When we diminish heat,
 the chemical action of bodies becomes languid ; and in some
 few cases,  when it is greatly7 diminished^ chemical action
 ceases entirely.  This is the reason why we cannot produce
 a greater  cold  with  salt and snow ;  and it is  plain that the
 power of nitrous acid and snow is limited in the same manner,
 as in  a very intense  cold the acid shews a diminution of its
 attraction for the water by crystallizing.   Query, Is this occa-
 sioned by the increased action of cohesive attraction, in conse-
 quence of the diminution of heat,  which  is an antagonist to
 it ?  Does a gentle heat promote chemical  action and union by
 weakening the cohesion of the aggregate, while a strong heat
 weakens, in many cases, even chemical union, by  introducing
 a repellency of the parts of the compound, of  different vola-
 tility ?*
   As the  general rule, that the chemical action of bodies, and
 their-union with one  another, is promoted  by moderate heat,
 has been long established upon the most extensive experience,
 ithas, given occasion  to some operations, which are often per-
 formed to^romote  the chemical  action of bodies fcpon  one
 another, such as, Digestion, Circulation,  and Cohoba-
 'tion.

  * The experiments of Mr. Berthollet, and of Mr. Seguin, shew some very
 remarkable changes, and alternations in the affinities of bodies, by a gradual
 increase of their  heat.   They may be explained by supposing that the
 attractions of different bodies vary in different proportions, by the same varia-
 tion of temperature ; so that in one temperature they are in equilibrio, and
inactive, but not in others.
                                                  ^EDITOtt:
    VOL. J.                   s s 
2C.J2       OF ELECTIVE ATTRACTIONS,

         OF  ELECTIVE ATTRACTIONS.
  The remarks which have been now made are equally appli-
cable to all cases of chemical action, and the union  ol  bodies
with one another.  But we must now give a little more atten-
tion to those numerous cases, in which a third body  frequently
acts on a compound of two ingredients, so  as to separate these
from one another, and join itself to one of the two.   An at-
tention to these cases is of the greatest importance  in chemis-
try ; for il is by our knowledge of them that we are  enabled to
obtain the decomposition of many compounds, the ingredients
of which cohere too strongly to be separated by heat, or by any
other  means.   Indeed, I may almost say,  that a complete
knowledge of  the  general laws of  this phenomenon is nearly
the whole of chemical science.
   The cases in which this method may be employed are very
numerous.  There are numerous  examples in chemistry, in
which the same body  has a chemical attraction for each of a
number of others, or a disposition to unite with them.  But
such is the nature of that power by which these unions are pro-
duced, that the body is  capable of being firmly joined to one
only of these others at the same time.  Whenever  two bodies
are joined by chemical attraction, a third can seldom, if ever,
be admitted into the compound. The disposition of these two
to unite with others, is diminished, and, so far as we can per-
 ceive,  it is diminished the more,  in proportion  to the force
 with which these two are joined together.  If  this force be
 strong, the attraction  of other  substances is rend^Kid ineffe#.
 ual; or,  if it have any effect, which I think it  mis in many
 cases in which this is not suspected, it is  only to weaken a
 little the force with which the two bodies  are joined,  but not
 so much as to separate them.   But if, on the other hand, the
 force with which two such substances are united, is weak, and
 we add a third, which has a  strong attraction for one of the
 two, it immediately unites itself to that one, and  separates the
 other, or, the other is let go, and separates.  Thus we form a
 new compound ; and this new compound  can, in many cases,
 be decomposed in its turn,  by  another  body,  in the same 
OR  DEGREES OF AFFINITY.
 manner.   Examples  of  this are  more  especially called
 elective attractions, or exchanges.   (See note  20,
 at the end of the volume.)
    Sir Isaac Newton has  not stated expressly any other
 supposition  to  explain this  phenomenon,  than  that  the
 added matter has a stronger attraction for one of the  in-
 gredients of the compound, than these have for one another.
 And no doubt this is one necessary cause ; and accord-
 ingly, we generally find it more difficult to decompose by
 heat, the new compound thus produced, than the former
 one, supposing the volatility of the volatile ingredient, and
 the fixedness of the fixed one, to be the  same in both cast s.
 But still this supposition alone,  of a stronger  attraction,
 stems hardly  sufficient;  for why  should  not the more
•strongly  attracted  body  unite  itself  to  that  ingredient
 wjiich attracts it, without dislodging the other, and thus
 produce a compound  of  three ingredients, two of which,
 though without attraction for one another,  might, how-
 ever, be combined by their attraction for the third ?   As
•this happens but very rarely, there  is reason to suspect that
 there is a cause of the decomposition of  the first compound,
 and this is possibly a repulsion  between the added body
 and that ingredient which it separates  and throws  loose,
 while.it unites  with the other.   There  are many phenom-
 ena in cnemistry that appear to suggest  such a  supposition
 and Sir Isaac himself throws out a hint to this  purpose, in
another part of his queries-.   But we shall have better op-
portunities hereafter,  to  point out some cases  in  which
such repulsions  appear to take place.   (See note 21,  at
the end of the volume.)
   It was said just now, that the knowledge of the different
cases of elective exchanges enables the  chemist to resolve
into their principles a great number of  compounds, which
it would be  impossible to decompose by  any other method,
and that they are,  therefore,  the  foundations  of  a  great
number of curious and useful operations.
   The celebrated  Mr. Geoffroy of the French academy,
perceiving what advantage it would give in chemistrv  to
have a distinct and comprehensive view of all the cases in
which such  exchanges may happen, first thought of reduc- 
5164         MR. GEOFFROY'S TABLE.

ing them into the form of a table, which presents them ttf
view  at once, and he called  it the Table of Elective
Attractions, or of exchanges, or of affinities.   It is not
proper at present to shew  and explain this table, because
you are not acquainted with the  facts to which it refers.
But to give you an idea of the manner in which it is  con-
structed,  observe, that it  has been found  by experience,
that a number  of different  metals  can be  dissolved in
aquafortis, in the same manner as salt is in water.   It has
likewise been discovered that these, by their different de-
grees of attraction for aquafortis,  are capable of separating
one  another, in a  certain determinate order, by  elec-
tive attraction.   Mr. Geoffroy,  in order  to  recal  these
facts to the mincL, constructed one of the columns  of his
table in this'irtanner:  he placed at the head of it nitric acicS*
or aquafortis, arid under it the different  metals which  rn^y
be  dissolved in that acid, placing iron uppermost,  next
under it copper, under copper lead, under lead quicksilver;
and under  that silver: thus,
                       Nitric acid
                       Iron
                       Copper
                       Lead
                       Quicksilver               ^>
                      Silver.
   This arrangmcnt  is equivalent to saying at  full 4ength,
that il, to a solution of silver in aquafortis,  we add a lit-
tle mercury, the silver will be separated  from the acid, and
we shall  have a solution  or  compound  of aquafortis and
mercury;  if we add to this solution a bit of lead, the  mer-
cury will fall to the bottom, and the liquor will be***a  solu-
tion of lead in aquafortis.  In  like  manner,  copper  will
separate the lead, and iron will separate the copper.  Also,
iron  wiil  separate lead, or mercury, or silver;  and  lead
will separate silver.   In short, any of the inferior metals
will, be Separated by any of those placed above it.   Such
a column represents a train of chemical operations.
   This was a very  ingenious  thought  of Mr. Geoffroy,
and his table was received with general  applause, and has
been exceedingly useful to the chemists.  It was construct- 
           OF  AFFINITIES....IMPERFECT.      265

 ed with that skill  and accuracy,  which might have been
 expected from his extensive knowledge  and great expe-
 rience  in chemistry.   But, being the first attempt of this
 kind, it could not be supposed to b •  entirely perfect; and,
 accordingly, succeeding chemists have made a number of
 objections to  the construction  of  different  parts of it.
 With respect to those objections, I  shall  only remark at
 present, that these attractions are capable  of being affected
 by some circumstances,  which, if they had been attended
 to,  would have  given chemists more distinct ideas on the
 subject, and might have prevented some of those disputes.
 One example of this is in the power of heat, whirh, when
 varied  in its strength or intensity, produces a considerable
 difference in the  force wherewith certain bcjdies  attract
 one another,  or remain combined.  This  happens remark-
 ably, when  one of these  bodies is fixed, and the other vo-
 latile.   If both have the  same degree of volatility, increase
 of heat will not produce (so far as we know) any variation
 of their attraction, unless  it  be perhaps to increase it.
 But when the one is volatile, and the other fixed, variation
 of heat produces a considerable difference in the  force of
 attraction.   There are  degrees  or  intensities  of  heat,
 which will weaken the attraction and  connection of two such
 bodies, and, in some  cases> will actually make them sepa-
 rate from one another; and, therefore, must make them very
 liable to be separated by the attraction of other  bodies, ^
 which  would  not otherwise have that power.    This is
 a fact, of which you shall  be fully satisfied hereafter, when
 we enter into a particular account of the objects of chem-
 istry*".
   There is  another circumstance which  occasions some
 variations, or apparent variations, of the  elective  attrac-

  * There are other cases, less remarkable, but equally constant, where it
appears that the effect of heat in weakening the attractions of bodies, does
not vary in the same degree, in different substances, which do not sensibly
differ in volatility.  And there are a few cases, where intermediate composi-
tions obtain, in complex subjects,  in certain temperatures, by  which the
mutual actions of the ingredients are often changed.   This frequently hap-
pens in watery solutions.
                                                   EDITOE 
 JG6 AFFINITY Al FECTED BY QUANTITY.

tions ; variations, which, when they were first discovered,
were so  unexpected, that they appeared to overthrow the
whole system.   The circumstance I mean is the  quantity
of an attracting substance, that is applied to another, or to a
compound*.   When the quantity of such attracting sub-
stance is great, its power over the other,  or the amount
and effect of its attraction, is thereby increased ; the large
quantity  of the substance acting at the same time, having
thereby tht* more power.   This,  in some cases, enables a
more weakly attracting substance to overcome the. attraction
of a stronger,  when the more weakly attracting substance
is used in much greater quantity than the stronger. There
are many examples  of this in chemistry.   And it appears
to be for this reason that an abundance of the matter of
heat, or  caloric,  overcomes more or less the cohesive at-
traction of bodies, and either expands them, or unites itself
with them in the form of latent heat, and then melts them
or converts them into vapour.  (See note 22, at the  end
of the volume.)
   Upon  the whole, Mr.  Geoffroy's table, considered as the
first thought,  and first attempt  in this  way, is  highly
worthy of praise.   Bur  such a multitude of discoveries in
chemistry have been made since his time, that his original
table gives now but a  narrow  and imperfect view of the
numerous facts which  a  table of this kind  can be made to
represent and  suggest.   Accordingly, several improved
and more comprehensive tables of elective attractions  and
exchanges have been drawn up  by different chemists ;  but
none has employed so much pains  on this subject, or given
such a comprehensive and copious table, as the late Profes-
sor Bergmann  of Sweden,  whose works on this and other
parts of  chemistry are translated  into our language.  And
since that time, Mr. Morveau  in the New  Encyclopaedia.
(See note 23, at the end of the  volume.)

  *  See Decompos. par 1'Acide Marin  de plusieurs sels vitrioliques er
nitre-ix, par M Cornette.  Mm. de I'Acad. 1779.
  See also the 'ater publications of Mr. Beithollet, on affinities, in which he
!.;s considered tl.is circurr.,7aiu:e with great discernment.
                                                  edi rorc 
             DOUBLE  EXCHANGES.          267

  Among the most remarkable additions which have been
made to these tables, since Mr. Geoffroy's first attempt,
are the cases of doubi»e elective attractions.  These
had been but little attended to, or understood  in  his time,
although they  happen in  many important operations  in
chemistry, and thr knowledge  of  them  is indispensably
necessary on mam occasions.
   By a double el< ctive attraction, or double exchange, is
meant,  the action of two compounds on one another, each
of  them consisting of two ingredients, and each of those
ingredients having an attraction for one of the ingredients
ol  the  other compound.   The consequence of applying
two such compounds to one another is, in many cases that
they are both  decompounded,  and two  new compounds
produced in their stead; each of the ingredients of the one
joining itself to that ingredient  of the other compoundfoi
which it has an attraction.
   As the knowledge of these cases is very useful, I began
in the year 1756 or 1757,  to represent them by  diagrams,
and to subjoin these  to the table of  elective attractions,
which I gave to the gentlemen who attended my lectures;
and some other chemists began soon after to do  the same
thing, with perhaps a different mode of expression. When
 I explain the table, I shall shew you some of the ways in
 which they may be represented.   I cannot just now enter
 more particular \  into this subject, as the different sub-
 stances which have attractions for one another are not yet
 known to you.
    Be-ore I conclude this part of our course, it will be pro-
 per to explain to you some terms of distinction which will
 frequently occur in the course of your reading, especially
 of the  foreign  writers.
    The union of aggregation is that where a number of par-
 ticles of the same  kind are retained in one mass, called the
 aggregate, while  the  particles  of which it consists are
called its integrant parts, seeing that their sum constitutes
the aggregate, the integer, or whole.  These terms express
very well the mere  relation of co-existence,  but I think
that the term attraction  of aggregation ill expresses the
bond of union.  It does not characterise it so well as the 
 268  AGGREGATION....COMPOSITION....INGR.

 common term of cohesion, which I should therefore prefer.
 It is thought proper, however, to distinguish this bond of
 union of particles of one kind  from that which essentially
 connects particles of different kinds, and is more peculiarly
 chemical.  The attraction of aggregation or cohesion may
 be overcome by mere mechanical  force, whereas the other
 requires elective or chemical attractions.
    The  attraction of composition expresses very7 precisely
 and perspicuously the force which unites  particles of dif-
 ferent kinds, composing of them,  so united,  particles of a
 kind  different from both.  The attraction of composition
 unites a  particle of  vitriolic acid  with  a particle of  fossil
 alkali, and these two united compose a particle of Glauber's
 salt.   Each of the  component  particles is called an ingre-
 dient,  and  the  united  particles form  a  particle  of the
 compound.  Particles of this kind, or ingredients, cannot
 be separated by  mechanical  force, but require the.action
 of heat, or of a third kind of ingredient, to effect a separa-
 tion.
   Of this kind of  union there  are different classes,  which
 chemists have attempted to distinguish  and mark by differ-
 ent names.
   When different  metals are  mixed together, we obtain
 particles which retain the former properties of the ingre-
 dients.    In  like manner, the mixture of different liquors
 is frequently attended by no change of chemical proper-
 ties.   The same thing obtains  in  the solution of salts in
 water, of resins in alcohol, of sulphur in oil,  &c.   Yet is
 the combination  of  chemical, and cannot be  undone by
 mechanical forces.    Such combinations are usually7 called
 mixtures.
   In other cases,  the properties  or sensible  appearance
 and chemical action of the ingredients are totally changed,
 and new ones induced.   Thus, aquafortis, and soap ley,
 are two of the  most  acrid and  corrosive substances in
 nature ;   their taste  is  intolerable,  and they  quickly
 destroy our organs ; but, united, they compose saltpetre,
 a mild and harmless salt.
   This  is true chemical combination, or composition; and
the substance produced is a true chemical compound.  And 
            MIXTURES....COMPOUNDS.        269

this combinafion is also distinguishable by mutual satura-
tion of the  ingredients,  whereas  a mixture  is possible
with any proportion  of them.
   These two kinds of combination are  not so  perfectly
distinct that no exceptions are to  be found  in  some  cir-
cumstances.   Thus salt and water, in a certain  determin-
ate proportion, form a saline crystal.   Here  is saturation,
but the chemical properties are scarcely changed.  1 in  and
copper mix in any proportion,  but  the mixture  has all its
sensible qualities exceedingly different from  those of  the
ingredients.   It is highly  elastic,  brittle, sonorous, bril-
liant, and dense;  whereas the  ingredients  were ductile,
soft, unelastic, and comparatively light.
   Still, however,  the  distinction is useful when attended
to.
   The older chemists gave the name mixt to  chemical
compounds  consisting  of  two ingredients, which we have
never been able to reduce to simpler ingredients.   Parti-
cles of a mixt, compounded with particles  of another mixt,
formed particles of a compound;  the union  of  two com-
pounds formed a decompound:  and the union of two  de-
compounds formed a superdecompound, &c. &c.
  Modern chemists no longer  use  those  terms,  but  say,
compounds of the first order, of the second order, and so
on.

           GENERAL OBSERVATIONS.

   This doctrine of  chemical  affinity is,  unquestionably,
the great and distinguishing principle of the science, as
the laws of  motion  are of mechanical philosophy. . The
great advances made  in the latter science  have derived
but little  assistance from the  speculations  of  ingenious
men about the foundation of the primary laws of motion,
and the intimate nature of moving  forces.  Indeed it  has
been greatly obstructed by these speculations, and by  the
attempts to  explain  all forces by impulsion.   Kepler and
Newton acquired their immortal fame by the  discovery
and exact description of the  secondary laws,  the great or
general facts, really observed in the operations of nature •
   vol. i.                r t 
■270       GENERAL  OBSERVATIONS.

which result,  indeed, from  the  primary laws of  motion,
and  are the modification of those laws, by the  circum-
stances of the particular cases.   Su h are the three laws
of Kepler, and the great law of gravitation, which in-
cludes all the three.
   I  apprehend that chemical science  is, in like  manner,
 obstructed by speculations about the principle of affinity,
 and, particularly, by the attempts of ingenious men to ex-
 plain the chemical operations by attractions and repulsions.
 Even in mechanical philosophy, attraction is a metaphori-
 cal  term; and it is a mere  figure of speech to sav, that a
 loadstone attracts  iron.    But the resemblance in  effect  to
 real  attraction is  very great; and I  mean  nothing by the
 term but this  resemblance.   I mean only that the thing
 attracted actually  approaches the attracter.   But,  in chem-
 istry, even this is not observed in all  the operations which
 we explain by  attraction.   In them all, the supposition is
 gratuitous, and,  in many cases it  is  false  ; for  often the
 density  of the compound is less than the medium  density
  of the ingredients.  Therefore there is not  that local motion
  and approach  which alone authorise us to consider the
  event as the  effect of an attractive force.    In chemistry,
  the metaphor, both in the  expression and the thought, is
  double.   Union of  things, formerly unconnected, resem-
  bles, in a certain degree, the bringing into contact things •
  formerly removed from each  other:  and  this approxima-
  tion resembles the effect of a real attraction, an actual put-
   ling the thing nearer to us with the  hand.
     1 may venture to say that no man ever got a clear and
   really explicatory notion of a chemical combination by the
   help of attractions.   When we have, in imagination, en-
   dowed a piece of copper  wire with an  attractive  force,
   regulated  by any law wTe  please, and think that  this will
   explain how it acts on a solution of nitrate of silver, or
   upon water,  the mere communication of the piece of  cop-
   per with the rest of a galvanic circle changes the whole
   operation, and it either oxydates, or deoxydates, accord-
   ing to  the  end of  the  pile with which it communicates;
   and now, the particles which  are supposed to exert an at-
   tractive force,  or  to attract, give  evidence of repulsion, 
GENERAL  OBSERVATIONS.         2~
 and vice versdf although no change of situation or distance
 been made in the particles attracted or repelled.
   The chemist will do more wisely, if he forego all specuia-
 tion about the ultimate internal action, and direct;.is  whole
 attention to the external phenomena.   Let him notice every
 general  fact in the phenomenon, and class thtm according to
 their extent, and carefully note the substances which are die
 subjects of these general phenomena;  these phenomena  give
 him all that he can discover of the nature of those substances.
 If he attempt to explain  the  internal  motion of the atoms,
 which are the  only true indications of the moving forces, he
 will  find it impossible.   He  may fancy.a certain attractive
 force and then deduce certain motions from its action.   But
 all the mathematicians in Europe are not  qualified to explain
 a single  combination by these  means.   When he speaks of
 attraction and affinity, let him  affix no further meaning to the
 terms than a  certain  faculty   of combining; and when he
 says  that the attraction of aquafortis  for potash is greater
 than for silver, let him mean  nothing  more than this,  that
 aquafortis combined with silver, will be separated by potash,
 but silver will  not separate aquafortis from potash.  We are
 altogether ignorant of the comparative intensities of the mov-
 ing forces,  and what forces co-operate, and what counteract
 each  other  in chemical  operations.    These  depend  as
 much on the circumstances and manner of action, as on the
 intensity of the forces.  The only explanation to be received
 of a chemical phenomenon is, to shew that it is a case of a
 more general phenomenon already known.  It  is thus only
 that  even a mechanical phenomenon is explained.   The lunar
 irregularities are explained by  shewing  that they are cases of
 universal gravitation.  The truly chemical explanation is ac-
 complished by  shewing that the circumstances of the pheno-
 menon are precisely, such  as  result from the acknowledged
 laws ^f that phenomenon from which we deduce the explana-
 tion.
  Let chemical affinity be  received as a first principle, which
 we cannot explain; any more than Newton  could  explain
gravitation, and let us defer accounting for the laws of affiniy, 
272          CHEMICAL  APPARATUS.
                                               •
till we shall have established such a body of doctrine as he has
established concerning the laws of gravitation.
  YA e mav sometimes have it in our power to assist the mind
in conceiving a chemical,phenomenon, by comparing it with a
mechanical phenomenon.   Thus, the chemical phenomena
arising from  galvanism may be conceived a little better, by
comparing them  with some magnetical phenomena.   The
crystallizati©n of salts may be illustrated  by comparing the
particles to little magnets.  But this is not explanation ; nor is
it even an illustration, unless on the supposition that we under-
stand something of the mechanical phenomena of magnetism.
  The late dissertations of Mr. Berthollet, on certain chemical
affinities, are full of information,purely chemical, and are more
free  from ill-judged applications of mechanism, than most
other chemical writings.   Yet they are not entirely free, and
occasion will  be taken to mention some  cases in which  his
explanations are incompatible with the  mechanical principles
which he certainly adopts. Mr. Seguin, also, has made some
very judicious observations, in his Dissertation on the chang-
es of chemical  Affinity  by   heat, published in different vo-
lumes of the Annales de Chymie,                   editor.
PART III.
      CHEMICAL  APPARATUS.

     AGREEABLY to the plan which I laid out in begin-
ning this  course,  we have finished  the  first  and second
division of the general doctrines of chemistry, and shall now
begin the third.   Under this I proposed to give an account of
the nature  and uses of the apparatus or instruments usually
employed in performing the  chemical operations and experi- 
•CHEMICAL  APPARATUS.         273
ments.  This is a subject which you are very  well prepared
to understand, by what has been already premised, in a gene-
ral way,  upon the subjects of heat and mixture.  And it is
necessary to take a view of it before we begin to consider the
objects of chemistry in a more particular manner.
   But under the title of the apparatus, or instruments, I do
not propose to take under consideration the nature and proper-
ties of water, of air and of earth, as  Dr.  Boerhaave has done,
Who has  given  elaborate  treatises upon these four subjects,
under the title of instruments of  chemistry.  It is true that
we apply heat to produce changes and effects upon different
bodies,  and that  when we employ it in this way, it may be
called, in metaphorical language, an instrument in our hands.
It is also true that we make use of water on many occasions,
as an assistant  by which we can  reduce some  bodies  into a
state of fluidity, and mix them more effectually together.  But
if, upon this  account, we must treat of  fire and of water as
instruments,  for the same reasons,  and with the same pro-
priety, I might bring most of the objects of chemistry under
the same denomination:  for, after we have learned by expe-
riments the  powers or qualities which belong to those bodies
we take advantage of this knowledge, and employ^hem ac-
cordingly to serve our purpose on different occasions.  Thus,
for gilding,  mercury is an instrument;  for separating some
metals from their ores, iron, and for others lead ;  various salts
for separating metals, and other bodies;  black flux for reduc-
tion.  But  to treat of iron, and lead, and quicksilver, under
the title of instruments of chemistry, would be a very improper
and preposterous method.  Dr. Boerhaave has been led into
this method by the metaphorical language of some of the old
systems of chemistry, in which it appears that earth, for what
reason I cannot tell,  has been likewise considered as an in-
strument and is accordingly treated of as such by the Doctor.
   For my part, under the title of instruments, (I mean what
is meant in common language) utensils  or  furniture neces-
sary for performing the chemical operations and experiments.
   When we reflect upon the different  particulars of which a
rhemical apparatus consists, we find it may be conveniently 
274
VESSELS.
divided into three parts.   1.  Vessels for containing the sub-
jects  upon which the effects of heat or mixture, or both to-
gether, are to be produced.   2.  Means of pioducing heat.
3. Means for applying it  in the  best manner to the subjects
on which it is to act, and for regulating its force.
  We shall consider the chemical apparatus according to this
arrangement, and first therefore describe the vessels.
  The variety which obtains among chemical vessels, in con-
sequence of which they are fitted for different purposes, de.
pends partly  upon the materials of which they are made and
partly upon their form.  I shall first mention the variety of
materials  which  we find it necessary to employ  in the con.
struction of chemical vessels,  and afterwards  describe their
differences in point of form.
  The remark with which Mr. Macquer sets out in treating
this subject is very just;  that were it  in our power to have
vessels completely adapted to chemical purposes,  we should
wish them  made of materials possessed of these qualities.
  1st.  We should wish them transparent, that we might see
distinctly what goes on  in them.
  2dly.  Qualified to resist the action of the most corrosive
substances.
  Zdly  To bear sudden alterations of heat without breaking
  4thly.   Strong, to confine elastic vapours.
  5thly.  To bear violent heat without melting or being other-
wise injured.
  But,  as there is no matter in nature possessed of all these
qualities,  chemists  are  under  the necessity  of constructing
their vessels of different materials, which have some of those
desirable qualities,  though they want the  rest; and in each
operation or process, they use vessels made of materials which
have  the  qualities more  especially necessary  in that opera-
tion. The different materials are glass, metal, earthen ware.
  Glass, when good, possesses the two first properties in suf-
ficient perfection.   It is transparent, and not easily touched
by corrosive substances.  But glass vessels neither bear sud-
den alterations of heat without breaking, nor are they strong,
»or do they endure a strong heat without melting.  The most 
VESSELS.                       275
troublesome imperfection is that inability to support sudden
alterations of heat and cold, which they have in common with
all brittle bodies, and which  was formerly explained.   The
way to diminish this imperfection in glass vessels, is to make
glasses which are to endure heat as thin as is consistent with
moderate strength.   Thus, the surfaces will be more equally
heated, and the  whole glass more flexible, so that the one half
may be heated, before the other with less danger.   Florence
flasks are extremely serviceable in  such operations, being so
thin, and of a glass  that is remarkably strong.   It is a  great
advantage too, to be spherical where the heat is applied, be-
cause this form  is easiest made thin, and equally so; and be-
sides, this  form receives heat  in a more equal and diffusive
manner.  It is  necessary too that the glass be well  annealed.
By  annealing glass, is meant the exposing it, while glowing
hot, to a red heat, which is allowed gradually to waste.  While
glass is of a low red heat, it has a great degree of soft tough-
ness, will bear the blow  of a hammer, and be dimpled rather
than break.  But if a thick glass be exposed in this hot  state
to cool air,  the outer surface cools and contracts, and.the soft-
ness of  the internal parts  enables  them to give way to this
contraction, like a ductile metal, and the glass grows a little
thicker.  When the outer surface has thus grown  cold and
hard, the inside commonly splits by contracting.   If it remain
whole,  all  the  parts are on the  stretch, and,  in this state, a
slight touch of flint, hard sand, or even hard  steel,  tears
this stretched surface with facility, and  the  sudden  shock
makes  the  vibration extend even to the outside, and all flies
in pieces.
   To remove this imperfection, the glass is made to cool ex-
tremely slowly, so that all may contract alike, and take the regu-
lar and natural arrangement of the parts. The workmen never
anneal funnels, because they say that the inside cools as fast as
the outside.
   When thus constructed, glass vessels are sufficiently secure
provided we be cautious to heat or cool them gradually, and
they are useful in a great variety of operations.   They are 
276         MATERIALS FOR VESSELS.
always preferred indeed where great heats or sudden changes
are not necessary, and where strength is not required.
   Metallic  vessels have the third and fourth qualities in per-
fection, which glass wants, viz. to bear sudden vicissitudes,
and to be strong.   But they are deficient  in all the rest, for
they are not transparent;  they are liable to be corroded, and
dissolved by many active substances ;  nor do they support a
violent heat, without melting,  or being destroyed by it.   The
most troublesome imperfection that  attends them is that they
are so liable to be corroded or rusted, and to taint the subjects
put into them.   This is unfortunately the case in such metals
as are most  usually employed, namely, iron or copper.   They
are in some measure improved by  tinning, but imperfectly.
Besides iron and copper in common  use, lead and tin are used
too in some particular operations, when a very moderate heat
only is necessary,  and when acrid substances are treated ;  as
in the manufacture of alum, or copperas, because they are less
liable to corrosion  and rusting.  For  very nice purposes, when
heat is necessary,  and all neatness and cleanness must be stu-
died with scrupulous care, as  in drying the gold after the de-
part, vessels of platinum are used.
   Earthen ware is the third and list matter employed  in the
construction of chemical vessels ; and the best kinds of it pos-
sess some of the desirable qualities  in a high degree of per-
fection, especially ^the power to withstand violent heats, without
melting, or otherwise sufft ring.  The basis, in general, is clay,
which, when good, has properties that rtnder it very conveni-
ent and useful for the formation of vessels.   These properties
are ductillity when moist, so as  to be easily wrought into any
form  ; and density, solidity,  firmness, are acquired by burn-
ing;  whik, at the  same time, it endures the most violent heat
without further  change.   These are the qualities of the very-
pure  kinds, as pipe-clay, Stourbridge, kc.  Others are impure
and melt or vitrify.  Of clay alone,  very dense and close ves-
sels may be obtained, with sufficient heat;  but they are ex-
tremely liable to crack.  This  is more or less prevented  bv
mixing the clay with substances that do nbt rarefy much  by
heat, and that divide, pr render it more porous.  Sand is a pro- 
VARIETY  OF FORMS.            277
 per substance, taking care that it be of a kind which does not
 dispose the pure clay to vitrification*   Powdered flint,  or
 quartz, are the best; also the fragments of clay vessels, which
 have already served, and become hard in the fire, are beaten
 into a coarse powder for the same purpose. Black lead is one
 of the finest ingredients for this  mixture:  but it makes  a
 porous composition,  which some melted matters readily soak
 through.  A greater or  larger  quantity  must be mixed ac-
 cording to the purposes intended.  (Vide Pott Berlin Mem.)
 Where the most perfect closeness of texture is required, porce-
 lain is chosen.  This is a composition, and will be described
 afterwards ; as will also Reaumur's porcelain, when we  con-
 sider the earths as subjects of chemical examination.
   For some very particular purposes, but of frequent occur-
 rence,  the  chemist  makes use  of vessels,  called  coples,
 formed of bone-ashes.
   The variety of vessels, in point of form, depends also on
 the uses for which they are intended.  To give a summary
 account of these uses, I shall  first remark, that, in the conduct
 of chemical experiments and operations,  we have occasion,
 either, 1st,  To mix fluids with one another, or with solid sub-
 stances, and often to promote their union and action on one
 another by  moderate heat;  or 2dly, To melt solids together
 with a strong heat 5 or, 3dly, To convert the whole or a part
 of the subject we are working upon into vapour.
   One of these three modes of operation is practised in every
 chemical process or operation ; and all the vessels we employ
 in our operations are fitted by  their form to one of these three
 intentions.   We may therefore divide them into,
   1st.  Vessels for making mixtures and solutions.
   2dly.  Vessels for fusions.
   Zdly.  Vessels for vaporations.
   These divisions comprehend all the vessels employed in the
operations of chemistry.  There is one other set of vessels,
however, necessary to the  chemist, and these are,
  4thly.   Vessels for the  preservation of his materials and
products.
   vol. 1.               v  u 
27S VESSELS FOR C1RCULATION....COHOBAT.

   First, therefore, of the vessels for  making mixtures and
 solutions.  There is  a little variety in  the  forms of those
 vessels, in consequence of some diversity in the nature of the
 subjects, and in the practice of the operations.   1st, We have
 frequent occasion to make mixtures in which there are some
 ingredients  considerably volatile, and part of  which would
 evaporate, were they exposed to the air.   It is also proper,
 in making many mixtures, to promote the action of the ingre-
 dients on one another by gentle heat, or by digestion ; which
 digestion cannot  be performed in a common  open vessel,  on
 account of the evaporation which, would take  place.  Vessels
 which serve  these purposes are,  the  Phiala  Chemica,
 Matrass,  or Bolthead, or vessels  approaching to that
 form, such as flasks.  The propriety of this  form is easily
 seen.  Their oblong shape and small orifice prevents the loss
 by evaporation, and allows  a considerable agitation, for pro-
 moting the mutual action of the contents, without any risk of
 spilling*  The bulb is made thin, to bear external or internal
 heat.
   But sometimes we apply7 greater heat to promote the action
 of the materials ; such a heat as raises them to vapours.   As
 these vapours must be condensed^ and made to return into  the
 vessel, the  chemist uses an apparatus  of vessels, called a
 Circulating Apparatus, or Pelican, from its distant
 resemblance  to a bird pecking  its breast.   The apparatus
 consists of a tall pear-shaped vessel, for receiving the subject.
 This has.a broad-headed, or turnip-shaped capital, fitted into
 its mouth.    The vapours rise into this head,  and are  there
 condensed,  and would  fall back through the rising vapours,
 and condense them also; but  this is avoided by making two
 pipes branch out  from the lower side  of the head, and reach
 pretty far down, where they are inserted, sloping into the sides
 of the upright body*   Thus, the fluid produced by condensa-
 tion in the head, runs down by the lateral pipes into the body.
 This operation is called Circulation.
    Cohobatiox is merely the pouring back  into the body of
 the matrass the liquor which has been collected,  by properly
 condensing the vapours.  Here we do by hand, and from time 
          DIGESTION....CEMENTATION.      279

to time, what the circulating apparatus performs without our
attendance.
   In these ways of applying heat to our mixtures, however,
we cannot raise it higher than the degree which the materials
can bear  under the ordinary pressure of the air.  I told yon
formerly that Dr. Papin, an ingenious physician and chemist,
has gone much farther, and has contrived a machine, by means
of which we can apply to some of these mixtures a  much
greater heat  than they can bear in  ordinary circumstances.
Such is his Digester formerly  mentioned.  In this machine
the operation called Digestion  is pushed to the greatest heat
possible.  You are  doubtless  aware that the operations with
Papin's digester must always be conducted with caution, that
accidents may not happen by the vessel's bursting.  The di-
gester must be made of copper, soldered with hard solder, and
its lid must be provided with a loaded valve, which will allow
the  vapour to escape when  its elasticity becomes hazardous,
By loading the valve at the rate often, twenty, thirty7, or forty
■pounds on a  square inch of the surface of the valve, we shall
raise the heat of watery mixtures to about 236°, 254°, 270°,
and 284°, of Fahrenheit's scale.   By managing the fire so that
the valve only puffs now and then, under these loads, we shall
know pretty  nearly the heat  of  the operation.   The digester
has long been used in Germany for  the purposes of cookery,
such as for making strong broths from bones and sinews and
the less delicate parts of meat.
   There is another operation, in which, on account of the na-
ture of the materials, which are less volatile,  a still greater
heat is applied to make them act.   The materials are laid
stratum si/per stratum, and pressed close together, into vessels
of a proper form,  made of earthen ware, or other materials
which will stand the red heat which is necessary for converting
one of the substances into vapour,  in  which state only it is
able to penetrate and  act  upon  the other in the manner we
want. This is a process chiefly practised in metallurgy. From
some resemblance between the arrangement of the materials,
and the laying of bricks  in  mortar, this practice has been
called Cementation. 
280     CRUCIBLES,...HESSIAN....AUSTRIAN.

  Other instruments employed by the chemist to facilitate
mixture and solution, are the mechanical instruments by which
solid substances are broken into  fragments,  or pounded to
powder, to prepare them  for being  dissolved or mixed with
others ; or, if  they are malleable, they are  beaten into thin
plates,  or reduced to filings ; all which is effected by  means
of mortars, sieves, files, the hammer and anvil, &c.
  In the last place, as it often happens that the matter we are
employed in  dissolving does not dissolve completely, but that
a sediment, or faeculency, or other undissolved matter remains
we purify the solution  by  filtration or by subsidence.
   We must next consider the vessels employed in the opera-
tions in which solid materials are melted together with a strong
heat.  These are Crucibles, which, by their taper form, and
the  triangular  form of the brim,  are  fitted for precipitation,
and for pouring out.  Besides the mere crucible, we must
have covers, to keep out  the coals or ashes.   A smaller cru-
cible, inverted on the mouth of that containing the materials,
is a very convenient cover.  They must generally be set on
pedestals of considerable height, that there may be abundance
of  glowing coals below as well as around them.  They are
lifted, for pouring, with a pair of crucible tongs, the ends of
which  are bent into a shape fitted for grasping  the crucible,
and holding it steadily.   The  contents are poured out into a
 mould, called an Ingot. Mould, or into a conical brass vessel,
 shaped like and old-fashioned  beer glass.   A common small
 brass  mortar  will answer the purpose verv well.   These
 moulds are rubbed with chalk, or are smoked, to prevent ad-
 hesion.
   The materials of crucibles are always earthy compositions,
 made with great care, as they are often exposed to great heats.
 There are but two varieties,in  common  use....the Hessian,
 and the Ipsian, or Austrian, or black lead.   The Hessian  are
 manufactured in immense numbers in the neighbourhood of
 Cassel.  They are composed  of  a very fine clay, mixed with
 a great proportion of sand, and are baked to a considerable
 degree of hardness.  They stand a most violont heat without
  vitrification.  Their great excellency is their compactness, not 
CRUCIBLES.                  281
easily penetrated by the melted matters.  Their defect, con-
nected with this compactness, is  a readiness to split,  when a
vein of  colder air sometimes gets  at  them while they are
intensely  hot.   The Austrian crucibles, composed of clay
mixed with black lead, can scarcely be acted on by any ma-
terials contained in them;  and they bear sudden  cooling
much better than the Hessian.  But they are so porous, that
some metalline glasses and flags soak through them like water.
They are of most use for melting the most refractory metals.
The ordinary method of preventing them from cracking is by
doubling them, that is, setting one within another, with pow-
dered clay between, or  by luting  without,  with a coating of
clay and sand.
   These vessels are employed for fusion, or for operations
that depend on it, when the  heat required is  considerable.
For sometimes others will do, as iron, for melting lead; glass,
for some salts, &c.
  We must next attend to vessels contrived for containing those
bodies,  or mixtures of  bodies, which are to be converted in
whole, or in part, into vapour.
   I had lately an opportuniy of remarking, that in operations
of this kind, we have one of  two ends in view.   We either
wish to dissipate the volatile parts, and to preserve the fixed only;
or we desire to preserve the volatile parts. The operationwe per-
form, is, therefore,  different in these two cases, and is called
in the first evaporation, and in the second distillation, or subli-
mation.   In evaporation,  the  vessels  are open ;  in the other
two, the vapour is confined,  and directed from the vessel in
Which it is formed into another, which is kept  cool, where it
is condensed again.  This is called Distillation, when the
vapours condense into a fluid: and Sublimation, when they
condense into a solid matter. The vessels employed in all oper-
ations in which vapour is produced, may therefore be arrang-
ed into those fitted for evaporations,  and those fitted for dis.
tillations or sublimations.
  The vessels employed  for  evaporations are  open vessels,
the materials and form maybe different according to the nature
df the subject.  If it is a mixture of water with other matters 
 282  VESSELS FOR EVAPORATION....BATHS.

 which have no corrosive quality with respect to metals, any
 metallic pan or pot, or other vessel, may serve. It the mixture
 be of a corrosive nature, the vessels must be of glass or earthen
 ware.   The best form of glass vessels for evaporations, is the
 globular ;  and,  where there is  an opportunity, they may be
 improved by hardening them in Mr.  Reaumur's way.  Or
 we may use vessels of the best and hardest kinds of earthen
 ware,  excellent ones for this  purpose  are  made  by Mr.
 Wedgewood.
  There are  some cases  of  evaporation,  in which the heat
 must be exactly regulated.  There are some substances which
 suffer from a brisk heat, as extracts, juices, or decoctions of
 plants ; and which it is  therefore proper to evaporate with as
 gentle  warmth as possible, by insensible or spontaneous eva-
poration.   If the utmost care be not taken, the extract, when
brought to its proper degree of concentration, is generally black,
 and has a burnt taste,  even although  the  heat appears far
below what would colour any vegetable substance.   Here we
 must make amends for the mildness of the heat, and promote
 the evaporation, by spreading out,such matters in as broad a
 surface as possible.  We therefore use, in this case, large flat
 dishes  of stone-ware, or porcelain; and the best way of using
these is to place  them in the mouth of a copper or iron vessel
 containing water. Thus, the steam of the water will heat them
 equally, and  secure them  from breaking,  and the heat will
never rh-: too high.  This manner of applying heat is called
 the balneum vaporis.   When the vessel is set in boiling water,
 it  is called balneum maria, and when in sand, it is called bal-
 neum arena.
  In some  cases of evaporation,  when the heats required to
 produce vapour  are very strong, and metallic vessels are im-
 proper, we use crucibles, as in evaporating some metallic sub,
 stances themselves.                                  «
  To these particulars,  with respect to the  conduct  of evapo-
 ration,  we  may add a method proposed not long  ago by Dr.
 Hales, for performing evaporation more quickly with a~entle
heat.   This is by blowing up showers of air,  as he expresses
himself, through the liquor.  This he  proposed,  in order to 
 5PIRIT....SUBLIMATE....FL0WERS....RESID. 283

 hasten the distillation of sea water;  and he found it effectual
 for discharging certain bad smells or flavours from milk, &c.
 An  instrument of this kind  is sold in  London, to sweeten
 stinking water at sea, which it does, by evaporating the putrid
 steams.  The instrument is  merely a long pipe, terminating
 in a broad streamer, like that  of a garden watering pot.  This
 is joined to the nozzle of a pair of bellows, and the streamer is
 immersed to the bottom of the fluid, and the bellows are work-
 ed,  to force the  air through  the holes, in a great number of
 small streams.  This contrivance  was applied, on a very great
 scale, to the evaporation of  water, in the manufacture of sea
 salt; but, instead of promoting the evaporation, it was found
 to check it prodigiously. It must do so, because, in this manu-
 facture,  a boiling heat is employed ; and  cold air showered
 through the boiling water, must both cool the water, and con-
 dense the* steam.   When we  come to consider this manufac-
 ture, we shall mention much more effectual methods of apply7-
 ing the abstracting power of air.
   Let us next consider the vessels employed in distillation and
 sublimation.   I said just now that we give these names to the
 operations  in which we direct vapour into  another vessel, or
 another part of the vessel, in which it is condensed  again by
 cold, and that the distinction  is taken from  the appearance of
 the condensed matter; if fluid, distillation ; if solid, sublima-
 tion.  I may also explain here a few more terms.  The term,
 Spirit, is often applied to the fluids obtained by distillation;
 aud  the Lerms, Sublimate, or Flowers, to the matters con-
 densed in sublimation ; the more  fixed matter which remains
 behind being commonly, in both cases, called the Residuum,
 and sometimes Caput Mortuum, when solid ai\d black.
  To return  to the vessels....Those employed in distillations
 have been suited  to three different ways in  which this opera-
 tion  has been conducted; and these three different ways got
 the names of distillation;  1. per  descensum, or downwards
 2. per ascensum, or upwards ; 3. ad latus, to a  side.
   The first is, when the heat is applied above the subject, and
the vapour is forced directly downwards,  and condensed in a
part of the vessel, kept cool below.  See in chemical books, 
284
DISTILLING VESSELS.
the manner of obtaining oil of cloves in this way.   But it is
laid aside, and there is no operation of this kind in use at pre-
sent, except the preparation or manufacture of tar, which re-
sembles this, and the separation of zinc and mercury from the
ore.
  Distillatio per ascensum, is where the vapour is allowed to
follow its  natural disposition, and  to ascend to some height
upwards from the subject, and is then directed aside into a
cold cavity, where it is condensed.  For this a variety of ves-
sels have been contrived, the most common of which is the
common copper still, so well known that it needs little des-
cription.  It  consists of a  body, a head,  and a refrige-
ratory, or cooler.   From the head issues a tube,  inserted
into the upper end of another tube, which  passes obliquely
downwards through the refrigeratory, but without communi-
cating with it.  This last tube is often formed into a' cylin-
drical spiral, like a cork-screw7, in order to increase the quan-
tity of cold surface, to which the passing vapour may be ap-
plied for condensation.   The liquor produced trickles down
the  sloping spiral, and is received  into a proper vessel.   Some
have them  with different heads;  and it is  also common  to
have a watet-bath-still adapted to the common one:  that is,
the  body of the still is set into another body, distant from it a
little way all round.   Water is kept boiling in the outer vessel,
and then a most regular and uniform heat  is applied to the
substances  contained in the inner body.   This  still  answers
for  the distillation of  all such subjects as are not disposed to
dissolve  or corrode the metal still, or worm.   But when we
have corrosive acrid substances to deal with, or when we have
a small quantity of  any  matter,  we use glass vessels.  And
those that resemble the common  still most in form,  are
cucurbits, with alembics  or capitals, and  a receiver.  Cu-
curbits are called bodies by the English chemists.   These
are  made higher or  lower.   The  two joints must be made
tight, by lute, or slips of linen smeared with clammy  matter,
and then lapped round, over the joints.
  But when  glass vessels  are proper,  either on account of
the  acrimony of the subject, or its small quantity, or our desire 
      CUCURBITS....RETORTS....ADOPTERS, &c. 285

  to see the phenomena,  the  vessels most simple, and gen-
  erally most convenient, and therefore most commonly used,
  are  the retort and receiver.   The distillation performed'
  in these is the distillatio ad latus.   It is more convenient in
  the  greatest number of cases, because there is only one joint.
  Retorts are made of different heights, and may be placed dif-
  ferently.  The materials are  commonly introduced into  re-
  torts by a funnel.   But for some operations,  it is proper  to
  have tubulated retorts ; that is,  having a tube formed at top,
  through which the subjects can be poured.   The tube is then
  shut by a cork, or a ground glass stopper.
    Glass  retorts  serve  for a  great variety of distillations,
  where moderate  heat is sufficient.   If a stronger heat be re-
  quisite, the  retorts must be coated with clay, which helps to
 prevent both breaking and collapsing; or they must be used
 of earthenware, like that of crucibles.  Green glass coated is
 better.  For particular  purposes, it  is  convenient to have a
 sort of iron retort, of two pieces.
    The ordinary form of receivers  has been globular, but a
 conical form,  ending with a hemisphere, is better.  Some-
 times a hole is bored in the neck, that the fumes may escape,
 which  would  otherwise burst  the vessel.  Some  receivers
 have a tube joined to their bottom.  These are  called spout
 receivers.   In operations upon some very volatile substances,
 the older chemists imagined that a simple receiver was not
 sufficient, and they used a series of them, each of which was
 terminated in a wide tube, which fitted into the neck of the
 next.   Such were called adopters.  They are never used
 now,  and a large  receiver is much better.   In very difficult
 cases of this kind,  some  of these  intermediate vessels are
 found indispensably necessary.    Mr. Peter  Woulfe, F. R. S.
 has contrived an  apparatus for  those  difficult distillations,
 which,  by proper modifications, will obviate  every  difficulty.
 A particular description of this apparatus would take up much
of our time, and therefore I refer you to Phil. Trans, vol. 57.
p. 517.
   After, or during some distillations, we have  occasion to
make use of  a  separatory, which is a glass vessel, of a tall
    vol. i.                    x x 
286          SUBLIMING VESSELS, &c.
form, having a spout rising from nearly the bottom, and rather
exceeding the vessel in height.   It is plain that  by this con-
trivance, the lower parts can be poured off, without disturb-
ing or being disturbed by the upper parts.
   For sublimation, the vessels employed are sometimes the
cucurbit and capital, because, in many sublimations, some li-
quor  comes before the dry matter  sublimes.   But in  such
cases, we more commonly  use a retort with a wide and short
neck and a receiver; forwhich purpose the conical form is much
the best.  For particular purposes we use an oblong or spheri-
cal glass vessel,....a Florentine flask, or even a common phial;
and sometimes the aludels, or allodials, which resemble the
antique receivers which are mentioned  above, are used in a
series.
   Having now finished the list of vessels which are employed
in chemical operations, there only  remain to be  mentioned
those necessary for the preservation of many of the products
of chemistry.   Many of those products are volatile, and re-
quirs to be shut up closely in vessels.  Others are affected by
the humidity of the air;  and it happens, at the same time, that
many of them are  very corrosive, and destroy common corks.
Such  can be preserved,  therefore, only in glass  vessels, with
glass  stoppers, that shut them close; and a number of such
phials are necessary.
   It is also proper to subjoin to the  enumeration of the ves-
sels, the mention of a means by which we alter them on some
occasions, by cutting off parts of them  to fit them better to
our purpose.  This is done by an iron knife or rod made red
hot.  A small scratch being first made on the edge of the
glass, the red  hot knife is  applied there, and presently splits
the glass in  that part.  By  keeping the rod applied, and turn-
ing the glass  very slowly  under it, this crack  may be con-
ducted  wherever we please, and the whole glass may be re-
duced to  a narrow spiral ribbon.   To  cut thick bottle  glass
quite round, we may tie round it four threads of cotton-wick,
and wet it with oil of turpentine, and set it on fire ;  and when
it is burnt out, sprinkle water on it, and it will  separate  im-
mediately7. 
MEANS OF  PRODUCING HEAT.      287
  Having given you  such remarks as  may  serve  to direct
you in the choice of the vessels employed in chemical opera-
tions  and experiments, we pass to the second part of this
section,  namely, the means for producing, and  the way  of
managing the heats which are required in those operations.
  The means which  the chemists, as well as others, employ
for producing heat upon  ordinary, and almost all occasions,
is the inflammation of combustible bodies, or the burning of
fuel.   But as there are several other sources of heat beside
this, most of which have been used occasionally for particular
purposes, and some of  which may be  capable of producing
singular effects;  I shall enumerate them shortly, and just no-
tice  what are the  purposes to which they have been  ap-
plied, or which  they may be capable  of answering in  che-
mistry.
   The means by which various degrees of sensible heat are
produced and communicated are, I think, seven:
   1.  That principle or  power in animals which keeps them
warm, and enables them to communicate heat to other bodies.
  2.  Friction or percussion of hard bodies.
   3.  Electricity.
  4.  Mixture.
   5.  Fermentation or putrefaction.
   6.  The sun's  rays.
   7.  Inflammation of fuel.
   1.  The heat produced  by the first of  these causes  is em-
ployed for no chemical purpose, except the regulation of ther-
mometers.  The heat of  the person who makes  them being
always the  same while in health, he  can  find  this  degree
on a  new thermometer, by putting it  in his  mouth for some
time.
   2.  Friction or percussion of hard bodies is applied to some
purposes in the arts and in common life ; as for kindling gun-
powder in the use of fire-arms, and to kindle fuel according to
the Indian practice;  but  it is seldom used  by the chemists
except occasionally for kindling fuel*.
     r                                            i
  * The experiments of  Mr. Picte't of Geneva, on the heat produced by
moderate friction in the air and in vacuo, are curious, and promise to lead 
288  HEAT PRODUCED BY FRICTION....ELECT.
  3. The third means I mentioned, electricity, produces the
electrical fire or flash which appears when  the electrical  fluid
is greatly condensed in its motion from one place to another.
We thus produce very violent heats....heats perhaps superior
in violence and intensity to any other whatever. This appears
from their effects, 'such  as melting gold, and iron,  and other
metals in a moment, and even dissipating them,  or convert-
ing them  into  vapour,  which penetrates  the surface of the
glass on which the experiment is made.  But we  can produce
these heats by  electricity,  in such quantity only as is sufficient
for heating very  small  masses of  matter.   The  reason why
we succeed  with  these is,  that  the heat  is  produced not
gradually,  but instantaneously ; so instantaneously, that there
is not  time for any part of it to be  lost by being communi-
cated  to other bodies before it  produce  its  effect.  Hence
money has been  melted  in the pocket, and a sword  in the
scabbard.  Dr. Lind melted steel under water by an electric
flash.
   Such  facts once suggested the notion that the  fusion occa-
sioned by  electricity was a cold fusion ;  but this is a mistake.
A wire too thick  to  be  melted may be made red hot.   Elec-
tricity, therefore, is a most powerful means for producing heat,
and produces  it in a very particular manner.  However, as
chemists have not much attended to these particulars, merely
as  sources of heat, the  instruments for  exciting  electricity
would be improperly considered as a part of the chemical ap-
paratus.
   4.  The fourth means of producing heat is the  mixing par-
ticular bodies.   This is not  applied  to use  in the practice of
chemistry for producing heats, because the heats produced in

to some further knowledge cf heat.  He published an account of them in the
Journal de Physique, and also in a small volume, Geneva, 1790.
  Also the great experiments made by Count Rumfordon the production  of
heat by the friction of metals, are peculiarly valuable, and lead to much
speculation concerning heat.  See Phil. Trans. 1798.
  It is worthy of notice that this production of heat is accompanied with
light,  even in cases where it cannot be ascribed to  combustion, as in the
collision or friction of pieces of quartz, and other siliceous and hard bodies.
                                                      EDITOR. 
  MIXTURE....FERMENTATION....PUTREFA. 289

 this way are all momentary and transient.   Some of them are
 used by  the  fire-eaters and  raree-show-men  for  suddenly
 kindling tow, &c.  They will be mentioned in  the course of
 our examination of chemical substances.
   5. The fifth means  I mentioned as sometimes employed,
 is by taking advantage of the heat generated in  the fermenta-
 tion or putrefaction of vegetable and animal substances.   Fer-
 mentation and putrefaction are changes to which animal and
 vegetable substances are liable, when preserved from exces-
 sive cold, and tempered with a moderate quantity of moisture.
 These changes come  on gradually, and require some time to
 complete them.   During this time, the contexture of the sub-
 stances is destroyed, their principles enter into new combina-
 tions, and the ostensible chemical properties of the matter are
 quite changed.  While this change  goes on, there is a con-
 stant emission of heat from the mass, which can therefore be
 employed to  communicate this  heat to other bodies.   The
 heat produced in  this way is not strong, but very lasting, and
 cheaply procured.  Hence it is  used for  promoting vegeta-
 tion ; and, in  Egypt, it has been long in use for  the  hatching
 of eggs, in particular parts of the country where they rear in-
 credible numbers of fowls (Vide Reaumur). And in chemistry,
 it has been often employed for the digestion of various mix-
 tures, to which purpose it is  remarkably adapted.   In some
 chemical  arts too, the heat thus  produced is  employed  as a
 digesting heat, as in the  manufacture of  verdigrease and of
 white lead.
   The subjects which have been  commonly  used are horse
 dung, and the oak bark of the tanners, either of which is capa-
 ble of maintaining a heat  equal to 120° Fahrenheit or more.
 The first of these acquires this heat soonest, and is capable of
 becoming hotter than the other ; but the intestine motion, and
 the generation of heat is  sooner over, commonly in about a
 month.   The heat produced by  the putrefaction of bark is
 more gentle, but more lasting, so as to continue, if properly
 managed, for  some months.  The particulars  to be attended
 to in using this means of producing heat, are,  first, that there
be a proper degree of moisture.  If there be either too much 
290  HOTBEDS OF  TAN....HORSE  DUNG, &c.

or too little, the fermentation will not go on properly, and little
or no heat will be produced.  Secondly, that a proper quan-
tity of materials be gathered together in one  compact heap,
The reason why a small quantity, or a heap that is not com-
pact,  will  not'answer the purpose,  is that the surface of the
mass, which is in contact with the surrounding air,  or other
cold bodies, bears so large a proportion to the contents, that the
heat is carried off as fast  as it is produced within.   But if i\\e
heap be considerable, and properly compacted, the dissipation
and waste of the heat is  diminished, and it is accumulated in
greater quantity in the fermenting materials.  In  some cases,
when  very large quantities of tender vegetable substances are
heaped up together, and  a due degree of moisture conspires
to promote this intestine motion, the waste of heat is so small.
and its extrication so copious and quick, that the mass is con-
sumed by it, as is well known to have often happened to ricks
of hay, when put up too green or wet, so that they contained
moisture enough to admit of putrefaction.
  6.  The heat of  the sun's rays.  These, when  a subject is
fully exposed to them in a sheltered situation, give a heat that
is not inconsiderable, equal even to that which is commonly
produced  by putrefaction.   (See note 25,  at the end of the
volume.)  The heat of the  sun's rays  may be usefully em-
ployed in  digestions, &e. but it is not so constant as  that ob-
tained by  putrefaction.   As  the  heat  thus produced  is not
attended by humid steams,  it is useful  also in drying  many
bodies, or dissipating their moisture.
  The rays of the sun have  a particular effect upon  many
coloured substances,  in discharging the colour.   They are a
severe trial of dyes.   Moreover, some bodies are  remarkably
changed in their composition by being exposed to the light of
the sun.
  Such are the effects of  the rays of the sun, in their ordinary
state  ; but they are remarkable on account of the amazing
violence and intensity to  which we can increase  their effect,
by  condensing them, or directing  a  great number to  fall
upon  the  same spot.  This  is effected  by concave mirrors,
•r convex lenses.  Instruments  of both  these kinds have 
           HEAT FROM THE  SUN'S RAYS.      291

 been executed, at different times, which produced effects that
 appeared astonishing when they were new;  such as melting
 in a moment many  earths and  stones, which were  reckoned
 before perfectly unfusible ; converting into smoke, gold, silver,
 and other metals, which were imagined before to be per-
 fectly fixed.   See some of those facts related by Boerhaave,
 and in the  Memoirs of  the Academy of Sciences of  Paris.
 The most famous of those instruments are a  speculum, made
 by the Viletti of Lyons, a father and two sons, and a lense,
 made  by the   celebrated  Baron  von Tchirnhaus*.   The
 diameter of the  speculum is  43 inches,  the  diameter of the
 focus half an  inch, the distance  three feet,  its  weight 400
 pounds French.  The lens belonged to  the  regent Duke  of
 Orleans,  and  with it were made the experiments related by
 Homberg and  Geoffroy,  in the Memoirs of the Academy.
 The diameter  of  it is 33  inches, the focus at the distance  of
 12 feet, and the diameter of the focus l£ inch.   The  effects
 of  the speculum were, however, much greater than those  of
 the lens,  and for this plain reason, that the image of the sun
 is formed by it at a small distance, and therefore not so  broad,
 and the rays are therefore condensed into less  space.   For
 this reason speculums have  more  effect, in general, than
 lensesf.   Specula of different sizes are not so different from
 one another in  power as  we might imagine.  This will not
 appear surprising, when we consider that, supposing the ex-
 actness  of workmanship equal, the rays will  be equally con-
 densed  by every speculum.  The diameter  or breadth of  a

   * Both of these ways of kindling fire were known to the ancients.  Euclid,
 in  his Catoptrics,  mentions axpressly  this effect of a spherical speculum,
 only mistakes the situation of the focus.  Plutarch also,  in his life of Numa,
 says that the vestal virgins kindled their fires Scaphiis.  The stratagem  of
 Strepsiades,  in Aristophane's comedy, for  burning the documents of his
 debts, by a globe of glass filled with water, gathering the sun's rays on them,
 is a proof of the other point.
                                                  EDITOR.

  f Lenses are  inferior, for another reason.  They absorb or intercept  ft
great proportion of the heat. This is evident from Scheele's experiments.
                                                  EDITOR. 
292      HEAT FROM THE SUN'S RAYS.
     speculum is always proportioned to the sphere on which it is
     ground.  A spherical surface collects the light  in the focus,
     only in so far as it coincides with the apex of  a paraboloid,
     and it happens to be but a small portion of a sphere that coin-
     cides with  it.   The speculum  is most  powerful....the  lens
     most convenient,  because  the speculum  directs the  rays
     upwards  towards  the  sun.   The  object  cannot  be  so
     conveniently  exposed  to  them in  that  situation,  and it
     interrupts light.  It is usual with lenses, to  employ another
     lens, of ^  of  the  diameter and  focal  distance, to collect
     the rays still more.  Thus  the  most  intense heats are  pro-
     duced.
       There are, in the Memoirs of the Academy, papers upon
     both these kinds of instruments, and  many projects are  pro-
     posed,  and  experiments  related, with a  view  to  construct
     them with greater  facility and  perfection, by  Mr. Buffon.
     This gentleman may be  said to have reinvented the  con-
     trivance put in  practice by Archimedes,  when he set fire to
     the ships of the enemy's  fleet, employed in  the siege of
     Syracuse, (Vide Encyclopedic)  This story was long con-
     sidered as one  of those  marvellous fables which are related
     of  the great men of remote antiquity.   But the  experiments
     of  M.  Buffon  have shewn that the thing is possible ;  and
     Tzetzes,  a  Greek author,  describes  the contrivance  of
     Archimedes, which proves exactly the same as the invention
     of Mr.  Buffon*.  I call it  invention,  as I do  not suppose he
     was acquainted with the Greek author,  otherwise he would
«    have acknowledged the hint he got from him.   Mr. Buffon

      * Zonaras mentions another instance of this contrivance.  Proclus burnt
    the ships of Vitellianus, at Constantinople, in the same way.  This was in
    the year 514. Kircher undertook to prove the truth of those ancient stories
    by experiment,  and, following the description by Tzetzes, of plain mirrors
    turning on hinges, (yiyyXv/xois)  he produced a very great heat by five
    mirrors, which he made to unite their reflection at 100 feet distance.  Vitellio
    quotes a passage from Authemius, the architect of Justinian, mentioning his
    having kindled  wood by  24 mirrors.  This work cf Authemius is now lost,
    and  never has  been printed.   Vitellio had no inducement  whatever to
    falsify.
EDITOR. 
BUFFON'S BURNING MIRRORS.
  employed a great number of small glass speculums, of  6, 7,
  and 8 inches broad,  framing them so together  that  their
  separate reflections  united at a convenient distance.  With
  40, he kindled deal  boards at 66 feet distance, and with 128,
  he did this at the distance of 150 feet.  With 45, he  melted
  tin at 20  feet  distance, and  with  117, he  melted silver.
  At another  time  he  kindled   wood  at  200  feet,  melted
  tin at  150, lead at  130, and  silver at 60, employing 168
  mirrors.   His  contrivance  had the eminent advantage  of
  directing the light downwards,  which made all experiments
  easy.
    7 We are  next to  consider  fuel,  as the most easy and
  manageable means of producing heat.   We can easily manage
  this source of heat, so  as to have all the different heats under
  command,  from the most mild and  gentle, to one approach-
  ing, in force,  to that occasioned by the collected rays of the
 sun.
   This variety in the effect of fuel depends upon the various
 nature  of the fuel itself,  and upon the management or com-
 mand of its inflammation. With regard to the variety among
 fuels,  it  is  very great.    But we shall notice only the most
 remarkable  differences among  the  fuels commonly used.
 They may be reduced to five divisions.  The first may com-
 prehend the fluid inflammables ; to the second we  may refer
 peat or turf; to  the  third charcoal of wood;  to the fourth
 pit-coal charred; and  to the fifth wood, or pit-coal, in  a
 crude  state,  and capable of yielding a copious and bright
 flame.
   1st.   The fluid  inflammables are considered as  distinct
 from the solid, on this account, that they are capable of burn-
 ing upon a wick, and become, in this way, the most manageable
 sources of heat;  though, on account of their price,  they are
 never employed for producing it in great quantity,  and are
 only used  when a gentle degree, or a small quantity of heat, is
 sufficient.   The species which belong to this class are spirit
of wine, and different oils.

   vol. i.                 Y y 
294   HEAT FROM  FUEL....SPIRIT OF WINE.
  The first of these,  spirit of wine,  when pure and free of
water, is as convenient and manageable a fuel for producing
moderate or gentle heats as can be desired.   Its flame is per-
fectly clean, and free from any kind of soot; it can easily be
made  to burn slower or faster,  and to produce less or more
heat, by changing the size or number of the wicks upon which
it burns ;  for, as long as these are fed with spirit, in a proper
manner,  they continue to yield  flame of precisely  the same
strength.   The cotton, or other materials of which the wick
is composed, is not scorched or consumed in the least, because
the spirit with which it is  constantly soaked, is  incapable of
becoming hotter than 174° Fahrenheit, which is considerably
below the  heat of boiling  water.  It is only the vapour that
arises from it which is hotter, and this too only in its outer
parts that  are  most remote from the wick, and where only
the inflammation is  going on, in consequence of  communica-
tion and contact with the air.  At the same time, as the spirit
is totally volatile, it does not leave any fixed matter,  which, by
being accumulated on the wick,  might render it foul, and fill
up its pores.   The wick, therefore,  continues to imbibe the
spirit as freely after some time as it  did at the first.  These
are the qualities of  pure spirit of wine as a fuel.  But these
 qualities belong only to a spirit that is very pure.   If, on the
 contrary, the spirit is weak, or contain water, the water, being
 less volatile, does not evaporate so fast from the wick, as the
 more spiritoua part; and the wick becomes, after some time,
 so much soaked with water, that it does not imbibe the spirit
 properly.   The flame becomes much weaker, or is  altogether
 extinguished.  When  spirit of wine is used as a fuel, there-
 fore,  it 6ught to be  made as strong,, or free from water as pos-
 sible.
   Oil, although fluid like spirit of wine, and capable of burn-
 ing in a similar manner, is not so convenient in many respects.
 It is disposed to  emit soot;  and this,  applying itself to  the
 bottom of the vessel  exposed to it,  and increasing in thick-
 ness, forms by degrees, a soft and spongy medium, through
  which heat  is not  so  freely and quickly transmitted.  This
  was observed by  Muschenbroeck, in his experiments upon the 
OILS USED  AS FUELS.           295
expansions of metalline rods heated by lamps.  It is true that
we can prevent this entirely, by using very small wicks, and
increasing the  number,  if  necessary, to produce the  heat
required.  Or, we may employ one of those lamps, in which
a stream of air is allowed to rise through  the middle of the
flame, or to pass over its surface with such  velocity as to pro-
duce a more complete inflammation than ordinary.  But we
shall be  as much  embarrassed in another  way, for  the oils
commonly7 used, being  capable of assuming  a heat  greatly-
above that of  boiling water, scorch and burn the wick, and
change its texture, so that it does not imbibe the oil so fast as
before.  Some have attempted a remedy, by making the  wick
of incombustible materials, as asbestos, or wire : but still, as
the oil does not totally evaporate, but leaves a small quantity of
gross fixed carbonaceous matter, this, constantly accumula-
ting, clogs the wick to such a  degree,  that the oil  cannot
ascend,  the flames become weaker, and, in some cases, are
entirely  extinguished.   There is,   however,  a  difference
among the different oils  in this  respect;  some being more
totally volatile and inflammable than others.   But the  best are
troublesome in this way, and the only remedy is to change the
wicks often, though we can hardly do this and be sure of keep-
ing always an equal flame.
  The second kind of fuel mentioned, peat,  is so spongy, that,
compared with the more  solid fuels, it is unfit to be  employed
for producing very  strong heats.   It is  too bulky for this....
We cannot put into a furnace,  at a time, a  quantity that cor-
responds  with  the quick  consumption  that must  neces-
sarily go on when  the heat is  violent.   There is  no doubt
a  great  difference  in this  respect  among different kinds
of this fuel; but this is the general character of it.  However,
when we desire to produce and keep  up,  by means of cheap
fuel, an extremely mild gentle heat,  we  can hardly use any
thing better than peat.  But it is  best to have it previously
charred, that is,  scorched,  or burnt to black coal.  The ad-
vantages gained by charring it will  be  presently explained.
When prepared for use in that manner, it is capable of being
made to burn more slowly  and gently, or  will bear without 
296     PEAT  CHARRED....WOOD CHARRED.

being  extinguished  altogether a  greater  diminution of the
quantity of air with which it is supplied, than any other of the
solid fuels.  Dr. Boerhaave found it extremely  convenient
and manageable, in his Furnus Studiosorum.
   3d.   The next fuel in order is the charcoal of wood.   This
is prepared by piling up billets of wood into a pyramidal heap,
with several spiracles or flues, formed through the pile.  Chips
and brushwood  are put into those below, and the whole is so
constructed, that, when kindled, it kindles almost over the whole
pile in a very short time.  It would burst out into a  blaze, and
be quickly consumed to ashes, were it not covered all over with
earth, or clay, beaten close, leaving openings at all the spiracles.
These are carefully watched; and, whenever the white watery
smoke, is observed to be succeeded by thin blue and transpa-
rent smoke, the  hole is immediately  stopped....this being the
indication of all the watery vapour being gone,  and the burn-
 ing of the true coaly7 matter commencing.   Thus is a pretty
strong red heat  raised through the whole mass, and all the
volatile matters are dissipated by it, and nothing now remains
but the charcoal.  The holes being all stopped in succession,
as this change of the  smoke is observed, the fire goes out for
want of air. The pile is now allowed to cool....This requires
many days ; for, charcoal being a  very bad conductor of heat,
the pile long remains red hot in the centre, and,  if opened in
 this state, would instantly burn with fury.
   Small   quantities   may  be  procured at  any  time,  by
 burning wood in close vessels. Little pieces may be very finely
 prepared, by plunging the wood in  lead melted  and red
 hot.
   This is the chief fuel used by the chemists abroad, and has
 many good properties.  It kindles quickly, emits few watery
 or other vapours while burning,  and when consumed, leaves
 few ashes, and those very light.   They are, therefore, easily-
 blown away so  that the fire continues open, or pervious to the
 current of air, which must pass through it, to keep.it burning.
 This sort of fuel, too, is capable of producing as intense a heat
 as can be obtained by any;  but  in  those violent heats it is
 quickly consumed, and needs to be frequently supplied. 
       WOOD AND PIT-COAL CHARRED.   297

  4th.  Fossil coals charred, called cinders, or coaks, have,
in many respects, the same properties as charcoal of wood;
as kindling  more readily in furnaces than when  they  are
not  charred,  and  not emitting  watery, or other gross
smoke, while they burn.   This sort of charcoal is even
greatly superior to the other in some properties.   It is a
much stronger fuel, or contains the inflammable  matter in
greater quantity, or in a more condensed state. It is, there-
fore, consumed much more slowly on all occasions, and
particularly when employed for producing intense melting
heats.  The only inconveniences that attend it,  are, that,
as it consumes, it leaves much more ashes than the other*,
and these much heavier too, which  are,  therefore, liable
to collect in such quantity as to  obstruct the free passage
of air through the fire ; and further, that when the heat is
very intense,  these ashes  are disposed to melt  or vitrify
into a tenacious drossy substance, which clogs  the grate,
the  sides of the furnace,  and the vessels.  This last in-
convenience is only troublesome, however, when the heat
required is very intense.   In ordinary heats, the ashes do
not  melt, and, though they are more  copious  and heavy
than those  of  charcoal of wood, they seldom choke up the
fire considerably, unless the bars of the grate be too close
together.
   This fuel, therefore, is preferable, in most cases, to the
charcoal of wood, on account of  its burning much  longer,
or giving much  more heat before it is consumed.  The
heat produced by equal quantities by weight of pit coal,
wood-charcoal, and  wood  itself,  are nearly in the propor-
tion of 5, 4, and  3.   The reason why both these kinds of
charcoal are preferred, on  most occasions, in experimental
chemistry,  to  the crude wood or fossil coal from which
they are produced, is that the crude fuels are deprived, by
charring, of a considerable  quantity  of water,  and some
other volatile  principles, which are evaporated during the

  * I don't know that this is the case witli the finest pit-coal.  I have seen
some of the finest  Newcastle coals which did not leave ,U.h of their weight
of ashes, and even these did not seem entirely consumed.
                                                  EDITOR. 
  298     ADVANTAGES OF CHARCOAL.

  process of charring, in the form of sooty smoke or flame.
  These volatile parts, while they remain in  the fuel,  make
  it  unfit  (or  less  fit)  for  many  purposes  in chemistry.
"» For, besides obstructing the vents with sooty matter, they
  require much heat to evaporate them ;  and therefore the
  heat of the furnace in which they are burnt is much dimin-
  ished and wasted by every addition of  fresh fuel,  until
  the fresh fuel is completely  inflamed, and restores  the
  heat to its former strength.   But these great and sudden
  variations of the heat  of a furnace are  quite inconvenient
  in  most  chemical processes.   In the greater number of
  chemical operations, therefore, it is much more convenient
  to  use charred fuel, than the same fuel in its natural state.
    There are, at  the same time,  some kinds  of fossil coal,
  which are exceptions to what  has now been delivered in
  general.   We meet with some of them that leave a smaller
  proportion of ashes than others, and  the ashes of some
  are  not so liable to melt in violent  heats.   There is one
  species too, such as the  Kilkenny coal of Ireland, and which
  occurs likewise in some  parts of this country, that  does
  not  contain any sensible quantity of water,  or other such
  volatile principles.  But this may be called  a sort of na-
  tive charcoal.  It has the appearance of .ordinary coal, but,
  when thrown  into the  fire,  does not emit smoke  or soot.
  It  merely becomes red,  gives  a subtile blue flame,  and
  consumes  like charcoal; only  it lasts  surprisingly  long,
  or continues  to give heat for a very long time before it is
  totally7 consumed.  But it cannot be made to burn so as to
  produce  a gentle heafc.  If not in considerable quantity,
 and violently heated, it  is soon  extinguished.
   In using this  kind  of fuel,  it is  proper to be  on  our
  guard against the dangerous nature of the burnt air which
 arises from charcoal of all  kinds.   Charcoal burns with-
 out  visible smoke.  The  air arising from it  appears  to
 the  eye as pure and as  clean as common air.    Hence  it is
 much used abroad by those, who are  studious  of neatness
 and  cleanliness in their apartments.   But this very  cir-
 cumstance  should make us more watchful against its effects
 which may prove dangerous, in the highest degree, before
 we are aware of it.  The air arising from common  crude 
DANGEROUS VAPOUR FROM BURNING F. 299

fuel is no doubt as bad, but the smoke renders it disagreea-
ble before it becomes dangerous.   The, first sensation is
a slight sense of weakness;  the limbs seem to require a
little  attention,  to  prevent falling.  A  slight  giddiness,
accompanied by a distinct feeling of a flush or glow in the
face and neck.   Soon after,  the person becomes drowsy,
would sit down, but commonly falls on the floor, insensible
of all about him, and breathes strong, snoring as in an apo-
plexy.   If the person is alarmed in time, and escapes into
the open air, he is commonly seized with a violent head-
ach,  which gradually abates.
   But when the effect is completed, as  above described,
death very soon ensues, unless relief be obtained.   There
is usually a foaming at the mouth, a great flush or suffusion
of blood over the face and  neck,  and every indication of
an oppression of the brain by this  accumulation of blood.
The  most successful treatment is to take off a quantity of
blood immediately, and throw  cold water on the head re-
 peatedly.  A strong stimulus, such as hartshorn, applied to
 the soles of the feet, has also a very good effect.
   To return to the fuels....
   The 5th and  last kind of fuel is wood, or fossil coal, in
their crude state,  which it is proper to distinguish from
 the  charcoals of  the  same  substances.  The  difference
consists in their giving a copious  and bright flame, when
plenty of air is admitted to them, in consequence of which
 they must be considered as fuels very different from char-
 coal, and adapted to different purposes.
    I  had occasion formerly to remark,  when  treating oC
 inflammation,  that  flame is  produced  from  those sub-
 stances only,  which are  either totally volatile when heat
 is applied to them, or which contain a  quantity of com-
 bustible matter that is  volatile or easily convertible into
 vapour by heat ;  and  that flame is nothing else but this
 vapour set on fire, or which becomes  inflamed as fast as  it
 arises from the body which affords it.
    Of this nature, therefore, is the flame  of wood  and f6ssil
 coal, when they are burnt in their  crude  state. These fuels
 contain a quantity of  inflammable matter that is volatile,
 and which, when a moderate and stifled heat is applied to 
 300 WOOD AND  PIT-COAL  NOT  CHARRED.

 them, evaporates in  the form of  oily  and  sooty  va-
 pours and smoke, and diminishes the heat instead of in-
 creasing it.   But if  they are exposed to a stronger heat,
 and air is freely admitted, the sooty vapours are suddenly
 set on fire, or become flame, and continue afterwards to
 burn as fast as they arise from the wood or coal, in conse-
 quence  of which  they produce a great heat.
   Such is the nature of flame and of flaming  fuel; and
 from this account of it, you will perceive that it is applica-
 ble only to certain purposes, and is unfit for the more com-
 mon uses in  experimental  chemistry.  For, in  the first
 place, the greater number of our experiments and processes
 do not require an intense and violent heat, but a moderate
 and continued heat, either  kept to the  same degree  for
 some time, or slowly and gradually increased.   This sort
 of heat is easily7  commanded by means of charred fuel,
 which burns faster or slower, according to the quantity of
 air allowed to pass through it,  and will  continue  to burn
 with a very small quantity of air  (but  more slowly) * as
 well as when much air is admitted.  Whereas flaming ffuel
cannot be managed in  this manner.  If  little air be ad-
 mitted,  it gives no flame, but sooty vapour, and  a diminu-
 tion of heat.   And if much air be admitted to make those
 vapours break out into flame,  the heat is too violent.
   These  flaming  fuels, however,  have their  particular
 uses, for which the  others  are far less proper.   For it
 is a  fact,  that flame, when produced in great  quantity,
and  made  to burn violently,  by mixing it with a proper
 quantity of fresh  air,  by driving it on  the subject,  and
 throwingit into whirls and eddies, which mix the air with
every part  of  the  hot vapour,  gives a most intense heat.
 This proceeds from the vaporous nature of flame, and the
perfect miscibility of it with the air.   As the immediate
 contact  and action of air is necessary to the burning of
every combustible body, so the air, when properly applied,
 acts with far greater advantage on flame,  than on the solid
and fixed inflammable bodies : for when  air is applied to
these last, it can only act on their surface, or the particles
of them that  are  outermost;  whereas flame being a va-
pour or elastic fluid, the air, by proper contrivances,  can 
      GREAT HEAT FROM FLAMING FUELS.  CO'.

  be  intimately mixed with  it, and made to act on every
  part of it, external and internal, at the same time.   The
  great power of flame, which is the consequence of this,
  does  not appear when we try small quantities of it, and
  allow it to burn quietly, because the air is not intimately-
  mixed with it, but acts only on  the outside, and the  quan-
  tity of burning  matter in the surface of a small flame is
  too small  to produce much effect.   But when flame  is
  produced in large  quantity, and is  properly mixed and
  agitated  with air, its power to heat bodies is  immensely
  increased.   It  is therefore peculiarly proper for heating
  large quantities  of  matter to a violent  degree, especially
  if the contact of solid  fuel with such  matter is inconve-
  nient.   Flaming fuel is used for this reason in many ope-
  rations performed on large quantities of metal, or metallic
  minerals, in the making of glass,  and  in the baking or
  burning of all kinds of earthen ware.   The potter's kiln is
  a cylindrical cavity, filled from the bottom to the top with
  columns of ware.  The only interstices are those that are
  left  between the  columns; and the  flame, when produced
  in sufficient quantity,  proves a torrent  of liquid fire, con-
  stantly flowing up through the whole of those interstices,
  and  heats the whole pile in an equal manner.
    Flaming fuel is also proper in many works or manufac-
  tories in which much fuel is consumed,  as in breweries,
  distilleries,  and  the like.   In such works, it is evidently
  worth while  to contrive  the  furnaces so that heat may be
 obtained from the volatile parts of the fuel, as well as
  from the  fixed ; for when this is  done, less fuel serves the
 purpose than would otherwise be necessary7.   But this is
 little attended to, or  ill understood,  in many of those
 manufactories.   It is not uncommon to see vast clouds of
 black smoke  and  vapour coming out of  their vents. This
 happens in  consequence  of their throwing too large a
 quantity of crude  fuel into the furnace at once.  The heat
 is not sufficient to inflame it quickly, and the  consequence
 is a great loss of heat.
  These are the principal varieties which occur in fuels,
so far as they are  fitted to produce more or less violent
heat,  or for heating bodies in different ways.   'I here  ar* 
J02  GENERAL STRUCTURE OF A  FURNACE.

differences too among them, in consequence of which they
produce other changes upon  bodies in contact with them,
than  merely heating them  mbr.e  or less.   This  depends
either on introducing something or taking something away.
But  I  shall notice this  when treating of the particular
bodies  which are disposed to produce effects in this way.
   The means of managing and commanding inflammation
are now to be considered under the  article furnaces.
   The necessary requisites of every furnace will occur, if
we consider  the  purpose of the contrivance in  general,
together with the nature  of fuel and inflammation.   Our
purpose, for instance, in the contrivance of furnaces,  is to
confine the heat as much as possible to the subject, that it
 be not wasted,  nor prove troublesome to the operator.
 There  is therefore  a chamber, or  cavity, to contain the
 burning fuel, called a Fire-place, the sides of which are
 of some thickness, and the materials such as withstand the
 heat.  There must likewise be a door,  or opening into this
 chamber, at which fresh fuel may be introduced.   Arid it
 must not  be  quite close  above ; it must have a Vent  to-
 wards  the upper  part,  to convey  away7  the smoke  and
 heated burnt air, otherwise the fire would  be quickly  ex-
 tinguished.   It  is next  obvious,  that any  fuel which we
 can  employ  in ordinary  furnaces  must produce ashes,
 which would collect and choke  up the  fire.   It is  ne-
 cessary,  therefore,  that the fuel lie on  a  grate  through
 which the ashes may fall in proportion as they are formed
 or collected in any quantity.   Under the grate it is con-
 venient to have another chamber for receiving these ashes,
  an  Ash-pit ; and at the side of  this  chamber,  another
  door to take them out,  and likewise  to  allow fresh  air to
  flow up into the furnace, to supply the place of that  which
  is passing through and going out at the vent.  These are
  necessary parts of every structure called a furnace ;  and it
  is  evident,  that in such a structure h.fiammation will go
  on, or fuel burn, with considerable  rapidity.   The  two
  requisites to the inflammation of combustible bodies, are
  a certain degree of heat once excited in the burning body,
  and a constant renewal  of the air in contact with it.    The
  first of these conditions is obtained when  the fuel is  once 
 DRAWING OF FURNACES AND CHIMNEYS. 303

 9et on  fire, seeing  that, in burning, it produces a great
 deal of heat, by which it is both kept hot itself, and com-
 municates heat to the surrounding  bodies.   The second
 condition is a  necessary consequence of the structure of
 the furnace, for no  sooner is  the  fire well lighted than
 there is a motion of the air upwards through the furnace
 with considerable velocity.   The air in the fire-place, by
 being heated, is expanded, made rarer and lighter, and it
 is therefore pushed upwards by the  cooler and heavier ex-
 ternal air,  which communicates with it by means of the
 door of the ash-pit and grate; and thus it will be  made to
 flow in  a stream through the vent.   Its place is supplied by
 fresh air rising up through the grate, and fuel.  No sooner
 does this fresh air touch the fuel than it is heated, rarefi-
 ed,  and. pushed up  in its turn,  so that there is a constant
 stream  of air flowing up through the furnace,  as  long as
 the fuel lasts or the furnace continues warm.   The burr-
 ing  of  any inflammable body in any manner whatever,  is
 always  attended with an ascent of air like this, and hence
 the ascent of smoke and flame ; but it is most remarkable
 in a furnace in  which fuel is allowed  to burn freely7,  be-
 cause the external air, not having admittance to the space
 above the fire, except through the grate and fire itself,  is
 more heated and rarefied when it has ascended through it,
 than the air which gets admission above an ordinary fire,
 which has a free communication with the surrounding air,
 and  is  constantly mixing with it.   It is  plain,  that the
 taller we make  the chimney or vent of a furnace, there is
 a longer column of hot and light air in it,  and therefore a>
 greater pressure upwards.   Air rushes  in  more violently
 by this increased preponderancy of the air of the laborato-
 ry ; this consumes more  fuel in the same time,  and there-
 fore excites a still  greater  heat.    In  the  iron smelting
 furnaces, they find the produce nearly in proportion to the
 number of cubic feet of air thrown into the fuel.
   The venting of chimneys depends on the same princi-
 ples, and the accidents to which it is  liable, or bad venting,
will be best remedied  by a person who understands these
principles.   Bad venting of chimneys is  chiefly of two
kinds, proceeding from different causes : 1st, A slow but 
;64 CAUSES AND CURE OF SMOKETCHIMN,

constant emission of smoke into the rooim   2d,  Violent
puffs  of smoke,  now and then only, and when particular
winds blow, the vent at other times  carrying up the smoke
well enoug^.  The causes of the first are, 1, Obstruction
or narrowness of the vent; 2, Too perfect closeness of the
room or house ; 3, Too great wideness of the vent below,
which allows too much air to get in above  the  fire, and
thus be less heated; or, 4, Its being seldom warmed  by
fire.
   The cure of  narrow and obstructed  vents  is obvious.
Over-closeness of the room or house, is cured by opening
the windows or doors, or making other openings at which
the air may enter.   A hole above the door of the room is
often effectual ; sometimes, however, it  is not, namely,
when there are well-going vents up stairs. In such cases,
the air of  the room is disposed to go up stairs to supply
them.  You will frequently observe that a candle held near
thetop of the open door indicates a current outwards, while
oneheldnearthebottom always shews acurrentinto theroom.
 The most generally effectual method that I know, is to  put
 a slip of wood into the sole of the window, which will keep
 the sash from shutting close down,  but does not allow any
 air to enter at that place.   The air enters by the passage
 which is always left between the upper and under sashes:
 but it comes in such a manner as  not even to disturb the
 flame of a candle held close by it.   The air flies upwards
 along the  glass, strikes against the soffit of  the window,
 and thence it spreads into the  room without offending any
 person.   In some  houses,  air  comes down  the  vents in
 which there are no fires,  or  in which the fires are weak.
  Hence we are troubled by our neighbour's smoke.   Too
  great wideness  of the vent is cured by filling up a part of it
  at each side of  the grate.
    The cause of the second kind of bad venting is various,
  and arises from situation : it  can only be cured by what
  will increase  the  current up the vent.   Contracting the
  vent below has some effect, by obliging the air of the room
  to come nearer to the fuel, by which it is more heated and
  expanded, and  is, therefore, driven up the vent with great-
  er force but it  is more certainly and completely remedied
  by something being done above. 
REGULATION OF THE HEAT....DAMPERS. 305

  A certain degree of this bad venting is sometimes pro-
duced by the bad situation of the fire-place, and door of the
room, with respect to one another.  In some cases it  is
cured by admitting air into the room at some particular
place or side of it, generally that which corresponds to the
wind which occasions the puffs.  Sometimes, however,  it
is almost incurable ;  as, when the wind beats on a higher
house to leeward.  This condenses the air,  and it endea-
vours to escape along any protected  route ; therefore down
the chimney,  and from thence out by the doors and win-
dows.
  After having thus formed a clear idea of the ventilation
of a furnace,  it will be easy to understand the methods  of
regulating the heat.   If we want it very moderate,  we di-
minish the supply of air, by diminishing the aperture either
above or below.   This is called Damping.  It is best done
by damping below, viz. by closing  the door of the ash-pit,
and admitting the air  through holes proportioned  to one
another, according to  some  convenient series, which shall
put it in our power to admit all the intermediate propor-
tions of air.   But in order to have holes, the apertures  of
which are so  proportioned, we must not make the  diame-
ters of them in these proportions.  This would give aper-
tures widely different from what we wanu   The areas or
apertures  of circles are as the squares of their diameters :
Therefore, having determined  on the proportion of the
different holes, we must make their diameters in the pro-
portion of the square roots  of those numbers.  I always
employ a series of holes in the continual proportion  1, 2, 4,
8, &c. and  by  using more or fewer  of them together, any
quantity of area can be employed.   The  construction of
this series  is  exceedingly simple.  (Fig. 7.)  Let the lines
AB and AC  contain an angle BAC, of 45 degrees.  Make
AD the  smallest  intended diameter.   Draw, in succes-
sion, DE,  EF, FG, GH, HI, IK, Sec. alternately perpen-
dicular to AB and AC.  Then the area of the hole whose
diameter is AD being 1, the lines  AD,  AE, AF, AG,
AH, AI, AK, will be the diameters of holes, whose areas
are 1, 2,4, 8, 16, 33, 64, (Euclid, 1. 47J 
 -306          FURNACES....ATHANOR.

    When the furnace is very air-tight in the doors and all
 the joints, and the fuel of an uniform good kind, the con-
 sumption of fuel will be perfectly uniform, and in propor-
 tion to the  area of  the open holes.   In the same propor-
 tion, as far as we can judge, will be the heat that is kept
 up.  But,  if the vent of our furnace have a  connection
 with the chimney of the room, it may be greatly affected
 by its irregularities. It should, therefore, be unconnected
 with it, standing open  under it.
   If we want to raise a greater heat, we must open more
 passages below the fire-place, or add a tube to the vent, to
 increase its draught.   We must  never  admit the air into
 the furnace above the fuel.    This, bypassing between the
 fuel and the subject, will abate the heat which it receives.
   Sometimes the furnace is set within a tall dome.   By
 warming the air  within this dome,  while the furnace re-
 ceives its air from below the dome, or independent of it,
 and the dome is shut on all sides,  and open only at the top,
 it is evident that the levity of the  air  in the dome must
 assist the levity of the air in the furnace, and thus increase
 the draught and the heat.   This  will not be  the case, if
 the furnace  receives its air by its  ash-pit, from the  dome,
 nor if there be any opening in the sides of the dome.
   We shall now  take a view of the variety that  is found
 among  chemical furnaces, or at least point out the most
 useful and remarkable kinds.  And, as the vessels were
 distinguished into three divisions, so the furnaces may, in
 the same manner, be brought under three similar heads, or
 divided into furnaces for digestions, fusions, and evapora-
 tions.
   First, therefore,  for  digestions,  we  may occasionally
 employ several different furnaces.  But those  which have
 been  particularly  contrived  for  this purpose,  are  the
 athanor,  with its sand heats, and the  lamp, and  lamp-
 furnace.   The athanor, or perpetual furnace, consists of
 a tower, round or square, having below, an ash-pit, grate,
 and fire-place.  The vent goes off laterally immediatelv
 above the grate, and is first conducted horizontally, passing
 under a row of vessels, or baths,  for digestions, and then
terminates in an upright chimney.   A little fire is kindled, 
      MELTING,  OR  WIND FURNACE.     307

on the grate, and the top  of the to^tn being shut, the hot
air passes along the  horizontal opening, goes up the chim-
ney, rarefies its air, and soon  produces a current.  The
cover is now taken off from the tower, and this is speedily
filled to the top with charcoal,  and then carefully shut up
above.   From this construction, it is plain that no fuel can
burn for want of air, but what lies immediately on the grate.
As it wastes, the charcoal above it subsides, comes into the
fire-place, where it burns in its turn, and the heat is kept
up as long as it lasts ;  and we can at any  time  lift off the
cover, and speedily throw in more  charcoal.   The lamp,
and its furnace, are often used for distillations, and shall
therefore be described  afterwards.
  The second, the Melting, or Wind-furnace, is among
the most  simple.  When  complete,  it  has all the parts
that  belong  to  furnaces in general,  and no more.  The
best, for the experiments of the philosophical chemist, are
made  of  plate-iron, well rivetted,  and lined with luting.
Much has been written about the most effectual forms for
producing intense heats,  in such small furnaces.   They
are  gravely recommended  to be elliptical, having  one
focus about the  bottom of the  crucible, and the other near
its top ;  but these are fancies, no way connected with the
nature of heat produced by fuel.  A great deal of  trouble
may be saved, by quitting these fancied forms, and con-
structing this furnace  of  a shape that is  the most easily
executed  with exactness.   The drawing here given you,
is a section  of  a  small air-furnace,  and I have found its
form and proportions as convenient as any.
   The ash-pit A B (fig. 8.) is of plate-iron, cylindrical,
and having on one side a door D, a little less  incurvated
than the cylinder, so that its latch bends it to the sides of
the aperture, making it shut and keep very close. Opposite#
to the door is a register-plate, with a series of holes, each
furnished with a brass stopper, nicely fitted to it.   On the
top of the ash-pit is a fiat ring, which carries the grate LK.
This being  made  a little  taper,  fits  exactly into the ring.
B E is the body of the  furnace, set on the  ring of the ash-
pit, and the joint  made nearly  air-tight,  by sprinkling the
surface of the ring with fine sand, or powdered chalk. The 
 308     MELTING, OR  WIND FURNACE.

 body is  a little wider above than  below, and is close all
 round.  On  its top stands  the dome E F, of a conical
 form, this joint being also secured with sand or chalk. On
 one side of the dome is a large hole G, for supplying fuel,
 and inspecting the  crucible.   This  is stopped with a plug
 of fire-clay.   Two handles must be fixed ta the dome for
 lifting it.  It terminates above in a wide tube or vent F H,
 to which additions may7 be made, when we would increase
 the heat.  The crucible is to be set on a brick pedestal of
 such a height that it may7 be  nearly in the middle of the
 body, and be  equally surrounded with fuel in every direc-
 tion.  It should be rather above the middle of the body,
 because the hot air is all driven upwards,  and it receives
 little heat from the fuel above it.   The taper form of the
 body  causes  the fuel to collect round it below as it  con-
 sumes.  The ribs of this, and indeed of all furnace-grates,
 should be  triangular, the narrowest side uppermost.   This
 does not allow the cinders to choke the bars, and it gives
 a steadier position to the pedestal.
    If the height B E be  12  inches, the external  bottom
 diameter should be 10 or 11, and the upper one 16.  The
 height of the  dome E F should be  eight inches.   Larger
 furnaces may be made in the same  proportion.
   The whole must be lined with  a proper  lute, (to be
 described  afterwards)  to the  thickness of  two inches.
 This is made to adhere by7 means  of a  great number of
 nails rivetted to  the external plate, having their points
 curled like so many hooks.
    Such, therefore, is the furnace more especially called the
 melting, or air-furnace, or wind-furnace.  The current of
 air necessary  to animate the inflammation to  that degree
 of violenct which  is often required, is  obtained  merely by
.the height of the vent.  But there are some cases now and
 then, when, in chemical experiments,  we would desire a
 rapidity  uf inflammation, and consequently an intensity of
 heat, even beyond what this structure  can easily be made
 to produce ;  and this is the <rse still more in many che-
 mical arts, which employ furnaces of  such a verv large
 size, that to  give them a sufficient height of vent for this
 purpose, would be  very inconvenient and even impractica- 
          WATER-BLAST, ASSAY-FURNACE.     309

  ble.   In such cases, we have recourse to mechanical, or other
  means of impelling the air through the fuel with sufficient ve-
  locity.  Such are bellows  of various  construction.  When
  these machines are of considerable _size,  they are generally
  wrought by the force of water.  But there is another way of
  employing the force of falling water, to impel air into furna-
  ces or fires, which is much more simple, and is said to produce
  as great, if not a greater effect.  This is by means of the Wa-
  ter-Blast, which Mr. Lewis describes in his Commercium
  Philosophico- Technicum.  The iEoLiPiLE may also be consi-
  dered as another way of employing water to drive air into fuel,
  but in a very different manner.  The aeolipile is merely a glo-
  bular vessel, having a small pipe coming out from it, something
  like the pipe  of a still, but terminating in a very fine bore.
  Water is made to boil violently in it by setting it on a chaffer
  of glowing coals.   A strong blast  of  vapou» flows from the
  pipe.   This is directed into the midst of the fuel, which  it
  animates  in the same manner as bellows would do ; chiefly
  I believe, by means of the great quantity of air which it drags'
  along with it from  all sides.
    After  the melting furnace, and other instruments for vio-
  lent melting heats,  the next we shall describe,  as being also
 often employ ed in melting certain substances, though not with
 so violent a heat,  is called the Assay-Furnace, so called,
 as it is employed chiefly in the examination of ores.   This is
 not so simple, by much, as the melting furnace.  The opera-
 tions to which it is adapted require that the subject and ves-
 sels be prevented from being in contact with  the fuel;  that on
 some occasions, it be exposed to a gentle current of air which
 has not passed through the fuel; and that the operator  have
 free access to see how the changes he proposes to produce are
 going on.  It has therefore an ash-pit and a door like the melt-
 ing furnace, and a fire-place and door, with a grate in the bot-
 tom, through which the ashes fall into the ash-pit.  This fur-
 nace is not round,  but square ; and  in the  middle of the  fire-
place is fixed a little arched oven, called a Muffle, of an ob-
long form, open before,  but shut at the far end.  The open
end of this oven is made to fit exactly the fire-place door, so as'
    VOL. T.               3 A 
310 ASSAY-FURNACE....MUFFLE, BLOW-PIPE.

to appear but one opening; and in looking in, we see nothing
but the inside of the muffle.  The two sides of  this muffle are
pierced with several openings like arches standing on its floor.
When the fire-place door is opened, the fresh air rushes in, in
order to  run up through these openings, and thus it plays on
the subjects exposed to the violent heat,  and burns them com-
pletely.  This air therefore must be fresh.   Above, there is a
dome or covering, and a tube.
  The effect of this furnace is to produce a considerable de-
gree of  heat under the muffle, and a constant stream  of air
through  it; and it  we want a stronger heat, and air is not ne-
 cessary, we close the door of the fire-place altogether.
   In  using  the furnaces  hitherto described, the best fuel is
charcoal, either of wood or  of  fossil coal.  But I observed
formerly7, that when violent heats are  required, and great
quantities of matter are to be lighted, it is better to use fla-
ming fuel, the  flame of which,  when'well mixed and agitated
with air,  is capable of giving a most intense heat.   As an il-
lustration and  proof of this last assertion,  I shall shew the ef-
fects of mixing air with the flame of a candle or lamp by means
of the blow-pipe.  This little instrument is used in small works
on glass,  as thermometers, &c. and in soldering metals;  it is
useful too to the chemist sometimes in adjusting vessels :  but,
besides,  it is also very often used as a very quick and ready me-
thod of exciting very intense degrees of heat, when he wishes
to examine  its  effects on a small quantity7 of matter, such as
 ores, placed upon a bit of charcoal.
   The blow-pipe was recommended by Cramer in his Ars Do-
 cimasiica ; but the  Swedish chemists were the  first who made
mnch use of it in examining mineral substances;  and one of
 them, Engestroem, published,  at the end of the Translation
 of Cronsted's Mineralogy into English, an account of the most
 convenient  manner of  using it, in which he made some im-
 provements.   It  is now an indispensable instrument for every-
 mineralogist :  and  various contrivances have been introduced
 for accommodating it for an imitation of almost every  metal-
 lurgical operation.   Engestroem  describes a box,  containing
 the blow-pipe and other implements, which he calls the Pocket 
 BLOW-PIPE, REVERBERATORY-FURNACE 311

Laboratory.  The advantages of  the blow-pipe are,—it
saves a great deal of time and expence, and  it enables us to
make a multitude of experiments with a small quantity of the
materials.  The blow-pipe is only fit for heating a small quan-
tity of matter, as the bulk of half a pea, or less.   Were we to
apply it to  larger masses, and to use  a wider pipe and much
larger flame,  it would not produce the effect desired ;  the air
would pass through the flame without being well mixed with it,
and would strike the matter we meant f> heat, in such a man-
ner as to cool it, or diminish the heat otherwise produced.
  Those who make thermometers and other small instruments
of glass with it,  often blow it with bellows, but the mouth is
most ready and  most under command.
  Thus we see that flame, mixed and agitated  with air, produ-
ces a most violent heat.  When therefore we  want an  intense
heat, and a great quantity of it,  it is best to use flame.   And
there are furnaces particularly adapted to the use of flaming fuel.
  Such is, first, the Reverberatory, called by the artists
who use it, the  Air-Furnace.  It is used  for melting of
iron, and for smelting the ores of copper and lead, to extract
those metals,  and for other purposes.
  It has an ash-pit, grate,  and fire-place, like other furnaces.
The fire-place door is large, and sloping downwards, that the
raw fuel may be thrown in more easily in abundance, and ga-
ther itself together on the grate.  But the flame produced is
not  allowed  to  mount straight upwards,  but is diverted  la-
terally, by the low arch, into a sort of oven.  The floor of this
oven is made considerably hollow in the middle, and the  mat-
ters to be treated are laid there.  Opposite to the entry into
this  oven from the fire-place, is another opening into the chim-
ney or vent.   The effect of this  construction is plain.  The
tall vent occasions a  current of air through the whole furnace,
from the ash-pit door to  the top of the chimney.   This blows
up  the fire :  and this,  by heating the air,  increases the  cur-
rent, and the fire soon becomes furious.  A volume of flame
rises from the raw fuel ; this is hurried into the lateral passage
mixed with air;  and the form of that passage causes the stream
of flame to dart violently down on the materials lying in the 
312  REVERBERATORY, OR  AIR FURNACE.

bed of the laboratory (so is the oven generally named).   I he
low roof of this place,  and some inequalities artfully made in
it on purpose, confine the stream of flame to the subject, and,
by tossing it into violent eddies on its surface, mix the air most
intimately with every part of the flame,  and thus completely
consume the whole combustible matter of the fuel, so that, in
a good reverberatory, not a particle of flame goes as far as the
bottom of the vent.  In the Journal de Physique, 1776, there
is  a description of a Reverberatory Furnace, which seems
very ingeniously fitted for a general experimental furnace.
   Thus  a most intense heat is produced on the matter lying
in the bed of the reverberatory.   There are openings made in
the side of it, just where the matter lies.  Here the workmen,
from time to time, open the door,  and put in their tongs and
rakes, and other instruments, to stir and manage the stuff ac-
cording to their views. Frequently, too, the nozzle of a pair
of bellows is  directed on the surface of the stuff, to burn it still
more completely, and blow the liquid slag and the dross from
off its surface to the other side, where a way is made for its get-
ting out.
   Another furnace, in which the flame of fuel is employed, is
the Glass Furnace.   The specialities of this depend on this
circumstance, that it is necessary for the workmen to have free
access to the matter, and, at the same time, to have a violent
degree of heat kept up.  An ordinary furnace, therefore, with
a high vent to produce a draught, and a door, or opening at the
side, will not answer;  because the cold air would be driven
in at this opening, and run up the  vent,  and thus cool both
the  fire-place  and  Vent.  A glass house,  therefore, is  so
constructed, that there is  a high building, of  a  conical,
or other form,  over the furnace,  which serves as a vent to in-
 crease the current of air, and is so wide all round, that there is
 room for the workmen.  The air within this building is heated
 considerably by the furnace standing in the middle of it, and
 matters  are so disposed, that the current of air thus produced
 is made to pass chiefly through  the furnace, by the ash-pit,
 which is under the floor of the  furnace and dome, and  has a
free communication with die external air, but none  with the 
GLASS-HOUSE  FURNACE.         313
dome.  No more is admitted to come in at the doors of the
building than what is merely necessary to render the heat to-
lerable to the  workmen, and to allow them breath'.   Fig. 9. y
will illustrate this description.  A is the grate for the fuel; B,
B,  B, the  pots;  D the holes where the  workmen dip their
rods into the pots ;  C the vault of the furnace ; and E the ash-
pit, communicating with the air without doors.  The whole
is set in the middle of a vast dome, F.  The air coming from
the ash-pit through the grate and fuel, ascends through the
working holes D into the dome, and thus makes a great co-
lumn of warm and light air, which rises through the opening
of the dome with great force.  Some of the holes Gopen into
ovens, in which the materials for glass are roasted, to drive off
their volatile parts.   Thus it appears, that the working holes
are the immediate chimneys or vents of the glass furnace. We
cannot otherwise obtain the heat that is necessary.  For if
there were chimneys, as in other furnaces, the air, rushing in
at the work holes,  would make  it cold in the place where we
want the greatest heat.   The workmen are therefore exposed
to almost intolerable heat.  But such heat  is not always neces-
sary.  Chiefly while the glass is preparing, and  in a state like
ebullition, much volatile matter is escaping from it.   Melted
glass being so viscid, it must be made to flow as thin as possi-
ble, that these volatile matters may make their way up through
it.   When the glass is ready for working, they  open some of
the doors of the dome.  This admits the fresh air,  in which
the men can breathe and work.  It abates the heat of the rising-
column of air in the dome.   This abates its ascensional force,
and, consequently, the heat of the furnace. This more mode-
rate temperature is called the settling heat, the frothing glass
being now quiet.   After this they increase the heat a little, to
bring the glass into a proper state for lifting.  This they call
the working heat.
  This, therefore, is  the furnace for melting together the ma-
terials of glass in large quantities.  If our purpose be only to
make small quantities, or compositions of different kinds of co-
loured glasses, or imitations of gems, or experiments of that
kind, we mav construct such a furnace, as  is described by Cra- 
 il4    FURNACE FOR  DISTILLATION.

 mer, at the end of his Art of Essaying, in which he says that
 a great number of experiments may be made at the same time
^vith grestTease and convenience.
   It remains now only to consider the furnaces for vaporisa-
 tion.
   The first of these I shall mention is what  Macquer calls the
 reverberatory7,  or for performing distillations that require the
 strongest  heat, in which the vessels employed, commonly of
 earthen ware, are exposed to  the  contact of the fuel; on ac-
 count of which it is commonly called the furnace for distilling
 with naked fire.  This furnace is described by Macquer.  It
 has nothing very different from the melting furnace, only there
 are two openings at the  fire-place ;  the one  is a door for the
 fuel; the other is circular, between the dome and  the body,
 intended for allowing the neck of an earthen retort to point out.
 This hole is formed by making a semi-circular notch in the
 upper part of the body,  and another in the lower part of the
 dome.  When these are set together,  the  neck of the retort
 finds room between them.  If this hole be shut by an earthen
 plug, the furnace is the  common melting furr.ace.   Likewise
 two iron  bars  must  be placed across the middle of the fire-
 place, to  support the bottom  of  the  retort;  or,  instead of
 these bars, we may  have a pedestal.   When this vessel is put
 in, the opening round its neck should be filled up with luting.
 It should have a high vent and  dampers.
   Ihe second  furnace for vaporisation is the furnace of the
 common still, having a vent that issues out at one side.
   3d, Other distilling furnaces are chiefly fitted for retorts, or
 vessels of glass, and  are  necessary for using these vessels, ap-
 plying the heat slowly and equally to the whole surface.  The
 particulars in which  they differ from others are only contri-
 vances for obtaining this end.   Therefore, their upper part
 immediately over the fuel is covered by a hemispherical pot of
 cast iron of some thickness, in the middle of which the vessel
 is placed and surrounded with water, ashes, sand, or other mat-
 ters.  Thus,  the pot having some  thickness and solidity, it
 heats slowly and equally,  and the matters with which the ves-
 sel is surrounded  transmit  this still  more slowly; and the 
DISTILLING FURNACES.  LAMP FURNACE. 315

whole, being once heated, though the fire may be a little une-
qual, is not  subject to sudden variations, which glass vessels
are unable to bear.   The matters interposed between the pot
and the glass vessel' have got the name of balnea, or baths.
The water bath, balneum marice; the sand  bath, balneum
arenas.  Iron filings are  sometimes used ; others have pro-
posed quicksilver, and  melted metals, as tin, lead,  and bis-
muth.   Sand is  the most  convenient of any.  The  older
chemists frequently  used no intermedium but  air, supporting
the retort on three bits of chalk, and setting over  it a little
dome of plate iron, notched, to admit the neck.  This they
called distilling in  capella vacu'i.   This is a foolish method;
because, unless  the space round the  neck of* the retort be
carefully  shut, which  it is  very difficult  to  do,  there is a
continual  current of air circulating round and escaping, and
the retort is very unequally as well as feebly heated.
   A lamp furnace,  and lamp, have been sometimes used for
distillations:  and they  are, no  doubt,  extremely  neat and
convenient, when a very moderate heat is sufficient.  But
spirit of wine is the only fuel we can  employ.  To manage
an oil flame requires more attention than a fire of charcoal or
coaks.
   Some have proposed  a train of furnaces, or  sand heats, for
the hot air, or heat  of an athanor*furnace,  to pass through?*-
but these are of little use.
   A variety of other furnaces are used in various chemical
arts, as enamelling, smelting metals from their ores, and re-
fining and separating them from one  another.  But a des-
cription of those  furnaces will not be instructive, unless in
giving a  particular account of  those arts themselves,  which
we cannot now do.   I have only given  you the chief varieties
commonly used by speculative chemists in the  way of inquiry
and experiment, explaining, as I  went on, their principle and
mode of action.   The same principles affect the performance
of all furnaces, and, therefore, this  knowledge of them not only
enables you to construct  such furnaces as  I have described,
but also to construct furnaces properly  accommodated to the
circumstances which occur in the practice  of the chemical 
316     BECHER'S GENERAL FURNACE.

arts.  I described the glass-house furnace,  because the great
dome is a part which does not occur in  the  furnaces of the
philosophical chemist:  and yet  an intelligent  chemist  may
find many cases  where it may be applied to other furnaces
employed in  a chemical manufacture.  It has been success-
fully employed to increase the  heat  in  small furnaces  for
smelting iron from the ore, and in very small lime-kilns.
  But beside the contrivance of these particular furnaces, as
they may be called, attempts have been made to  contrive what
may be  called a  General  Furnace, that is, one  applicable
to all the chemical  operations.   The attempts of this kind
which have  been attended with any  remarkable degree of
success, are not  many in number.
  The first, so far as I know, which drew any attention, was
that of  Becher, the famous German chemist.   He published
a pamphlet, to which he gives the mysterious title of Tripos
Hermeticus Fatidicus; the Chemical Fortune-telling Tripod;
by means of  which, to wit, an adventurer in alchymy was to
learn, by trial of his own skill, whether he should be success-
ful  or not.  The pamphlet contains the description  and draw-
ings of a furnace, consisting of four or  five different pieces,
which can be joined  variously together, so as to adapt the
furnace more or less completely to the purposes of digestion,
or fusion, or the essaying and refinement of  metals, or dis-
tillation, either with  earthen retorts and a strong heat, or the
common still, or in glass vessels set in warm sand, or in hot
water.  You  may see the  drawings and description of this
furnace at the end of Dr. Shaw's English translation of Boer-
haave's chemistry, Dr.  Shaw having copied it from Becher
without mentioning his name.   The  different  parts of this
furnace are made of earthen ware,  or a sort of clay, such as
fire-bricks are made of; but our potters here are  not accustom-
ed  to such work.
   This furnace is  constructed with  much ingenuity  and
judgment, and does honour to this early chemist.   When ac-
curately made, so that the joinings of  its different parts can
be made tight with a little  chalk, I believe  that  it may be
 i-asily managed  in all its  functions.  But such earthen ware 
       FURNACE MADE OF CRUCIBLES.     317

is very subject to crack.   This may be secured  by hoop-
ing.  But frequent placing and shifting the pieces on each
other, soon chips the edges; the joinings grow open ; and
then there is no regularity in the heats,  and veins of cold
air  get in and crack our vessels.  A pottery,  also,  which
is sufficiently strong for the furnace, will transmit the heat
so much as to become extremely disagreeable ; and it be-
comes very troublesome to keep receivers cool enough for
condensing the vapours.
   Another contrivance,  by which  we can  more readily
provide ourselves in this country with a general furnace, or
a set of furnaces for chemical experiments, is described by
Mr.  Lewis  in his Commercium    hilosophico-Technicum.
The  author of this contrivance was a goldsmith.   Being
daily accustomed to use crucibles, it occurred to him that
those made  of black-lead, some of which are of a large
size, might easily be converted into furnaces, which would
serve for  chemical experiments  on small  quantities of
matter.   The  black-lead crucibles have the advantage of
being so soft, that it is easy to  bore holes through them,
to  cut doors in the sides of them,  or to cut or grind off
any part of them to adapt them to  such a purpose.   And
they also bear unequal heat, or sudden, changes, with far
less danger of cracking or splitting than earthen ware does.
Mr.  Lewis, therefore* teaches how the  largest of them
may  be converted into  furnaces  for different  purposes,
either singly in some cases, or by joining two of them
together in others; and,  in others, by additional tubes, or
other pieces which he adapts to them.   As he gives exact
descriptions and  delineations  of all this, I  need  not be
more particular here, but refer you to his work.  The
contrivance is  certainly ingenious,  and affords a short and
ready method  for providing  small furnaces.  But they
have their faults.  They are not sufficiently durable, black-
lead being consumed or destroyed by the  repeated  action
of  violent  heat.   Neither do they contain enough of fuel •
nor can they be regulated with sufficient-exactness.
    I  have therefore endeavoured   to  contrive a general
furnace for myself, which might  be free  from those im-
perfections,  and I shall now describe it.   It has the adv..
    VOL. !>•                  •'  B 
318    DR. BLACK'S GENERAL FURNACE.
     vantage of being far more simple than those I have already
     mentioned.
       Plate-iron is by far the  best material for the outside of
     an experimental furnace: but, as its  metal communicates
     heat very fast, this must be cut off by a proper lute lining.
     I have so  far succeeded in this  respect, that my furnace,
     though only two  inches thick  in  the middle,  will  not
     scorch paper applied to its  outside, when it is  melting
     iron within.   I make it of the simplest rectilineal shapes,
     because workmen find great difficulty in executing curved
     and uncommon forms; and  not  one of a score  of  them
'     will do it with accuracy.   Indeed,  those highly praised
     forms seem to  me of very little importance in most cases.
     The only case  in which I find it of value, is in the  labo-
     ratory of a reverberatory  furnace,  where something may
     be done in this way for mixing the hot air and flame,
     and making it play with  force on  the subject to be
     treated.
       The body or fire-place is the  only part of this furnace
     that requires  description;  the ash-pit, with its door  and
     registers and  grate,  being  constructed as   in any  other
     furnace.   It will  be  easily understood by considering the
     section represented in fig. 10.
       The base, represented by the dotted  line ABC,  and
     the top KLM, are oval plates of iron, the longer diame-
     ter  AC, being to the shorter as three to two nearly.   The
     base and top are  equal, so that  the  sides KA,  MC,  are
     upright, the whole body forming an oval cylinder.  DEF
     is half of the  hole in the  bottom, which is  occupied by
     the grate fixed on the top of the ash-pit.   GHI  is half of
     the  mouth of the furnace, which receives a still, or a sand-
{    pot for distillation, with a retort.  This is a little nearer
     to the front K of the top, than  the* grate hole  i3 to the
     front A of the bottom, so that the luting is thicker  below
     than above.  Near the back M of the furnace, is a smaller
     hole P for the vent.  The luting at Q. and R  is so formed,
     that the  cavity of the furnace does not greatly differ from
     a cylinder,  except in  so  far as  the vent  PO  does  not
     communicate with it abruptly,  but  is gradually curved
     downwards, as represented in   the  figure,  making  the 
LUTES.                   .319
middle of the cavity more roomy backwards, by which
means it contains  a greater quantity.of  fuel.  S is the
section of the luting, which forms a sort of arch, or bridge,
contracting the entry of the  vent.   An  iron pipe is set
on  at P,  to  increase  the draught of  the chimney.
The fuel is put into the furnace  by the aperture P, and the
sloping form  of  the  cavity7  causes it to  distribute  itself
pretty uniformly.
   When the furnace  is used for smelting, the ciucible is
set on a pedestal standing on the grate, and the fuel is
placed round  it with great ease, the mouth of the furnace
being open.   This is then shut up by a stopper made on
purpose, or by a flat fire-tile simply laid on it.
   When we would distil with a naked fire, the retort has
its bulb resting on a ring which  hangs on the mouth of the
furnace by three hooks, and the neck of the retort lies  over
the front of the furnace.  The space round the retort at
the mouth of the furnace is  closed,  as much as is neces-
sary, by  two or three pieces  of tile, shaped so as nearly to
fit the  bulb of the retort when they are laid  on the mouth
of the furnace.   A quantity of light  ashes are now to be
laid on these tiles, and heaped up so as to  cover the  bulb
and part of the neck of the retort.  I find that this produ-
ces a very gradual diminution of the heat as it recedes from
the fuel,  and is less liable to crack the retort, by inequality
of heat, than  any contrivance I know.  Scarcely any pro-
cess occurs which this furnace does not answer with great
ease.
   We have  now said enough on  the subject of furnace.
There is only one article more of the apparatus which
remains  to be mentioned,....I mean the Lutes.
   In using the furnaces which  I have described  as  most
convenient for experimental chemistry,  (I  mean those
made of plate  iron) it  is necessary that the iron be defended
from the  heat by lining or lute, as we  called it, on the
inside; and such lutes are necessary on other occasions in
chemistry; as when we have occasion to close the joining
of vessels  with one another, or to  give a coating to retorts,
or  even  to crucibles,  which is sometimes done.   The ma-
serials employed for these purposes have their general de- 
320
LUTES.
nomination from  clay, of which some of the most useful
are partly composed,  though there are some  that do  not
contain any of it.   They7 may be divided into such as con-
tain animal or vegetable matter, of the glutinous or adhesive
kind, and such as are  composed only of earthy substances.
The first are used for closing the joining of vessels, when
the heat we mean to apply is not to be strong, nor the va-
pours to be  produced corrosive.  The  second serve  for
the lining of furnaces, or for closing the joinings of vessels,
in operations in which the vapours are very corrosive, or
in which a  strong heat must be employed, which would
scorch, or burn and destroy any animal or vegetable glutin-
ous matter.
   The joinings  of vessels with  one another,  which we
have the most frequent occasion  to close up by means of
lutes,  are those of retorts with receivers.   And we may
remark, in the first place, with* regard to these, that there
are not many operations  in which it is necessary to make
the joining  perfectly close, except  when  the  receiver is
provided with an air-pipe.  On the contrary7, it is danger-
ous, on account of the air which must be allowed to escape
in some manner.   Therefore  we  are  not anxious to con-
trive the most close  and compact.   They are sufficient,
and better,  if  they be moderately so, and, in some cases,
when we think the lute too close, we  even obviate it by a
pin-hole.   The animal and vegetable lutes,  employed in
this way, are glue and chalk mixed in thin paste, and spread
on slips of  paper, or gum Arabic and chalk,  used in the
same  manner, or flour and water, or a bladder, or lintseed
 meal, or fat lute. Mr. Lavoisier recommends, for joinings
 which  we desire to be air-tight,  but which are not to be
 exposed to  heat, the  following: to  16 ounces of bees-wax
 add 1 4 or  2 of  turpentine, and keep it for  use.   When
 used, soften, and make it tough, by warming and working
 between the fingers ; then put it on the joint in little rolls,
 ;md make it close ; and,  lastly, cover it with  slips of wet
 bladder laced with pack-thread.   But, if the joint is liable
 to be warmed or heated during the  operation, you  must
 take fat lute.  This  is made of raw pipe clav and lintseed 
                        LUTES.                    321

oil, beaten together very hard, to the consistence of a stiff
adhesive paste.
   Of the second kind  of lutes,  called the Fire-Lutes, a
great variety have been proposed, and some of them com-
positions of many ingredients, some of which I have tried,
but find none equal or superior to clay and sand, viz. sand
3, or 4, or 5, or 6, to clay 1.  These are for luting vessels
together, and for coating.   But in lining furnaces, I use a
double lining; 1st,  a charcoal lute ; 2dly, a fire-lute.
   It may appear paradoxical to  speak of  a charcoal lute
for lining a furnace.   But I find that a layer of powdered
charcoal, beaten up, or kneaded,  with as little water as
will give its particles adhesion enough to  attach  itself to
the metal sides of the furnace, by means of cautious beat-
ing, forms a firm stratum, which is the most imperfect con-
ductor of heat of all that I have tried.   When this layer of
charcoal is defended from the action of the air by a layer
of fire-lute,  composed  of one part of fine clay, and three
or four parts of sand, carefully put on, and  consolidated by
gently beating it from day to day, till it no longer receives
an impression from the mallet, it will last as long as any
part of the  furnace.   Its durability  will be greatly im-
proved, without much  change in its conducting power, by
using, instead of pure water, water made  muddy by about
l-20th of pipe-clay. If finely powdered charcoal be kneaded
with l-5th of pipe-clay, it may be kneaded and formed
into any shape, and will be so impervious to heat, that a
bit of it may be held in  the  fingers,  within an inch of
where  it is  red hot.   Such a composition is, therefore,
 very proper for the doors of furnaces, and for stopples  for
such apertures as must be frequently opened and shut.
   Thus have I taken a survey of the apparatus, and have
thrown in such remarks upon the manner  of using several
parts of it, as  could be delivered in a general way.  It is
not necessary at present, and it would be tiresome, to enter
into  further details.   You will have the opportunity of
observing the  conduct of  a variety of operations which
will be performed during the  rest of this course ; and such
as desire to make this part of chemical knowledge  an
 object  of their attention,  will find  instructions  for per- 
322                CONCLUSION.
   forming every particular process in the numerous books
   upon the Practice  of  Chemistry....Macquer, Boerhaave,
   Lemery, Wilson, &c.
      We have  now gone over all the articles that can  with
   propriety be  called  general, affecting every branch of che-
   mical inquiry, or requiring to be well understood, in order
   that we may prosecute this inquiry with skill and success.
   Our next occupation will be to examine the chemical pro-
   perties, and  mutual relations, of that immense  variety of
   bodies which nature presents to our view.  The natural
   philosopher,  or mechanician, looks at the same object that
   we do, but with very different purposes. He only inquires
   whether the  body be moveable, and what force  will move
   it; if it will move altogether, when touched in any one part;
   or if it will  separate  like  sand  or water.   He inquires
*  whether it  be hard, soft, liquid, or elastic ; and the inte-
   resting properties which he finds in one, he finds  in all ;
   and they are  not only general,  but obvious.   To the me-
   chanician, a pound of gold, and a pound of free-stone, are
   nearly the  same.
      The natural historian  sees much greater varieties in the
   objects around  him ;  but,  if he strictly7 keeps  within the
   pale of his science, he sees only the surface ;  he attends
   to the colour, the shape, the configuration, the structure,
   and perhaps the strength of the connection of the different
   parts.
      To the mechanician and the natural historian, gunpow-
   der is little more than  an assemblage of black grains^ of a
   certain weight  and firmness.   What  a different and  inte-
   resting object is it  to the chemist, when he examines it by
   his instruments, mixture and heat!  He can get aquafortis
   and  potash out of it, and brimstone, which all the powers
   of mechanism  cannot separate from it, and which the na-
   tural historian  cannot discover in it.  If the chemist apply
   his other great instrument of research to it, what wonderful
   effects and productions will he now behold !   A force ap-
   pears, of which it gave not the least indication,  and which
   exceeds all  the powers of machinery.  Scorching  heat,
   dazzling light,  and deafening noise,  strike him at once ;
   his gunpowder is dissipated,  not into its ingredients,  for 
CONCLUSION.               323
neither aquafortis, potash, nor brimstone, are to be seen;
and, instead of a cubic inch of gunpowder, he finds a vast
volume of elastic air.
  Thus do  those two instruments of knowledge, those two
ladders of science, give us access to the most remarkable,
and the most interesting properties and relations of bodies.
Every step  we take  gives us  a new and useful  acquain-
tance.  If,  therefore, you have found interest and instruc-
tion in what has hitherto occupied your attention in this
place, I  think that I can assure you of still more in the.
application of the knowledge you have acquired.
SND OF THE FIRST VOLUME^
  PRIN'FtD BY B. CRAVES,
No. 40, North Fourth-street. 
 
NOTES  AND  OBSERVATIONS
BY  THE  EDITOR.
                 [Note A. p. 58, 2d line.]

    THE reader will surely be  sensible that the  sources ot
 error  in the experiments for ascertaining  this fundamental
 point are numerous, and are with difficulty avoided.  We are
 not entitled to make a decisive inference from the experi-
 ments, unless they be numerous, and their unavoidable irregu-
 larities be as frequently on one  side as on  the other of the
 general inference.  It was-a natural expectation that equal
 increments of bulk should accompany equal  increments of
 heat;  yet we  cannot  say that this has a good foundation.
 Ignorant of the intimate nature of heat, we are ignorant of the
 manner and the force with which it is accumulated round the
 particles of a body.   But, on the other  hand, the observation
 of the  variations of bulk afford us the best means of discover-
 ing  those hidden circumstances.   Philosophers acquiesced
 too readily in the opinion that the  increments  of bulk  were
 proportional to those of heat.  The experiments of Dr. Brook
 Taylor, made with  great  judgment, seemed to confirm this
 opinion.  Those of Rhenaldini  did not contradict it.  Sir
 Isaac Newton's experiments, proceeding on  this supposition,
combined with a most  ingenious  supposition  concerning the
     vol. i.               3 c 
 326       NOTES AND  OBSERVATIONS.

 progress of heating and  cooling, exhibited a happy7 coinci-
 dence of facts, depending on principles totally different, which
  seemed to put this opinion beyond  all doubt.   Dr. Black's
 experiments in 1760 perfectly coincided with it.
    But Mrr De Luc, one of the most assiduous,  and perhaps
 most scrupulous philosopher of Europe, had more than doubts
 about the certainty7 of this correspondence.   He  had noticed
 the  observation of Beaume,  that water  not only increased
 greatly its bulk in freezing, but that its  contraction by cold
 ceased  before its conversion into ice, and that it even expand-
 ed a little.  He verified this,  and ascertained its quantity by
 careful experiments,  of a very simple and unexceptionable
 kind, namely,  by  comparing the contractions of a quicksil-
' ver and a  water thermometer.  This being the case,  there
 would be  great uncertainty in the indications of  temperature
 by a water thermometer, or one containing any water in its
 composition, seeing that there must be a condition of greatest
 density ; and therefore, on each side  of the corresponding
 temperature, the water will have the same bulk in very dif-
 ferent temperatures.   The  extent of this uncertainty is un-
 known.  The  curious  experiments  of Captain  Williams at
 Quebec, in which he froze water in bomb-shells,  shew that
 before it congeals, it  expands prodigiously and gradually.
 This  must be inferred from the expulsion of the iron  plug,
 and protrusion  of a long cylinder of  ice,  in cases  when  the
 shell did not burst.  It were worth examination  what would
 be the progress of  its  expansion,  while cooling undisturbed,
 below the  ordinary  freezing temperature.  But are we certain
 that during this expansion of water in temperatures below 38°,
 the water is giving out heat in the same manner as during its
 contraction, in cooling from a state  of higher temperature?
 Mr.  De Luc now saw the necessity of  following out this
 comparison  through the  whole range  of the thermometer.
 For, since  there is a temperature a little above that of melting
 ice, in which a change of heat makes no change in the bulk of
 water, it necessarily follows that equal variations  of tempera-
 ture on both sides of this degree of heat, will be accompanied
 by variations of bulk,  which continually increase.  Accord- 
NOTES  AND OBSERVATIONS.       3.27
 mgly, he found that while the mercurial thermometer increas-
 ed  its  bttflc equably, the increments of the bulk of the water
 continually increased.   But, farther, when he endeavoured to
 determine the proportion of this augmentation of the succes-
 sive increments  of the  water,  corresponding to equal incre-
 ments of the mercury,  he found that when the temperature
 approached to that of boiling water, these augmentations of
 bulk increased with increasing rapidity, and were remarkably
 irregular.   He found the same thing in spirit of wine and
 other  fluids.   Some fluids, however,  such as mercury, olive
 oil, lintseed oil, oil of camomile, and others, contract in freez-
 ing, and are not subject to all this irregularity.  In such fluids,
 the acceleration of their expansion by heat begins at the very
 departure from the state  of solidity.  Hence  Mr.  De  Luc
 concludes  that their expansion is more corresponding to the
 changes of heat, in proportion as they  are farther from either
 the freezing or the boiling temperatures.
   Mr. De Luc's reasoning on this subject is  in some degree
 hypothetical, depending on his notions of the relation of bodies
 to heat.   But he has established beyond dispute the fact, that
 the  successive augmentation  of bulk  by equal increments of
 heat are continually increasing.   Mr.  De Luc makes one in-
 ference which cannot be  controverted,  and is of importance to
 the philosopher; namely, that is the  best fluid  for a thermom-
 eter, which  has the greatest interval  between its freezing and
 boiling  temperatures ; because  the  successive augmentation
 of bulk towards the middle of its range will the most nearly
 correspond with the augmentations of heat.  Were it not that
 there is now no difference of opinion, this circumstance alone
 would justify the preference which De Luc gives to mercury
 above all other fluids.  Its deviations from equable expansion
 by equal changes of temperature, in  the cases which most fre-
 quently occur in our experiments, are  so trifling, and indeed
 so uncertain, that they need scarcely be attended to.   They
are called uncertain, because it is plain that the nature of the
experiments for ascertaining the real  medium temperature,
arisin" from the mixture of a hotter and a colder fluid, seems 
328       NOTES AND OBSERVATIONS.

unsusceptible of the accuracy necessary for determining such
small quantities with precision.
  The annexed  table of Mr. De Luc's comparison of ther-
mometers filled w.ih different fluids cannot but be acceptable.
  He had determined the expansion of mercury by very care-
ful  experiments; and  found that  in the middle temperature
between those of freezing and boiling water, that is, at 40° of
Reaumur's thermometer, the mercurial thermometer stood at
38,6; and the  others, whose scales were all of equal parts,
con raining 80 between these two temperatures, stood  as fol-
lows :
         Mercury,     -      -     38,6    15 : 14
         Olive and lintseed oil,    37,8    15 : 13,4
         Oil of camomile,     -    37,2    15 : 13
         Water saturated with salt, 34,9    15  : 11,6
         Rectified spirits of wine,  33,7    15:10,9
         Water,    -     -     -   19,2    15  :  4,7
   The second  column expresses  the proportion of the con-
 traction from the temperature of boiling water, to the middle
 temperature 40°, to the contraction from 40°  to 00°.


            [Note B.p. 60, 3d line from bottom.]

   This method of measuring intense heats is extremely refi-
 ned, and could occur only to such a mind as Newton's.  It is,
 however,  in some respects hypothetical and gratuitous, be-
 cause it proceeds on a supposition which I apprehend cannot
 be demonstrated, namely, that the momentary emanations of
 heat, whether in the way of communication by contact, or by
 simple emission, or in both ways, are proportional to the ex-
 cess of the temperature of the cooling body over that of the
 surrounding or receiving bodies.   We know nothing of the
 connection of  bodies with heat, from which this law of varia-
 tion can be properly inferred.   But, on the other hand, when
 we observe the progress of heating and cooling, in cases where
 we have other evidences of the quantities gained or lost, we
 find it wonderfully correspondent to this law;  so much so in- 
NOTES  AND OBSERVATIONS.       329
deed, that we cannot perceive any deviation from it which may
not be accounted for by the circumstances of the case.   This
method, therefore, is a very valuable acquisition to the philo-
sopher.


                    [Note 1. p. 81.]

   It is somewhat strange,  that neither Boerhaave,  who ex-
presses  great curiosity about alf the relations of bodies to heat,
nor the  great number of ingenious men who were much occu-
pied with the studies connected with those relations, were in-
duced, by the original experiment of Fahrenheit on a mixture
of water and mercury,  to examine so important a question as
that concerning the proportion of heat in different substances
having the same temperature.  The experiment lay unnoticed
or was copied out by many compilers, without taking the trou-
ble of thinking about it.   Dr. Martin,  also, though engaged in
experiments which were very closely  allied to this  question,
did not attend to it.  The philosophers on the Continent seem
agreed  in giving the honours of the discovery of the unequal
distribution of heat among bodies  of different kinds,  to
Professor Wilcke of Stockholm, who read a dissertation on it
in the Royal Academy in  1771.  (Swed. Trans, vol. 35. p.
93. New Swed. Trans. 1782, Part Second.)
   But Dr. Black had seen its importance long before, and had
discovered many cases of very unequal distribution.  In some
small .note books, written while a student of medicine in Edin-
burgh, he sets down queries, founded on  Fahrenheit's expe-
riment, and projects of experiments, on a variety of substance;.
which had exhibited puzzling appearances in this respect.  He
had made several of these before 1757,  and, among these, some
which must have given rise, in his thoughts, to his'celebrated
theories of fluidity and vapour.
   Mr. Watt of Birmingham was, as far as I can learn, the first
who considered this subject steadily, and in a system.   He
was strongly incited to this examination by the great object •
which then occupied his attention,....the improvement of the 
 330       NOTES  AND OBSERVATIONS.

steam engine.   From Dr. Black's lectures he had learned the
doctrine of latent heat, and saw that a great obstacle to the per-
formance of that noble engine  was the waste of steam by inef-
fective condensation.   It was, therefore, an interesting object
to him, during his first speculations, to learn the quantity thus
wasted by warming the cylinder and other parts of the  appa-
ratus ; and  some of his  earliest attempts to improve the en-
gine  consisted  in employing cylinders made of substances
which required little heat for warming them.   He made many
experiments on various substarfces, in order to learn their pro-
perties in  this respect.  Though rude, they were sufficient for
his purpose; and  the  examination would have been carried
much  farther, had he not contrived the  separate condenser,
which made this property of bodies of little moment to him.
   Mr. Watt's experiments were all communicated to his pre-
ceptor, who took the warmest interest in them ;  and this cor-
respondence gave  him the opportunity of knowing the uncom-
mon genius  and  the great worth of his pupil, and was the be-
ginning of a friendship which lasted through life, and which,
for the merit of the parties, and for mutual confidence, res-
pect, and attachment,  has had few equals.
   Before  the year 170.5, Dr. Black had  made many experi-
ments en the heats communicated to  water by different solid
bodies, and had completely established their regular and steady
differences in this respect.  He mentioned  these in his lectures
in the  University of Glasgow, and was occupied with the ex-
periments for the same purposes when he was called to Edin-
burgh.  He was much assisted in the experiments by7 Mr.
Irvine, who afterwards lectured from the same  chair.  The
registers of  these experiments  remain among Dr. Black's pa-
pers,  most of them dated, and partly  written by himself, and
partly by  Mr. Irvine.  They  are all previous to 1770.  Mr.
Irvine made many experiments on the same subject,  besides
those iii which  he assisted Dr. Black ; and  particularly  on
those fluids,  which, without any remarkable chemical action,
produce heat by  mixture ; such as wait r and vitriolic acid, wa-
ter and spirits of wine, &c.  And it was this train of experi-
ments  which led Dr. Irvine  to that ingenious and plausible 
NQTES  AND OBSERVATIONS.
Oi>l
method of determining the point of absolute privation of heat,
which he publicly taught in his lectures, and communicated to
his philosophical friends.   His fundamental proposition was,
that" the heat which appeared in mixing vitriolic acid and wa-
" ter is the difference between the sum of the absolute heats of
" the two ingredients,  and the absolute heat of the  mixtuire,
" while the heats,  which each of them separately required for
" an equal variation of temperature, had the proportion of their
" respective absolute heats."  Therefore, having discovered,
by such experiments, the difference, and the ratio of the abso-
lute heats of the ingredients, we can find those absolute heats,
and the  temperature  at whieh those heats commence, or in
which the ingredients contain no heat at all.
   As  Dr. Black and Dr. Irvine now lived at a distance from
each other, and each was assiduously occupied with his official
duties, and interested in acquiring a deserved reputation, their
mutual communications were less frequent than formerly, but
they were by no means given over.  Dr. Black's speculations
were  anxiously directed to one favourite object,....the theory
of fluidity and vapour;  and Dr. Irvine's to the discovery of
the absolute heats of bodies.  It was in the prosecution of this
investigation that Dr. Irvine found himself obliged to use some
appropriate terms  for expressing his peculiar views of the sub-
ject.   That quality by which some substances required more
heat than others to produce the same elevation of temperature,
had different names at different seasons during his progress.
Affinity pleased him a  long while, and was thus employed both
by Dr. Black  and  Dr.  Irvine. Both also used the terms capa-
city, faculty for receiving heat,  appetite for heat,  &c. &c.
Capacity  was at last adopted, as the best suited to all the oc-
casional modifications  of this property which appeared in the
experiments,   It  was  the familiar language among the stu-
dents of chemistry, both in Glasgow and Edinburgh;  and I
have  not the least doubt but that the Swedish gentlemen, of
whom there were several at Glasgow and Edinburgh between
1763 and 1770, carried home with them both the doctrines
and  language which  were familiar to  them.   These facts,
which are all consistent with my personal knowledge, having; 
332       NOTES  AND  OBSERVATIONS.
been myself on the spot,  are of some use for ascertaining the
value of many claims of originality  and priority which have
been too warmly disputed.  The great  merits of a Wilcke, a
Bergmann, a Lambert,  are not diminished, although we learn
that Dr. Black and Dr. Irvine had preceded them in some of
their ingenious researches.
  Before concluding this note,  it will not be unacceptable, if
the general view  of this doctrine of the specific capacities of
bodies for heat be communicated, in a compendious manner,
and the method of applying the principle to the chief questions.
which  may be proposed, be briefly described.
  It is assumed,  in the  first place, that the proportion of the
quantities of heat required for changing the temperature of the
two bodies equally, is constant.   This is not unlikely ; but it
is by no means demonstrated :  Nay,  the experiments of Dr.
Black and Dr.  Irvine  on bees wax,, spermaceti,  &c. directly
prove  the contrary ; in all cases at least where  the body be-
comes gradually softer and softer before melting. We are not
certain  but  that something of  this may obtain  in most sub-
stances.   Lichtenberg thinks that water itself becomes more
perfectly fluid by incr-asing its heat.   But the change observ-
ed by  him was so trifling as  not to cause any sensible varia-
tion in its comparative capacity,  in all the  temperatures be-
tween  freezing and boiling.
  Dr.  Black estimated the capacities, by mixing the two bo-
dies in equal masses, but of different temperatures; and then
stated  their capacities as inversely proportional to the changes
of temperature  of each by the mixture.- Thus, a pound-of gold,
of the  temperature 150°, being suddenly mixed with a pound
of  water, of the  temperature  50°,  raises it to 55° nearly :
Therefore the capacity pf gold is to that of an equal weight of
water as 5 to 95,  or as 1 to 19 ;  for the gold loses 95°, and the
water  gains 5°.
   It will be most convenient to compare all bodies with water,
and to express the capacity of water by unity, or to  call it 1.
 Let the quantity of the water be W,  and its temperature w.
 Let the quantity of the  other body be B, and its temperature
b. Let the temperature of the mixture be m.  The capacity of 
NOTES AND OBSERVATIONS.      333
 t> •  W X tn__w
    lS b Xb—m °r» when the water has ^en the hotter of
 the two, the capacity of B  is ^X^—m In other word
   •                           ii X ui—b.
 multiply the weight of the water by its change of temperature.
 Do  the same for the  other substance.  Divide the first pro-
 duct by the second.   The quotient is  the capacity of the other
 substance, that of water being accounted 1.
   These experiments require the most scrupulous attention
 to many circumstances which may affect the result. 1% The'
 mixture must be made  in a very extended surface, that it
 may quickly attain the medium temperature.   2. The stuff
 which  is poured into the other should have  the tempera-
 ture of the room, that no change may happen in pouring it
 out of  its containing vessel.   3. Ihe effect  of the vessel
 in which the  mixture is made must  be considered.  4. Less
 chance of error will be incurred when the substances are of
 equal bulk.   5. The change of temperature of the mixture,
 during  a  few successive moments,  must  be  observed, in
 order to obtain the  real temperature at  the beginning.
 6.' No  substances  should be mixed which produce  any
 ehange of temperature by their chemical action, or which
 change their  temperature, if mixed when  of the same
 temperature.    7. Each substance must be compared  in a
 variety of temperatures, lest the  ratio of the capacities
 should  be different in different temperatures.
   When all these circumstances have been duly attended
 to, we  obtain the  measure of  the  capacities of different
 substances for heat.
   Philosophers  do  not seem agreed as  to the meaning
 affixed to  this  term.  It is frequently used in the sense of
.mere capaciousness,  or  room  for the  admission of heat.
 But this is a very needless consideration, and  unconnected .
 with all chemical science. Heat is not  merely admitted
 into bodies, but absorbed or  drawn into  them, in conse-
 quence  of some tendency to  combination, whatever  the
nature of this union may be ;  whether mere adhesion, like
capillary attraction, or the mixture of metals or of watery
fluids ; or like the solution of  saline crystals in water, and
the mixture of brines with more water :  or like the union
VOL. i. 
 334      NOTES AND OBSERVATIONS.

 of salt and wavr  in  crystallization ;  or like the union of
 acids and alkalis.   In all these examples,  there  is some
 physical action, and  this  is continued till an equilibrium
 of the acting forces is effected.  This is very different
 from mere capaciousness, and it even receives no explana-
 tion by the help of capaciousness.
   ■ The  only  thing expressed by the numbers above men-
 tioned is the variability of temperature by heat.
    Dr. Crawford,  in his  essay on animal heat, has given
 tables of  the capacities of a  great number  of substances.
 But a want of attention  to some of the above-mentioned
 cautions,  particularly the sixth *, has led him into many
 mistakes ;  and therefore, in a second edition he has made
 very great changes in h'i8 measures.   But even his l|econd
 measures seem exceedingly gratuitous and uncertain. Mr.
 Kirwan has also ascertained the capacities of a considera-
 ble number of bodies.  His table is published in Magellan's
 dissertations.  Bergmann has  given another table in hi-, es-
 says on elective attraction.  Baader has given another in his
 Dissertationmon heat; and, lastly, Gadolin has given a large
 list, in his Theory of the specific Heat of Bodies, Abo. 1784.
    Wilcke calls the number arising from  our formula, the
 measure of the specific heats.    But if the quantities of the
 substances are reckoned by their bulk, then the  resulting
 numbers  are the  measures  of what he  calls the  relative-
 heats.  The relative heats may be obtained  by multiplying
 the specific heats by  the  specific gravity of the substance
 compared with water in  the experiment.   Thus, the rela-
 tive heat of gold  is 1, or equal to that of water ; while its
 specific is  only 0,0526,  or -fe.  The  comparison  is most
 frequently  made  by measures in bulk, in which case  the
 formula gives the relative heats.   But Wilcke thinks the
I measures  by weight more proper, because the question
 seems to relate chiefly to the peculiar nature of the matter
 itself, independent of all consideration of space.

   * Dr. Crawford was particularly interested in the specific heats of the
 substances useJ. by animals f,r food.   Most of these generate heat or cold,
 when mixed with water. Flour of all kinds does this in a remarkable degree
 Even different kinds of flour generate heat, by mixing them suddenly to-
 gether in great quantities 
         NOTES  AND  OBSERVATIONS.      335.

   Dr. Irvine, Dr. Crawford, and most of the other writers
on .this subject, have considered those numbers as expres-
sive of the proportions of the absolute quantities of heat,
or cause of heat, contained in bodies, when they are all of
one temperature.   Indeed, it is this opinion which gives
the chief importance to the whole doctrine of specific heats.
But this  opinion  is just, only on the supposition  that the
measures, obtained by the above described experiments
and calculation, are constantly the same, whatever the tem-
peratures may be in which the experiments are made.  Dr.
Irvine's ingenious method of discovering the temperature
of absolute privation, evidently presupposes this constancy
of specific heat;  or, if  they are not constant, it supposes
that we know the whole law of variation.   Now both of
these assumptions are  highly improbable.   In none of the
progressions of natural operations that we are acquainted
with, do  we find this constancy.   It is much more analo-
gous to other phenomena, to suppose, that, in the tempera-
tures near to that of absolute privation, the quantities of
heat  necessary  for producing equal elevation gradually
diminish, and this, perhaps, without end, like the distance
of the hyperbola from its assymptote. It is equally probable
that the  law of diminution may be different in different
substances.   This will cause the measures of specific heats
to change their proportions continually; and therefore the
specific capacities observed in temperatures, all of which
are far removed  from that of the entire  absence of heat,
give us no means of obtaining the proportions of the accu-
mulated sum of all the heats which has been received into
the substances.   It follows from this,  that even although
it should  be granted to Dr. Irvine, that  the heat which
emerges,  in  mixing vitriolic acid and  water,  or in  the
freezing of water, is  the  difference between the absolute
heat of the mixture, or the ice, and the absolute heats of
the substances before mixture, or of the  water before
freezing,  still we cannot ascertain those absolute heats, or
the temperature of no  heat.
   Accordingly7, it appears that it has  been only in a very
few cases that Dr. Irvine found a tolerable coincidence of
his determination of  this extreme  cold : and  the deter- 
•336       NOTES AND OBSERVATIONS.

 mination  by means of mixtures, differed enormously from
 those obtained by  means of congelation ; and  still  more
 from those obtained by means of the condensation of va-
 pour.   The observations of Gadolin on these experiments
 are highly instructive and judicious.
    Upon the whole, it appears that, although the informa-
 tions which we have  acquired by those  experiments  must
 be, on many occasions, very useful, yet they give us little
 assistance for determining either the  proportions, or the
 real quantities of the  matter or cause of heat contained in
 bodies, or the temperatures in which any body contains no
 heat at all.   It  is  not even certain  that adjoining bodies
 will all acquire one temperature.   In  short, we are  as yet
 too little acquainted with the  nature of heat,  and the laws
 of its connection with other substances, to pronounce with
 confidence on any of these questions.  We  must be better
 informed concerning the mutual relation of the particles of
 this supposed substance,  and concerning  the  powers or
 forces which combine it with  other matter.   There must,
 in every quiescent or permanent condition  of a body with
 respect to heat, be an equilibrium between  all these  forces
 and the forces which connect the particles of other mat-
 ter.  This is a  subject more  of a mechanical than chemi-
 cal  nature.  The chemist will not perhaps  derive  much
 advantage from a perfect  knowledge of the  subject ; and
 he will contribute more to the advancement of his own
 science by overlooking this research, and attaching himself
 to the general though complex facts that occur in his ex-
 periments.   Establishing a general fact in chemistry is
 obtaining a fixed point, from which.we can set out on any
 farther search.   In this respect, the  tables of specific or
 relative heats, or of capacities,  are real additions to che-
  mical science, and will often  assist us in the explanation of
  phenomena still more complicated, although we still remain
  ignorant  of the  principle that unites them  all.  Newton
  established a noble body of astronomical science, although
  he professed total ignorance  of the  nature of gravity.  Dr.
  Black, ever more desirous of communicating  truly che-
  mical knowledge than refined philosophical doctrines, has
 £-nerally avoided all such disquisitions in  his lectures. 
NOTES AND OBSERVATIONS.      357
 These were intended for the information of even such as
 had no previous scientific education: and it was always
 with some reluctance that he engaged in abstruse specula-
tions.


                    [Note 2, p. 83.]

   It must be acknowledged, however, that this proportion
 is not supported by any very precise experiments, although
 such  are  neither  abstruse  nor  difficult.   There  are
 even  circumstances  well known to us, which cannot but
 have a notable influence on the heating and cooling of bo-
 dies.  We know that heat is much more speedily commu-
 nicated to the remoter parts in some bodies than in others,
 when the heating substance is applied to one extremity of
 the  body.  Dr. Black takes particular notice of this distinc-
 tion in a subsequent part of his lectures.   This being the
 case, it must follow that bodies which conduct heat slowly
 along their own substance, must  also imbibe and emit it
 slowly, because their superficial parts transmit slowly what
 they have received  from the heating cause to the interior
 parts.  The imbibition and emission of heat will also depend,
 for the same reason, on the conducting power of the heating
 or cooling medium.
   But, besides this obvious cause of difference, there are
 circumstances of a more abstruse nature, which render the
 truth of this proposition very doubtful, and make it neces-
 sary to have it evinced by very unquestionable experiments.
 There are two ways in  which heat  is received or emitted
 by a body.   First, by communication in contact with the
 body from which it comes, or into which it goes.   The
 transmission  arises from the want  of that equilibrium of
 connecting forces, which produces a permanent condition
 of things.   Now, since we find that different bodies  con-
 tain different proportions of this moveable heat, it is plain
 that the equilibrium  now  mentioned must be broken in
 different  degrees by the .addition or abstraction  of  the
 same absolute quantity of heat; and therefore the quantity
 which will escape or come in, in  equal times, so as to pro- 
338       NOTES AND OBSERVATIONS.

duce  a new equilibrium, will  be different,...that is,  that
heat will not be absorbed or emitted with equal celerity
by all bodies.
   In the next place,  heat is unquestionably emitted from ^
bodies also in the manner of light, or by what may be called
radiation.  This is performed with immense  velocity, so
as to seem instantaneously darted to any distance.  A late
writer indeed has  asserted, in Nicholson''s  Philosophical
Journal for November 1800, that heat is propagated only by
communication  through  the air,  and  therefore  slowly7.
But t:.:s  is contrary to most distinct and varied experiment.
The ver  curious observations of Dr. Herschell shew that
heat is ra.iiated in the same manner as light, (Phil.  Trans.
1800,J and is refracted and reflected also in a similar man-
ner.   It may here be observed, by the  way, that this ob-
servation of the constant ratio of the sines of incidence and
refraction, is the strongest argument  that has yet been ob-
tained for the materiality of heat.  For this law of motion
is competent only to a material particle, having  mobility
and inertia, and acted on by  transverse accelerating forces,
in the same manner  that a cannon ball is acted on by gra-
vity, and by the force of projection.   Mr. Scheele  is con-
sidered  as the  first  person  who observed and established
this radiation of heat, and its separability from light. But
it was observed  much  more than  a  century  ago  by Mr.
Robert Hooke, and exhibited by him to the Royal Society
at the meeting of May 15, 1682.  (Birches History, IV.
 137.) Mr. Pictet, of Geneva, also has made some remark-
able  observations and experiments, which  shew  that heat
 is  continually emitted from all bodies warmer than the
 medium, although by  very  small degrees.   This  is con-
 firmed by the direct observations of Dr. Herschell.
   Now in this case, as we are entirely ignorant of the forces
 which expel this heat from  the body7, and of  their laws of
 action, and of the proportion of the heat radiated to that car-
 ried off by contact, and whether this proportion be constant
 or variable, it seems unwarranted to assume  this proposi-
 tion as  true, without any direct proof  from  observation.
 We have no such proof, nor indeed any observation of the 
NOTES  A<ND  OBSERVATIONS.       339
comparative celerity from different bodies, sufficiently pre-
cise to give us distinct knowledge either of its truth or
falsehood.
                    [Nate 3, p. 96.]

   We must not omit taking notice of a very singular dis-
tinction, which Count Rumford has endeavoured to esta-
blish  between  the conducting power of solid and fluid
bodies, namely, that while heat is propagated  along solid
bodies, from particle to particle, according to certain regu-
lar laws, fluid bodies are heated in their  remote parts only
by the actual transference of  the  particle which received
heat from the hot body when in contact;  and that, without
this motion of the heated particles, occasioned by their ex-
pansion, and consequent levity, the remote parts of a fluid
would  never be warmed.    The  ingenious and  worthy
author has supported  this opinion by a series of curious
and  well concrived experiments, the results of which are
most  unexpected and surprising.  They are to be seen in
the 7th of his essays.
   Dr.  Black was much  struck with the novelty and im-
portance of this opinion, and began to examine it with that
cautious exactness which he had always  practised; but the
great decline of  his health and strength soon checked his
progress, and the few notes on the subject which are to be
found among his papers, are insufficient  for giving any no-
tion of his sentiments.
   It  is clearly established by the experiments of  Count
Rumford, that when the internal motion,  producible  by the
inequality  of specific gravity,  occasioned  by heating the
lower parts of a fluid,  is prevented, the progress of heating
is almost insensible, and  is no more than what we may ex-
pect by means of the solid matter of the  vessel, and.  by
radiation.   It  is  also  certain, that  our  knowledge of the
relation or connection of a particle of matter to heat, gives
us no information of the manner how heat, attached to one
particle, is shared with or communicated to another.   It
i*s the perfect equality of action in all directions, which fits 
 340      NOTES AND OBSERVATIONS.

 a particle of matter for being the particle of a fluid; while
 a certain  polarity,  arising from  an inequality of action in
 different directions, would fit it for being the particle of a
 solid.   It is not at all unlikely that this difference may so
 change this attraction (or whatever name is given to the
 relation)  of a particle for heat, that  a superfluity of heat
 may have a tendency7 to quit the pole of one particle for the
pole of another, in a solid body, while there is no such ten-
 dency with respect to a particle  of a  fluid, having no po-
 larity.  Heat may continue to accumulate spherically round
 the latter, expanding the molecula consisting of such par-
 tides, without leaving it for another.  We see a difference
 of relation not less distinct and remarkable, in the case of
 liquids and  gases.  Liquids, like solids,  expand in  arith-
 metical progression,  acquiring equal  additions of bulk by
 equal additions of heat; whereas  gases, such as common
 air,  carbonic  acid, &c. expand in geometrical progression
 by  equal  elevation of temperature.   An  increase  of 30°
 nearly doubles the bulk of ordinary  steam; another 30° dou-
 bles this bulk ; or 60° quadruples the  original  bulk nearly.
 It may be added, that the phenomena of galvanism  shew
 us  still another  modification of the chemical relations of
 atoms, by which  a  quality or power of producing effects is
 propagated  to any distance, without our being able to con-
 ceive that any material substance is transported along this
 line of visible activity.
   There  is nothing, therefore, in the  opinion offered with
 great caution by Count  Rumford,  that is  incompatible or
 inconsistent with our  most distinct notions  on these sub-
 jects.   It is a fair field of inquiry, most ingeniously laid
 open to philosophers.   It even promises to assist us in ac-
 quiring juster notions of chemical action, than those which
 at present satisfy  chemists: and they would do well to study
 it mathematically,  as P. Boscovich has done.  This would
 soon make  them exclude some modes of action  which are
 in fashion, and by thus limiting the  attention, would sooner
 bring us into the  right path. 
NOTES AND  OBSERVATIONS.      341
                    [Note 4. p. 138.]

   The celebrated Bergmann having learned and adopted
Dr.  Black's theory of  fluidity,  imagined that this absorp-
tion  of heat in a latent state would afford an excellent me-
thod of examining the  question so much debated by philo-
sophers,  " Whether heat be  ponderous or not ?"   The
experiments  which had  been made  for this purpose have
been  very anomalous, by  reason of  many unavoidable
sources of inaccuracy,  in all cases where great quantities
of sensible heat are accumulated in bodies, in order to
make its weight perceptible.  Currents of air are produc-
ed, which affect the scales of the balances employed. The
great heats expand the arms of the balance,  which destroys
the equilibrium,  and affects the measurement.   The very
expansion of the body  affects it,  by  changing  its specific
gravity.
   Bergmann proposed to weigh,  not the sensible heat, but
the latent heat, which is accumulated in considerable quan-
tities, in such a manner as  to be free from errors.  He
proposed to weigh a mass of water, of the temperature 32°
or 33°, in a room having this  temperature, and then to freeze
it completely ; and, having again brought the  ice  to the
temperature 32°, to weigh it again in  a room having that
heat.  Thus all the causes of error seem to be avoided.
   This judicious experiment has  been  made with the most
scrupulous care by Dr. Fordyce,  (See Phil. Trans. 1785.)
with about three and a half ounces of water, contained in
a thin glass globe hermetically  sealed.   Its  weight was
examined  by a balance  which was  sensible to the ToWh
part of a grain, and it was repeatedly  found TVh of a grain,,
nearly yyforeth  of the whole, lighter when in the fluid state.
Similar experiments have  been  made, having  the  same
result, by Sir Benjamin Thomson, (now Count Rumford)
by Mr. Gouvenain at Paris, and  by others.  Dr. Fordyce-
also found that  the ice became Teth  of a grain heavier,
when cooled down to 22°.   This might be expected from
the contraction of bulk,  and from the downward current
-d" air which so cold a mass  would occasion.
vol.. r.
-.I r. 
342.      NOTES  AND OBSER\ A 1 IONb.

   Dr. Black seems disposed  to  ascribe this unexpected
effect to a repulsion  for the heat in the  earth.   This,
however, considered as the weak exertion of the repel-
lency which he supposes to be a property7 of the atoms of
heat, is not consistent  with  such laws  of relation, whether
attraction or repulsion, as are necessary for the corpuscu-
lar combinations of matter. These must decrease at least
in the inverse triplicate ratio of the distances, and be alto-
gether insensible  at any7 measurable  distance.  But it is.
much more reasonable tO' ascribe the small diminution ob-
served, the l*6th part of the 17000th part of the whole, to
errors  unavoidable  in such  experiments.   Accordingly,
Count Rumford,  some years after,  repeating the experi-
ments On  much  larger quantities of material's,  could ob-
serve  no difference whatever.   Nor  should we expect to
find heat  ponderous.   When we  recollect that it comes
with the same velocity as light from  the sun, and yet
exerts no  sensible impulsive force, we  have no reason tc
suppose  the quantity of matter to have a sensible weight.

                                   •
                    [Note .i.  p.  189..]

   The  worthy author of these lectures was always more
anxious to communicate what  may be  called a clear and
confident knowledge of the doctrines of  pure chemistry,
than to lead his pupils iiito abstruse or refined speculations
 on the unseen and unknown immediate causes of chemical
combination.  He considered every question of this kind
 as rather out of the' pale of chemical science;  and so it
 certainly is.  Whenever we speculate about the  attractions
 and  repulsions of particles,  as the  immediate agents in
 effecting the chemical changes, we are no longer chemists,
 but mechanicians.  We are  considering questions about
 local motion, and the mathematical determinations of the
 effects- of moving forces.   Not  only  is  the occupation not
 chemical, but the questions themselves  give little addition
 oL chemical knowledge.   This, at  least  in the  present
 state of the science, is promoted chiefly, if not solely, by
 ■i more successful generalization of chemical phenomena. 
NOTES AND OBSERVATIONS.       343
This generalization is, in fact,  the establishing of general
laws, and marking with greater accuracy their distinctions
and limitations.   But the chemical phenomena which are
to be classed are all of a  very complicated kind; and it is
of  infinitely more consequence  to  know  the full extent,
and all the modifications  of the  law, than to know the hid-
<len process of nature in  any one of the phenomena; just
as the noble system of physical astronomy, which  Newton
was able to class under one general fact (universal gravi-
tation in the inverse duplicate ratio of the  distance) was
of more  importance than the explanation how this  fact was
effected.
   In all cases, therefore,  Dr. Black contented himself with
presenting his subject in the simplest form, suggesting the
simplest notion of it. Nothing could be more simple than
his doctrine of latent heat.   The experience of more than-
a century had made us  consider the thermometer as a sure
and an accurate indicator of heat, and of all its variations.
We had  learned to distrust all others.   Yet,  in the lique-
faction and vaporization of bodies, we had proofs uncon-
trovertible of the entrance of heat into the  bodies.   And
we could, by suitable processes, get it out  of  them again.
Dr. Black said that it was concealed in them,..../ater,....it
was as much concealed as carbonic  acid is in marble, or
water in  zeolite,....it was  concealed till Dr. Black detected
it.   He  called it Latent Heat.   He did not mean by
this term that  it was a different  kind of heat from  the heat
which expanded  bodies,  but merely that it was concealed
from our sense of heat, and from the thermometer.
   It has always  surprised me  thar a doctrine so plain,
simple, and unassuming,  should be misunderstood.   For
it must be misunderstood to be contradicted in any shape.
Dr. Black would  not have thought any more about a sub-
ject already so plain, nor would he have paid any attention
to the condition or  state  in which this heat exists in the
bodv, had he not been  called upon by those who said that
he had introduced an unnecessary principle into the doc-
trine of heat, and had he not heard it asserted that the heat
was no more  latent than while  it  expands a body.   He
knew this ; but asserted that the indication of its prgpcnce', 
344      NOTES AND OBSERVATIONS.

though undisputable,  was not heating, or expanding, but
melting a body, or converting it into vapour.  It was only
in order to answtr this challenge that he was* led to consider
its  peculiar condition, by which its  mode of  action was
rendered  so different from what it was before.   His first
observation was, that it was no longer so free  and move-
able as before, and that it was only the additions made  to
the heat necessary for mere fluidity which were now move-
able, and affected the  thermometer.   He now saw good
chemical  reasons for considering this  concealed  heat  as
united with  the substance of the body, in a way very much
resembling many7 chemical combinations.   A warm body
carried its  warmth about with it.  It shared  its warmth
with other bodies, just  as a brine shares its salt with more
water.  In  liquefaction  and vaporization  it attached  to
itself a determined quantity, and no more  than the proper
quantity, just as water does of salt, or as salt does of water
in forming  a crystal.    The appearance, and many proper-
ties and modes of activity7, are changed by this union, just
as in other chemical combinations.   He was therefore led
to consider heat as chemically combined,  in all its modes
of  existence,  whether  in  expanding, in melting, or in
boiling off a fluid.  He considered heat in a certain pro-
portion as an ingredient of all liquids, and, in another pro-
portion, as an  ingredient in all vapours.
   But all this was refused,  and the combination of heat
was denied, merely because the touch of a cold body could
take out this heat,....because the combination is not perma-
 nent, like the generality of  chemical combinations ; and
 it was asserted that this great quantity of heat was merely7
 received into the body7, and lodged there, without affecting
 the thermometer, merely because the body, by its change
 of form, had more room for it.
    Dr.  Black  now saw strong reasons for  adhering still
 more to  the conception he had now formed of its mode of
 existence in -the fluid and vapour. He now considered heat
 as the active cause both of fluidity and of vapour, producing
 those new  modes of aggregation by  a true chemical com-
 bination with  the particles  of the body.   To the  reasons
 adduced by Dr. Black  for this opinion I would add one ot 
NOTES AND OBSERVATIONS.     343
two more, which I think are incompatible with the notion
of mere capacity.
   When a piece of dry sponge lies in the  exhausted re-
ceiver of  an air-pump,  and we admit the air into the re-
ceiver, it soon fills all the pores of the sponge.  This has
capacity for  its  reception, in the purest meaning of the
term.  The sponge  exhibits no appearance of change upon
this reception of air into its pores.  But, let us pour water
into the receiver, the sponge has the same capacity or room
for the water.   But how different the appearance !  The
sponge swells to twice its former bulk.   And, although
we understand  very little  of the corpuscular'actions of
bodies, we comprehend a good deal of this process.   We
know that the sponge does not merely admit the water into
its pores ;  it attracts it, and will even raise the water to a
considerable height, if one end only of the sponge be dip-
ped into the water.   We even comprehend a little of the
manner in which this attraction produces the expansion of
the sponge.   There is a force, which is mutual between
the sponge and the  water, acting at the  mouth of the pore,
urging the water into it, pressing it on the water  already
in, in the same manner as if a little piston were pressing on
the water in  the mouth  of the pore, and therefore distend-
ing the pore.  We can  even calculate the limit to the effect
of this force, and what  rigidity of the pore will put a  stop
to the accumulation.   We know that this attraction  will
keep  the  bodies united.   And we say, with the utmost
propriety,  that  they are united....combined....and we  call
this particular mode of  union adhesion.  And lastly, we say
that the water is the cause of this distension of the  sponge.
It would be very awkward to say that the capacity of the
sponge is the cause of this distension.  It is very true  that
the cause of it is not the water, but the force which unites
the water and the sponge.   But the water is the occasion
or cause, in the  same manner that  fixed alkali is the cause
why lime  is separated from its solution in an acid.  If the
experiment had  been made with a piece of dry wood in
the place qf sponge, we know that it would have swelled
with  a force sufficient for splitting the  firmest rocks.   It
would surely be ridiculous to ascribe this to the capacity 
3+6      NOTES  AND  OBSERVATIONS.
of the wood.  If any person, unaccustomed to discussions
of this sort, finds a difficulty in conceiving how attraction,
which rather seems fitted for contracting the sponge, pro-
duces its expansion,  he may see this effect of undoubted
attraction exerted between tlie particles of mercury.  Lay
two or three small globules of mercury7 on a smooth  plate
of glass :  There can be no doubt of the mutual attraction
of the atoms being the cause of the  round form of the
drops.   Lay another light plate of glass on these  drops of
mercury: Its  weight will flatten them  a little.   Lay on
some small weights:  This will  flatten them still more.
The addit on  of more weight  will increase this effect.
Now lift off the  weights one after another.   The drops
will  gradually contract their breadth again, by raising the
upper plate farther from the under one.   Here it  is  plain
that the mutual attraction of the  particles forces the plates
asunder.
   This  reasoning is applicable, in the strictest  manner,
to  the  phenomena now  under consideration ;   and it
is even  more  forcible  in the case of  some of them,....in
vaporization,  for example.   Here, we  know  that a due
supply of heat will cause an  ounce of  water to  burst a
bomb-shell.   Can any person ascribe  this to  the  capacity
of the steam?  This capacity  is not yet  acquired, nor will
it be again acquired, unless the  due supply of heat  be at
hand.   The particles of water acquire  the  vaporous ar-
rangement which fenders them so capacious, only  in conse-
quence of the  mutual action between them and the  matter of
heat.   In short, it is plain that  the absorption of heat is
not the consequence, but the cause of the enlarged capacity.
 Strictly speaking, they are concomitant events,  and it is
 the  combining force that is the immediate and operative
 cause of both.
   Those who assign  the  enlargement of capacity as the
 cause of the absorption of heat, in a state which  does not
 affect the thermometer, are by no means precise  and uni-
 form in the meaning which they affix  to the term.   They
 must be sensible that mere capaciousness, or room, will not
 apply to the phenomenon of melting ice, in which the room
 is undoubtedly diminished in theliquefaction, and even for 
NOTES  AND OBSERVATIONS.
some time after.  For, while the water, just formed, con-
tinues to accumulate heat  around its  particles, the bulk
does  not increase;  on the  contrary, it diminishes.   In
their conception of capacity is  included the notion of a
disposition to absorb heat; that is, of an attraction for heat;
or, to express  it more  philosophically, the notion of the
existence of a  force, which unites the matter of heat with
that of the  ice.  What is this but the union or combina-
tion  which Dr. Black  at last contended  for?  Chemistry
presents a considerable variety of such combinations, and
there are ..iany, especially of the gases, which are as little
permanent, and yield as readily to a difference in the propor-
tion of the ingredients, as  th.ose  combinations of  bodies
with heat.  'Nay, we shall find still another combination of
heat  with otheu matter, which has all the permanency that
can be desired, and which  the most pertinacious partisan
of capacity  must admit as a true chemical union.   I mean
the gaseous form.   Oxygen gas is  supposed to consist  of
the basis of vital air, united with the substances  of heat and
of light.  It requires a certain temperature, and the  pre-
sence of a proper third body, to produce a decomposition -.
and here we find a prodigious quantity of heat accumulated.
Capacity will  explain this accumulation, in the very same
manner that it explains all  the others.
   I am disposed to think that there is a gradation of union7
between the particles of matter and heat, which  very^much
resembles the  gradation which late observations have  dis-
covered in  the  combinations of oxygen.  The  degrees  of
oxy dation, as it is called, are as progressive, and  as distinct,
as  those exhibited in the effects of heat;  and they  are all
produced by the  gradual  introduction or abstraction  of
oxygen.  There seems to  be,  first of all, a union similar
to that of mere adhesion.   The heat thus united has been
called free,  moveable  heat.   Perhaps  the  union accom-
panied  by expansion,  or which  produces expansion,  is
something more close, and  it may be that only  the super-
fluity is free and moveable.   This  mode  of  union  is  evi-
dently independent of the others,  and may be superadded
to them id!.  Then comes the union which is characterised
bv fluidity.   This it-followed by the  union of vaporization, 
848      NOTES  AND OBSERVATIONS.

and this by the union which forms a gas.   Perhaps a just
conception of these changes would be more easily sugges-
ted, were we always to employ the word temperature for the
degree of the scale, and heat for the matter combined; and
to call the heat concealed or latent, in a liquid, and in va-
pour, the  melting heat, and the boiling heat; thus  distin-
guishing them from  the melting temperature, and boiling
temperature.
   In  short, this dispute seems  merely occupied about the
propriety of a term:  And even in this humble sense, Dr.
Black's employment of  the term combination  is more pro-
per than the  employment actually made of the term capa-
city.   About the propriety of the term latent heat, as the
expression of a mere fact, there# can be no dispute.
   In  Dr.  Black's examination of Dr.  Cleghorn's method
of explaining the phenomena of fluidity and vapour, he was
obliged to  consider  the  uniting forces, not as a chemist,
but as a mechanician.   He  disliked this occupation ex-
ceedingly;  and the  remarks on  Dr. Cleghorn's hypothesis
are almost the only examples to be found in the manuscript
from  which he lectured.  But  observing  this way of con
sidering chemical  subjects to  be gaining ground, after
having been  so long and so profitably banished from che-
mistry, he was induced to say more on the  subject, in some
particular courses.  Some notes  of those occasional lectures
remain.  I recollect a conversation, in which he explained
to me some of his notions of this kind.  He said that this
way" of considering  the phenomena was only mispending of
time, because it produced no clear  conception, and gave
no confident knowledge.   He said that he sometimes
thought himself successful;  but some other views  con-
vinced him that he had only been surrounding, in imagina-
tion, the particles with material, active atmospheres, which,
notwithstanding their activity,  and the  mutual repellency
of their particles,  must be  conceived to penetrate each
other, without derangement of their forms; in short, with-
out the very activity which  he had  assigned  them.   lit
could recollect his being affected in the same way by read-
ing Dr. Gowen Knight's ingenious essay on magnetism.
The  first  steps of  th" investigation seemed  to give very 
NOTES  AND OBSERVATIONS.      349
clear notions;  but he found them afterwards involved in
continual  inconsistencies.   He  had observed  the same
thing in all attempts of this kind ; and he therefore avoided
them.   He was convinced of  a sort of rivalship between
cohesive attraction and many chemical attractions, by which
they counteracted and weakened each other.   But  this
was  only  a way of conceiving, or rather of expressing, a
plain fact; for when he considered them  purely  as attrac-
tions, there could be no opposition between them.
   This is a very just account of the matter.   Indeed, when
we recollect that the  united efforts of  all the mathemati-
cians of Europe have  not  yet given a demonstrable and
accurate account of the motion of  three particles attract-
ing each other, we can expect little success  when we want
to determine the motion of millions attracting at  once.
   But, although we  cannot,  perhaps, explain phenomena
by this way of considering them, we can discover, in some
cases, the insufficiency of explanations which are in vogue.
Nothing is more common than the notion that  fluidity is
produced by heat, merely by its  destroying the attraction
of cohesion.  Observing that  all the corpuscular actions
 are weakened by an  increase of distance,  and that  heat
 expands bodies, we think it  plain that the expansion may
 be continued till the particles get beyond the sphere of cohe-
 sion, and that fluidity is the necessary consequence.   Dr.
 Black very  properly observes, that the attraction of  cohe-
 sion is not destroyed; because we  see that a fluid gathers
 into drops.   He  grants that  it is weakened, and seems to
 think that the attraction, weakened to a  certain degree, is
 the constitution proper to fluidity7.   But this concession is
 not necessary, and this weakened attraction is not enough
 for fluidity.
    It is by no means certain that the attraction of two parti-
 cles of water is less than that of the same  particles in the
 form of ice.  The only effect of this attraction  which can
 affect their motion,  is their being pressed  together by it,
 as a piece of iron is pressed to the magnet.  Now, we know
 that water  is  equally fluid at  the bottom of the ocean as at
 the surface.  At the bottom, the particles are pressed with a
 verv great force.probahlv equal, nay, certainly superior, to
     vol.  p.                  * ? 
350      NOTES  AND OBSERVATIONS.

the cohesion of ice.  The easy separation of the particles
of water is owing to this perfect mobility from one situa-
tion among the others to another situation.  In shifting
them from their places, we are scarcely  opposed by any
thing besides their inertia.   We act but on one, or but on
a very few at a time.  It is quite different when we attempt
to separate the parts of a solid.  But to understand this dif-
ference, we must think a little more of the nature of fluidi-
ty and solidity7.  I do not mean to enter into the laws of  at-
traction, &c. but merely to reflect on what circumstances
are  necessary for  rendering an  assemblage of  matter a
fluid,  such as  we observe water to be.  A fluid is an  as-
semblage of matter,  the particles of which  yield to the
smallest impression,  and, by  thus yielding,  are moved
among themselves with the greatest  facility.   A body7,
having a base perfectly flat and smooth,  and lying on a
horizontal  plane,  having no  friction,  can be  pushed  to
any side, in the same manner as if it hung in the air, what*
ever its weight and pressure may  be, because it must  be
indifferent as to any situation on that plane.  In like man-
ner, in order that  a particle of  a fluid may be so easily
moved from its place among the lest, all that is necessary
is, that all new  situations   shall  be perfectly  indifferent.
That this maybe the case, the action of a particle must be
equal in every direction at the same  distance ;  no matter
how strong this may be, or whether it attract or repel.   It
is quite plain that if this action be thus equal in every di-
rection,  it  can require no force to remove an  adjoining
particle from one situation  to  another.   Nor will  it re-
quire any  force to keep the  particle  in this new situation,
because this situation is indifferent to it, there being no ine-
quality of  action urging it to either side.   The attraction
and repulsion may vary according to  any law, by a change
of distance from the particle ;  but it is equal in every di-
rection at  that distance.  An assemblage of such particles
must have all the well-known  appearances of a fluid.  It
will fcpather into drops perfectly spherical, because any other
form would cause an  inequality  of  the compound action
on any one particle, to which inequality of action the par-
ticle would yield,  and it would shift its place.   Such  an 
          NOTES  AND  OBSERVATIONS.     451

assemblage will,  if heavy,  acquire  a horizontal surface,
ckc. &c. It may be demonstrated that such a particle must
either be a single  atom of matter, or must be a sphere con-
taining numberless atoms  symmetrically  disposed.  The
spherical form is  necessary for fluidity, not for the reason
commonly assigned, that they may touch in few points, and
easily slide on each other.   These would not constitute a
fluid ; for  a parcel of spheres, perfectly smooth, could be
piled up in a pyramid.  It is necessary merely in order to
produce this equality of action in every direction.
   Solidity, on the  other hand,  requires  a very different
law of  action  among the  particles.   All situations must
not be indifferent.  A particle must affect a particular situa-
tion among the adjoining particles, must require some
force  to  change  its  present situation, and  when this is
changed a little, it must have a tendency to  regain it, and
must  actually  regain  it  when the   disturbing force is
removed.
   To effect this, it is only necessary  that a particle  attract
or  repel  another more strongly7 in one direction  than in
another, at the same distance ; and if the action of a par-
ticle be thus unequal, an assemblage of such particles can-
not be fluid.    The particles will unite in one attitude rather
than in another.   Force is necessary for changing  this ar-
rangement ;  and if this change be very small, it will be re-
sumed when the force is removed.
   All this will be  illustrated by7 attending to  the appearan-
ces exhibited by a parcel of small bits of iron wire, (sup-
pose one-tenth of  an inch long) scattered at random on the
surface of quicksilver.  They will lie in any position, and
the  touch of a single hair will change their position at plea-
sure. But now bring a great and powerful magnet near them,
they will all  instantly change  their respective  attitudes
and positions.   Each will  turn its ends towards  the ends
of the adjoining particles,....they will run together into cer-
tain curious groupes.   The mere touch of a hair will not
change these new  positions  and attitudes ; or, if it should
change them a little, they will resume them when the "hair
js withdrawn.   The reason of all this is merely, that the
neighbourhood of the magnet makes  them all little -magnets.; 
 352      NOTES AND  OBSERVATIONS.

 and now  each little bit attracts more  strongly by its ends
 than by any other part,  and in one direction much more
 than in  another ;  so that there is a particular position and
 attitude, which each will assume and  maintain with a cer-
 tain minute force.   Remove  the magnet,  and they all
 return to their former state of indifference and mobility.
   The constitution  of a solid body resembles this.   The par-
 ticles have such an inequality7 of action, resembling the polarity
 of magnets.   It is by this,  operating in the most favourable
 circumstances, that most solid bodies acquire a symmetrical ar-
 rangement of parts.   Thus do salts concrete from their solu-
 tions in a crystalline form.
  Here then we have a narration of facts which cannot be dis-
 puted.    A fluid must consist of particles,  which, while they
 attract each other, will take any situation indifferently.   A so-
 lid consists of particles which have a quality resembling the
polarity of magnets; and we understand in some small  de-
gree, on what dynamical  principles these constitutions  de-
 pend.
  Now,  let heat be  applied to a solid body, and let it continue
to expand  it  till its  cohesive attraction is destroyed, what is
the effect?  It now is an assemblage  of  incoherent particles.
 But  this is not a fluid ; it is a dust, an impalpable powder,
which may be heaped up like so much flower; or it is a par-
 cel of slightly adhering  particles, like wet sand, or half-boiled
barley.   But it is not fluid.   Heat must  do  a great deal more
to make  it  a fluid.  It must render each particle equally active
in every  direction, destroying its former polarity.
  Thus, we see clearly  that the particles  do not first fall asun-
der, then take the arrangement of fluidity, with an enlarged ca-
pacity; and lastly, admit the heat into the  enlarged pores. The
arrangement of fluidity requires the previous change of the
law of action ; and this  must be produced by the heat. There.
fore  Dr.  Black was  fully7 warranted to say that fluidity was im-
mediately effected bv heat.
  What has been said of  fluidity is also true  with respect to
the vaporous form of aggregation,  and need not be repeated.
  We know very little of the corpuscular constitution of bo-
i. es: yet we know som?thins;, ar.d  can make some useful in- 
NOTES  AND OBSERVATIONS.       353
 terences.   We know that all the corpuscular forces diminish
 fast by an increase of distance : and we can demonstrate, that
 the inequality of action on which solidity depends,  decreases
 much faster.   We  see  this remarkably in  magnets.  At a
 small distance the action of a magnet,  in different directions,
 is very unequal; but as we recede, the inequality of action di-
 minishes exceedingly.  We can demonstrate strictly,  that if a
 body, acting in this unequal manner, be surrounded by an as-
 semblage  of  smaller particles,  which act equally at the same
 distance at least, the inequalities in the  action of the  nucleus
 will be prodigiously diminished by the superadded equable ac-
 tion of this sort of atmosphere.  Nay, we can shew that this
 atmosphere may be so disposed around the particle as to take
 away  entirely the polarity or inequality  of action.  We see,
 therefore, that the accumulation of heat round a solid particle,
 must, independent of the expansion caused by it, diminish the
 cohesion of solidity, and cause the union  to approach,  at least,
 to the nature of the union of fluidity.   The great quantity  of
 heat that is accumulated round the particle in the liquefaction
 gives still more weight to this argument.   We see, too, how
 the abstraction of heat, and the contraction of bulk,  tend  to
 bring the unequally acting parts nearer  to each other,  and to
 give  occasion for the cohesion of  solidity.  When a  mass  of
 water has cooled below the freezing temperature, nothing in-
 duces the freezing with so much certainty as the touching of
 it with the point of one of the fine spiculse which have  been
 formed by freezing.   It  is evident that  the  polarity  of this
 spicula, by its action on a particle surrounded by a small quan-
 tity of heat, must dispose the heat to emerge in  fine streams
 from certain points of it,  by destroying that equilibrium of
 heat around it,  which fitted the particle for the fluid aggrega-
 tion.   I am far from offering this as a complete explanation ;
 but I say that if heat consists of particles much finer than those
of tangible  matter, and if it be capable of being constipated
round them, it cannot but produce effects of this very kind, and
that  it is possible for it so to accumulate round them, as com-
plexly to effect the changes that we observe. 
354      NOTES  AND  OBSERVATIONS.

  The change from  the liquid to the vaporus form is much
more easily conceived, but does not afford such cogent proofs
of the important part which is  acted by heat in producing the
change.  But there is something curious in the law of expan-
sion that seems to obtain  in vapours.  Equal augmentations
of temperature (which seem to arise from equal  additions of
the  matter of heat) produce nearly equal multiplications of bulk.
This seems  to» indicate an equal density of heat in all tempe-
ratures.  For, if the atmosphere of heat be like other elastic
atmospheres, equal additions of the matter of heat should pro-
duce additions of bulk, which continually7 increase.
  There is another circumstance in the vaporous combination
of heat, which I think of immense importance to the philoso-
phy of chemistry7, namely, its being counteracted by mechani-
cal  force.   Not only7 can the elasticity or mutual repellency be
counteracted by7 pressure, but the chemical union of heat and
the matter of a body can be prevented by it.   The instant that
the  pressure on the  surface of boiling water is removed,  the
union takes  place; for steam  is produced, and the tempera-
ture of the water is diminished.  Hold the ball A of the pulse-
glass represented by fig. 3. in the mouth.  The liquor will
boil in the ball B ; and A will feel cold.  Grasp B in the warm
hand, and the ebullition is then stopped by the elasticity of the
steam, which is allowed to retain this form by the warmth of
the hand.   A immediately grows warm also.  Now remove
the hand from B, and let it be wetted with some strong spirits
of wine.  This  produces a cold which condenses the steam in
B,  removes its pressure,  and the ebullition begins again.   At
the very same instant A feels vastly cold.    Thus was the com-
bination of water and heat in A effectually prevented, although
the mutually acting substances are contiguous as before.  Add
to  all this, that when the  vapour is produced by the union of
heat with tangible matter, pressure will put an end to it, the
vapour becomes water, and the heat flies off. We cannot form
 a conception of any  thing opposing a mechanical  force, but
another mechanical force.  Hence we must infer that the cor-
 puscular powers or qualities which ultimately produce the che-
 mical phenomena, are not different from the forces which  pro- 
NOTES AND OBSERVATIONS*       3SS
duce and modify local motion, and are really similar to those
which produce the fall of heavy bodies, and the communica-
tion of motion by impulse.   There  were strong reasons for
thinking so,  independent of this fact.  Capillary attraction,
adhesion,  cohesion, and some of the effects  of electricity and
of light, furnished surmises of this kind.   But the prevention
of vaporization by external  pressure is the most direct proof.
The conjecture of Sir Isaac Newton,  expressed in one of his
reflections at the end of his treatise on Optics, seems fully ve-
rified ; and it becomes highly probable that  all bodies consist
of atoms endowed with one and the same  power of attraction
and repulsion, varying according to a determined law of the
distances.  The ingenious essay on this subject by the Abbe
Boscovich merits therefore the careful perusal of every philo-
sophical chemist.
                      [Note 6. p. 196.]

   There is something very curious and unaccountable in the
manner in which these vapours are precipitated.  It is not in
round drops, as one should naturally expect.  Were this the
case, the precipitation would be almost instantaneous.  A
drop, whose diameter is the thousandth part of an inch, would
acquire the velocity of nine or ten feet per second; whereas
we see clouds hover at a very small elevation, for many hours,
and they can  thus be transported from the sea, lake, river, or
marsh,  from which they are raised,  far into the country,
where such supply  cannot be had any other way.  Thus is
water carried to the tops of mountains, where it is condensed,
supporting their vegetation,  and  giving the surplus  to the
ground,  there to percolate, and  supply us with springs and
streams'of fresh water.  A cloud formed of solid drops would
always exhibit a rainbow when opposite to the sun ;  and when
between us and the sun,  would exhibit coloured rings alto-
gether unknown in the ordinary course of things.
   The precipitated or falling  mist is all in  the form of  thin
transparent bladders or vesicles.   This was affirmed by Der- 
356       NOTES AND  OBSERVATIONS.

ham and others, but without  any authority, or  indeed  any
assertion that  they had  observed them.  M. Saussure,  in a
journey of observation  among the Alps, found himself  in a
thick mist, which was almost stagnant.  He was astonished at
the size of the drops, as he thought them, and at seeing them
swimming slowly  by him  without falling  to the  ground.
Some of them were larger than the largest pease.  Catching
them in his hand, he found them to be nothing but bladders
inconceivably  thin.   He  now  found them all to be of  this
structure.  He is the first person, so far as I know, who has
described them.  I  have  many  years ago observed that the
particles of  mist visible in the receiver of an air-pump, are
bladders.  I inferred this from their size, and from their
very slow descent through  air  so much  rarefied.    I  was
much surprised at them,  but imagined that this was peculiar
to the  precipitate formed  by sudden rarefaction.   But I  now
see that M. Saussure's  description is general.  The  mist of
boiling water,  when between the eye  and a candle, and be-
tween  U3 and  the sun,  exhibits the halos which are produci-
ble by  vesicles only.  This is easily demonstrated by the laws
of optics
   This constitution of a* mist or fog, so necessary for accom-
plishing the  great purposes  of nature, is to me most unac-
countable.  I cannot conceive how the formation of  such  a
vesicle commences, or what is the a'rial substance which
distends it.   It is an air in a state of slight compression; for
the film of water round it is endeavouring to contract.  I see
such bubbles sometimes arise from lintseed oil, when boiling
to free it from its  watery part.  This I understand;  but its
origin has no  resemblance to the production of a mist.  The
vesicles may be very distinctly seen in the evaporation from
a dish of very warm coffee without milk.   Set it in the sun,
and, in  viewing it, turn the back  to the sun.   When  thus
viewed through a magnifying glass, the vesicles are seen rising
 in great abundance ; some of  them so large and heavy, that
 they fall down immediately7, and roll about like so much dry-
 dust on the surface of the coffee, appearing like brilliant spots 
NOTES  AND OBSERVATIONS.      357
on the dark ground.   (See M. Saussure's Essai sur PHydrom-
ctrie, page 285.)

                      [Note 7. p. 202.]

  If heat be  the sole agent in producing the vapour, I cannot
see how the  air above the evaporating body can be  uniformly
transparent.    Although the  vapour  may  have  the  same
elasticity with the air, it is very improbable that it  has either
the same weight,  or the same  refracting  power, and it must
always surround the evaporating body with an unequable atmos-
phere, like a mixture of oH and water shaken together.  The
necessity of renewal of air, the stop to evaporation by stagna-
tion and even the effect of extended surface, are all inexplicable,
if heat alone be the agent.  These facts are strong arguments
for believing that the air also  acts by its dissolving power.
The prevalence of bad smells immediately before  rain, is a
fact very similar to the precipitation of one substance by means
of another.  Dr.  Hutton's  observation,  that two  masses of
very  damp air, of  very  different temperatures, precipitate
water on mixture, is a fact  similar to a  like precipitation of
nitre from  two saturated solutions of different temperatures,
but is inexplicable by the other theory.   Dr. Black thought
very7 highly of M. Saussure's essais sur Vhydrometrie,  and of
his general method of philosophising, which is perspicuous,
logical, and modest.

                      [Note 8. p. 207.]

  I do not think that  I can conclude this article as  I ought,
without taking notice of some facts and circumstances, which
affect Dr. Black's  claim to be considered  as the  discoverer of
the peculiar  combination of heat, or the cause of heat, with
different substances, by which they are made to exist in the
form of a liquid or of an expansive  vapour.   His title to^this
discovery, and to  the theoretical  doctrine founded  on ir, hafc
been called in question.   I am not certain that it is  generally
jrknowledged on  the continent, or that it is even  generally 
* 3SH       NOTES  AND OBSERVATIONS.

  known that he has occupied himself with this research.  1 see
  it treated in the compilations of many of the foreign chemists,
  with more or less ability, as the discovery of Mr. Wilcke of
  Stockholm,  and  by others as coming from Mr. De Luc, and
  ■>v many as  a part of the great discoveries of Mr. Lavoisier.
  I find this last opinion very general, and I see clearly whence
  this arises.  Mr. Lavoisier and Mr.  De  la Place published
  the first of their valuable  dissertations on the relations and
  measures  of heat in 1782.  Of the doctrines contained in
  these dissertations, this doctrine of combined (or, as Dr. Black
  calls it, latent) heat, is  the basis, and it is involved in all the
  chief propositions established by these philosophers.  Yet the
  name of Dr. Black is not  mentioned.   These  dissertations
  seem to be  received as the original source of  the whole doc-
  trine,  and  to  be considered  as the result of their labours
  alone.
     Although Dr. Black was duly conscious of his own  merit as
  a civ.■mist and a philosopher, there never was a man less assum-
  ing, or less eager to obtrude himself on the public attention.
  Even his discovery  of fixed air, and his  ingenious theory7 of
  quick-lime, would not perhaps have been committed  to the
  press, had they  net formed the subject of his inaugural disser-
  tation,  when he received  his  medical degree.   His doctrine
  of latent heat, and the theory of its combination with all kinds
  of matter,  in the forms of liquid and of vaporous aggregation,
  although incomparably more ingenious, and of more extensive
  and important application to the explanation of natural pheno-
  mena,  were known to the  world only through the lectures de-
  livered to  his own numerous students.   Scarcely did a year
  pass over,  after his settlement  in Edinburgh,  in which some
  corner of this doctrine was not nibbled off, and appropriated
  by others ; generally  by his  former  pupils.   He was often
   urged in the most pressing manner by his friends to publish,
   rind was told that the whole of it would,  in this way, be soon
   honestly portioned out among different persons, as the fair re-
   sult of their own enquiries; and that if he should at last publish,
   he would only be considered  as a copyist.  Yet nothing could
   preval on  him  (so much uid he dislike  authorship) to vindi- 
NOTES  AND  OBSERVATIONS.      359
■cate his claims, till  he learned that his friend Mr. Watt was
likely  to suffer by his silence, because others were handing
about  as their own,  some very peculiar views which he had
taken of the subject,  dependant on Dr. Black's doctrine. The
Doctor was so far roused by this,  that he promised to Mr.
Watt that he would print an account of these speculations with-
out loss of time.  But this  went no further than writing two
octavo pages, and noting with his pencil some particulars for
continuing  this account.
  In the mean  time, others were appropriating to themselves
various parts of the  doctrine of latent  heat, or,  without per-
ceiving the connection of the whole, were acquiring reputation
by ingenious explanations of particular phenomena, which de-
pended on this doctrine.   Thus, in or about the year 1772,
Professor Wilcke of Stockholm read in the Koyal Society in
that city, a dissertation, in which he shewed that ice absorbed,
while  melting,  72 degrees (by7 the Swedish scale) of heat.
Magellan published  a work,  in which he asserts, in a very au-
thoritative manner, that Wilcke is the author of the discovery.
Others represent Dr.  Cullen as the author, because his sur-
prising experiments  with aether in vacuo, exhibit the most re-
markable example of the absorpsion of heat.   But Dr.  Cullen
never  laid  the  smallest claim to the  doctrine.  Many affirm
that Dr. Irvine and  Dr. Crawford share at  least the honours
of the discovery with Dr. Black,  and that they had more just
conceptions of the phenomenon than their preceptor.
  The claims of these gentlemen are however, refused by that
eminent naturalist Mr. De Luc, in his Meteorohgie, publish-
ed in 1786.  But this refusal is not in order to secure the dis-
covery to its worthy and unambitious author.  Mi:.  De Luc
condescends, indeed, to say  that Dr. Black was the first who
attempted to measure the quantity of heat that is absorbed du-
ring the melting of ice and the boiling of water.  But he consi-
ders himself as the  first author of the doctrine that heat com-
bines with  the particles of a  solid body, and that the result of
this combination is the liquefaction of the solid, and its exist-
ence in a fluid form ; heat or fire being one of its constituent
oar is; and that, in  like manner, another combination of fire 
J60       NOTES AND OBSERVATIONS.
7vith  the particles of bodies converts them into elastic vapour,
of which fire is a constituent part.   He refers the reader for
proofs to his publication in 1772, under the title of Recherche*
sur les modifications dt V atmosphere, of which this theory, mo-
dified in a very particular manner, makes a principal part.  He
professes to have formed this theory in the years 1754, 1755,
and  1756,  and quotes some observations or experiments, of
that date, on which he affirms it to be founded.  He considers
Dr.  Black's doctrine of latent heat as making but a part of his
own theory, which extends to every case of evaporation, while
Dr. Black's relates only to the heat absorbed in ebullition, and
combined in elastic steam.
   Mr. Watt,  warmly attached to his highly respected friend
Dr.  Black, was surprised at this faint and inadequate conces-
sion of Mr. De Luc, and was grieved that the Doctor's inac-
tivity had given room for it.'  Knowing that every7 publication
of so eminent a philosopher as Mr. De Luc would strongly
impress  the public mind, and  would leave it uninformed of
what Dr. Black had done, he wrote to Mr. De Luc a letter,
in which he asserts that Dr. Black had done much more than
merely TRY to measure the quantity of heat absorbed, &c. ; that
he had first discovered and demonstrated that heat is absorbed,
and is combined xvith bodies when they are rendered fluid or va-
porous, &c; and he insisted that this letter should be inserted,
at the end of the next volume of the Meteorologie.  Accord-
ingly it is there inserted,  in page 503 of the appendix.  Mr.
De  Luc expresses his regret that his words had been taken in
a sense  different from that in which he had employed  them,
and professes the highest regard for Dr. Btack.   But he still
retains his claim  to discovery and  to priority.
   There is nothing of which  I am more fully convinced than
 that Dr. Black is the sole author of the doctrine of latent heat;
 and, when the following particulars, leading to that conviction,
 which my duty to my much respected teacher and friend im-
 pels me to lay before the reader, are duly considered,  it will
 remain  with him to say whether  he coincides with me in the
 belief,  that whatever portions of this doctrine may have been
 seen, or guessed at, on the continent, or in these kingdoms. 
NOTES  AND  OBSERVATIONS.       361
diey have emanated originally from Dr. Black, probably through
the medium of  students attending his lectures, as  they did
from every part of Europe.
   Dr.  Black was so little solicitous to secure his title to disco-
very, that  I got very little information to this purpose among
the notes from which he lectured.  But, searching among his
old memorandums and discarded papers, I found several small
note books of an uncommon form, such  as I remembered to
be much used by the students  in the University of Glasgow,
and were sold in the shop of the University printer.  These
little note  books were filled with memorandums of thing*
which  had engaged the attention of the young philosopher ;
and these were mixed with notes of occurrences, and names
of persons, which were all familiarly known to me,  I being a
native  of the place.  By these circumstances, the dates of
many memorandums may be ascertained within a month ; and
they shew the whole to have been written in 1754-5-6-7, while
the Doctor was a student at Glasgow and Edinburgh, and du-
ring the first year of his Professorship in Glasgow.  I shall
insert  a few of such notes as regard the present subject.
   No.  IL Page 1.  " Does not the heat produced in slacking
" lime, and in the solution of lime and caustic alkali in acids,
" arise from the sudden  fixing of the fluid part in a firm state ?
17 taking them from a state of  fluidity*.
   Pa<re 2.   " Intense cold produced by ice dissolved in  an
" acid explained,  and the principle applied to  crystallization.
" Is not ice crystallized water? Is it not similar to a solution
" of a saline crystal? A greater alteration of heat should  be
« produced by mixing a neutral salt with ice or snow, than by
14 dissolving it in water.
   No. III. Paget.  " One way of estimating the quantity of
44 heat which is transferred and disappears in the production
<4 of vapour, or which is expended, and contributes to the for-
 44 mation of vapour, but cannot be discovered in  the vapour
" by the thermometer.
  " Place a phial with water, close corked, in a stove or oven,
» where it will  acquire a  degree of heat above the boiling
« point: Then,  by  some contrivance, open it suddenly, and 
*362        NOTES AND OBSERVATIONS.

 " observe how much has been converted into vapour, and what
 " heat i ■  ■ ..tains."
  N. J3.  In the net page Dr. Black describes the method of ex-
 amining the thermometric scale, so  as to decide whether the
 scale of equal expansions corresponds to equal augmentations
 of heat.   Mr. De Luc seems  to consider himself as  .ue first
 who examined this question also.
   No. IF. Page 6.  u Nitrous  aether boils in  vacuo, and
 u grows  vastly cold.  Is not  the  cold produded merely by
 44 the  evaporation of the  elastic parts ; or does it emit  air,
 **■ which requires heat for its elastic appearance ?
   Page 20. u In the effervescence of mercury in nitrous acid,
 44 whence comes the great heat which must certainly be re.
 " quired; for so much, elastic matter  must rob  it, as aether
 44 boiling in vacuo does ?    ,
   44 All solutions must be considered as liquefactions in  con-
 44 sequence of heat, forfthere is no fluidity without heat.  Salt
 44 and water, when mixed, melt with much less heat than what
 " will melt ice, and  vastly less than what  will melt salt.
   Page 30.  4t Does it not appear, on a review  of cooling
 ** mixtures, that a refrigeration will always happen, when salt,
 " in a crystalline form,  (that is, combined with water) or water
 " in a crystalline form, (that is, frozen) is  made fluid by salts,
 44 or in spirit of wine.'  According  to this  supposition,  salt
 M mixed with  ice should produce  much more cold than salt;
 *' mixed with water that is as cold  as the ice.
   44 Will not the progress of cooling be retarded while a solu-
 u tion of salt crystallizes ? Try if this happens while fats be-
 *>4 come solid.
 (  44  Do  not these salts 'produce the greatest  cold which are
 44 dissolved by7 the smallest quantity  of water?
   Page 32.  44 Will a boiling fluid raise  the thermometer in
 I* proportion to the  pressure on its surface ? Will  this be of
 44 am- use in discovering the absolute proportions of heat?
    No. f.  r4 ".In-;1"*  greater heat when a caustic lev is mixed
 " with an a^id,  than when a mild  one  is ;  none is tal<en offbv
 "■• the elastic effervescence.
    Page  15. 4' Will distillation in vacuo save fuel?  - 
          NOTES  AND  OBSERVATIONS.      363

  Page 16.  44 Is not heat produced in solutions,  when a fluid
44 matter is converted into a  solid ? Calcined Glauber salt and
44 water ; but not when solid matter is rendered  fluid.
  Page 24.   44 Is not the  cold produced  by mixing vitriolic
44 acid with sal ammoniac owing  to the production of elastic
44 matter ?
  No. VI.  44 Is  the heat which is disengaged from water in
44 becoming solid in a crystal;  equal to what must be employ-
44 ed to disjoin it from the  water ? How  may this be tried ?
44 In a salt it is not all disengaged, for the brine is still salt ,-
44 and when  it effloresces, more must be employed to produce
44 vapour.44
   By these  memorandums, I think that it must  be evident to
any reader,  that the doctrine of latent heat, as  a necessary
ingredient in all  fluids and  elastic vapours,  has  not been an
opinion suddenly formed by Dr. Black, or deduced from a
single phenomenon.   Nor  has  it arisen from a general,  but
vague view  of all the phenomena of liquefaction and vapori-
sation. It has long occupied the mind of the ingenious author,
and his surmises have been noted down,  that they might not
be  forgotten, while present studies,  or  the circumstances of
his situation, hindered him from putting his conjectures to
the test of experiment. He might truly say, therefore, as in
page 156, that he  scarcely remembers the time when some
suspicion of this combination did not possess his mind. The
theory has been the matured result of many observations,  and
frequent meditation.  The  reader will not wonder that  Dr.
Black's  mind was  not sooner  ma*de up upon  this subject,
when he recollects that he was elected Professor of Medicine,
and that chemistry was only a lectureship.  His medical stu-
dies  mats have occupied most of his time;  and  the  attention
which he gave to his patients  was so anxious and unremitting,
that one should have thought that he had not an hour left for
anv other duty.  Mr. Watt thinks that he had completed his
proofs of the absorption and emersion of heat in liquefaction
and congelation in 1758.   But, in an old and rejected sheet
of lectures,  which some other circumstances oblige me to re-
fer to this  year, he treats the subject still  as a  general view, 
364       NOTES AND OBSERVATIONS.

and makes no mention of the  quantity of heat which enters
into the formation of water.  I think that this would not have
been omitted, had the experiments been already made.  I am
persuaded, however, that the extension of the doctrine to the
ebullition of fluids followed very soon after its  establishment
in the cases of liquefaction and  congelation.  The fundamen-
tal phenomena, and the experiments for determining this part
of the theory,  appear  to  me to be much more obvious and
impressive than in the  other case.  I am so much of this opi-
nion new, that I  wonder, as often as  I think of it,  that the
heat latent in steam was not detected  even ages ago.   At all
events,  it is  fact, that the doctrine  of latent heat, precisely
as it is contained  in these lectures, was explained and demon-
strated by Dr. Black in 1761, and in every subsequent course.
   I must now take  the liberty of saying that I was particular-
ly hurt, when I was informed  of  the cold, and,  as  it were,
reluctant concession of Mr. De Luc, 44 that Dr. Black wa.-.
u the first who /»«</Attempted  to measure the quantity of heat
44 which ice absorbs  during its liquefaction," &c.  because I had
long been made to  expect  from Mr. De Luc a very  different
account indeed of Dr. Black's labours.   At this time (1786),
I had been for several  years the colleague  of my former pre-
ceptor, and I took  the liberty to join in the  importunities of
his friends, who urged him to print an account of his doctrines.
His  health  was  very delicate  about this time,  and  we
despaired of his being able to undertake  the task.  We were
consoled,  in some  degree, by  hearing that Mr. De Luc had
repeatedly offered to take this trouble on himself.  We were
given to understand that,  three  or four years before  this,  the
Doctor had been  prevailed on to communicate  the necessary
materials to that  gentleman for this purpose, and that the  ac-
count would speedily appear, in a great  work which he was
about to publish ;  and  it was said that Mr. De Luc was par-
licularly desirous to have this opportunity of maintaining Dr.
Black's right, in opposition to the pretensions of Dr. Craw-
ford and others,  who  had endeavoured to rob him of his  de-
served fame. 
          NOTES AND OBSERVATIONS.       365

   The account which did at last appear was  indeed a matter
of surprise and disappointment: For it was understood  that
Mr. Watt had been allowed to communicate to Mr. De  Luc
the leading points of his theory, and even to repeat with  him
the experiments necessary  for its demonstration.  Yet not a
word is said by  Mr. De Luc of the attempts of Wilcke,  Dr.
Crawford,  and  Dr. Irvine,  to explain the  phenomena in
another way, except in  order to refute them  by his own new
theory of watery vapours.   Nay, the explanation which  Mr.
De Luc vouchsafes to give, after inserting Mr. Watt's letter,
and acknowledging that his words were susceptible of the in-
terpretation given of them , is merely condescending to say
that it was a great satisfaction to him to learn, by the letter of
Dr. Black's friend, Mr. Watt, that his own system of the com-
binations of fire, as a constituent part of certain substances, had
had Dr. Black for a defender at that time, as it has since had in
Mr. Lavoisier ;  and that he -willhave a greater confidence  now
in his opinion, since it is common to  him with naturalists of
their  rank.
   This is really too much.  Had Mr. De Luc forgotten his
repeated offers  and requests  to become Dr. Black's editor,
and his importunity to be instructed in the particulars?   Had
he forgotten his being a witness, in 1782, of some of the prin-
cipal experiments on the latent heat of fluids, when boiling
under the pressure of the atmosphere, and also in vacuo, and
the important deduction made from them by Mr. Watt ?
   Let it not be  thought that I make these assertions respect-
ing Mr.  De  Luc's acquaintance with Dr. Black's doctrine,
on the sole authority of what  I heard in  conversation  with
Dr. Black's friends.  I have now lying before me the unfim,
shed scroll of a long letter  of Dr. Black's handwriting to  Mr.
Watt, in which  every particular that  I have mentioned is al-
luded to.   Mr.  De Luc's repeated solicitation to be his  edi-
tor,....his intention of doing him  justice,  by supporting his
claims against the  encroachments  made  on  it,....assurances
from Mr. Watt  that Mr. De Luc will be a zealous friend,....
Dr. Black's  expressions of confidence in Mr.  De Luc's geniiw
and honour,....specificaf.ion of the things  m  be  .•omnrmnica-
  VOL. T.                  3 H 
366       NOTES  AND OBSERVATIONS.

ted,  and the experiment to be shewn him.  The letter is da-
ted January 30,1783, but is not finished.
  Let us now consider a little the authority on which Mr. De
Luc founds his claim to the  discovery of the absorption and
combination of heat or fire with the particles of other matter,
so as to become a constituent part of every fluid and  of every
vapour.
  For this the reader is referred to the Recherches sur les Mo-
difications de VAtmosphere, published in  1772.  The authority
referred to in that work for the latent heat of  melted ice,  is
an observation said to  have  been  made in 1754, that there
was no  rise  of temperature in a  vessel  containing ice melting,
till the whole was  become fluid. I believe that this has been no-
ticed by every naturalist, since the time that Dr. Hooke re-
commended melting snow for settling a point in  the scale  of
temperatures; for the propriety of this method of ascertaining
a determinate temperature rests  entirely on the truth of this
observation.  Both Fahrenheit and Mairan,  after remarking
that the temperature rises to the melting degree, affirm that
it remains fixed there till all be melted.   But neither of these
philosophers infer from this fact that the ice is continually ab-
sorbing heat, and that the heat so absorbed may be measured
by the time  of  the liquefaction.   Neither  does  Mr. De Luc
mak- any such inference: Aor does he afterwards employ this
inference as a principle in  explaining any  phenomenon.   Nor
does he shew how this absorbed heat may be made  to emerge
again, and  warm other bodies.
   This inference, this conclusion drawn from the  constancy
of  the  temperature, was reserved for Dr.  Black ; and  he
proved that it was a just inference, by shev.ing  that the same
quantity of heat is afterwards obtainable from the water, when
it freezes. Mr. De Luc noticed the fact indeed, that, during
the  liquefaction,  the temperature was constant: But it  took
no  hold  of his  mind.  It  seems to have been forgotten  till
 177~.  I do not  see therefore that  he can found his opir.ioi
 or his theory on  this observation.  It does not receive or ad-
 mit any exrdamiiion  fio;,: his  dnguinr theory of watery va-
 pours. 
NOTES  AND OBSERVATIONS.       367
  How different is all this from the procedure of Dr. Black ?
The notes which I have inserted shew that a certain opinion
had  long lurked in his mind concerning the constitution of
fluids ; and that from time to time it had gathered strength,
in consequence of his observing classes of phenomena, which
would be just what we see them,  on the supposition that heat
is a constituent part of all fluids, and that fluidity is the effect,
and the only  sign of this particular combination of heat with
other matter. The proposed experiment with the corked phial
is the one which was made long after by Mr. Wa^tt, with Pa-
pin's digester.  The conjecture  about the  intense cold in
freezing  mixtures is precisely what  was afterwards found to
be just.   In-short, Mr. De Luc's observation was just one
of those  Remarks that are frequently made by an ingenious
man habituated to speculation: but it lay for years unimpor-
tant and neglected, among  other memorandums ;  whereas
there was a prepossession, or an  impression on the mind of
Dr. Black, which directed his attention to certain classes of
facts, and was nourished and strengthened by subsequent re-
marks and reflections, till a great body of evidence at last in-
cited him to a serious discussion, which  terminated in con-
viction.
   I have already said  that the arguments for the vaporous
combination of heat are much more obvious and impressive.
I am not surprised, therefore, that so ingenious and inventive
a mind as that of Mr. De Luc very early  conceived this with
some degree of confidence and precision; and yet, even on
this head,  Mr. De Luc is satisfied with general observations.
In  § 676, he  says distinctly enough,  that44 Fire quits the fuel
44 to unite with the water, converts it into a vapour, and escapes
44 with it."  Farther, in \ 684, he generalizes the fact,  by as-
cribing all evaporation to the union of fire with water;  and in
§ 693, he explains on this principle the cold produced by eva-
poration.  But, as  Mr.  Watt very properly observes in his
letter, Mr.  De Luc has made no experiment to demonstrate
any part of this theory, or to determine how  much heat is ab-
sorbed or emitted.  Surely, till something like this be done,
the whole theory is nothing more than  an ingenious but vague 
368       NOTES  AND OBSERVATIONS.

conjecture, resting entirely on general observations, and des-
situte of that precision which is required for the foundation of
a theory.   Muschenbroeck had formed the same  conjecture
long before (See his Natural Philosophy, translated by Sigaud
de la Fond § 1455, &c.)  He says, that  the fire  enters the
water, throxvs it into ebullition, and that the greatest part of the
igneous matter escapes; but a portion remains combined with
the water, andgives-the vapour its expansive form and force.
As for the accessary circumstances  of Mr. De Luc's theory,
and his manner of conceiving fire as an expansive fiuid, acting
the part of a carrier to the water, and the mode of its efficiency
in taking up or depositing the water  with which it is com-
bined, I admit that this is the entire property of Mr. De Luc.
I content myself at present with agreeing with Mr.  Watt, that
no experiment is adduced by the author, by which any part of
the theory can be said  to  be  demonstrated.  I have not the
least notion how this theory of vapour is to be connected with
that of liquefaction.
   In the mean time, I request the reader to recollect that the
Recherchcs,  &c. were  not published till   1772.  The main
question is, whether Mr. De Luc held this doctrine in  1754
or 1756?  I  rather  think that he  did not.   He  only says
(Meteorologie,  app. p. 512.)  that in his Recherches he made
use of observations which he had made in 1754 and 1755, on
the phenomena exhibited by  freezing water and melting ice,
and of those made in 1756 on vapours : and he  acknowledges
that, with  respect to the first, he went no farther at that time.
Now,  the proofs of absorption, combination,  and  emission,
and the mode of reasoning from those proofs, are  so similar
in the two cases, that I cannot conceive how a naturalist who
draws conclusions from the one does not find himself incited,
nay obliged, to make the same inferences from the  other.   I
presume that Mr. DeLuc, incessantly occupied with meteoro-
logical studies, has  been considerably7  later in forming the
theory, explained in some degree in the researches, and has
found that these old observations or remarks also  were ex-
plained by it, and might therefore be adduced as proofs.  But,
can a person be thought to have any clear notions of the sub: 
NOTES  AND OBSERVATIONS.       369^
ject, who says in § 676, that44 water extinguishes fire, because
" it  has an affinity with it; and that air cm the contrary ani-
44 mates fire, because it has no affinity with it ?"....who says, that
" by the pressure of the air, the fire  is confined in the com*
44 bustible or the hot body, and kept in prison, but, when this
44 compression is  removed,  the  fire immediately  darts out,
" and is dissipated ?"....who  says, § 684, " that  the particles
44 of fire, being in continual agitation, knock on the particles
14 of bodies, separate them, and in their escape from the body,
"• carry off some watery particles  with them, and thus become
44 the vehicle of the water, and constitute what we call watery
44 vapour ?"  This is in no respect different from  the  vague
writing of Muschenbroeck, who is always groping about, and
catching at any thing that will bear an  application to his sub-
ject ; and then gives us a strange mixture of mechanism and
chemical affinity, which both mechanicians and chemists will
only smile at.  By collecting such vague and occasional ob-
servations of the ancients, Mr.  Dutens  found in their writings
all the discoveries of the two last centuries: and Mr. De Luc
may find in his Recherches sur les Modifications deVAtmosphere,
every discovery made since 1772.  Such writing, by its want
of precision, becomes a net which will catch  both fishes and
fry.   Nothing can escape it.  And shall  this be compared
with the clear and precise propositions of Dr. Black ?....These
may be false; but we  know  exactly  what they are, and can
form the most distinct notions  of them, so as to subject them
to the test of experiment, which will at once establish or refute
them.
   I  have  said that the Recherches did not appear till  1772.
Dr.  Black ere  this had given at  least ten or twelve  annual
courses, in which his doctrine of latent heat was demonstrated
and  explained.  He had many pupils  from Geneva, some of
them very well informed young gentlemen, and very able to
understand this  doctrine.   I  remember a Mr.  Chaillet at
Glasgow  in  1763, and a Dr. Odier in  Edinburgh, who (at
least the latter) corresponded with Mr. De Luc, and commu-
nicated several of his valuable  observations  to Dr.  Black.
 But we never heard of such opinions being held by any other 
 370      NOTES AND  OBSERVATIONS.

 person besides our own professor.   I should think it not un-
 likely, that a philosopher of Mr. De Luc's ardour and assi-
 duity, who held a correspondence with the learned in all parts
 of Europe, would be pretty soon  informed of Dr. Black's
 speculations.   Indeed there was a surreptitious and imperfect
 publication made of them at London by Nourse in 1770, en-
 titled Enquiry into the General Effects of Heat, with Observa-
 tions  on the Theories of Mixture, in which all the  general  or
 leading points of this doctrine were expressed with abundant
 distinctness and precision; and the honour of the invention is
 given in very express terms, to Dr. Black, professor of chem.
 istry at Edinburgh ;....44 a man," says the compiler, 44 whose
 " modesty is  even superior to his merit."   These speculations
 might find their way to Stockholm in the same manner.  I re-
 member  a very accomplished young gentleman from thence
in 1768 or 1769,  (I, think his name was Willems or Willem-
son) who  was much  in  the company of  Dr. Black  and his
friends.  He was occupied solely with  chemistry, and parti-
 cularly with metallurgy.   Besides, manuscript copies of Dr.
 Black's lectures were to be had for a moderate price, and were
purchased by many  students.
  Should I be thought by some readers  to have  trespassed
unreasonably  on their time and patience,  I sincerely ask their
forgiveness.  But I trust that I shall have the thanks of all
those  who have had the happiness of attending the  lectures  of
our amiable and excellent Professor.  I hope also  that others
will grant that I have only endeavoured to discharge my duty
to his honoured memory.  A more proper occasion will pre-
sent itself, in the  consideration of another article, for taking  .
 some  notice of the  silence of Mr. Lavoisier, and  Mr. de  la
 Place on this head * when I trust it will appear in what high
estimation Dr. Black's speculations on this subject were held
by those gentlemen, and how anxious they were to keep them
from the public eye. 
          NOTES AND OBSERVATIONS.       371

                     [Not* 9. p. 208.]

  This,  however, is a very gratuitous  opinion,  and has al-
wa.s been admitted with great hesitation.   Dr. Hooke exhi-
bited at  a meeting of the Royal Society in 1682, an experi-
ment, in which it appeared that a niirror of foiled glass reflect-
ed the heat of a fire very feebly, although it reflected the light
with great brilliancy.   Mr. Scheele  shewed  that a plate of
clear glass, held between the face and a glowing fire, interrupt-
ed fou a while the whole heat, but transmitted the light with-
out sensible diminution:  And Dr. Herschell has shewn, in
the most convincing manner, the complete separability of light
and a determinate portion of the heat, by means of a difference
in their refrangibility.

                    (Note 10. p. 210.]

  Dr. Darwin directed  a stream of air through a small aper-
ture of a tube intensely heated, into a box, which had an op-
tical apparatus in the opposite  side, by which he could see any
light at the point of the pipe with great advantage.  Having
staid some time in the dark, to render his eye very sensible
to light, he looked in, and caused the injection of heated air to
be made ; but he could not perceive the smallest light. He then
placed a slender bit of  gold at the distance of half an inch
from the hole.   When the heated air was again injected, the
gold acquired a very bright glow in a few seconds, although
the heating air was altogether  invisible.   Dr. Herschell's cu-
rious observations shew us that,  by aproper optical apparatus,
a body may be  intensely heated by the rays of the sun, with-
out being at all illuminated ; and it seems  to require an expe-
riment like  Dr. Darwin's to decide whether this  heat would
cause it to emit light.   Also,  seeing.that it is affirmed by the
French chemists that oxygenous gas is the sole source of the
heat and light which appear in combustion, it would seem to
be doubtful whether Dr. Darwin would have seen the gold ig-
nited, had it been heated by a stream of azotic gas instead of
common air. 
 372      NOTES AND  OBSERVATION?.

                     [Note 11. p. 214.]

   This porosity of clay is susceptible of considerable variety,
by different modes  of preparation.  It is prepared for the
better kinds of pottery, by beating it into a uniform, and al-
most fluid mud with water, and  allowing this to settle at lei-
sure in a pot or cistern-  By this treatment, all small stones,
and even coarse sand,  fall to the bottom.  When so much of
the water has evaporated as to leave the clay fit for being form-
ed in the mould,  or on the wheel, and able to support itself
in shape, it may be dried in the air and sun, without shrinking
remarkably.  When a vessel of this consistency is baked with
a moderate red heat, such as is given to bricks for inside work
it  is so porous that the water will run through it in a few mi-
nutes, but without taking away the smallest portion of it. This
is very easily understood.  It is owing to the great space which
had been occupied by  the water.   The subsequent contrac-
tions of such baked clay by stronger heats are much greater
than those of clay which has been compressed by beating or
kneading, after a greater portion of the water has evaporated
from the prepared  mud.  I find,  by some careful experiments
with the  finest clays,  that a difference in this respect induces
a very sensible difference in the total contraction of baked clay,
even when very hard.  I mention this circumstance, because,
though sufficiently7 obvious and intelligible,  it does not seem
to have been duly attended to by Mr. Wedgewood and others
who employed this kind of thermometer ;  and  it introduces
considerable uncertainty in the comparison of  experiments
made by different  persons.  The different  contractibility has
a sensible relation to the specific gravity of the baked clay,
(varnished, to prevent imbibition in the trial),  but I have not
been able to ascertain its degrees with any tolerable precision.
I found very unexpected anomalies.

                     [Note 12. p. 227.]

   Having occasion, in autumn 1774, to go. down and inspect
a drain in a coal-work, where an embankment hnd been made 
NOTES  AND OBSERVATIONS.       373
 to keep off a lateral run of water,  and, crawling along,  I laid
 my hand on a very luxuriant plant, having a copious, deep-in-
 dented, white foliage quite unknown to me.  I inquire d of the
 colliers what it was ?  None of them could tell me.  It being
 curious,  I had a sod  carried up to the day-light, to learn from
 the workmen what sort of a plant it was.  But nobody had  '
 ever seen any like it.  A few days after,  looking at the sod,
 as it lay at the mouth of the pit, I observed that the plant had
 languished and died, for want of water, as I imagined.   But
 looking at it more attentively,  I observed that  a new vegeta-
 tion was beginning, with little sproutings from the same  stem,
 and that this new growth was of a green colour.  1 his instantlv
 brought to my recollection the curious  observations of Mr.
 Dufay  ; and  I caused the  sod to be  set in the  ground and
 carefully watered.  I was the more incited to this,  because I
 thought that my fingers had  contracted a sensible aromatic
 smell by handling the plant at this time.  After about a week,
 this root set out several stems and leaves of common  tansy.-
 The workmen  now recollected that the sods had been  taken
 from an old cottage  garden hard  by, where  a great   deal
 of tansy  was  still growing  among the  grass.  I now  sent
 down for  more of the same  stuff: and  several sods  were
 brought up, having the same luxuriant white foliage.   This,
 when bruised between the fingers,  gave no aromatic smell
 whatever.   All these  plants withered and died down, though
 carefully watered, and, in each, there sprouted from the same
 stocks   fresh stems,  and  a copious foliage, and produced,
 among  others,  common tansy, fully impregnated with the or-
 dinary juices of that plant, and of a full green colour.  1  have
repeated the same experiment with great care on lovage, (le-
visticum vulgare) mint, and caraways.  As these plants throve
very well  below, in the dark,  but  with  a blanched foliage,
which  did  not spread upwards, but lay  flat on the ground:
in all of them there was no resemblance of shape to the ordi-
nary foliage of the plant. All of them died down when brought
into day-light;  and the stocks then produced the proper plants
in their  usual dress, and having all their distinguishing smells.
      VOT,. T.                  - T 
174       NOTES  AND OBSERVATIONS.

  From such  experiments, I thought myself entitled to say
that  the  sun's  rays  not only produced the green  fiecula of
plants, but also the distinguishing juices, and particularly the
essential oils.   The improvements which have been made in
chemical science since  that  time, have,  I think,  fully7  con-
firmed my conjecture.  The separability  of light and heat,
first  noticed by Dr. Hooke, and exhibited by him to the
Royal Society,''at the  meeting of  March 16, 1682;  (See
Birche^s Hist,  af the RayalSociety, vol. IV. p. 137.) and, af-
ter being forgotten,  again  brought forward by Dr. Scheele,
has been lately confirmed in the most complete manner by Dr.
Herscheil's valuable experiments, in which he found the calo-
rific  rays less refrangible than the lucific   It was no unreason-
able   supposition, therefore,  that  the sun's  rays  contained
phlogiston, since we see that they contained move  than light.
Nor was my conjecture without support from what was then
known.  The sun's rays blacken the  vitriolic acid ; and from
vitriolic acid much blackened we can  obtain sulphur. His rays
render colourless nitrous acid ruddy  and fuming, in the same
manner as a drop of spirit of wine would have done.  They
destroy vegetable  colours.   They blacken luna cornea, and
other metalline salts or oxyds,  in the same manner nearly as
tatty steams and vapours  do.  These are strong  indications
of some material emanations from the sun, and are in no man-
ner explicable  by any mechanism of elastic undulations. These
phenomena cannot be conceived by the mind as the effects of
an  undulation.  Thev  resemble communications  of matter.
But, indeed,  the proportionality of the sines of incidence and
refraction  has always appeared to me to give the strongest
proof, that the  sun's rays  are  emissions of moveable,  inert
matter, -affected, as all other matter is affected,  by what we
call  moving forces.
   Th-e new doctrines in chemistry7 make no change in the in-
ferences from these  phenomena.  It is not admitted that phlo-
giston is communicated to nitric acid by the sun's rays when
thev render it fuming ; but they are said to  detach oxygen.
The whole train of chemical phenomena induce us to conclude
that oxygen is disengaged, either by being detached by some- 
NOTES  AND OBSERVATIONS.       375
thing now  combined with the remainder, or that the thing
furnished by the rays makes part of the gas now formed.
  When I first heard of Scheele's curious experiment of throw-
ing the  prismatic  spectrum on the nitrate or muriate of sil-
ver, in  order to compare  the blackening powers of the diffe-
rently colouring rays, it  revived  my chemical speculations,
from  which I had long desisted ;  and, in beginning to repeat
his experiment, an  accidental circumstance suggested to nyy%
mind an experiment which I thought decisive of the question,
" Is light an emanation of matter from the sun, or is it the un-
" dulation  of an  elastic medium ?t4  I proposed  to form  an
image of the sun on the nitrate of silver, by means of rays col-
lected by7 a metalline speculum, and made to pass through a
glass  vessel filled with clear and colourless nitrous  acid.  On
the supposition that the preparation of silver is blackened, and
nitrous  acid rendered  fuming, by  the  same principle in the
sun's  rays,  I concluded,  that  when they  had performed one
of those tasks, they could  not be in a condition to perform the
other, at least in the same degree.  I accordingly began the'
experiment,  and found a  remarkable diminution of the effect
by the interposition of the acid.  But I  could not  make  any
very confident inference from this, having long before that
time found that the separation of heat and light by a transpa-
rent body,  such as Scheele's pane  of glass, was chiefly owing
to absorption of the heat by the glass, which, after it was sa-
turated, transmits the heat copiously, along with the light.
It therefore required a  train of  experiments to  determine
how much  was owing to this cause.  Before I could  accom-
plish this object, the failure of my health, in the spiingof 1787,
put a  full stop to all active investigations and pursuits.
  My thoughts on this subject have again been roused by the
observations of Messrs.  Bockmann and Ritter, in Germany,
and Mr. Wollaston  in England.   They have observed that
the muriatof silver was strongly darkened by  rays of the sun
which are  more refracted than even the violet rays.  The
muriat was affected in a space  lying beyond the violet light, in
the same manner as Dr. Herschell  had observed the thermo- 
376       NOTES  AND  OBSERVATIONS

meter affected in a spase equally in the dark, beyond the red
rays of the spectrum.
   Thus it appears diat there are rays which illuminate, rayr-
whicn warm without illuminating,  and ravs which affect bo-
dies i hemicallv' without producing  either light or heat.  This
observation bids fair for giving us  more knowledge of the na-
 ture of combustion.  And,  if this  newly discovered principle
 shall be found necessary for this most remarkable phenomenon
 of nature, it may very well be called the phlogiston, in what-
 ever way it contributes to the effect. And these facts seem to
 recommend the prosecution of my experiment.  It would be
 proper to compare the  darkening power of the rays  coming
 through  nitric acid with that of rays coming through as much
 water.   The ray s produce  a remarkable effect on the  first,
 and none on the last.
    But, v hatever may be  the result of such experiments, it
 remains certain, that the rays of the sun,  and the light of day,
 affect plants in the manner described above ; and in some way
A>r other are necessary to  the production  of their most com-
 bustible  ingredients.  The plants may be said to  feed on the
  light by their green leaves, as much as they do on the juices
  of the soil by their roots.  As the roots are protruded all
  around through  the yielding  soil, in  quest  of nourishment
  from the earth, so do the plants direct their growth, and turn
  the  upper disk of their green  leaves to the light,  and to  the
  light alone.

                      [Note 13. p. 230.]

    This theory,  so opposite,  as  Dr. Black observes, to  the
  theory of Stahl, is i.ot so recent as is generally7 imagined.  It
  was seen, in all  its  extent  and importance, by Dr. Robert
  Hooke, one of the greatest geniuses and most ardent inquirers
  into the operations of nature, who figured  during  the latter
  half of the. seventeenth  century,  a period full of  great  disco-
  veries.
     Dr. Hooke proposed this th-ory in considerable detail in
  Id-, 3iicragraphia, published  in  1665  ; (sft p. 103.) ; and ir. 
NOTES  AND  OBSERVATIONS.       377
his Lampas, published in 1676:  and he makes it  an impor-
tant doctrine in his treatise  on Comets, and in many passages
of hi^ Cutlerian Lectures.   He promises to take it into seri-
ous consideration, and to publish a full exhibition of it.  The
allusions made to it in his Lectures make  it evident that  he
had continued to make some desultory additions to his first
conceptions.   His Lampas  contains a most accurate  explana-
tion of flame,  which cannot be surpassed by any performance
of the present day.
   In the Micrographia he  states the theory in the following
words :
   1. The air in which we  live,  and breathe, and move, and
which encompasses and cherishes all bodies, is the universal
solvent of all sulphurous (synonymous, at that time, with in-
flammable) bodies.
   2. This action it performs not till the  body be sufficiently
 heated,  as we  observe in other solutions.
   3. This action of dissolution produces the great heat -which
 we call fire.
   4. It  acts with such  violence  as to agitate the particles of
 the diaphnaous body, air,  and to  produce that elastic pulse
 called light.   (See his own hypothesis concerning the propaga-
 tion of light.)
   5. This action,  or dissolution  of inflammable  bodies, is
 performed by a substance inherent in,  and mixed with the air,
 that is like, if not the very same, with that which is fixed in salt-
 petre.
   6. In this dissolution of bodies by the air, a part of the bo-
 dy, uniting with the air, is dissolved or turned into air, and
 escapes and flies about.
    7. As one part is thus turned into air, so another  is mixed
 with it,  but forms a coagulum, or precipitation, some of which
 Is so light as to be carried away with the air, while other gros-
 ser and  heavier matters remain behind, &c. &c.   This latter
 article is frequently employed in other parts of his  writings,
 and is sometimes called a grosser compound, mixed with mat-
 ters terrene,  and  originally  insoluble in air,   and incom-
 'mstih'e. 
 i78       NOTES AND OBSERVATIONS.

  Can any7 thing more be wanting  to  prove that  this is the
same with the modern theory of combustion ?  Nothing but
to shew that this coagulum contained the air which had for-
med it, by shewing an increase of its weight, or by  separating
it again.   But the eager mind of Hooke, attracted  by every
appearance of novelty, was satisfied  with the general notion of
a great subject, and immediatelv quitted it in chase of some
other interesting object.  Had he not  been thus led off by a
new  pursuit,  this wonderfulman would not only have antici-
pated,  put completed many of the great discoveries of the last
century.  It was a bold conception, and only a vigorous mind
could entertain it for a moment,  that the vast heat of combus-
tion  was contained in a few grains  of  air.   Yet this was his
opinion,  as  appears by the  explanation which he gives, in
various meetings of the Royal Society, and in his lectur- s on
comets, of the deflagration of combustible bodies with saltpe-
tre,  and of fiery motion.
  In the  treatise. called Lampas,  he  observes that this his
treatise, published eleven years before, had been very7 favour-
ably  received, and  that he had not  seen  any valid objection
offered to it.   It  was in  this interval that Dr. Mayhow, at
Oxford,  published his Book De Sale Nitro, et Spiritu Nitro-
a'e'reo,  in which he  holds precisely  the same doctrine.  But
his exhibition  of it is obscure,  complicated,  and wavering,
mixed with much mechanical nonsense, of wedges, and darts,
and  motions,  &c.  according to the  fashion  of  the  times.
Hooke's conception of the subject,  on  the contrary, is clear,
simple, and steady7.  The only addition made by Mayhow are
some observations on the increase  of Weight observed in the
preparation of diaphoretic antimony, &c.  Hooke, explaining
at a meeting of the Royal Society, some tricks of the plumb-
er's  workmen,  who  called the litharge which formed on the
surface of melted lead dross, and took it with them as their
perquisite, says expressly,  that they can  make dross of the
whole, and that it is more than the  lead  by7 all  the  air which
 was its menstruum.  But Mayhow wrote  on the  subject ex-
 pressly, and it appears in the title of his book.  He is remem-
 bered, while Hooke is forgotten ; because no one would think 
NOTES  AND OBSERVATIONS.       379
 of looking  into the Micrographia for chemical information.
 The theory comes in  by chance,  to explain the  indestructi-
 bility of charcoal in close vessels by heat.  Mayhow also made
 many very ingenious  experiments on the air which had con-
 tributed to inflammation, and has anticipated both the manipu-
 lations and  the discoveries of modern pneumatic chemistry.
   I do not know a more  unaccountable thing in the history of
 science, than the total oblivion of this theory of Dr. Hooke,
 so clearly expressed,  and so likely to catch attention.   No
 notice, that I know of, had been taken of it, till I mentioned
 it in the supplement  to the Cyclopaedia Britannica, in  the
 sketch given of Dr. Herschell's discovery of the radiation of
 solar heat,  in the artichY Thermometrical SpectrumP  I made
 the observation in 1798,  having then  got a copy of Hooked
 Lampas, and other tracts,  which are  as curious as they  are
 rare.   And I then requested Sir Joseph  Banks to order a
 search among Dr. Hooke's  papers, in the  possession of the
 Royal Society.   I am persuaded that many of his speculations
 on this subject will be found among them; and I earnestly
 request an inspection of them to be made.
   I trust that the reader is  not displeased with this my en-
 deavour to do justice to the well founded claim of our illus-
 trious countryman.   Had Dr. Hooke's fortune allowed him
 to multiply  experiments, to associate  in his labours a knot of
 eminent assistants, and to form a party in the  Society, ready
4at all times to meet  and to document every occasional dis-
 covery, I make no doubt but that he would have stood next to
 Newton in philosophical rank : but44 his lot forbade."  Other
 eminent men would willingly have benefited by his genius, but
 would not be his seconds.   They used  him extremely  ill;
 and this increased his distrust and jealousy.  He does  not
 seem to have had the  least ambition to head a sect, although
 he sometimes boasts  too much of his own inventions.  Not
 incited by this ambition,  all his efforts  were solitary,....merely
 to quench his own ardent thirst for knowledge.   His temper,
 soured by ill usage, and by the infirmities of  a  most feeble
 and sickly constitution, made him quarrel with all his friends
 in the  Society; and,  in so doing, he  was guilty of some  in.

                                                    U 
       380        NOTES AND OBSERVATION'S.

       gratitude ;  for  he had been encouraged, protected, and even
       borne with, with some tenderness, on account of his genius
       and his infirmities.  But all these things contributed  to the
       neglect of his writings, and this took away the chief incitement
       to  the prosecution of  this  ingenious  theory.  He did  not,
       however, altogether give it up; for some of his occasional
       employments of it allude to recent observations.  I am  per-
       suaded that valuable  thoughts on  this subject are still to be
       found in the chest of papers committed  to the keeping of the
       Society by Mr.  Waller, after he had  extracted  fr< m them
       what he has  published as  Dr. Hooke's posthumous works.
       Dr. Hooke died in peace ;  and his memory was honoured by
       his colleagues in the Society.   Mr. Lavoisier fell a sacrifice
       to the ambition of those whom he had associated with him in
       his researches, all of whom either were at the time of his
       death, or became, in a few months, ministers, or generals, 01
       persons in high  public functions.   France, indebted  to  this
       man for the only honour that it has  acquired during the event-
       ful period of  the revolution, condemned him to death, for
       putting -water into the  tobacco, and other things hurtful to the
       health of the citizens.
                         ........Quis, talia fando,
                       Temperet a lachrymis !........

                           [Note 14. p. 232.]

         Wheu the dish is filled to the brim, so that the air  may
       have free access  the burning fuel,  and we take care,  at  the
       same time,  to hinder it from  being thrown into  eddies, by
       setting  a glass cylinder round it, at a small distance, allowing
if»v    'he air to get  in between it and the dish, we shall observe the
       iiame occupy7  the whole circumference of the  dish.   But if a
       small bit of paper be lying on the middle of the spirits, or even
       -.uppurted at a small height above it, we see that it is not burnt,
       •hough surrounded by  flame.  It is only at the outer surface
       of  the ascending vapour, that  the  inflammation is going on.
       What is-  "ithm  cannot burn; because there is no air in con-
       tact with it.   f f ve hold a glass plate above it, so as to hinder 
NOTES AND OBSERVATIONS.       381
the flame  from rising high, and we then look down through
the glass, we shall see that none of the internal part is burning.
But if we introduce a pipe through this film of flame to the mid-
dle, we shall see a stream of flame arise from the endof this pipe,
and we are ready to believe, that there is a stream of inflam-
mable  matter coming from the pipe.  But it is only air which
comes out, and kindles what it touches.  This flaming s'ream
from the pipe is also only a cylindrical film of flame like the other.
These observations and remarks enable us to explain the up-
right and tapering form of all flames.   Flame rises, because it
is a hot vapour, which expands the contiguous air, which there-
fore rises by its specific levity, and carries up with it this shining
vapour.  Its form  is tapering, because  it is luminous  only
where  it is really burning,  that  is, in contact with the  air.
The exterior stratum is burnt before it gets far up.  The next
rises till it overtops the  outer one, now burnt; and it burns in
its turn, getting a little higher before it is consumed. The third
stratum must get above the second, before it begin to burn;
and it adds a little more to the height of the flame : And thus
the strata burn attop, in succession, and the centre one closes
the pyramid.   Dr. Hooke gives a most accurate description
and explanation of  all the phenomena of flame,  illustrated by
a figure, in the Lampas, p. 5. proving it to be a shining film of
inflamed vapour.
   In consequence of this superficial constitution of a flame,
whenever we cover it with any thing which prevents the access
of fresh air, it is instantly extinguished. But an inflammable body
that is sojid, and fixed  in a great heat, may be superficially ex-
tinguished in this way.  But  if the cover be taken off imme-
diately, the surface, which appears  black at first, soon grows
red again, by the heat of the interior parts, and the combustion
recommences.
   Combustible bodies which have  an  intermediate degree
of volatility burn with a flame more  or less violent.   But
when a considerable portion is flaming at once,  it rarely hap-
pens that the combustion is complete in the interior parts of
the flame.   Much of this vapour is only scorched, or charred,
and is hurried awav in this condition by the current of air ge- 
332       NOTES AND OBSERVATIONS.

nerated by the heat..  This charred  vapour is what we call
smoke, and soot; both of which are  still highlv inflammable.
The sure way7 of producing a more complete inflammation is
to prevent a great quantity of surface from flaming at once.
Thus, a very  small wick is found to burn without smoke or
soot; and Argand's lamp, which admits a current of air into
the centre of the flame, produces the same effect in an eminent
degree.  I employed such a lamp in 1767-8.  It consisted of
two concentric circles of wicks of rush pith stuck on pins.  The
air came up in the centre, and between the circles.   This was
the lamp of a distilling apparatus, and stood on the laboratory
table of Glasgow college  for two y7ears.   Quitting all chemi-
cal pursuits in 1769, I thought no more of it.

                    [Note 15. p. 254.]

   For Sir Isaac  Newton, in stating  this violent conflict  of
forces, in order to account for the turbulent motions really ob-
served, did not proceed,  as many of his followers have done,
and suppose the force to be great, because he had a violent mo-
tion to  account for.  He first prepares the way, by  proving,
that,  in the corpuscular  actions of bodies, we observe forces
which are inconceivably great.  He shews that the sum total
of the force  with which  one grain of glass attracts  the light
which it refracts, is not less than the force with which a wire
would be stretched by a hundred thousand millions of millions
of pounds of matter hanging by it.  This is surely competent
to the production of the most violent explosion that chemistry
presents ts our wondering view.   Observe too, that it did not
escape the remark of Newton, that this identical force is con-
cerned in all the most violent effects of chemical mixture. They
are accompanied by the extrications of heat and of light. When
we come to consider some of these motions in detail, we shall
be better able to appreciate the wonderful sagacity and pene-
tration of this extraordinary man.  We shall see with dis-
tinctness why argentum  and  mercurius  fulminans, while they
explode with a force  certainly7 greater than  that of gun-
powder, cannot propel a bullet with half the force which it can
impart. 
            NOTES AND OBSERVATIONS      383

                     [Note 16. p. 255.]

    It will prevent many mistakes if we adhere strictly to this
 meaning of the term ; if we consider it merely as an abbrevi-
 ated way of saying that the particles of bodies do really thus
 come together.   We shall find abundant employment in  as-
 certaining with precision all the cases in which this action is
 to be observed,  the limits to its generality, and the peculiar
 modifications of it, which distinguish certain classes of this
 phenomenon, and the cases or substances which exhibit these
 distinctive  modifications ;.~.and this employment will advance
 our chemical knowledge incomparably more than any specula-
 tion about  its cause.  At the same time, I see some advan-
 tages attending the use of the term chemical affinity.   It dis-
 tinguishes very compendiously the phenomena of combination
 (which are  the chief objects of chemistry) from the phenomena
 of cohesion, adhesion, eapillary attraction, &c. which are affect-
 ed by forces perfectly similar, but are not so charactei istically
 chemical.   But further,....attraction surely is the drawing eme
 thing toward another.  Now there are many instances where
 the bulk of the compound exceeds the sum of the bulks of the
 ingredients.  The particles, therefore,  have not been  drawn
 towards each other.   Affinity does not, in general, imply any
 similarity.  When it is  so used, we are sensible of some de-
 gree of figurativeness or metaphor.
   But whether we employ the word attraction or affinity, we
 must be careful to attach no other idea to it but that of a fact,
 a determination of each particle of a certain substance to unite
 chemically with each particle of another certain substance,  so
 as  to constitute particles of  a compound of both.  We run
 great risk of mistake, if we adopt any notion of the mode of
 action by which  this union is effected.  The late discoveries
 of the chemical phenomena called galvanic,  seem to derange
 all  our notions  of  the immediate agent  in chemical union or
separation. 
 384       NOTES AND  OBSERVATIONS.

                      [Note 17. p. 255.]

   I think that it will be of service to remark, once for all, that
 we have no denominations for these forces which we conceive
 as the immediate agents in natural  phenomena, except the
 phenomena themselves,  which we conceive them to produce.
 We call them the force of gravity, cohesion, refraction,  &?c.
 In a few cases, we name them by the substances in which they
 seem to reside, as electricity, magnetism. This circumstance
 alone shews that we have no direct knowledge of the. nature
 and mode of action of those forces ; they are not objects of our
 observation;  and  our knowledge  of their supposed  effects
 that is, our knowledge of the phenomena, is all  that we shall
 ever acquire of the forces or supposed causes.  When we go,
 or think that we go, a little farther, and call the force of cohe-
 sion an attraction, we mean nothing more than that the effect
 resembles the effect of a real attraction, or pulling a thing to-
 ward us with our hand.

                     [Note 18. P. 257.]

   In employing th" words stronger and weaker, as expressing
 qualities of the chemical attractions, we must be careful not to
 use them precisely in the same way7 as in mechanics.  They
 must be used merely as an expression of prevalence, but not as
 expressing a measurable  quantity, to which we give the name
 of intensity, otherwise we shall run the risk of mistake in our
 conceptions of those quantities. Aquafortis separates the parts
 of iron ;  and we express  this also, by saying that the attraction
 of iron for aquafortis is stronger than its cohesive attraction
 for its own parts.   We say the same thing of potash,....and we
 say that the  attraction of aquafortis for potash is stronger than
 its attraction for iron, because potash separates aquafortis from
 iron.   These expressions might oblige us to say that the co-
hesive force  of iron is less dian that of potash, did we not know
the contrary.  The prevalence is all that we should conceive to
 be expressed ;  and this rather arises from the peculiar mode
«,f ac?;on, tha?1 from the greater  intensity,  ,Perhaps there is 
          NOTES AND  OBSERVATIONS.      385

nothing in many of those chemical changes analogous to what
we call resistance and counteraction in mechanics.  We see,
indeed, some cases in which we cannot doubt it, as in the pre-
vention of vaporisation (and perhaps of congelation in some
cases) by external pressure.
                     [Note 19. p. 260.]

   Dr. Black sometimes explained the effect of stagnation, and
 the propriety of agitating the  mixture,  at length,  but has not
 committed his view of the matter to writing.  I recollect dis-
 tinctly the explanation which he gave in  1765 at Glasgow.
 A tall beer glass being filled with water,  a  long funnel was
 put into it, whose pipe reached to the bottom, and as much of
 a saturated and deep-coloured solution of blue vitriol was
 poured  in, as  occupied about an inch at  the bottom  of the
 glass. It was set on a bracket, having a white paper behind it.
 This was done  some days before this lecture:  and the stu-
 dents were made to observe how the colour slowly ascended
 upwards, growing fainter and fainter as it was farther  up the
 glass. In this lecture Dr. Black observed, that this gradation
 of colour shewed that the different horizontal strata of the
 fluid  were  in different states  of saturation, and because the
 colour did not change by sensible steps, but gradually, the dif-
 ference between two contiguous strata in respect of saturation
 was infinitely small.  Therefore, since the attractive force  of
 a stratum was so much the greater, as it contained less of the
 salt,  and as the adjoining stratum contained  more, it  must
 follow that, in any  horizontal  section, the force drawing the
 salt upward must be extremely small, and the motion extreme-
 ly slow;   and that the only method to make it proceed faster,
 is to bring the parts which  contain least salt into  contact with
those  which contain most.  As  a proof of his doctrine, he
kept the glass clear of all  disturbances,  on the bracket;  and
three  months after, the  colour had  not risen three inches.
Indeed it did not reach  near the top after  three years.   Dr.
 Black explained the slow communication  of  heat along the 
 386       NOTES AND OBSERVATIONS.

 parts of solid bodies, and the effects of stagnation in the atmos-
 phere, in the same manner.

                    [Note 20. p. 263.]

   It may be  said here, that this manner of generalising the
 events is gratuitous, and that we have no other test of the pro-
 portional strength  of the chemical forces concerned in these
 phenomena but the phenomena themselves.  We  call that a
stronger force which separates two ingredients united by the
 force which we term the weaker.  It must be  acknowledged,
 that this generally has been the evidence on which the propor-
tion of the attractions has been established;  and it is legiti-
mate, and is equally true in mechanics and every  department
of physics.   But it is not our sole authority, even in the che-
mical actions of bt dies.  It is perhaps true without exception,
that, when these different combinations (which may be thus
changed in succession by a third body) can also be destroved
by the varied force  of one agent, such as heat, we  shall find
that those ingredients which manifest the strongest elective at-
tractions  really require the greatest  exertions  of this general
agent to separate  them.   The  exceptions seem, at first, to be
numerous and remarkable ;  but when the co-operation of dif-
ferent degrees of fluidity and volatility are considered, almost
the whole of the exceptions are found to come under the ge-
neral rule.

                    [Note 21. p. 263.]

  When Sir Isaac Newton took that cursory view  of chemical
composition and  decomposition which  he  has expressed in
the queries annexed to  his Optics, he  saw well hov. vain an
attempt it would prove, to reason minutely on those forces, on
 mechanical principles.   For he  had already found,  that his
 own utmost efforts could only  approximate to the  solution of
the problem of three particles acting on  one  another.   Nor
have the united efforts of all the  mathematicians  of Europe,
 since his time, produced an accurate solution of this celebra-
ted problem.   What then can we hope  for,  when  we must 
NOTES A-ND OBSERVATIONS.       387
consider at once the combined action of myriads of particles?
He contented himself, therefore, with considering the pheno-
menon of chemical combination as the only  accessible  mark
of the acting force, and recommended the study and compari-
son of these as the only means of improving  the science.  It
was, as yet, far removed from first principles; and the  simplest
phenomenon presented to our view is a very complex event.
The only proper method of proceeding is to compare them,
and class them according to their extent and generality, in
the hopes of finding points of resemblance, which would form
a law still more  general, and therefore nearer to the first prin-
ciples of t||e  science.  The separation by a third body is a
very general law, and it did not appear necessary, or even pro-
per, to speculate on its cause.
   Notwithstanding the eager endeavours of his followers to
account for the separation, or for the extreme rarity of a com-'
bination of three ingredients in a series of elective attractions,
no satisfactory explanation has yet been obtained.  Few che-
mists were mechanicians: and those  who were eminent ma-
thematical philosophers, such as Keil, Bernouilli, and  a few
others, were but sorry chemists* Boscovich is the first, who,
by considering corpuscular  force in the most general and ab-
stract manner, has shewn how the action of two particles may
be totally annihilated by the action of a third,  and this, even
although the action of this third on either of the other two, be
in any degree weaker than the mutual action of those  two.  He
shews that this will depend as much on the kind, as on the ge.
neral magnitude or intensity of the force.
   It may perhaps assist the imagination a little, to point out
some examples of the same kind which we  can explain to a
certain degree.  When a piece of iron is hanging by the north
pole of a magnet, if we  present  the  south pole  of another
magnet to the place of junction, the iron immediately drops.
Moreover,  if we hang to a  magnetic pole a piece of iron as
large as it can carry, and then attempt to hang another such
piece beside it,  the first will certainly drop from it.   Again,
we know that a metal plate, negatively electrified, attracts a rod
of metal very strongly.  We know that a plate positively elec- 
388       NOTES  AND OBSERVATIONS.
 trified does the same. Yet we also know, that a charged plate
 of glass, one side of which is most highly positive  and the
 other as highly negative,  exerts scarcely any attraction on the
 metal rod.  And we understand in a considerable degree how
 these things happen.  A still more apposite illustration may
 be had by the following curious phenomenon in  magnets,
 which had not been  noticed till I mentioned it in the article
 Magnetism,  of the Cyclop. Britan. § 29.
                         Having laid two good  magnets, A, B,
         E            of equal power,  on the table, vdth the
         D            north pole of A near the south pole of
         —j-           B, and  the two magnetsj{fbrming  a
    •^  __      -B     straight line, take a very small compass
?-------U  '=------!—i needle,  such as are frequently  mount-
                      ed for wearing  on a watch chain, and
 place it at C.  The needle will immediately place itself parallel
 to the magnets A and B, and its poles will be in the same di-
 rection with theirs. Draw it out perpendicularly from the two
 magnets, and you will find its polarity gradually diminish,  as
 you may perceive by  its vibrations growing slower. When at
 a certain distance, at  D, its  virtue seems entirely gone ; for it
 will remain in any position that is given it.   Draw it still far-
 ther off,  and you will perceive  its virtue returning again, but
 its polarity is reversed.   At E, as far from D as D  is from
 C, it  will stand in the position contrary to that of C.
   While the needle is at  E, gently draw the magnets  farther
 asunder,  both equally  from the middle point  between them,
 and keeping them still in the same line.  When at  a certain
 distance, the virtue of E is  again destroyed, and it will retain
 any7 position.   And,  on separating them still farther,  E reco-
 vers the same polarity that  it had in C.
   From this experiment, we see  how much the mutual rela-
 tions  of particles endowed with powers  like polarity, de-
 pend  on the precise  position,  and the peculiar constitution
 df the subject which affects them.   If, instead of one little
 magnetic needle which we place successively at C, D, and E,
 we employ several, turning on pins stuck into little pedestals,
 we shall observe  them all affected,  and attracting each other 
NOTES  AND OBSERVATIONS.      289
by the ir nearest poles ; and when the distance of A and B is
properly changed, the virtue of the little needles is also chang-
ed, every pole repelling what it formerly attracted.

                     [Note 22. p. 266.]

   In all these cases, the progress of chemical science has been
greatly obstructed by the attempts to reason from higher prin-
ciples.   We know, and ever  shall know, so very little about
the mechanism of chemical attraction, that we can only frame
hypotheses, and then reason from such hypotheses.  No ac-
cession of knowledge is obtained by this procedure.   For our
hypothesis, in order to have any influence on our mind, must
be founded on a certain extent of  resemblance to the pheno-
mena; and its whole value depends on that resemblance. We
have, the phenomena themselves ; and their coincidence is our
only  general law.   The hypothesis is only a fanciful picture of
this law :  it is therefore useless.  It does not support the phe-
nomena ; but it is founded on them, if it be good for any thing.
 In the present case Mr. Berthollet has wisely avoided reasoning
from any supposed properties of chemical attraction ; and de-
 duces all his general laws from extensive coincidences of facts.

                      [Note 23. p. 266.]

 •  Besides the authors  mentioned by Dr.  Black, many useful
remarks and general  views may be gathered from the tables of
other eminent chemists.   Marherr de affnitatibus Corporum,
 Vienna:, 1762,....and particularly WenzePs Doctrina Ajfinita-
 turn, Dresden, 1777,....and Kirxoan on the specific Gravities and
 attractive Powers of different Substances, London, 178 ^....de-
 serve a careful  perusal.   The table in Greri's Chemistry in-
 cludes almost every article, and every particular view that has
 been taken of the subject.  Wenzel endeavours to prove, that
all the distinguishing attractions of different substances are only
modifications of the general attraction of gravitation ; and are
modifications arising from the figure and structure of the ulti-
mate particles of each substance.  But Wenzel  had not  the
    vol. i.                :* *■ 
390       NOTES AND OBSERVATIONS
mathematical  knowledge  necessary  for such a task.  Other
authors have had the same notion.   And,  no doubt, the dis-
tinguishing actions  of different substances may have this ori-
gin,....that is,  may  depend on the figure and structure of the
particles, although the primary atoms, of which these consist,
have  but one law of action.   This is the  great principle of
Abbe Boscovich's theory.  But I think it  strictly demonstra-
ble that no figure or structure will constitute such particles as
we observe, if the action of the primary atoms be an attraction
in the simple ratio of the square of the distance inversely: and
I am certain that no repelling particles, or repelling body, can
ever arise in this way.

                    [Note 24. p. 268.]

   This most interesting law of chemical combination may be
conceived, if not explained, in the following manner:
-A.       TA       T*J    When a compound consisting of
   V-X^".....J4-'......."W*   the ingredients A  and B is mixed
   i    \     /   i    with another,  of which the ingredi-
   <p      >*v     3    ents  are ^ anC* "■"**' tlierc are two am^"
       yS   n.     j    nities or forces supposed to be in ac-
   L/^   £-   \j    tion ; one connecting A with B, and
 q»**""   "*>-'"*      q  tne other connecting C with D.
   But let us also suppose that A has an affinity to C, or attracts
it as well  as B, and that B also attracts D.   Then,  in the mix-
ture we have two other forces, one connecting A with C, and
the other connecting B with  D. We may express these forces
by the symbols AxB,  CxD, AxC, BxD.
   The two  first may be called maintaining forces, because they
tend to keep  things in their present condition.   The forces
AxC and  BxD  may be called divellent,  because they are
conceived to  have a tendency to separate A from B, and  C
from D.  This supposition is founded on the authority  of the
 general fact of separation of  two ingredients, by presenting a
 third substance which has affinity to one of them.
    From this manner of conceiving the subject, it  is inferred,
 that it the sum of the forces  A X C and BxD, exceed that  of 
NOTES  AND OBSERVATIONS. .      391
the forces A x B and C x D, we shall have a double exchange,
and two  new compounds.   This may happen,  therefore,
although the attraction of A for B should exceed its attraction
for C, provided it do  not exceed it so much as the attraction
of D for B exceeds its attraction for C.
   Since it is a general fact in chemistry, that  substances act
most  powerfully  in their simple state, it may appear strange
that C, when already united with D, should  overcome  the
stronger attraction of A for B, which  it cannot  do alone.
This  shews us, by the way,  that the  prevalence in chemical
action depends rather on the manner and concomitant  cir-
cumstances of the action, than on the measurable intensity or
magnitude of a particular force.  Dr. Cullen used to represent
this cfiemical fact by the diagram in the margin,  where the
numbers placed between the  substances  express the supposed
attractive forces  exerted between the substances.   This dia-
gram suggests the notion of bodies attached  to the ends of
two rods  or levers, AD and BG, moveable round their in-
tersection, E.  Were this the case, it is certain that the at-
traction of A for C, and of B for D,  tends  to separate A
from B, and C  from D:  and what is asserted above  will
happen,....the levers  will close  between AC and BD;  and
A 'will apply.itself to C, and B to D.  Dr.  Black first em-
ployed thffMfrrrram :  but he gave it up, because it suggested
a  noti(mlrcP^^nical, but mechanical.   Levers can have no
place here.   It* suggests  also  an erroneous  notion.  The
levers produce the effect, only in consequence of a connection
which they establish  between A and D, and between B  and
C.  Now, in by  far the greatest number of cases in which  a
double exchange is observed, we know of no such connection.
He used  to express the cases more in the  style of chemical
phenomena,  by saying, that, in order  to have a double  ex-
change, the partiality of D for B must  exceed that of  A for
B.  But still this is merely a figurative expression of an  un-
known cause.
   It  is not easy to conceive any mode of operation which will
clearly produce the observed effect.   It confirms, I  think,  the
considerations mentioned  in note 21,  p.  263; and  it  re- 
392       NOTES  AND  OBSERVATIONS.

 ceives some illustration from the magneiical phenomena men-
 tioned there.
   When  we  say  that a  double  exchange happens when
 AxC+BxD  exceeds AxB-fCxD, we seem to  say some-
 thing like instruction.  But, in truth, it is only an inference
 of this greater partiality from the observed effect.   We have
 generally no other  argument for  saying  that  the  excess of
 D's attraction for B above its attraction for C, is greater than
 the excess of A's attraction for B above its attraction for C.
 The fact is that'we have  inferred  all these  partialities only
 from  the observed double exchanges.
    Still, however, these double affinities are  among the most
 interesting chemical laws, and of immense use for operating
 changes which  are otherwise impossible.  Therefore a co-
 pious collection  of the cases in which they are  observed, will
 be a  vast accession to chemical science, especially if judici-
 ously arranged.
    Did we know  every, case in which one body can detach ano-
 ther,  so as to make  up a table of single affinities complete, we
 might then, by comparing them in parcels of four, tell before-
 hand whether there  will be  a double exchange:  but our
 knowledge is not yet  so far advanced.
    No where are the double exchanges so  frequB|n±.as in the
 spontaneous fermentations of animal anj^y^  BSeujuices.
 A fertile imagination, by employ ing fou*»J^SPnJthis way,
 and  assigning a convenient order of their: action, Jnay bring
 out almost any  ultimate  result he pleases.   It  is, therefore, a
 delicate part of the science.  There are some very judicious
 observations on the marks and characters of double affinities,
 in Dr. Lubbock's inaugural dissertation, De Principio Sorbili,
  which  a philosophical chemist should be very clearly acquaint-
  ed with; otherwise he is liable to many great mistakes.
 -t ,W:e must multiply, as much as possible, the  cases of double
  exchange by experiment;  and we must  apply to eath case
  numbers which will agree with the observed prevalence.  We
  must correct and  recorrect  those numbers,  so that the sa,me
  number may always belong to the same  substance,  and yet
  £uit its place in every case whejre that substance is one ingre- 
NOTES  AND OBSERVATIONS.       393
dient in a double exchange.  This is a long and difficult task;
but till it be accomplished, there is no certainty in the inge-
nious explanations  given  of many7  complex phenomena *
especially where a change of temperature makes a change  in
the chemical attractions.

                     [Note 25. p. 290.J

   It may be carried much farther than this, by carefully pre-
venting dissipation.  We see  what  considerable  heats are
produced  in the common  garden glasses,  and in  green-
houses, merely by preventing the  air from absorbing it and
carrying it off.   Mr.  Saussure (Voyage sur les Alpes,  II.
932. J made a little box, lined with fine dry cork, whose sur-
face was nicely and uniformly charred, to make it black, and
also more spongy  and, unfit for conducting heat.    It was
covered by a very fine and thin plate of glass. A thermome-
ter being laid on the bottom,  and the box set in  the sun's
ray's, when the temperature of the surrounding air was 75°
Fahrenheit, the inclosed thermometer rose to 201°  in a few
minutes.   The account is somewhat equivocal, in  the way
of expressing the change on the thermometer; and I see some
foreign v/rj^^conclude that it rose to 229°, 17° above boil-
ing watjtf     |^_
   I c<-fl         Apparatus of this kind, employing three
very thirH       Bflint-glass,  which transmits much more
heat (acco^Kig^l^Ierschell's experiments)  than any other
species of glass.  They were nearly of similar shapes, arched
above:  and there was about one-third of an inch of an in-
terval between them.   They were set on a cork base,  pre-
pared like Mr. Saussure's, and set on down, contained in a
pasteboard cylinder.   With this apparatus I have often,  in
a clear summer day, raised the thermometer to 23C
once to 237° of Fahrenheit's scale.  Even when set
bright fire, it  raised the thermometer  to  a boiling
When I took the apparatus into a damp cellar, before I set
the glasses in their  place, so that  the air between them was
damp, I found that when exposed to the same degree of the
sun's light, I never  could raise the thermometer above 208°;
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