S ?;>v» v-; ■•■•'.•".r>:'!i{ii»»R»«^: • '■!'••;•. ,} 'i • 11 ::.•-.. i ■. i .-•■{■: m n »? »jn »;•;t! ! ■: • »I iii ; .';':'. -i.' > j ti t i i.t I i: > I ; f •" ?*• '■ <' ■ • : ',■■ ',r ■;aft , >fl-f»-i P|i:. iltitS g&B'j. ■ ' i-L>Hc BSk&m ftpt m:\ t&fc r^Piiil SURGEON GENERAL'S OFFICE LIBRARY. Section-. No. 113, W. D.S.G.O. No.'Z ^V (4 u « « i it' r f ,"?* «• < A MANUAL OF CHEMISTRY; CONTAINING A CONDENSED VIEW OF THE PRESENT STATE OF THE SCIENCE, WITH COPIOUS REFERENCES TO MORE EXTENSIVE TREATIES, ORIGINAL PAPERS, &c. INTENDED AS A TEXT-BOOK FOR MEDICAL SCHOOLS, COLLEGES AND ACADEMIES. By LEWIS C. BECK, M. D., PBOFESSOB OF CHEMISTRY AND BOTANY IN THE UNIVERSITY OF THE CITY OF NEW-YORK, AND IN RUTGERS COLLEGE,. NEW JERSEY—MEMBER OF THE ROYAL PHTSICAL SOCIETY OF EDINBURGH ; OF THE LINN.SAN SOCIETY OF PARIS ; OF THE NATURAL HISTORY SOCIETY OF MONTREAL ; OF THE PHILADELPHIA ACAD- EMY OF NATURAL SCIENCEI ; OF THE NEW-YORK LYCEUM ; OF THE ALBANY INSTITUTE, &C. &C. THIRD EDITION, ( '■■' '^-> ILLUSTRATgp WITH NUMEROUS WOOD CUTS. • */W NEW-YORK: '. i W. E. DEAN, PRINTER & PUBLISHER, 2 ANN STREET. COLLINS, KEESE & CO., 254*PEARL STREET. MDCCCXXXVUI. Entered According to Act of Congress, in the year 1838, BY LEWIS C. BECK, In the Clerk's Office of the District Court for the Southern District of the State of New-York. Ann e\, .*• T9 T. ROMEYN BECK, M.D. L.L.D., PROFESSOR OF MATERIA MEDICA AND MEDICAL JURISPRUDENCE, IN THE UNIVERSITY OF THE STATE OF NEW YORK, &c. &c. THIS VOLUME IS INSCRIBED, AS A SLIGHT TRIBUTE OF RESPECT FOR HIS SCIENTIFIC ATTAINMENTS, AND OF GRATITUDE FOR THE MANY PROOFS OF KINDNESS WHICH HAVE BEEN RECEIVED FROM HIM BY HIS BROTHER, THE AUTHOR. PREFACE TO THE THIRD EDITION. In preparing the Third Edition of this Manual, I have adhered to the original plan of the work, which was to set forth in a concise and perspicuous manner the principal facts of the science to which it is devoted ; and at the same time to furnish the student with such references to other works and memoirs as would give direction to his enquiries in cases of doubt or difficulty. In this as in the preceding editions, I have constantly consulted the elaborate treatises of Berzelius, Then'ard, Thomson, Henry, Brande and Turner. I should state, however, that the work of the late Dr. Turner has been used more freely than any other, and may in some respects be considered the basis of the present Manual. The method of arranging the subjects is essentially that proposed some years since by Professor Brande; which I have adopted from the conviction, that although not entirely free from objections, it is upon the whole more easily acquired by the student, than any which to my knowledge has hitherto been followed. In the description of individual substances, brevity has been con- sulted as far as was consistent and I have in many instances em- ployed the style usually pursued in Natural History. The atomic numbers and symbols introduced into the last edition have been con- tinued with little alteration in this ; and in the case of the simple bodies and of the more important compound ones, they are so placed at the head of each article that their meaning will at once be under- stood and cannot occasion the least embarrassment to the student. Thus, B is the symbol for Boron, and 8 is its atomic number; Ba. is the symbol for Barium, and 68-7, its atomic number. When bodies combine atom to atom, as in the case of Protoxide of Nitrogen, the symbol is O-fN, and the atomic number of this compound is 08-f-N 14=22 ; but when more than one atom of an element is 1* VI PREFACE. contained in a compound, a figure indicates the number of atoms. Thus 50+N denotes Nitric Acid and indicates that it consists of 5 atoms of Oxygen combined with one atom of Nitrogen. The atomic number of this compound is 50-f-lN=54 In those cases where the symbol is complex abbreviations are sometimes employed. Thus, Aq. for water, and Ac. for acid, wiU be occasionally noticed in the descriptions of the salts. The improvements in the present edition consist in the introduc- tion of many interesting facts discovered within the last four years, which are inserted in their proper places: the descriptions and wood cuts of the most useful articles of apparatus, some of which will be found in the body of the work, while others, with definitions of chemical terms, and tables of atomic weights, of specific gravi- ties, and of weights and measures, constitute an appendix. The materials for the latter additions have been chiefly drawn from the last volume of Berzelius' Traite de Chimie, Faraday's work on Chemical Manipulation, and Reid's Practical Chemistry. It only remains for me to acknowledge the important aid which in the preparation of this work and in its subsequent revisions, I have received from my brother Dr. T. Romeyn Beck ; and to express my obligations to my brother Dr. John B. Beck for the revision and cor- rection of a part of these sheets asthey passed through the press. University of the City of New- York, September, 18§8, CONTENTS. Page Definition op Chemistry, ..... 13 CHAPTER I. ATTRACTION. Section I. Cohesion—Crystallization, ... 14 Section II. Affinity, ...... 22 Section III. Of the proportions in which bodies combine—Ato- mic theory, ....... 28 CHAPTER II. CALORIC OR HEAT. Section I. Nature of Caloric, .... 35 Section II. Communication of Caloric—Radiation, Conduc- tion, ........ 36 Section III. Distribution of Caloric, . . .44 Section IV. Effects of Caloric—Expansion, Liquefaction, Va- por^ation, . . . . . . .47 Section V. Specific Caloric, ..... 60 Section VI. Sources of Caloric, • . . .61 CHAPTER III. Light, ........ 67 CHAPTER IV. Electricity.—Galvanism, ..... 72 CHAPTER V. Magnetism.—Electro-Magnetism. Thermo-EIectricity, . 87 INORGANIC CHEMISTRY. General Remarks, table of elementary bodies, explanation of the Nomenclature, &c. ...... 100 V11I CONTENTS. CHAPTER VI. Supporters of Combustion, or Electro-negative Bodies. Section I. Oxygen, . . . • • • 10* Section II. Chlorine—Protoxide of Chlorine, Peroxide of Chlorine, Chloric Acid, Perchloric Acid, . . .105 Section III. Bromine—Bromic Acid, Chloride of Bromine, 110 Section IV. Iodine—IodoBs Acid, Iodic Acid, Chloijjodic Acid, Bromides of Iodine, ..... 112 Section V. Fluorine, . . . • .115 CHAPTER VII. Non-metallic Combustibles, or Electro-positive Bodies. Section I. Hvdrogen—Water, Deutoxide of Hydrogen, Muri- atic Acid, Hydriodic Acid, Hydrobromic Acid, Hydrofluoric Acid, . . . . . . . .116 Section II. Nitrogen—Atmospheric Air, Protoxide of Nitrogen, Deutoxide of Nitrogen, Hyponitrous Acid, Nitrous Acid, Ni- tric Acid, Nitro-Muriatic Acid, Chloride of Nitrogen, Iodide of Nitrogen, Ammonia, Salts of Ammonia and the foregoing Acids, ...... 129 Section III. SuLPHUR4-Hyposulphurous Acid, Sulphurous Acid, Hyposulphuric Acid, Sulphuric Acid, Chloride of Sulphur, Bromide of Sulphur, Iodide of Sulphur, Hydrothionic Acid, Hydrothionous Acid, Salts of Ammonia and the acids con- taining Sulphur, ...... 149 Section IV. Phosphorus—Oxide of Phosphorus, Hypophospho- rous Acid, Phosphorous Acid, Phosphoric Acid, Pyrophospho- ric Acid, Salts of Ammonia and the acids containing Phos- phorus, Chlorides of Phosphorus, Bromides of Phosphorus, Iodide of Phosphorus, Hydruret of Phosphorus, Bihydruret of Phosphorus, Sulphuret of Phosphorus, . . 163 Section V. Carbon—Carbonic Oxide, Carbonic Acid, Salts of Carbonic Acid and Ammonia, Chlorides of Carbon, Chloro- carbonic Acid, Bromide of Carbon, Iodide of Carbon, Hydru- rets of Carbon, Parafline, Eupione, Naptha from Coal Tar, Napthaline, Paranaphthaline and Idrialine Camphogen or Campene, Citrene, Coaljand Oil Gas, Fire Damp, &c, Chlo- ride of Hydrocarbon, Bromide of Hydrocarbon, Iodide of Hy- drocarbon, Cyanogen, Cyanic Acid, Cyanuric Acid, Chlori- contents. ix Page des of Cyanogen, Bromide of Cyanogen, Iodide of Cyanogen, Hydrocyamc Acid, Hydrocyanate of Ammonia, Cyanide of Sulphur, Sulpho-cyanic Acid, Bisulphuret of Carbon, . 172 Siction VI. Boron—Boracic Acid, Bichloride of Boron, Fluo- boric Acid, Sulphuret of Boron, .... 195 Section VII. Selenium—Oxide of Selenium, Selenious Aeid, Selenic Acid, Chloride of Selenium, Bromide of Selenium^ Hydroselenic Acid, Sulphuret of Selenium, Phosphuret of Se- lenium, ....... 198 CHAPTER VIII. METALS. General properties of the metals, .... 202 Section I. Potassium—Oxides, Chloride, Bromide, Iodide, Hy- druret, Nitruret, Sulphuret, Phosphuret, and Cyanide of Po- tassium, Alloys and Amalgams, Salts of Potass!, . 215 Section. II. Sodium—Oxides, Chloride, Iodide, &c, Alloys and Amalgams, Salts of Soda, Disinfecting Liquid, . . 230 Section III. Lithium—Oxide, Chloride, Salts of Lithia, . 240 Section IV. Barium—Oxides, Chloride, Bromide, &c, Salts of Baryta, ....... 242 Section V. Strontium—Oxides, Chlorides, Iodide, &c, Salts of Strontia, ....... 247 Section VI. Calcium—Oxides, Chloride, &c, Chloride of Lime, Salts of Lime, ...... 250 Section VII. Magnesium—Oxide, Chloride, &c, Salts of Mag- nesia, ....... 256 Section VIII. Aluminum—Oxide, Chloride, &c, Salts of Alu- mina, ........ 261 Section IX. Glucinum—Oxide, &c, .... 264 Section X. Yttrium—Oxide of Yttrium, . . . 265 Section XI. Zirconium.—Oxide, &c, . . . 266 Section XII. Silicium—Oxide,Chloade, Bromide, Fluoride, &C.267 Section XIII. Thorium—Oxide of Thorium, . . 270 Section XIV. Manganese—Oxides, Acids, Chloride, &c, Salts of Manganese, ...,.* 272 Section XV. Iron—Oxides, Chlorides, Bromides, Carburets, &c, Alloys, Ferrocyanic Acid, Salts of Iron, Ferrocyanates, 278 Section XVI. Zinc—Oxide, Chloride, Iodide, and Sulphuret, Alloys, Salts of Zinc, ..... 288 Section XVII. Tin—Oxides, Chlorides, &c, Alloys, Salts of Tin, ........ 293 X CONTENTS. Page Section XVIII. Cadmium—Oxide, Chloride, &ac., Alloys, Salts of Cadmium, ... . 97 Section XIX. Nickel—Oxides, Alloys, Salts of Nickel, . 299 Section XX. Cobalt—Oxides, Chloride, &c*6alts of Cobalt, 302 Section XXI. Arsenic—Arsenious and Arsenic Acids, Chloride of Arsenic, Bromides, Iodide, Hydruret, and Sulphurets, Al- loys of Arsenic, Arsenites, Arseniates, . . . 305 Section XXII. Molybdenum—Oxides of Molybdenum, Molybdic Acid, Chloride of Mblybdenum, Fluoride and Sulphuret, Molybdates, ....... 313 Section XXIII. Chromium—Oxides of Chromium, Chromic Acid, Chlorochromic Acid, Fluochromic Acid,Sulphuret and Phosphuret of Chromium, Chromates, . . . 316 Section XXIV. Vanadium—Oxides of Vanadium, Vanadic Acid, Chlorides of Vanadium, Sulphurets, &c, . . . 320 Section XXV. Tungsten—Oxide of Tungsten, Tungstic Acid, Chlorides of Tungsten, &c, . . . .322 Section XXVI. Antimony—Oxide of Antimony, Antimonious and Antimonic Acids, Chlorides of Antimony, Sulphurets, Alloys, Salts, t ..... 324 Section XXVII. Uranium—Oxide and Sulphuret of Uranium, Salts of Uranium, ...... 328 Section XXVIII. Columbium—Oxide of Columbium, Columbic Acid, Chloride of Columbium, Fluocolumbic Acid, Sulphuret of Columbium, . . . . . . 330 Section XXIX. Cerium—Oxides and Sulphuret of Cerium, Salts ofCerium, ....... 331 Section XXX. Titanium—Oxide of Titanium, Titanic Acid, Bi- chloride, and Bisulphuret of Titanium, . . . 333 Section XXXI. Tellurium—Tellurous Acid, Telluric Acid, Chlorides of Tellurium, Telluretted Hydrogen, . . 334 Section XXXII. Bismuth—Oxide, Chloride, Bromide, Iodide, arid Sulphuret of Bismuth, Alloys, Salts, . . . 337 Section XXXIII. Copper—Oxides, Chlorides, Sulphurets, and Phosphuret, Alloys, Salts,*. - . . . . 340 Section XXXIV. Lead—Oxides, Chloride, Iodide, and Sulphu- rets, Alloys, Salts, ...... 346 Section XXXV. Mercury—Oxides, Chlorides, Bromides, Io- dides, Sulphurets, and Bicyanide of Mercury, Amalgams, Salts of Mercury, ...... 351 Section XXXVI. Silver—Oxides, Chloride, Bromide, Iodide, Sulphuret, and Cyanide of Silver, Alloys, Salts of Silver, 359 Section XXXVII. Gold—Oxides, Chlorides, Bromide, Iodide, Sulphuret and Phosphuret of Gold, Alloys, . . 364 CONTENTS. XI 367 370 370 371 371 Page Section XXXVIII. Platinum—Oxides, Chlorides, and Sulphu ret, Sulphate of Platinum, Alloys, . Section XXXIX. Palladium—Oxides of Palladium, Section XL. Rhodium—Oxides of Rhodium, Section XLI. Osmium—Oxides, &c. . Section XLII. Iridium—Oxides, &c. . CHAPTER IX. VEGETABLE SUBSTANCES. Preliminary Remarks, Section I. Vegetable Acids—Acetic Acid, Pyrol igneous Jftid, Kreosote, Acetates, Oxalic Acid and the Oxalates, Oxamide, Tartaric Acid and the Tartrates, Racemic or Paratartaric * Acids, Citric Acid and the Citrates, Malic Acid and the Mal- ates, Benzoic Acid and the Benzoates, Benzule, Gallic and Pyrogallic Acids and the Gallates, Ellagic Acid, Aspartic, Boletic Camphoric, Carbazotic, Chloroxallc, Caincic, Crame- ric, Croconic, Igasuric, Indigotic, Kinic, Laccic, Lactucic, Meconic, Mellitic, Moric or Moroxylic, Mucic or Saccholic- tic, Pectic, Phosphoric and Hydrocyanic, Rocellic, Suberic, Succinic, Ulmic, Valerianic, and Zumic Acids. 374 Section II. Vegetable Alkalies—Morphine and its Salts, Nar- cotine, Codeine and Narceine, Cinchonine, duinine, Strych- nine, Brucine, Emetine, Veratrine, Sanguinarine, Atropine, Buxine, Corydaline, Crotonine, Curarine, CynapineA Daph- nine, Daturine, Delphine,Digitaline, Essenbeckine, Eupato- rine, Hyoscyamine, Nicotine, Solanine, Violine, 389 Substances somewhat allied to the preceding, but not alkaline— Amydalin, Asparagin, Bassohn, Caffein, Cathartin, Chloro- phyle, Colocyntin, Columbin, Conein, Cytisin, Dahlin, Dra- cin, Fungin, Gentianin, Hematin, Imperatorin, Inulin, Legu- min, Liriodendrin, Lupulin, Madaria, Medulin, Olivile, Pic- rotoxin, Piperin. Plumbagin, Polychroite, Populin, Rhubar- barin, Salicin, Sarcocoll, Scillitin, Suberin, Tiglin, Ulmin, Zanthopicrite, Bitter Principle, Extractive Matter, 396 Section III. Substances which in relation to oxygen contain an excess of hydrogen—Oils, Fixed Oils, Volatile or Essen- tial Oils, Camphor, Coumarin, Resins, Amber, Balsams,Gum, Resins, Caoutchouc, Wax, Alcohol, Alcohol in Wine, &c, Ether, Ethers of the first Class, Sulphuric Ether, Sulphovinic Acid, Phosphoric Ether, Ethers of the second class, Muriatic, Chloric, Hydriodic, Fluoric and Fluoboric Ethers, Ethers of Xll CONTENTS. Page the third class, Nitric and Acetic Ether, Bituminous substan- ces, Bitumen, Naptha, Petroleum, Asphaltum, Mineral Pitch or Maltha, Retinasphaltum, Pit-coal, Glance Coal or Anthra- cite, ........... 400 Section IV. Substances, the oxygen and hydrogen op which are in exact proportion for forming water—Sugar, Man- na, Sugar of Grapes, of the Maple, of Beets, &c, Honey, Mo- lasses, Starch or Fecula, Amidine, Hordein, Gum, Mucilage, Lignin or Woody Fibre,.......411 Section V. Substances which do not belong to either of the preceding sections—Colouring Matter and Dyes, Tannin, natural and artificial, Gluten, Yeast, Vegetable Albumen, 413 Section VI. Fermentation—Panary, Saccharine, Vinous, Ace- tous, and Putrefactive Fermentations,.....416 CHAPTER X. ANIMAL SUBSTANCES. Preliminary Remarks: . Section 1. Animal Acids—Uric, Pyro-Uric, Purpuric, Erythric, Rosaic, Amniotic, Lactic, Formic, Caseic, Sebacic, Choles- teric, Stearic, Margaric, Oleic, Phocenic, Butyric, Caproic, Capric, Hircic, and Cetic Acids,.....419 Section II. Oleaginous Substances.—Train 03, Spermaceti Oil, Dippel's Oil, Hogslard, Suet, Butyrine, Phocenine, Hircine, Adipocire, Cholesterine and Ambergris, .... 422 Section III. Substances which are neither Acid nor Oleagin- ous—Fibrin or Animal Gluten, Albumen, Gelatin, Urea, Su- gar of Milk, Sugar of Diabetes,.....424 Section IV. The more complex Animal Products—The Blood and its constituents, Respiration, Animal Heat, Saliva, Pan- creatic Juice, Gastric Juice, Bile, Biliary Calculi, Chyle, Milk, Eggs, Humours of the Eye, Tears, Mucus, Pus, Sweat, Urine, and Urinary Concretions, Solid parts of Animals, Bones, Teeth, Shells of Eggs, Lobsters, &c, Horn, Tendons, Hair, Wool and Feathers, Muscle,.....425 APPENDIX : Containing definitions of Chemical Terms, descriptions of Ap- paratus, Tables of Atomic Weights, of the Specific Gravities of Bodies, of Weights and Measures, &e. &c.....436 MANUAL OF CHEMISTRY. DEFINITION OF CHEMISTRY. Chemistry* has been variously defined. By Dr. Black it was de- nominated " the science of heat and mixture." Modern chemists, however, have given it a wider range. Thomson calls it the science which treats of those events and changqp in natural bodies, which are not accompanied by sensible motions.— Syst. of Chem. i. 18. Brande considers it "the object of chemistry to investigate all changes in the constitution of matter, whether effected by heat, mixture or other means."—Man. of Chem. 1. According to Berzelius, "chemistry is the science which makes us acquainted with the composition of bodies and with the manner in which they act upon each other."— Trait, de Chim. i. 31. Chemistry borders closely in many instances upon Natural Philoso- phy ; but the distinction can be easily drawn. It is the office of natural philosophy to investigate the sensible mo- tions of all bodies; whereas chemistry studies the constitution and qualites of these bodies. The natural philosopher contemplates whole masses and ascertains their properties ; while the chemist notices the operations of their par- ticles, observes their reciprocal actions and seeks to discover all the changes that may occur. Thus in examining our atmosphere, when studied as a whole, its weight, pressure, density and elasticity, are subjects falling within'the province of natural philosophy; but when we endeavour to discover the elements of which this air is composed, the changes which it under- goes, by heat or combination, and the phenomena which attend these changes, we are within the boundaries of chemistry. The business of the chemist, therefore, is to interrogate nature, and thus to make himself acquainted with the ultimate constitution of bodies. When this is effected, he is furnished with the means of imi- tating her in some of her most interesting operations, and thus in many instances, of contributing largely to individual as well as to national wealth and prosperity. Chemistry is divided into— I. The general forces, or powers productive of chemical phenome- na and the laws which govern them; or the general theory of the science. II. The particular effects which are produced in different bodies by * The term chemistry is probably derived from the Greek word Chemia^ ori- ginally applied to the art of making gold and silver. The Arabians by prefixine the article gave it the name of Alchemy.—Thorn son'a History of Chemistry 14 ATTRACTION — COHESION. the agency of these general powers ; or the chemical history of indi- vidual substances. The general powers, or as they are sometimes called, general pro- perties of matter, or imponderable substances, are Attraction. Heat. Light. Electricity. Magnetism. CHAPTER I. ATTRACTION. The term attraction is employed to express that unknown principle which causes distant bodies to approach each other, and to resist a separation with some degree of force. Attraction may be, I. Remote; when it acts on masses of matter at sensible distances; as in gravitation, electricity, and magnetism. II. Contiguous ; when it acts on masses of matter at insensible dis- tances ; as in cohesion and in chemical affinity. Contiguous attraction is of two kinds, viz.: homogeneous and hetero- geneous, or cohesion and affinity. The former takes place between bodies of the same nature, and the latter between those which are different. SECTION I. COHESION. Synonymes. Attraction of Aggregation—Cohesive Affinity—Corpus- cular, or Molecular Attraction—Homogeneous Affinity. Cohesion may be defined to be that force or power by which parti- cles or atoms of the same kind are brought into contact and retained in that situation. Cohesion is exerted in different bodies with different degrees of force. In solids, its force is exerted with the greatest intensity ; in liquids, it acts with much less energy ; and in aeriform bodies it is doubtful whether it exist at all. Thus water in a solid state has considerable cohesion, which is much diminished when it becomes liquid, and is entirely destroyed as soon as it is changed into vapour. The force of cohesion in solid bodies which is denominated tenacity, is measured by the weight necessary to break them, or rather to pull them asunder. Heat is excited at the same time, a good illustration of which occurs in the process of wire drawing. [An abstract of Mr. Rennie's elaborate experiments on the cohesive force of various solids will be found in Ure's Chem. Dictionary.] COHESION. 15 In liquids the force of cohesion is demonstrated by the spherical figure which they assume when suffered to form drops. The drop is spherical, because each particle of the fluid exerts an equal force in every direction, drawing other particles towards it on every side, as far as its power extends. To the same cause is owing the property possessed by all liquids of remaining heaped up above the brims of the vessels which contain them. Other examples of cohesion. ' 1. Similar portions being cut off with a clean knife from two leaden bullets and the fresh surfaces being brought into contact with a slight turning pressure, the bullets cohere, almost as if they had been original- ly cast together. 2. Fresh cut surfaces of India rubber adhere in a similar manner. 3. Two pieces of perfectly smooth glass or marble laid upon each other adhere with great force. It has been supposed that in some of the above cases the attractive force is confined to the surfaces of the masses, and it has been called adhesion. Mr. Ruhland has given a table exhibiting the weights which were found necessary to separate equal surfaces of different bodies from the same liquids. [Gorham's Chem. i. 6. Ann. of Phil. vii. 20.] Dr. Thomson considers adhesion as exhibiting the characteristic marks of chemical affinity and as affording a particular case of the action of that power. Cohesion is weakened by the following causes: L By heat. When a fusible body is exposed to the influence of heat, its volume is at first augmented, and this increase of bulk is in conse- quence of the separation to a certain degree of its constituents. Co- hesion is thus lessened, though not destroyed; and hence if heated zinc be struck with a hammer, much less force will be required to disinte- grate it, than if it were at the ordinary temperature of the air. If the heat be continued, the particles of the metal will be so far removed from each other, as to allow of free motion, and it will become liquid. By raising the heat still higher, while the metal is not in contact with the air, all cohesion will be removed and it will be resolved into vapour or an elastic fluid. II. By mechanical violence. Under this head may be arranged seve- ral processes which are highly useful in the laboratory, as— Pulverization and trituration ; by which substances are reduced to powder, generally performed by means of pestles and mortars. Levigation; a process similar to trituration, except that the rubbing is assisted by the addition of a liquid in which the solid matter under operation is not soluble. Granulation; effected either by pouring the substance while in fusion into cold water, or by agitating it in a box. Sifting ; employed for the purpose of obtaining bodies in powder of an equal degree of fineness throughout, performed by instruments termed sieves. III. By the influence of a more powerful attraction. Thus if a mass of limebe.immersed in vinegar, it gradually lessens and finally disappears. Here an attractive force has been introduced which is superior to the 16 CRYSTALLIZATION. attraction of the particles of lime for each other, and consequently they are separated. The effects of the exertion of cohesion are, 1. To unite the particles of bodies in a confused manner, without any regularity of form. 2. To bring them together in a determinate mode so as to form regu- lar geometrical figures or crystals. The latter is the most fre- quent. CRYSTALLIZATION. When we diminish in any manner the cohesion of a solid body so as to render it liquid or gaseous and afterwards remove the cause of this change, the body returns to its former state and the molecules ar- range themselves in a determinate manner, or in the form of crystals. Hence when a body passes from a gaseous or liquid state to that of a solid, it crystallizes ; but if this passage be too rapid, the crystallization will be confused. The most common agents employed to crystallize bodies are water and heat ; alcohol is also sometimes made use of in certain analyses. I. Water. This agent is employed in two ways, viz. a. To dissolve the body by the aid of heat and then to allow the solution to cool. In this case it will generally be only necessary to continue the process of evaporation until a drop of the solution when placed upon a cold body shows a tendency to crystallize ; or at least until a film or pellicle appears on its surface. This proves that the at- traction of the saline particles for each other is becoming superior to their attraction for the water. In this case, crystallization takes place because hot water generally dissolves a greater quantity of salt than cold water, and when it has become saturated a portion is necessarily deposited as the water cools. There are, however, a few exceptions to the Jaw that salts are more soluble in hot than in cold water, and Mr. Graham suggests that the efflorescent salts generally belong to this class.—Phil. Mag. and Ann. i. 7. b. To leave the cold solution to spontaneous evaporation. In this case the water by evaporation is brought to the point at which it is unable to hold the salt in solution. In both of the above cases, crystals generally retain a portion of water, which is termed the water of crystallization, and the salts con- taining it are denominated hydrous salts. These salts when heated liquify and undergo a process which is called watery fusion. Alum offers a familiar example. In a few cases, however, salts do not retain any water of crystalliza- tion and they are then termed anhydrous salts. Salts, in crystallizing, frequently enclose mechanically within their texture particles of water, by the expansion of which, when heated, the salt is burst with a crackling noise into smaller fragments. This phenomenon is known by the name of decrepitation; and it is observed to be most powerful in those crystals which contain no water of crys- tallization ; as the nitrates of baryta and of lead. Some salts part with their water of crystallization by a simple ex- posure to dry air, when they are said to effloresce ; but there are other salts which deliquesce or attract water from the atmosphere. Car- CRYSTALLIZATION. . 17 bonate of potash is a deliquescent salt; carbonate of soda an efflores- cent one. [For a table of the action of atmospheric air, on some of the most common salts, see Parkes' Chem. Catechism, 231, 8th ed.] II. Heat. There are two methods of employing this agent, viz. a. To expose the body to heat until it has melted, then to let it cool slowly and without agitation, till a crust has formed on its surface, to pierce this crust and decant the liquid contained in the interior. We then obtain the outer portion in the form of a solid crystalline bed, sometimes resembling a geode. This effect can be finely exhibited by treating in such a manner sulphur, or lead, bismuth and some other of the semi-crystalline metals. b To reduce the body to a state of vapour and to condense it gradu- ally. This however, is not always practicable, because few of the solids can be volatilized. The process is generally called Sublima- tion. Exp. Put powder of corrosive sublimate or arsenic in a dry flask, obstruct the mouth slightly and apply heat. The vapour rises and forms crystals in the upper part. More heat is required in the case of arsenic than in that of corrosive sublimate. HI. Alcohol, This is generally employed by applying heat and then allowing the solution to cool as above. The great utility of this agent depends upon the fact, that a few salts only are soluble in it. Hence it is often made use of to separate such salts from others with which they may be combined. It is scarcely necessary to observe that in such cases the purest alcohol must be employed. [For a table of substances soluble in alcohol, see Henry's Chem. 11th ed. ii. 653.] Several circumstances affeeting the process of crystallization deserve to be noticed. Among these are, 1. Rapidity of evaporation. When the heat is high and the evapo- ration very rapid, the crystallization is confused. 2. The access of atmospheric air. This under certain cicumstances produces instant crystallization in some saline solutions. Those which are the most remarkable on this account are carbonate and sulphate of soda. Hot saturated solutions of these salts in well corked phials may be cooled down without the deposition of any crystals, but as soon as the corks are withdrawn, crystals begin to form and at the same lime the temperature rises. Dr. Thomson has satisfactorily proved that the water of crystallization of the salt which crystallizes,*gives out its latent heat, and that this evolution is the cause of the increase of temperature observed. *—Ann. of Phil. xix. 169. * The theory of this singular phenomenon has not yet been well settled.— That it does not depend on atmospheric pressure is proved by the fact that the solution maybe cooled in open vessels without becoming solid, provided its surface be covered with a film of oil, as first shown by Gay Lussac ; and, as Dr. Turner states, that the experiment also succeeds without the use of oil by causing the air of the flask to communicate with the atmosphere by means of a moderately narrow tube. Mr. Graham supposes the effect of air to arise from a certain chemical action upon w ater. He has shown that gases which are more freely absorbed than at- mospheric air, act more rapidly in producing crystallization. And it would seem from his experiments that the rapidity of crystallization, occasioned hy the con- 2* 18 CRYSTALLIZATION. 3. The nature of the liquor in which the crystals are formed. Dr. Wohler has ascertained that this often influences the fundamental or primitive forms of crystals. Thus he says that when a small portion of solution of sulphate of iron is poured into a solution of alum and the whole allowed to crystallize, the sulphate of iron assumes the octa- hedral form of the alum, although these octahedral crystals contain scarcely a trace of alum.—~Edin. New Phil. Jour. i. 189. 4. The immersion of some foreign body into the saline solution. This serves as a nucleus or attracting point around which the parti- cles as they are deposited may be attached. Hence many of the salts are found concreted around sticks or twigs, and the crystals of sugar are arranged around threads. A crystal of the same kind as that held in solution also answers the purpose. 5. Light. Of this, instances are observed in the bottles of camphor placed in druggists' windows, where the crystals are always most co- pious on the side exposed to the light. Chaptal found that by using a solution of a metallic salt, and shading the greater part of the vessel with black silk, capillary crystals shoot up the uncovered sides, and that the extent of the exposed part is distinctly marked by the limit of crystallization. The phenomenon termed saline vegetation, consist- ing in the creeping of the salt around the edge of the vessel, is also re- ferred by Chaptal to the influence of light. For the perfect success of this experiment, the edges of the vessel should be smeared with oil. 6. Electricity. It has been repeatedly remarked that saline solu- tions which have not yielded crystals, after having been sufficiently concentrated and left undisturbed for several days, have suddenly de- posited an abundant crop, during, or immediately after a thunder storm. Dr. Ure has shown that negative electricity facilitates, and positive electricity retards the formation of crystals.—Brande's Jour. iv. 106. •Crystallographers have observed that certain crystalline forms are peculiar to certain substances. Thus, calcareous spar crystallizes in rhombs, fluor spar in cubes, and quartz in six-sided pyramids; and these forms are so far peculiar to those substances, that fluor spar is never found in rhombs or six-sided pyramids, nor does calcareous spar or quartz ever occur in cubes. Crystalline form may, therefore, serve as a ground of distinction between different substances. It is tact of gaseous matter, is proportional to the degree of its affinity for water- Upon this principle also, he accounts for the fact that solutions of sulphate of soda which have not been boiled are less affected by exposure to the air than well boiled solutions ; for the former still retain most of their air, and do not absorb air so eagerly on exposure as solutions which have been boiled.—Phil. Mag. and Annals, iv. 215. On the other hand. Dr. H. Ogden contends that the access of air is not neces- sary to the process of crystallization. He asserts that it often occurs when the vessel is closed, by mere agitation, without opening it, (a fact which I have also observed,) and to this he ascribes the results of Mr. Graham- He also shows that this peculiar property of resisting crystallization is not confined to any genus of salts in particular, and he enumerates several alkaline, earthy and metallic salts which will exhibit it. Among these are the sulphate, carbonate, acetate, and phosphate of soda, the tartrate of potash end soda, ferrocyanate of potash, sulphate of magnesia, the muriates of lime anil baryles and the sul- phate of copper.—New Edin. Jour, xiii.309. CRYSTALLIZATION. 19 accordingly employed by mineralogists for distinguishing one mineral species from another ; and it is very serviceable to the chemist as afford- ing a physical character for salts. A notice for this subject therefore should form a part of every treatise on chemistry. The surfaces which limit the figure of crystals are called ihe planes or faces, and are generally flat. The lines formed by the junction of two planes, are called edges, and the angle formed by two such edges is a plane angle. A solid angle is the point formed by the meet- ing of at least three planes. The planes whieh terminate a prism are called terminal planes; those at the sides lateral planes; the face on which a crystal is supposed to stand is called a base. When the end of a crystal is formed by two'planes inclined to each other like the roof of a house, it is said to be culminated; if three or more planes meeting in a solid angle, terminate the prism, they form a pyramid which is called the summit; the planes which form the sum- mit are called acuminating planes, and the edges produced by their junction, edges of the pyramid. When an edge or solid angle is cut off and replaced by a single new face, it is said to be truncated ; if by two or more new faces, it is bevelled. A very short prism is called a table. The forms of crystals are very various. They are divided by crys- tallographers into what are called primitive, primary, derivative, or fun- damental forms, and into secondary or derived forms. The number of primary forms is differently stated by different au- thors, according to the system which they adopt. The most simple, however, is that which reduces these forms to the following, viz. 1. The cube. 2. The tetrahedron. 3. The octahedron. 4. The six-sided prism. 5. The rhombic dodecahedron. 20 CRYSTALLIZATION. 6. The dodecahedron with isosceles triangular faces. 7. The rhomb. These primitive forms by further mechanical analysis may be redu- ced to three integral elements. 1. The parallelopiped, or simplest solid having six surfaces, paral- lel, two and two. 2. The triangular or simplest prism, bounded by five surfaces. 3. The tetrahedron or simplest pyramid, bounded by four surfaces. The secondary forms are supposed to arise from decrements of par- ticles taking place on different edges and angles of its primitive forms. Thus a cube, having & series of decreasing layers of cubic particles upon each of its six faces, will become a dodecahedron, if the decre- ment be upon the edges; but an octahedron, if upon the angles: and by irregular, intermediate, and mixed decrements, an infinite variety of secondary forms would ensue. There are some appearances in crystallography to which the above explanation will not apply. A slice of fluor spar, for instance, ob- tained by making two successive and parallel sections, may be divid- ed into acute rhomboids; but these are not the primitive forms of the spar, because by the removal of a tetrahedron from each extrem- ity of the rhomboid, an octahedron is obtained. Thus, as the whole mass of fluor may be divided into tetrahedrons and octahedrons, it be- comes a question which of these forms is to be called primitive, es- pecially as neither of them can fill space without leaving vacuities, a structure not adapted to form the basis of a permanent crystal. To obviate this difficulty, Dr. Wollaston suggested that the integrant, par- ticles of all crystals might be considered as spheres or spheroids, which by their mutual attraction have assumed that arrangement which brings them as near as possible to each other; at;d this view of the subject has been confirmed by the experiments of Mr. Daniell. [ Wol- laston, in Phil. Trans. 1813. Daniell, in Brunde's Jour. i. 24.] But although the preponderance of evidence is rather on the side of the spherical form of atoms, this opinion is,attended with difficulties which in the present state of our knowledge cannot be obviated. Such are some of the facts developed by Isomorphism.—See Thomson's Inorga- nic Chem. i. 16. The primitive forms of crystals can be ascertained—1st, bv mechan- ical division or cleavage; and 2d, by the action of fluid menstrua. For our knowledge of the latter we are wholly indebted to Mr. Dan- iel! ; and the fact is well shown by plunging into a tumbler full of cold CRYSTALLIZATION. 21 water, a shapeless mass of alum, the surface of which becomes in a few days, eaten and carved out into a variety of regular crystalline forms. The process called cleavage consists in separating thin layers or slices from the sides, edges or angles of a crystallized substance in a given direction. Many crystallized substances are very obviously composed of thin plates or laminae, which by a careful operation may be separated from each other, without presenting the appearance of a fracture. The planes in which these laminae are applied to each other, are called the natural joints of a crystal. The direction in which it may be cleaved is called the direction of cleavage. Sometimes a crys- tal is cleavable only in one direction, and it is then said to have a single cleavage. Others may be cleaved in two, three, four or more direc- tions, and are said to have a double, treble, fourfold cleavage, and so on according to their number. It was at one time supposed that substances of different composition never assumed precisely the same primitive form. But the researches of Professor Mitscherlich have proved, that certain substances are capable of being substituted for each other in combination without in- fluencing the crystalline form of the compound. This discovery has led to the formation of groups, each comprehending substances which crystallize in the »ame manner, and which are hence said to be isomor- phous. One of the most instructive of these groups includes the salts of arsenic and of the phosphoric acid. Thus the neutral phosphate and biphosphate of ammonia correspond to the arseniate and biarseniale of ammonia; and the biphosphate and biarseniate of potash have the same form. Indeed each arseniate has a corresponding phosphate, hav- ing the same form, the same number of equivalents of acid, alkali and water of crystallization, and differing in fact in nothing except that one series contains arsenic and the other an equivalent quantity of phosphorus. Several other similar groups occur, and while their stur dy is of great importance to the chemist, their existence should serve as a caution to the mineralogist, not to place exclusive reliance on crys- tallographic character. In some instances certain groups of crystals approximate in their forms without becoming identical. To this approximation the term plesiomorphism, has been applied.—An illustration occurs in the sul- phates of strontia and baryta, the primary forms of both salts are rhombic prisms, very similar to each other; but on measuring the inclination of corresponding sides in each prism, the difference is found to exceed two degrees. The scope of this work forbids a more detail- ed view of these subjects, and I would therefore refer those who are desirous of a full account of the present state of our knowledge upon the subject of isomorphism, and the allied branches of enquiry, to Mr. J. F. W. Johnston's Report on Chemistry, made in 1832, to the British Association for the advancement of Science. It is of great importance in the examination of crystals to measure their angles with precision; for this purpose an instrument has been in- vented, called a goniometer, of which there are two kinds, the common and the reflective. The reflective goniometer, invented by Dr. Wollaston, is the most useful of these instruments. It enables us to determine the angles even of minute crystals, with great accuracy: a ray of light reflected from the surface of the crystal being employed as radius, instead of the 22 AFFINITY. surface itself.—See Phillips' Introduction to Mineralogy, and Brande's Chemistry. References—On Cohesion—Muschenbroeck's Experiments on the cohesive force of solid bodies, in Thomson's Chem. iii. 94. On the means employed by the chemist to prepare the particles of bodies for chemical ac- tion, in Chaptal's Chem. applied to the Arts, i. 51. Boscovich's explana- tion of the phenomena of cohesion, in Thomson's Chem. iii. 96. Arnott's Physics. On Crystallography—Hauy Traite de Cristallographie, i. Brooke's Introduction to Crystallography. Cleaveland's Mineralogy. Philip's Introduction to Mineralogy. Shepard's Mineralogy. Moh's Miner- alogy, i. Leblanc's method of obtaining large artificial crystals, in Thomson's Chemistry, iii. 98. Some valuable directions for crystallizing salts are also to be found in the Encyclopaedia Britannica, iv. 443. Remarks upon Daniell's theory of Crystals, Ann. of Phil. xi. 125, 129. 287. SECTION II. AFFINITY. Syn. Chemical Affinity—Chemical Attraction—Heterogeneous At- traction. If oil and water, or water, oil and mercury, be agitated together, they do not act on each other, but soon separate and exhibit their ori- ginal characters ; they do not combine and have no affinity for each other. But if olive oil and asolution of potassa be agitated together, they form a milky fluid in which neither of them is recognized ; they unite and form a chemical combination; and bodies which unite chem- ically are said to have an affinity for each other, and those which do not unite under any circumstances in which they have been placed, are correctly said to have an affinity for each other. Affinity is defined to be that force by which are united the particles or atoms of bodies of different kinds. Like cohesion, it is only effec- tive at insensible distances; it is mutual and reciprocal between those oodies which it combines—thus a cannot be said to have an affinity for b, while b has none for a. Affinity produces, 1. In some cases a compound not materially altered in Us properties. £«„e^^^JProposition, solution is usually offered as an illustration, though it may be doubted whether this is a good example of affinity Solution is an operation by which asolid body combines with a fluid in such a manner that the compound retains the form of a permanent and transparent fluid. In this case the fluid is termed a menstruum. or°Susira:S4e:dlSUngU1Shed ^ * ™n mechaniCal mi*ure „ Jff' niffuSe a 9uantily of magnesia in water, the mixture is turbid and finally the magnesia is deposited. If to the turbid mixture a few drops 0f nitric acid be added, it will become transparent, and the mZ nesia can no longer be separated by any mechanical process or by reft 1 he nitrate of magnesia is held in solution by the water. AFFINITY. ' 23 Solution is promoted, \ a. By diminishing the cohesion of the particles of the body to be dis- solved: Exp. Place a lump of marble in a wine glass, and a small quantity of the same, previously reduced to powder, in another; pour upon each, dilute muriatic acid; the powdered marble will dissolve much more rapidly than the lump. b. By mechanical agitation. Exp. Put a crystal of tartaric acid into a wine glass containing in- fusion of litmus or of cabbage. The acid if left at rest, produces only a slight effect in its immediate vicinity, but if the liquor be stirred, the whole will become red. c. By heat. There are a few exceptions to this statement which have already been adverted to. (p. 16.) In most cases of solution there is a certain point at which the force of affinity between the solid and fluid will be balanced by the cohesion of the solid, and beyond which solution will not proceed : this point is called saturation, and the resulting compound a saturated solution.' 2. Affinity produces in most cases a compound whose properties differ essentially from those of the components. Exp. Burn phosphorus in oxygen gas or atmospheric air, the result- ing compound is phosphoric acid. Exp. Pass atmospheric air or oxygen gas into a vessel of nitric ox- ide ; the resulting compound is nitrous acid. Affinity sometimes also produces, 3. A change of state. Exp. Two glass vessels, one filled with ammoniacal gas, and the other witi muriatic acid gas, when brought into contact produce solid muriate of ammonia. Exp. To a solution of muriate or nitrate of lime add sulphuric acid; solid sulphate of lime will be formed. Exp. Crystals of sulphate of soda and nitrate of ammonia triturated together become a liquid. Exp. Gunpowder, when exploded, is resolved into various gases. 4. A change of colour. Exp. Add liquid ammonia to a solution of nitrate or sulphate of cop- per—a rich blue colour is produced. A few drops of sulphuric acid render it colourless. Exp. Sulphate of copper and acetate of lead rubbed together in a mortar assume a green colour. Exp. A few drops of tincture of galls added to a very dilute solution of sulphate of iron produce a deep black. In each of these cases, the change of colour is owing to the formation of a new chemical compound. 24 AFFINITY. 5. A change in specific gravity and temperature. Exp. Sulphuric acid and water when combined have the specific gravity of the compound greater than that of the mean. Exp. Surround a phial with some tow and place a piece of phospho- rus within the tow and against the phial. The phial being half full of water, add a small quantity of sulphuric acid; the heat produced will be sufficient to fire the phosphorus. 6. Intense ignition. Exp. Add a few drops of sulphuric acid to a mixture of chlorate of potash and sugar. Exp. Add sulphuric acid to phosphorus and chlorate of potash un- der water. There are several circumstances which influence and modify the op- eration of affinity. 1. A previous state of combination. This generally diminishes, and often prevents chemical action. 2. Cohesion. This, as has been already remarked, often acts as an antagonist power to chemical affinity. Hence the utility of mechani- cal processes which diminish this force. Solid antimony is but slowly acted upon by chlorine gas, but when in the state of fine powder, it takes fire as soon as it touches the gas. Hence also liquidity fa- vours chemical action, the cohesive power being comparatively so tri- fling as to present no appreciable barrier to affinity. There are, however, some instances in which two solids act chemically on each other. 3. Caloric. This has a very important influence over chemical ac- tion : sometimes increasing it, at other times destroying or subverting it. Thus bodies which unite at one temperature refnse to combine or remain combined at another temperature. Lead shavings do not de- compose cold nitric acid, but when heat is applied, rapid decomposition ensues. An increase of temperature favours chemical action by its ef- fect in overcoming the force of cohesion ; but this explanation is not of universal application. 4. The electric state of bodies. Those bodies which are in the same electric state do not combine, those in different electric states do com- bine. Indeed, so intimate is the connexion which subsists between electricity and chemical action, that they have been supposed by some to depend upon the same power. Hence the Electro-Chemical Theory of Davy, which will be more fully noticed hereafter. 5. Specific gravity. When two bodies have different specific gravi- ties, they tend to separate. If their affinity is very feeble, they cannot be made to combine. Oil and water, and mercury and water, are familiar examples. The influence of specific gravity over chemical action is, however, quite limited. 6. The intervention of a third body. This sometimes increases and sometimes destroys chemical action. Oil and water are made to unite upon the addition of an alkali. Alcohol added to a saturated solution of sulphate of soda or nitrate of potassa combines with the water, and the crystallization of the salt instantly takes place. This has been AFFINITY. 25 called disposing or predisposing affinity, which to say the least, is an extremely vague term. 7. Mechanical action or compression. This frequently modifies chem- ical action in a great degree, particularly in the case of gases and liquids. Thus water when under high pressure combines with a greater quantity Wcarbonic acid than when the pressure is less The compound of carbonic acid and lime, known under the name of chalk, may be decomposed by the simple application of an intense heat; but under strong pressure, a heat may be applied sufficient to melt the chalk with- out expelling the carbonic acid. It is this principle, (the influence of pressure in opposing chemical decomposition,) that is the foundation of Dr. Hutton's ingenious Theory of the Earth.—Henry's Chem. 11th ed. i. 65. 8. Quantity of matter or mass. The influence of quantity of matter over affinity appears to be now generally admitted. If one body a, unites with another body b, in several proportions, that compoand will be the most difficult of decomposition which contains the smallest quantity of b. Of the three oxides of lead, for instance, the peroxide parts most easily with its oxygen by the action of caloric ; a higher temperature is required to decompose the deutoxide, and the protoxide will bear the strongest heat of our furuaces, without losing a particle of its oxygen. The influence of quantity over chemical attraction may be further illustrated by the phenomena of solution. When equal weights of a soluble salt are added in succession to a given quantity of water, which is capable of dissolving almost the whole of the salt employed, the first portion of the salt will disappear more readily than the second, the se- cond than the third, the third than the fourth, and so on. The affinity of the water for the saline substances diminishes with each addition, till at last it is weakened to such a degree as to be unable to overcome the cohesion of the salt. The process then ceases and a saturated solu- tion is obtained. Quantity of matter is employed advantageously in many chemical operations. If, for instance, a chemist is desirous of separating an acid from a metallic oxide, by means oi' the superior affinity of potassa for the former, he frequently uses rather more of the alkali than is suffi- cient for neutralizing the acid. He takes the precaution of employing an excess of alkali, in order the more effectually to bring every par- ticle of the substance to be decomposed in contact with the decompos-' ing agent. But Berthollet has attributed a much greater influence to quantity of matter. His views, however, do not appear to be supported by facts.— Berthollet Chem. Slat. Turner's Chem. Davy's Elements. Affinity is of two kinds— 1. Simple. 2. Elective. Simple affinity is the union of the constituent atoms of a compound without causing decomposition. It is sometimes also called combina- tion. Thus the combustion of carbon in oxygen gas produces by the mere union of these two elements carbonic acid. So also sulphuric acid and potassa by mere combination form sulphate of potassa. 26 AFFINITY. Elective affinity is of two kinds, simple and compound, or single and double. An important law of affinity, and which indeed is the basis of almost all chemical theory, is that the same body has not the same force of affinity towards a number of others, but attracts them unequally. Thus when sulphuric acid is added to a solution of nitrate of lime, ^composed of nitric acid and lime,) the lime leaves the nitric acid and combines with the sulphuric, forming a sulphate of lime. This is an example of what is termed in chemistry, a simple decomposition. The lime in this case is considered as making an election of the sulphuric acid in preference to the nitric ; and this affinity has been called single elec- tive affinity. When one of the substances falls down in the state of powder, it is termed a precipitate. Other illustrations of single elective affinity. Exp.9 AAA. sulphuric acid to muriate of soda, muriatic acid is disen- gaged and sulphate of soda remains. Exp. To a solution of nitrate of silver, add mercury ; the nitric acid will in a short time leave the silver and combine with the mercury. To the nitrate of mercury thus formed, add lead, the nitric acid will leave the mercury and unite with the lead. To this last add copper, and nitrate of copper will be produced. Upon the discovery of this important law, it occurred to Geoffroy, a French chemist, that tables might be constructed, which should exhibit the relative forces of the attraction of any body towards others. The substance, whose affinities are to be thus expressed, is merely placed at the head of a column separated from the rest by a horizontal line. Beneath this line are arranged the different substances for which it has ' any attraction, in an order corresponding with that of their respective forces of affinity; the substance which it attracts most powerfully being placed nearest to it, and that for which it has the least affinity at the bottom of the column. The following series exhibiting the affin- ities of sulphuric acid for the alkalies and alkaline earths, will serve as an example. Sulphuric acid. Baryta, Strontia, Potassa, Soda, Lime, Ammonia, Magnesia. Double elective affinity. This kind of affinity takes place when two bodies, each consisting of two principles are presented to each other and mutually exchange a principle of each ; by which means two new bodies or compounds, are produced, of a different nature from the ori- ginal compounds. In this case it frequently happens, that the compound of two principles cannot be destroyed, either by a third or a fourth sepa- rately applied ; whereas if this third and fourth be combined, and placed in contact with the former compound, a decomposition or change of AFFINITY. 27 principles will ensue. Thus when limewater is added to a solution of sulphate of soda, no decomposition happens, because sulphuric acid attracts soda more strongly than it does lime. If nitric acid be applied to the same compound, still its principles remain undisturbed, because the sulphuric acid attracts soda more strongly than the nitric. But if lime and pitric acid previously combined, be mixed with the sulphate of soda, a double decomposition is effected and two new compounds are produced. These changes may be expressed by the following diagram, contrived by Bergman. Nitrate of Soda. Sulphate ( Soda. Nitric acid. ) Nitrate of .) [ of Soda. f Sulphuric acid. Lime. ) Lime. Sulphate of Lime. On the outside of the vertical brackets are placed the original com- pounds, (sulphate of soda and nitrate of lime,) above and below the horizontal lines, the new compounds produced, {nitrate of soda and sulphate of lime,) the upper line beingstraight indicates that the nitrate of soda remains in solution, the dip of the lower line, that the sulphate of lime is precipitated. A piece of sheet lead immersed in a solution of sulphate of zinc pro- duces no change, because the sulphuric acid has a stronger affinity for the zinc than for the lead. Neither does aceticacid produce any change. But when acetate of lead is added, two new compounds, viz. sulphate of lead and acetate of zinc are the result. In the above cases, as in many others, we can easily explain the de- compositions which take place. There are two distinct sets of affini- ties, one tending to prevent any change of composition, as between the sulphuric acid and the soda in the former case, and between the nitric acid and the lime ; termed by Kirwan the quiescent affinities. Another set tending to produce a decomposition, as between the nitric acid and the soda, and the sulphuric acid and the lime, are termed the divellenl affinities, and which in this case are the most powerful. But other cases of double decomposition cannot be explained in this man- ner. As for instance, carbonate of baryta and sulphate of potassa mu- tually decompose each other.—R. Phillips Quart. Jour. i. 80. Dulong. in Phil. Magazine, 41. References. The article ' Chemical Decomposition,' in the Supple- ment to the Encyclopedia Brilannica, by Dr. Thomson. Turner's Che- mistry, section on Affinity. Murray's Chemistry, Book 1, on Attraction. Davy's Elements of Chemical Philosophy, lire's Chemical Dictionary, Art. Attraction. Brande's History of Chemistry. Henry's Chemistry, 11th ed. 1. Bishop Watson on the various phenomena attending the solu- tion of salts, in Phil. Trans. 1770, 235, and in the Chemical Essays, v. 43. Gay Lussac on the solubility of the salts in water. Ann. of Phil, xv. 1, 28 LAWS OF COMBINATION. SECTION III. OF THE PROPORTIONS IN WHICH BODIES COMBINE ; AND OF THE ATOMIC THEORY. V, In the chemical combination of bodies with each other, the follow- ing circumstances deserve to be mentioned. I. Some bodies unite in all proportions ; for example, water and sul- phuric acid, and water and alcohol. II. Other bodies combine in all proportions, as far as a certain point, beyond which, combination no longer takes place. Thus water will take up successive portions of common salt, until at length it becomes in- capable of dissolving any more. In cases of this sort, as well as in those included under the first head, combination is weak and easily de- stroyed, and the qualities, which belonged to the components in their separate state, continue to be apparent in the compound. It is necessary however, to remark, that these two deductions, though they appear to be warranted by a general survey of the pheno- mena, are not absolutely and strictly true ; for though some acids, for instance, appear to unite with water in every proportion, yet there are certain relative qualities of water and acid which form the most ener- getic compounds, distinguished by their permanency and peculiar pro- perties.—Henry's Chem. i. 43. III. Some hodies unite in one proportion, or in a few proportions only. Chlorine and hydrogen combine in no other proportions than those con- stituting muriatic acid. On the other hand, carbon and oxygen unite in two proportions; oxygen and nitrogen in five proportions, &c. The greatest number of compounds that any two substances are known to produce, is six, if we except those noticed in the preceding para- graphs. The combination of bodies that unite in this manner, is regulated by the following laws: I. The composition of bodies is fixed and invariable. A compound substance so long as it retains its characteristic properties, must always consist of the same elements united together in the same proportion. Muriatic acid, for example, is always composed of 35.45* parts by weight of chlorine, and one of hydrogen ; no other elements can form it, nor can its own elements form it in any other proportion. Water, in like manner, is formed of 1 part of hydrogen and 8 of oxygen ; and were these two elements to unite in any other proportion, some new compound, different from water, would be the product. The same observation applies to all other substances, however complicated, and at whatever period they were produced. Thus, sulphate of baryta, whether formed ages ago by the hand of nature, or quite recently by the operations of the chemist, is always composed of 40 parts of sul- phuric acid and 76.7 of baryta. This law, in fact, is universal and permanent. Its importance is equally manifest. It is the essential * I adopt, with few exceptions, the equivalent numbers of Dr. Turner. LAWS OF COMBINATION. 29 basis of chemistry, without which the science itself could have no ex- istence.— Turner's Chem. II. The relative quantities in which bodies unite, may be expressed by proportional numbers. Thus 8 parts of oxygen unite with one part of hydrogen, 16 of sulphur, 35.45 of chlorine, 40 of selenium, and 108 parts of silver. Such are the quantities of these five bodies which are disposed to unite with 8 parts of oxygen ; and it is found that when they combine with one another, they unite either in the proportion ex- pressed by those numbers, or in multiplies of them according to the third law of combination. Thus sulphuretted hydrogen is composed of 1 part of hydrogen and 16 of sulphur, and bisulphuretted hydrogen, of one part of hydrogen, to 32 of sulphur; 35.45 of chlorine unite with one of hydrogen, 16 of sulphur, and 108 of silver, and 40 parts of se- lenium with 1 of hydrogen, and 16 of sulphur. From these examples it is manifest, that bodies unite according to proportional numbers; and hence has arisen the use of certain terms, such as proportions, combining proportions, proportionals, or equivalents to express them. Thus the combining proportions of the substances just alluded to are— Hydrogen ...... 1 Oxygen ....... 8 Sulphur .......16 Chlorine ^y......35.45 Selenium * ....;. 40 Silver........108 This law also applies to compound bodies. Thus water is compos- ed of one proportional or 8 parts of oxygen, and one proportional or 1 part of hydrogen, and hence its combining proportion is 9. The pro- portional of sulphuric acid is 40, because it is a compound of one proportion or 16 parts of sulphur, and three proportions or 24 parts of oxygen. And this law applies not only to compounds of two elemen- tary bodies, but to the salts; the latter of which indeed, furnish the most striking illustrations of this subject. III. When one body A unites with another body B in two or more pro- portions, the quantities of the latter, united to the same quantity of the for- mer, bear to each other a very simple ratio. These ratios of B may in all cases be represented by one or other of the two following se- ries :— 1st.—A unites with 1, 2, 3, 4, 5, &c. of B. 2d.—A unites with 1, 1 1-2, 2, 2 1-2, &c. of B. The following table exemplifies the first series.— Water is composed of .... Hydrogen 1 . . Oxygen 8 ) 1 Deutoxide of hydrogen .... Do. 1 . . Do. 16 \ 2 Carbonic oxide ......Carbon 6 . . Do. 8*1 Carbonic acid ...... Do. 6 . . Do. 16 \ 2 Nitrous oxide ..•.., Nitrogen 14 . . Do. 8~] I Nitric oxide....... Do. 14 . . Do. 16 | 2 Hyponitrous acid...... i Do. 14 . . Do. 24 ^3 Nitrous acid ...... Do. 14 . . Do. 32 [4 Nitric acid ....... Do. 14 . . Do. 40 J 5 It will be observed that in all the above cases the ratios of the oxygen 30 LAWS OF COMBINATION. Protoxide of iron consists of Iron 28 Oxygen Peroxide , Do. 28 Do. Protoxide of lead Lead 103.5 Do. Deutoxide Do. 1035 Do. Peroxide Do. 103.5 Do. Arsenious acid Arsenic 37.7 Do. Arsenic acid Do. 37 7 Do. Hypo-phosphorous acid Phosphorus 15.7 Do. Phosphorous acid Do. -. 15.7 Do. Phosphoric acid . Do. 15.7 Do. 8)1 12 \ li are expressed by whole numbers. In water_ihe hydrogen is combined with half as much oxygen as in the deutoxide of hydrogen, and hence the ratio is as one to two. The same may be said of carbonic oxide and carbonic acid. In the compounds of nitrogen and oxygen the latter is in the ratio of 1, 2, 3, 4, & 5. This ratio ai3o extends to the combi- nations of combustibles with each other and to the salts. It may be il- lustrated by a very simple experiment, first made by Dr. Wollaston ; let a given weight of bicarbonate of potassa be thrown-into a tube over mercury, and diluted sulphuric acid sufficient to cover it, be in- troduced into the tube, when a certain volume of carbonic acid gas will be disengaged ; let an equal weight of the carbonate of potassa be treated in the same way and it will be found to give off exactly half as much carbonic acid gas. The second series is exemplified in the following compounds 8 12 12 \ li 16*2 12(1* 20)2$ 4*4 12 Mi 20J2J Both of these series, which together constitute the third law of com- bination, as Dr. Turner remarks, result naturally from the operation of the second law. The first series arises from one proportion of a body uniting with 1, 2, 3 or more proportions of another body. The second series is a consequence of two proportions of one substance combining with 3, 5, or more proportions cf another. Thus if two proportions of phosphorus unite both with 3 and 5 proportions of oxy- gen, we obtain the ratio of 1 1-2 and 2 1-2 ; and should one proportion of iron combine with one of oxygen, and another compound be formed of two proportions of iron to three of oxygen, then the oxygen united with the same weight of iron would have the ratio, as in the table, of 1 to 1 1-2. And the compounds of lead and phosphorus with oxygen, af- ford examples of the same kind. Turner's Chem. 4th ed. IV. Gases or airs unite in the most simple ratios of volume or bulk. This important fact was discovered by Gay Lussac. Thus I volume unites to 1, or 1 to 2, or 1 to 3, &c. In combination by weight there is no simple multiple ratio between the weight of the elements in the first compound ; the oxygen for example, is not equal to, or twice or thrice, &c. the weight of the nitrogen in nitrous oxide, or of carbon in carbon- ic oxide; it is only when there is a second compound formed of the same elements, that the new proportions of the body which has been added, become a multiple of the first. But in combinations by volume, the bulk of one of the gases in the first, as well as in the olher com- pounds, is always equal to, or is some multiple of that of the other: thus, 100 of. oxygen combine with 200 hydrogen. 100 ammonia " " 50 carbonic acid. 100 ammonia *' " 100 carbonic acid. 100 nitrogen "- " 50, 100, 150, 200. and 250 volumes of oxygen. LAWS OF COMBINATION. 31 Another curious fact iptablished by Gay Lussac is that the diminu- tion of bulk, which gases frequently suffer in combining is also in a very simple ratio. Thus the four volumes of which ammonia is con- stituted, (3 volumes of hydrogen and 1 of nitrogen,) contract to one half or two volumes when they unite. There is a contraction to two- thirds in the formation of nitrous oxide, to one half in the formation of sulphuretted hydrogen, and to one half in that of sulphurous acid &c. [For a more full exposition of this law see Henry's Chem. i. 55.] V. The respective quantities of any number of alkaline, earthy or me- tallic bases required to saturate a given quantity of any acid, are always in the same ratio to each other, to whatsoever acid they be applied. This law appears to have been discovered by Richter of Berlin, in 1792, and has been fully confirmed by succeeding chemists. For an illustration of it let us take potassa and soda for the bases, and sulphuric acid for the acid. Having found by experiment that two parts of soda will saturate as much of the acid as three of the pot- ash, their power of saturating every other acid is in the ratio of two to three. Thus if we should ascertain by experiment that it required 4 parts of soda, to neutralize any given quantity of nitric acid, we should know without experiment that it would require 6 parts of potash to neutralize the same amount of nitric acid. Two parts of soda are therefore said to be equivalent to three of potash. This rule applies as well to all the acids as to the bases, and greatly facilitates chemical in- vestigations. Thus if we have 100 bases and 50 acids, we have only to apply each of these bases to any one of the acids, and one of the bases to all the acids, to ascertain the amount of all the other bases necessary to saturate all the acids. Hence we need only perform 149 experiments instead of 5000 whieh we should be obliged to do without a knowledge of this law. By arranging the numbers indicating the relative combining weights or equivalent quantities of different substances on a moveable scale, and writing against them the names of the substances they respectively represent, Dr. Wollaston constructed a Scale of chemical equivalents, an instrument stamped with the accuracy and ingenuity of its author, and of great value to the practical chemist.— Phil. Trans. 1814. In this instrument the slide is a line of numbers on which equal dis- tances denote equal ratios. The distance between 50 and 100, for example, is the same as that between 1 and 2, because 50 : 100 : : 1 : 2. Opposite to the numbers on this scale, are written the names of the bodies of which the numbers themselves are the equivalents ; then the distances between these bodies are in like manner the measures of the ratios of their combining quantities, and will be the same with the dis- tances between the numhers. But the chief value of the arrangement is, that by means of the slide, we can at once solve a great number of cases which arise out of combinations and decompositions, the solution of which in the ordinary way would require a tedious number of com- putations. As the proportional numbers merely express the relative quantities of different bodies which unite together, it is of no consequence what figures are employed to express them, provided the relation is strictly observed : Thus Dr. Thomson makes oxygen 1, so that hydrogen is 8 times less than unity, or 0.125, carbon 0.75, and sulphur 2. Dr. Wol- laston in his scale of equivalents, fixes oxygen at 10, by which hydro- 32 ATOMIC THEORY. gen is 1.25, carbon 7.5, &c. But the greatest.number of chemists call hydrogen unity and therefore oxygen 8. This is much the most simple, and has been adopted in the scale of equivalents constructed by Profes- sor Henry and myself, and also in the present work. A very complete scale of chemical equivalents has been drawn up by Mr. Prideaux of Plymouth, Eng.— Phil. Mag. d> Annals, viii. 430. The utility of being acquainted with the important laws which have here been given is almost too manifest to require notice. Through their means our knowledge of the composition of bodies is much sim- plified. The exact quantities of bodies necessaiy to produce a desired effect can also be at once determined with certainty, and these laws thus become highly useful both in the practice of the chemical arts and in the operations of pharmacy. The same knowledge, moreover, af- fords the best guide to the analyst, by which to judge of the accuracy of his results. Thus a powerful argument in favour of the accuracy of an analysis is derived from the correspondence of its results with the laws of chemical combination. On the contrary, if it forms an exception, it may be considered doubtful, and we may hence be led to detect an error, which might otherwise have escaped notice. It may be observed that these laws are deduced from experiment, and the student should be careful to keep them distinct from the theory which has been proposed to account for them. Whatever may be the fate of this theory, it cannot affect these laws, though the progress of discovery may render it necessary that they should be modified. ATOMIC THEORY. The foregoing are the principal facts, which the experimental ex- aminations of chemical combinations and decompositions have unfold- ed. To account for these facts a theory has been invented which is de- nominated the Atomic Theory; and which derives much probability from the complete solution it affords of the laws of attraction. For the full developement of this theory we are indebted to the labours of Mr. Hig- gins and Mr. Dalton. The atomic theory proceeds on the supposition that every body is an assemblage of minute, solid particles, without considering whether a farther division of them be possible or not. The question of the in- finite divisibility of matter is not therefore involved ; the theory only assumes that matter is not, in fact infinitely divided. These undivided particles are the atoms in question. It is also assumed that these atoms differ from each other in weight, whether this difference is owing to specific gravity or size, or both together, is not material.— Thomson's First Prin. It is further supposed that though we appear, when we affect a chem- ical union, to operate on masses, the combination only takes place be- tween these ultimate particles or atoms.* To apply this theory,; let us take the compounds of nitrogen and oxy- * An ingenious method of illustrating this theory has been invented by Pro- fessor Hadley of the college of Physicians and Surgeons of the Western District. It consists of a box containing a number of cubical blocks, which are variously coloured to represent, the principal elementary substances. By these represen- tatives of atoms, the laws of combination may be strikingly presented to the stu- dent, and many of the more complex chemical decompositions rendered appa- rent. ATOMIC THEORY. 33 gen, which are five, differing much from each other: viz.: nitrous ox- ide, nitric oxide, hyponitrous acid, nitrous acid and nitric acid.^ It we take a given quantity of nitrogen, say 14 grains, and combine it with 8 grains of oxygen, we form nitrous oxide ; with 8 more, we have ni- tric oxide; with 8 more, hyponitrous acid; with 8 more,nitrous acid ; with 8 more, nitric acid. Now supposing that these several compounds are formed by the union of a certain number of atoms of oxygen, the latter number varying in the several compounds, the first compound is probably formed of an equal number of atoms of each element. It is plain that one atom of nitrogen can combine with no less than one atom of oxygen, because the atoms are, by the hypothesis, indivisible, or at least undivided. Nor is it probable, that, in forming the first compound, one atom of nitrogen combines with any more than one of oxygen ; for this union is the most simple and hence the most natu- ral. The first compound then being formed of one atom of nitro- gen and one of oxygen, it is plain that no new compound can be form- ed, until we add at least one atom of oxygen ; and hence the reason is evident why in all the higher combinations the quantity of oxygen is just twice, or thrice, or four times that in the lowest, there being respectively just twice, or thrice, or four times as many atoms of oxy- gen. ... , In ca«es where two bodies unite only in one proportion, it is assumed that they unite atom to atom ; and hence if we determine the relative weights of the masses, which enter into combination, we can easily determine the relative weights of the atoms; the ratio being the same. Thus chlorine and hydrogen combine only in one proportion, forming muriatic acid. The volume of each is the same, but the weight of the volume of chlorine is 35.45 times greater than that of the volume of hydrogen ; hence the atom of chlorine is to that of hydrogen as 35.45 to 1. . The weight of the atoms of all other bodies is ascertained in the same manner. Thus an atom of carbon is six times, and an atom of sulphur 16 times heavier than that of hydrogen ; and this is precisely the reason why they unite with each other in the proportions express- ed by those numbers. What are called proportional or equivalent numbers, are therefore, in fact nothing else but the relative weights of atoms.* The two assumptions, which indeed constitute the basis of this theo- ry, would be considered gratuitous, were it not for the complete expla- nation which they afford of the laws of definite proportions.—They are further supported by the fact, that the weight of an atom comes out the same when deduced from different premises. This theory is also confirmed by the observations of Dr. Wollaston, in his essay on the finite extent of the atmosphere.—[ Phil. Trans. 1822. ] And another argument which appears to amount almost to demonstration, * These terms have been proposed to avoid theoretical annunciations; but the term atom originally proposed by Mr. Dalton is generally employed in this work as being the most convenient. And I think Dr. Thomson has correctly re- marked that " unless we adopt the hypothesis with which D«lton setout, name- ly that the ultimate particles of bodies are atoms incapable of farther division, and that chemical combination consists in the union of these atoms with each other, we lose all the new light which the atomic theory throws upon chemistry, and bring our notions back to the obscurity of the days of Bergman and Berthollet." —History of Chemistry. 34 ATOMIC THEORY. is deducible from the peculiar connexion noticed by Professor Mits- cherlich, between the form and composition of certain substances. —Ann. de Chim. et de Phys. xiv. 172, xix. 350. and xxiv. 264 and 365. . . There is a seeming objection to the atomic theory in those cases where one proportion of one body combines with one proportion and a half of another. In such cases, however, it is supposed that two atoms of the one are combined with three atoms of the other, by which the exact ratio is preserved, and the idea of a fraction of an atom avoided. If the atoms occupied the same space when in a gaseous form, they might be represented by volumes, and their proportional numbers would be identical with their specific gravities. This, however, is not the case. The atom of hydrogen, and several other gaseous substan- ces occupy twice the space of an atom of oxygen : but in such cases it is easy to calculate the specific gravity by multiplying the atomic weight by half the specific gravity of oxygen. It might at first view appear that the atoms and volumes could be made to correspond, if we considered water as a compound of two atoms of hydrogen and one atom of oxygen. This has been proposed by Sir H. Davy, and advocated by other chemists: but it increases in- stead of removing the difficulty. Thus sulphuretted hydrogen, on this supposition, must be considered as consisting of one atom of sul- phur and two atoms of hydrogen, while it is composed of one volume of each of the constituents. Muriatic acid gas would be formed of one atom of chlorine and two atoms of hydrogen, though constituted in like manner of one volume of each gas. And the same remark would be applicable to hydriodic acid, hydrocyanic acid, and indeed to most other compound gases containing hydrogen. For this and other rea- sons which might be given, water is considered a compound of one atom of hydrogen and one atom of oxygen, though the volume of oxy- gen is only one half that of the hydrogen. The learner will now see the reason why the atomic weight of oxy- gen is fixed at 8 compared to hydrogen 1, although its specific gravity is 16 when compared to the same substances as unity. References.— Turner's Chemistry, lire's Chem. Dictionary, Art. Equivalent. Professor Olmstead on the present state of Chemical Sci- ence, Silliman's Jour. xii. \ ; a paper containing a lucid explanation of the Laws of Combination and of the Atomic Theory, from which some of the ideas in the above section are obtained. McNevin on the Atomic The- ory. Thomson's First Principles. Dalton's new system of Chemical Philosophy, v. 1. Berthollet's Chemical Sialics. Some interesting papers concerning the Atomic Theory, by Berzelius, Dalton and Thomson, will be found in t/ie 2d, 3d, 4lh and bth volumes of the Annals of Philosophy. Berzelius Traite de chimie. Davy's Elements of Chemical Philosophy. Daubeny on the Atomic Theory. Prout's Bridgewater Treatise. HEAT OR CALORIC —NATURE OF CALORIC. 35 CHAPTER II. HEAT OR CALORIC. The term Heat, in common language, has two meanings ; in the one case it implies the sensation experienced on touching a hot body; in the other it expresses the cause of that sensation. To avoid any ambigui- ty that may arise from the use of the same expression in two such dif- ferent senses, the word caloric (from the latin color, heat) has been adopted to designate exclusively the principle or cause of the feeling of heat; but to prevent repetition, the terms caloric and heat are both occasionally employed to designate the cause in question. This subject will be treated of in the following order: 1. The nature of caloric. 2. Its communication. 3. Its distribution. 4. Its effects. 5. Specific caloric. 6. The sources of caloric. SECTION I. NATURE OF CALORIC. Upon this point philosophers are divided into two sets; the one main- taining that caloric is a mere property of matter; the other that it is a distinct substance. The former was an old opinion, but was adopted by Sir H. Davy, and is at the present time quite generally received in France. The latter was maintained by Sir Isaac Newton ; and as this affords a more easy explanation of most of the chemical phenomena, it will be adopted in the present treatise. Caloric, on the supposition of its being material, is a subtle fluid, the particles of which repel one another, and are attracted by all other substances. It is imponderable ; that is, it is so exceedingly light that a body undergoes no appreciable change of weight, either by the ad- dition or subtraction of caloric. It is present in all bodies, and can- not be wholly separated from them. For if we take any substance whatever, at any temperature, however low, and transfer it into an at- mosphere, whose temperature is still lower, a thermometer will indi- cate that caloric is escaping from it. That its particles repel one an- other, is proved by observing that it flies off from a heated body; and that it is attracted by other substances, is equally manifest from the ten- dency it has to penetrate their particles and be retained by them.— Turner. Caloric has also been defined as the agent to which the phenomena of heat and combustion are to be ascribed. So far as chemical agen- cies are concerned it is, 1st. An antagonist power to cohesive attrac- tion ; and, 2d. It concurs with, and increases elasticity. Caloric in many of its properties, resembles light. As for example, in those of refraction, reflection and radiation, and in the repulsion 36 COMMUNICATION OF CALORIC. whieh exist between its particles. This latter property, indeed, is so striking that caloric is often treated of under the appellation oi calorific repulsion. As the presence of caloric produces the sensation of heat, so its ab- sence produces that of cold. This last sensation, therefore, is not to be considered as produced by any particular agent, but as altogether a negative expression. Caloric exists in two states, viz.: free or sensible, and, latent or com- bined. In the former state it is capable of exciting the sensation of heat and producing expansion in other bodies, and to it the term caloric of temperature has also been applied. By the term temperature, we are to understand the stale of a body relatively to its power of exciting the sensation of heat, and occasioning expansion; effects, which in all probability, bear a proportion to the quantity of free caloric in a given space, or in a given quantity of matter. Thus what we call a high temperature may be ascribed to the presence of a large quantity of free caloric; and a low temperature to that of a small quantity. We are unacquainted, however, with the extremes of temperature; and may compare them to a chain, of which a few of the middle links only are exposed to our observation, while its extremities are far removed from our view.—Henry's Chem. i. 82. In the latter state, caloric exists either in combination with bodies or in something resembling it. Under these circumstances, it does not possess its distinguishing properties—cannot be discovered by our senses or by the thermometer, and produces important and sometimes permanent changes in the bodies with which it combines. The dif- ference between these two states may be shown by the following expe- riment. Exp. Place two vessels of thin glass, the one containing water and the other a small portion of sulphuric acid, in contact with the bulb of an air thermometer. Allow them to remain in this situation for a short time, to show that the thermometer is not affected by them. Now empty the sulphuric acid into the water, when it will be seen that the liquid in the thermometer descends, owing to the expansion of the air by the caloric which has been rendered free by the mixture. The com- mon fire syringe offers another striking illustration of the difference be- tween combined and free caloric. References.—For a notice of facts and reasonings concerning the na- ture of Caloric, see Thomson's Chemistry, vol. 1 ; and Library of Useful Knowledge, Art. Heat. Professor Hare's paper, on the materiality of Heat, and the discussion on this subject between him and Prof. Olmstead. Silliman's Amer. Jour. vols. iv. xi. xii. xiii. SECTION II. COMMUNICATION OF CALORIC. Caloric is radiated in all directions, and moves with great velocity. It is also absorbed by certain bodies, and when it has thus entered it makes its way through the body. In the latter case its motion is com- paratively slow. Hence caloric is communicated, 1. By Radiation. 2. By Conduction. COMMUNICATION OF CALORIC. 37 RADIATION, When the hand is placed above a heated ball, or a fire, the sensation •of heat is perceived ; and the same thing also takes place when the hand is placed below the source of caloric. Now as nothing intervenes in the last case but the air, and as this fluid, when caloric is commu- nicated to it, expands and rises, the impression cannot be owing to the transmission of hot air to the hand ; but rather to the action of the particles of caloric which are supposed to be thrown offin all directions. Bodies capable of discharging caloric in this way are called radia- ting ; and the principle which is thus projected is denominated Radi- ant Caloric. Radiant Caloric is reflected. This fact can be shown in a familiar manner, by standing at the side of a fire in such a position that the caloric cannot reach the face di- rectly, and then placing a plate of tinned iron opposite the grate and at such an inclination as permits the observer to see in it the reflection of the fire ; as soon as it is brought to this inclination, a distinct im- pression of heat will be perceived upon the face. If a line be drawn from the heated substance to the point of a plane surface from which it is reflected, and a second line from that point to the spot where it produces its effect, the angles which these lines form with a line per- pendicalar to the reflecting plane are equal to each other, or in phi- losophical language, the angle of incidence is equal to the angle of re- flection.— Turner. This may be illustrated in a still more striking manner, by the ex- periment devised by Pictet, which is, to take two concave mirrors of planished tin or copper, and to place them at a distance of from 9 to 12 feet apart, with their concave surfaces towards each other. If now a hot ball or a small basket of coals is placed in the focus of one mirror, it will instantly affect a thermometei in the focus of the other; or if a piece of phosphorus be substituted for the thermometer, it will soon be kindled. Now these phenomena cannot be explained upon the supposition that the heat of the ball or of the coals is communicated directly to the thermometer through the medium of the air which intervenes ; for upon this hypothesis, a point intermediate between the two mir- rors would be of a higher temperature than that directly in the focus, which is not the case. There must, therefore, be something emanating from the ball or the coals which is reflected by the mirrors, and is con- centrated also into a focus, so as to affect the thermometer and the phosphorus. When the heated ball is placed near the mirror, rays of heat fly off from it in straight lines in all directions ; some of these strike the mir- ror, by which they are instantly reflected again in straight lines to the opposite one, where they are also reflected, and are thus brought to a point where the heating effect is produced. The distance at which this takes place, depends of course on the form of the mirror ; and the effect is much greater when burning charcoal is employed instead of the heated ball. But the thermometer is affected even when a glass of boiling water is substituted for the heated ball. 4 38 COMMUNICATION OF CALORIC. Radiant Caloric is absorbed. This may be shown by placing a thermometer before the fire or any heated body, when the mercury will be seen to rise in the stem. And it has been ascertained that the intensity of effect diminishes according to the squares of the distance from the radiating point. Thus the thermometer will indicate four times less heat at two inches, nine times less at three inches, and sixteen times at four inches, than it did when it was only one inch from the heated substance. It is therefore evident that a hot body placed in the air is the centre of a multitude of calorific rays ; that these rays traverse the air almost without resistance; and that when they fall upon the surface of a solid or liquid substance they are either reflected from it and receive a new direction, or lose their radiant form altogether, and are absorbed. With regard to caloric then, three powers may be recognized as belong- ing to bodies, viz. 1. The power of emitting or radiating caloric ; 2. The power of absorbing ; and 3. The power of reflecting caloric. These powers appear to depend chiefly upon the temperature, and polish upon the surface of the body ; their nature having but little influence. Temperature. The higher a body is heated, the greater is its radia- ting power ; for then the caloric of the body has greater tension, or in other words, makes more effect to escape. Polish of surface. When even a large quantity of radiant caloric falls upon a metallic body highly polished, the body is scarcely heated ; from which we infer that all the caloric has been reflected. But if we blacken the surface of this body and expose it again to the same amount of caloric, it becomes considerably heated; and in this case nearly all the caloric will be absorbed. Polish influences the radiating, as well as the absorbing and reflect- ing power. This is strikingly illustrated by the experiment of Mr. Leslie. He coated one side of a canister with lampblack, covered a second with writing paper, applied to a third a pane of glass, and left the fourth bright and polished. The canister was now filled with boil- ing water and placed in the focus of the mirror. When the metallic surface was presented to the reflector, the impression upon the ther- mometer amounted to 12° ; from the glass surface it was equal to 90° ; from the papered side 98° ; and 100° from the lampblack. Colour of surface. The influence of colour over the radiating and absorbing powers of bodies, so far as solar heat, or terrestrial heat ac- companied with light is concerned, has long been observed. Dr. Stark of Edinburgh has, however, recently shown that this extends also to simple heat. The bulb of a delicate thermometer was successively sur- rounded by equal weights of differently coloured wool, was placed in a glass tube heated by immersion in hot water to 180° and then cooled to 50° in cold water. The times of cooling were 21 minutes with black wool, 26 with red wool, and 27 with white wool. Similar results were obtained with flour of different colours. On the other hand, when dif- ferent coloured wools, were wound upon the bulb of a thermometer and exposed within a glass tube to hot water they rcse from 50° to 170° in the following times,—black wool in 4' 30", dark green in 5' scarlet in 5' 30", white in 8'.—Phil. Trans, for 1833. Part. ii. Ratios of the radiating, absorbing and reflecting powers. The powers of bodies to radiate and absorb caloric are directly proportional to each COMMUNICATION OF CALORIC. 39 other. Those which absorb with the most ease also radiate with the greatest facility. If in the usual arrangement, the bulb of the thermo- meter be coated with tin foil, or even gold leaf, the impression of the radiant caloric will be exceedingly slight; if the bulb be naked, the effect will be much greater, but if it be coated with lampblack, the ac- tion on the instrument will reach its maximum. Now experiments have proved that lampblack absorbs more than glass, and glass more than the metals, which is exactly the order of their radiating powers. —Leslie's Inquiry. The powers of radiation and reflection are inversely proportional to each other. Those bodies which reflect the most, radiate the least, and tho^e which radiate the most, reflect the least caloric. Thus metals reflect heat with great facility ; but if a glass mirror be substi- tuted for the metallic reflector, the positions of the hot body and the thermometer being the same, the effect will be considerably diminish- ed, and if the surface of the glass be coated with lampblack, no action upon the thermometer will be perceived. The vast superiority of metallic bodies over glass in reflecting ca- loric may be proved by a very simple experiment. In the focus of the commonest tin reflector, if at all approaching to a concave or para- bolic form, when held before a large fire, a considerable degree of warmth will be perceived ; whereas the best glass mirror of the same dimensions will hardly collect in its focus heat enough to be felt. On the other hand, glass radiates more caloric than the metals, and lampblack more than glass. If the surface of the metallic mirror be furrowed or roughened, or if it be covered with a thick film of amal- gam, its power to reflect caloric will be diminished, while the power to radiate will be proportionably increased. Metallic bodies possess this property in different degrees, and it will be seen, by inspecting the table of Mr. Leslie, that those which reflect the most, radiate the least, and that the converse of the proposition is equally true. It also appears, as might be expected, that the reflecting and absorbing pow- ers of bodies are in the inverse ratio to each other. Rate of radiation through different bodies. In ordinary cases heat is radiated through the medium of the air; and no sensible radiation takes place when the whole apparatus is plunged into water—although the experiments on this point are not free from objection. It radiates through all gaseous bodies tried, and it does not appear that the rate of radiation is much influenced by the surrounding medium. The rate is the same at least, in air, and in hydrogen gas; and oxygen and nitrogen appear to have the same properties in this respect as air. Mr. Leslie has shown also that the rarification of the surrounding air di- minishes somewhat the radiating energy of surfaces ; but the radiation diminishes at different rates in different gases. He has given a table showing the diminution of the power of radiation in air and in hydro- gen gas of different degrees of rarity; but perhaps the experiments require to be repeated. It has been supposed that solid bodies are impermeable to radiant caloric, and an experiment is described in some chemical works as a conclusive proof of the difference between caloric and light. It consists in placing a lamp or burning coals in the focus of a mirror, and a pane of glass between it and the opposite mirror : the rays of light will pass through the glass, but the rays of caloric, it is said, will be completely intercepted. But this distinction can scarcely be maintained. It is true indeed that the thermometer is not so much 40 COMMUNICATION OF CALORIC. affected as it would be were no screen interposed, and the glass itself becomes warm. These farts prove that the greater part of the calori- fic rays are intercepted by the glass. But the thermometer is affected to a certain degree, and the question is, by what means do the rays reach it 1 Professor Leslie contends, that all the rays which fall upon the glass are absorbed by it, pass through its substance by its con- ducting power, and are then radiated from the other side of the glass towards the thermometer—an opinion, which Dr. Brewster has ably supported with an argument, suggested by his optical researches.— [Phil. Trans, for 1816, p. 106.] The experiments of Delaroche, on the contrary, [Biol. Traitede Physique]\ead to the conclusion that glass does transmit some calorific rays, the number of which, in relation to the quantity absorbed, is greater as the intensity of the heat increases. This general result has been confirmed by various other philosophers. Mr. Leslie has advanced the idea that radiant ealorie is not thrown off from hot bodies and darted through the air to distant bodies, but that the air itself is the medium of its transmission. According to this hypothesis, the stratum of air immediately in contact with the heated body acquires a portion of its high temperature, by which rt is expanded and made to press upon the next stratum of particles; this in like manner recedes, and thus an undulation or chain of aerial pulsations is produced. " The mass of air, without sensibly chang- ing its place, suffers only a slight fluctuation as it successively feels the partial swell; but the heat attached to this state of dilatation is actu- ally transported, and with the swiftness of sound."—Leslie's Inquiry, p. 140. But the experiments of Delaroche and others, offer insuperable objections to the adoption of Leslie's theory, and indeed go to prove that there is but little difference between the radiation of heat and light. Apparent radiation of Cold. When a cold body, as ice or a vessel containing a mixture of snow and salt, is placed in the focus of one of the mirrors, instead of the hot body, the bulb of the thermometer in the focus of the other mirror will indicate a diminution of tempera- ture below that of the surrounding air. This fact led some philo- sophers to advance the opinion that cold was a material substance, and subject, like heat, to the laws of radiation and reflection. But this, as well as the other phenomena of radiant caloric, will be ex- plained in the section on the distribution of temperature. It should be observed, that in all these experiments upon the radiation of calo- ric, the Differential Thermometer, described in Section 4, should be em- ployed. References. Leslie's Inquiry. Pictefs Essay. Delaroche on Ra- diant Heat, Ann. of Phil. ii. 100. Ritchie, in Edin. Phil. Jour. xi. 281. Prof. Powell in Repertory of Inventions, iv. 394.—Also Ann. of Phil. xxiv, xxv, and xxvi. Powell's Report on Radiant Heat to the British As- sociation, 1832. CONDUCTION. Another method in which heat is communicated, is by what has been aptly termed conduction, which differs considerably from radia- tion. The most striking point of difference, however, is the velocity with which caloric is transmitted. When caloric is radiated, its mo- COMMUNICATION OF CALORIC. 41 tion is rapid, nay almost instantaneous;—when it is conducted, its mo- tion is slow. These two modes of communication, however, are sel- dom perfectly independent of each other. When caloric is radiated through the air, a portion is also conducted by the air which comes in contact with the rays. And again, when caloric is conducted through a solid body, for example, radiation at the same lime lakes place. The difference in the velocity of transmission will explain why, in the for- mer case, only a small portion of heat is conducted, and in the latter case why so large a quantity is radiated. Caloric is conducted through solids, liquids and aeriform substances; in each of these bodies, however, there is some peculiarity in the man- ner of its conduction. Conducting powers of Solids. When a metallic bar, is exposed at one end to the heat of a furnace, the heat is gradually transmitted through its whole length, and after a short time the other extremity cannot be touched without danger of our being burnt. But this may be done with perfect safety with a rod of glass or of wood. Hence bodies are said to differ in their power of conducting caloric, and some are called good and others bad conduc- tors. Metals are the best conductors of caloric;—but even among these there is considerable difference. According to the experiments of In- genhouz, silver and gold are the -best conductors, next copper and tin, which are nearly equal, and lastly platinum, iron, and lead, which are much inferior to the others. Glass, pottery, clay, &c. have much less conducting power than any of the metals. This is the reason why glass is so liable to break when suddenly heated or cooled; one part of it is receiving or parting with its caloric before the rest expands or contracts, and hence the co- hesion is destroyed. It is on this account that the manufacturers re- sort to a process called annealing, which consists in putting the glass while hot, immediately after being formed into the required shape, into an oven strongly heated, where the glass remains till it becomes cold by slow degrees. Exp. Put a lamp under the centre of a sheet of copper, and at equal distances from the centre place a piece of silver, copper, iron and por- celain, of the same size and thickness, having on each a small bit of phosphorus. That on the silver will be first kindled, showing that it is soonest heated; in other words, that the caloric has passed most quick- ly through it. Next comes the copper, then the iron, and lastly the Eorcelain, the phosphorus on which will remain a long time, without eing kindled. Next to these bodies in point of conducting power, come the dried woods, which also differ materially from each other. According to Professor Mayer, the conducting power of these is inversely as their specific gravities. {See Ais table in Webster's Brande.) The difference between the conducting power of the metals and wood may be shown as follows : Exp. Take a smooth cylindrical tube, or still better, a solid piece of metal, about an inch and a half in diameter and eight inches long; wrap a clean piece of writing paper round the metal so as to be in close contact with its surface. When thus prepared it may be held for a considerable time in the flame of a spirit lamp without being in the 4* 42 COMMUNICATION OF CALORIC. least affected. Wrap a similar piece of paper around a cylindrn* cal piece of wood of the same diameter, and hold it in the flame ; it will speedily burn. When the paper is in close contact with the metal, the heat which is applied to it in one particular part car not accumu- late there, but enters into the metal, and is equally diffused- thrispgh its substance, so that the paper cannot be burned or scorched, until the metal becomes very hot : but when the paper is wrapped round wood, the heat that is applied in one particular part, not being able to enter ii to the wood with facility, accumulates in a short time in sufficient quantity to burn the paper.—L. U. K. Art. Heal. Charcoal is also a bad conductor : but feathers, silk, wool and hair are worse than any yet mentioned. Count Rumford has made experi- ments with a view of determining the conducting power of substances of this nature, from which it appears that those have the least conduct- ing power, in which the fibres are finest and most condensed, provi- ded the interstitial air is not expelled by the condensation.—Rumford's Essays. The substances which form the warmest articles of clothing are those which have the longest nap, fur or down, on account of the air which is involved, resisting the escape of the natural warmth of the body. The imperfect conducting power of snow arises from the same cause, and hence its utility in preventing the surface of the earth from being injuriously cooled in many parts of the world. While the temperature of the air in Siberia has been—70° F. it is affirmed that the surface of the earth has seldom been colder than 32° F. Despretz has given the following table of the comparative conduct- ing powers of the principle metals and some earthy bodies. The re- sults have been confirmed by Prof. Forbes, except as it regards plati- num, which he places between iron and tin.— Traite Elemenlaire de Physique. 1837. Gold, (greatest conducting power,) 1000.0 Platinum, - 981.0 973.0 898.2 374.3 363.0 303.9 Lead, ...... 179.6 23.6 Porcelain, - - - - - 12.2 Earth of bricks and furnaces, - 11.4 The conducting powers of bodies have been investigated by Fourier, by the aid of instruments called a Thermometer of Contact, and a Ther- moscope of Contact, from the firsi of which some curious facts have been ascertained .-.-See Henry's Chem. i. 108. Ann. de Chem. et de Phys. xxx vii. 291. Conducting powers of Liquids. Liquids may be said, in one sense of the word, to have the power of communicating caloric with great rapidity, and yet they are very im- perfect conductors. The transmission of caloric from particle to par- ticle does in reality take place very slowly ; but in consequence of the COMMUNICATION OF CALORIC. 43 mobility of their particles upon each other, there are peculiar inter- nal movements which under certain circumstances may be occasioned in them by increase of temperature, and which do more than compen- sate for the imperfect conductingpower with which they are really en- dowed. When certain particles of a liquid are heated, they expand, and thus become specifically lighter than tho>e which have not yet received an increase of temperature ; and consequently, according to a well-known liw in physics, the colder and denser particles descend, while the warmer ones are pressed upwards. It therefore follows that if caloric enters at the bottom of a vessel containing any liquid, a double set of currents must be immediately established, the one oi hot particles ris- ing towards the surface, and the other of colder panicles descending to the bottom. Now these currents take place with such rapidity, that if a thermometer be placed at the bottom, and another at the top of along jar, the fire being applied below, the upper one will begin to rise al- most as soon as the lower. Hence, under certain circumstances, ca- loric is rapidly communicated through liquids. But if, instead of heating the bottom of the jar, the caloric is made to enter by the upper surface, very different phenomena will be obser- ved. The intestine movements cannot now be formed, because the heated particles have a tendency to remain constantly at the top; the caloric can descend through the fluid only by transmission from parti- cle to particle, a process which takes place so very tardily, as to have induced Count Rumlord to deny that water can conduct at all, and to assert lhat liquids were heated exclusively by their transporting or carrying power.—(Rumford's Essays.) The incorrectness of this opin- ion, however, appears to be quite satisfactorily shown by the experi- ments of Dr. Hope, Dr. Thomson and Dr. Murray ; though they all admit that water, and liquids in general, mercury excepted, possess the power of conducting caloric in a very slight degree.* The transporting power of liquids can be very satisfactorily shown by putting into a vessel of water some small pieces of amber which are in specific gravity so nearly equal to water as to belittle influenced by gravitation. The lowermost part of the vessel being subjected to heat, when thus prepared, the pieces of amber rise vertically, and on reaching the surface, move towards the sides of the vessel, which are colder from the influence of the external air ; they then sink and rise again as be- fore. When the boiling point is nearly attained, the particles beingnearly of one temperature, the circulation is retarded. The portions on the surface are converted into steam before they can be succeeded by others; but the steam thus produced cannot rise far, before it is con- densed. Hence the vibration and singing observed at this time.— Hare's Minutes. Exp. The slow conducting power of water can be shown by cement- ing an air thermometer into a glass funnel, and covering the bulb of the instrument with water. If now a small quantity of ether be pour- ed upon the surface of the water, it may be fired without sensibly af- fecting the fluid in the stem of the thermometer. * Some ingenious experiments in confirmation of the theory of Count Rum- ford, by Lieut. W. M. Mather, are detailed in Silliman'a Jour. xii. 368. 44 DISTRIBUTION OF CALORIC. Conducting powers oj jEriform Bodies. It is extremely difficult to estimate the conducting power of aeriform fluids. Their particles move so freely on each other, that the moment a particle is dilated by caloric, it is pressed upwards with great ve- locity by the descent of colder and heavier particles, so that an ascend- ing and descending current is instantly established. Besides, these bodies allow a passage through them by radiation. Now the quantity of caloric which passes by these two channels is so much greater than that which is conducted from panicle to particle, that we possess no means of determining their proportion. It is certain, however, that the conducting power of gaseous fluids is exceedingly imperfect, pro- bably even more so than that of liquids.— Turner. References. Soquet on the power of Fluids as conductors of Heat, in Repert. of Arts, \st ser. xiii. 277. Dalton ontlie same subject, in Repert. of Arts, 2d ser. ii. 282. Dulong and Petit on the Laws of the commu- nication of Heat, in Ann. of Phil. xiii. 112. Murray's Experiments on the conducting power of Liquids, Syst. of Chem. i. SECTION III. DISTRIBUTION OF CALORIC. Different theories have been proposed to account for the tendency of bodies to acquire an equilibrium of temperature. I shall content my- self with an exposition of the theory of Professor Prevos{, of Geneva, which though not wholly free from objection, is now generally adopt- ed. It is altogether founded upon the phenomena of radiant calo- ric. There appears to be no point of temperature at which radiant calo- ric is not given out by bodies. Ice, which is so cold when in contact with the hand at ordinary temperatures, becomes hot if it be transport- ed to a chamber whose temperature is 20° below zero, and a mass of melting ice then presented to the thermometer will cause the fluid to ascend as well as the vessel of boiling water presented to it at the or- dinary temperature. Again, a mixture of snow and salt cooled down to 20° below zero, becomes a hot body if it be transported to an atmos- phere of 40° below zero. In all these cases, as in our own sensations, there is nothing absolute; all is relative. We are therefore forced to the conclusion that all bodies, at all temperatures, radiate caloric; but that the radiation is of unequal intensity, according to their nature, to their surfaces and to the temperature to which they are brought. The constancy of the temperature of a body depends then upon an equality in the quantity of radiant caloric which it emits and receives during the same time • and the eqilibrium of caloric which takes place among several bodies by mutual radiation, depends upon the perfect compensation of the in- stantaneous changes which are effected between all and each of them This is the ingenious theory suggested bv Prevost, and which com- bined with the peculiar properties of different surfaces explains all the phenomena of radiant caloric. A few experiments will more fully illustrate this interesting subiect Place a thermometer in a chamber which has an equal temperature DISTRIBUTION OF CALORIC. 45 in all its parts, and allow it to remain until it becomes of the same temperature. In the same chamber have an opake disk of any nature and form whatever, also at the same time temperature. If now this disk be presented at a greater or less distance lo the bulb of the ther- mometer, no effect will be produced upon it. The reason is obvious. Before we employed the disk, the bulb received at each instant from the walls and from the air of the chamber, a certain quantity of heat by radiation and reflection, and itself at the same time, sent back by this double mode an amount exactly equal, since its temperature remained the same. Now when we present the opaque disk to the bulb of the thermometer, we intercept for each point of the bulb all the calorific rays which are comprised in the cone formed by this point, and the whole surface of the disk. But, in exchange, the same point receives from the disk a certain number of rays comprised in the cone just mentioned ; and in consequence of the supposed equality of tempera- ture, this number is exactly equal to that which came from that por- tion of the wall upon which the disk is projected. Thus after the in- terposition of the disk, each point of the bulb receives still as much heat in the same time, as it received previously; and as the quantity which it gives out is not changed, it is evident that its temperature and that of the bulb will remain the same. A different result, however, will be observed, if there be presented to the thermometer a disk whose temperature is either higher or lower than that of the chamber ; for then the number of calorific rays radia- ted or reflected by this disk in a given time will be, in the former case greater, in the latter, less than that which came from the portion of the wall which it conceals. The same explanation will apply in the cases where two mirrors are employed. Place a thermometer, the bulb of which is blackened, in the focus of one mirror, and allow it to become of the same tempera- ture as the surrounding air, and in the focus of the other mirror, place any body which is of the same temperature. The thermometer will not be affected. The reason will be readily perceived. When the passage of the rays through this last focus was free, there arrived at this point from all parts of space a certain number of calorific rays, which, afier they have crossed, fall upon the second mirror, are re- flected from it to the first, and finally are concentrated upon the bulb of the thermometer. These rays are indeed intercepted by the opaque body which we have placed in the focus, but as this is supposed to be of the same temperature with the surroundingspace.it transmits by radiation and reflection a number of rays exactly equal,rwhich falling upon the second mirror are reflected to the first, and finally to the ther- mometer; and hence no change is effected. But this will not be the case when the body placed in the focus is either of a higher or lower temperature than that of the surrounding space and of the thermome- ter. For then the thermometer, after the interposition, will receive through the medium of the mirrors more or less than it received be- fore, and also more or less than it loses in the same time either by re- flection or by radiation. Whence it follows, that the temperature will be elevated in the former case, and reduced in the latter. These views are fully confirmed by experiment. For as has al- ready been shown, if we place a hot body in the focus of one mirror, the thermometer in the focus of the opposite one will indicate an increase of temperature ; and on the contrary, if we place a piece of ice or a mixture of snow and salt in the fociis of one mirror, the thermometer 46 DISTRIBUTION OF CALORIC. in the focus of the opposite mirror will indicate a diminution of tem- perature. All these phenomena can be explained if we only admit that all bodies, however low their temperature, radiate caloric, and in this there is nothing surprising, since our ideas of heat and cold are all relative. Without this admission we should have to adopt the opinion advanced by some philosophers, that cold as well as heat is radiated; an opinion which is not necessary, nor even warranted by the facts.— Biot. Precis Elementarie, ii. 642. The theory of radiation as thus unfolded, has been succesfully em- ployed by Dr. Wells, in explaining the phenomena of Dew. By his numerous and well directed experiments he has amply proved, that the formation of dew is owing to the radiation of caloric from the ground, and he conceives that all the previous explanations fail in not accounting for the production of cold in the dewed body. He has shown that the degree of cold on the surface of grass, &c. is in propor- tion to the quantity of dew which is formed; that dew appears in the greatest quantity upon those substances which radiate the most calo- ric ; that the radiation is the greatest, and the dew most copious in calm and serene nights; that the process is diminished or suspended by high winds and by the presence of clouds ; and that in those cases the temperature of the grass, &c. is the same as that of the air.— Wells' Essay on Dew, fyc. A similar explanation has been applied by Dr. Wells to various ap- pearances connected with dew, and among these not the least curi- ous is that of the formation of ice during the night in Bengal, when the temperature of the air is above 32° F. This has generally been ascribed to cold produced by evaporation ; an opinion which has been adopted by Davy, Leslie, Thomson and others. Dr. Wells, however, has shown conclusively, that it cannot be owing to this cause ;'but that it depends upon the radiation of heat to the heavens. For it is observed that ice is chiefly formed in Bengal during the clearest and calmest nights, when the greatest cold from radiation is observed on the surface of the earth ; and that clouds and winds pre- vent its formation by preventing, or at least diminishing, the radiation of heat.* The study of the laws of radiant caloric has lead to important im- provements' in the construction of fire places ; a subject which en- gaged the attention of Count Rumford.—See Rumford's Essays. References.—Prevosl's Researches sur la Chaleur. Walls' Essay on Dew, xxv. Concerning the cause of evaporation a difference of opinion exists among chemists. It was at one time supposed to be owing to a chem- ical attraction between the air and water. But Mr. Dalton has shown that caloric is the true and the only cause of the formation of vapour. As our limits do not permit us to enter into these discussions we shall refer the reader to— Turner's Chemistry ; Henry's Chem ; Dalton, in Manchester Memoirs, v.; Ure, in Phil. Trans. 1818; Biot's Traite de Phys. 1. ; Faraday on the existence of a limit to Vaporization, Phil. Trans. 1826. The presence of watery vapour in the atmosphere, is owing to eva- poration which takes place from the water on the surface of the earth. As this evaporation goes on to a certain extent, even at low tempera- tures, it is probable that vapour is always present in the air, though its quantity is subject to great variation, in consequence of the changes of temperature to which the air is constantly subjected. But even when the temperature is the same, the quantity of vapour is still found to vary ; for the air is not always in a «tate of saturation. At one time it is excessively dry ; at another it is fully saturated; and at other times it varies between these extremes. Instruments have been constructed to determine this variable condition of the atmosphere as to saturation, which are called Hygrometers. These consist for the mast part of some substance, such as human hair, or a fine slip of whale bone, which is elongated by a moist atmosphere, and shortened by a dry one. The ex- treme points are attained by placing it, first in air artifically dried, and then in air rendered as humid as possible. The degree of expan- sion or contraction is rendered more sensible by connecting it with an axis, which moves a circular index, like the finger of a clock. Mr. Leslie, by a slight modification of his differential thermometer, makes it serve the purpose of an hygrometer : for ifone of the balls be covered with silk, and then moistened with water, the rate of evaporation will be shown by the degree of cold produced, as indicated by the descent of the liquid in the opposite leg of the instrument. The drier the air, the quicker will be the evaporation, and the greater the effect in mov- ing the liquid within the instrument.—Henry, i. 280. But the most perfect instrument of this kind, is that invented by Mr. Daniel], for a particular description of which the reader is refer- red to Daniell's Meteorological Essays—or to the Library of Useful Knowledge, art. Pyrometer and Thermometer, where several modifica- tions of the Hygrometer are also described. For information on the subject of Hygrometers,- see alro Prof. Forbes' Report on Meteorology, made to the Brit. Ass. in 1832. I should also mention that Mr. A. 60 SPECIFIC CALORIC. Hayes has described a dew-point Hygrometer in—Silliman's Journal, xvii. 351. SECTION V. SPECIFIC CALORIC. Equal weights of the same body at the same temperature, contain the same quantity of caloric. But equal weights of different bodies at the same temperature, contain unequal quantities of caloric. Thus if we add a pint of water at 100° F. to a pint of the same liquid at 50°, the mixture will have a temperature of 75°, or the mean between the two; that is, the 25° which the hot water has lost, has been just sufficient to raise the cold by as many degrees. But if one pound of mercury at 185° F. is mixed with a pound of water at 40°, the mix- ture will have a temperature 45° only; or if the experiment be re- versed by having the water at 18&° and the murcury at 40°, the mix- ture will have a temperature of 180°. In the first case, 140 degrees lost by the mercury served to heat the water by five degress, and in the second, five degrees lost by the water sufficed to raise the temperature of the mercury by 140°. It hence appears that 28 times more caloric is required to raise the temperature of" water through one or more de- grees, than for heating an equal weight of mercury to the same ex- tent. By mixing water with various other substances it has been found that it requires different quantities of caloric to heat them equally. Thus on the addition of warm water to a jar containing a pound of wa- ter at 50°, the temperature will rise, say 10°; on adding a similar quan- tity of warm water to another jar containing a pound of spermaceti oil, it will rise 20°; and the same addition to a jar containing a pound of powdered glass will cause a rise of 50°. Here then it will be observed that eqaul quantities of caloric added to water, oil, and glass, have raised the temberalure of the first 10, of the second 20, and of the third 50 degrees. Now it is clear that if we wished to raise them all to the same temperature, say 50°, we must add twice and a half as much to the oil, and five times as much to the wa- ter. The quantities of caloric which they are capable of receiving, are therefore as glass 10, oil 20, water 50; or taking water as the stand- ard, and calling it 1000, they are water 1000, oil 500, glass 200. The term employed, to designate this remarkable difference, by Dr. Black, who first observed it, was capacity for caloric. But as this term is apt to lead to erroneous impressions concerning the cause of this dif- ference, that of specific caloric has been substituted for it, and is now generally employed. - When substances can be mixed together, as in the instances above mentioned, their specific caloric can be determined, by ascertaining the relative quantities of caloric which is requisite for heating them by an equal number of degrees. Water is commonly employed as one of the substances, and the specific caloric of other bodies is usually compared with that of water. In some cases, however, the bodies under examination cannot be in- timately mixed. When the specific heat of a solid mass of metal is to be examined, it may be heated throughout to a certain degree, and then surrounded by water, at 32°, observing the increase of temperature SOURCES OF CALORIC. 61 which is gained by the Water, and calculating the specific heat as be- fore. This was the method of Wilcke, of Stockholm.—[ Thomson's Chem. i. 100.] Lavoisier and La Place substituted ice for water, plac- ing, by means of an apparatus called the Calorimeter, the heated body in the centre of a quantity of ice, and determining the caloric evolved, by the quantity of ice melted in each instance.—Lavoisier's Ele- ments. It has been ascertained by Petit and Dulong, who have recently in- vestigated this subject, that the specific heats of bodies are greater at high than at low temperatures. They have also deduced from their researches the law, that the atoms of all simple bodies have precisely the same specific heat.—[Ann. of Phil. v. 13.] This, however, is to be considered at present, merely in the light of an ingenious speculation, derived from a train of reasoning, a defect in any part of which must be fatal to the conclusions. See Henry, i. 160; Dalton's New Syst.ii. 280 ; Also, Professor Bache's Strictures on the Table of Petit and Du- long, Jour, of the Acad, of Nat. Scien. Phil. Jan. 1829. The determination of the specific heat of gases has successively en- gaged the attention of some of the most profound and ingenious chem- ical philosophers. Among the most valuable observations on this subject, are those of Delaroche and Berard, De La Rive and F. Mar- cet. In the experiments of the two latter philosophers which are the most recent, they appear to have avoided sources of fallacy which were not provided against by those who preceded them in these investiga- tions. From a review of their experiments, they consider the follow- ing conclusions as legitimately deducible. 1. That under the same pressure, and with equal and constant vol- umes, (the elasticity alone varying,) all gases have the same specific heat. 2. That, all other circumstances remaining the same, the specific heat diminishes at the same time as the pressure, and equally for all gases, according to a progression but little convergent, and in a much less ratio than that of the pressures. 3. That each gas has a different power of conducting heat.—Hen- ry's Chem. i. 163. References. For tables of the Specific Heats of some Gases, see Li- brary of Useful Knowledge^ Art. Heat, or Thomson's Outline of Heat, xviii. Meikle onthe Specific Heat of Air. Edin. New Phil. Jour SECTION VI. SOURCES OF CALORIC. The sources of caloric may be reduced to six, viz. 1. The sun, 4. Mixture, 2. Friction, 5. Electricity, 3. Percussion, 6. Combustion. 62 SOURCES OF CALORIC. 1. The Sun. The heat produced by the sun is found to differ according to the surface exposed, and the colour of the surface. Franklin found that when pieces of cloth of various colours were exposed upon snow to the light of the sun, they sunk deeper, and consequently acquired heat, in proportion to the darkness of their colour. This experiment was repeated with more precision by Sir H. Davy, with similar re- sults. The temperature produced by the direct action of the sun's rays sel- dom exceeds 120°; a higher temperature, however, may be produced if we prevent the heat communicated from Jaeing carried off to sur- rounding bodies. But when the sun's rays are concentrated by means of a burning lens, intense heat is produced, provided they are directed upon some body capable of absorbing ana retaining them. Some lenses have been constructed of extraordinary power, and among them may be mentioned those of Tschirnhausen and M. de Trudaine, by which many of the most refractory substances were readily fused. [For a description of these and other powerful lenses, see ChaptaVs Chem. app. to the Arts,and Parkes' Chem. Essays.] According to Count Buf- fon, however, the only way by which the sun's rays can be made to produce an intense heat at a great distance, is by the combination of a considerable number of plain mirrors, so disposed as to throw nu- merous images of the sun upon the same spot. By an instrument con- structed upon this principle he was enabled to melt the metals and metallic minerals at the distance of forty feet, and to kindle wood when at the distance of 210 feet.—Parkes' Chem. Essays. 2. Friction. Fires are often kindled by rubbing pieces of dry wood smartly against one another. So also when parts of heavy machine- ry rub against one another, the heat excited, if the parts in contact are not well greased, is sufficient for kindling wood. The axle-tree of carriages has been burned from this cause, and the sides of ships are said to have taken fire by the rapid descent of the cable.—[Parry's 2d Voyage, N. Y. Ed. 212.] Count Rumford observed, that in the bor- ing of cannon, by the friction of the borer, a very large quantity of ca- loric is rendered sensible. To ascertain its quantity, he fixed a solid cylinder of brass in a trough filled with water, and having adapted the borer to it, connected with the machinery by which it is turned, it was made to revolve in the usual manner, at the rate of 32 times in a minute. Heat was soon excited, and of course raised the temperature of the metal, and of the surroundijg water. In an hour the tempera- ture had risen from 60 to 107° ; aUd in two hours and a half the water was brought to boil, the quantity of this water being 18 lbs. ; the appa- ratus itself, which was of course raised to the same temperature, weigh- ed 15 lbs. From this experiment it may be safely concluded, that the access of atmospheric air is not essential to the evolution of caloric ; an infer- ence confirmed also by the experiment of Pictet—who constructed an apparatus by which friction could be excited in an exhausted receiver. The thermometer rose higher than when this.fricton was going on in open air.—Rumford's Essays. Pictet's Essay on Fire. It was hence concluded by these philosophers that caloric was not a material substance, but a kind of motion. The same opinion was adopted by Sir H. Davy. But although these facts present a difficulty in the adoption of the hypothesis that caloric is material, the other SOURCES OF CALORIC. 63 phenomena can be more satisfactory explained upon the latter supposi- tion, and hence it is quite generally adopted by the chemists of the day. —See references under the first Section of this Chapter. 3. Percussion. The heat excited by percussion is equal, and in many cases superior, to that evolved by friction. When a piece of iron is smartly and quickly struck with a hammer, it becomes red hot; and another familiar illustration is the production of heat by the compres- sion of air in the common fire-syringe. No heat, however, has been observed to follow the percussion of liquids, nor of soft bodies which easily yield to the stroke. • 4. Mixture. We have already given some examp.es of the effect of mixture in producing heat; as in the case of the mixture of sulphuric acid and water, and the mixture of sulphuric acid with the chlorate of potash and sugar. [See page 24.] So also some gaseous bodies, which, when united together, form solids, asammoniacal and muriatic acid gases, evolve a considerable degree of heat. Sometimes how- ever, the temperature of the mixture is reduced, and the sensation of cold produced ; as is remarkably the case in some of the freezing mix- tures. It may be laid down as a rule, to which there are few exceptions, that when the compound formed by the union of two bodies is more fluid or dense than the mean fluidity or density of the two bodies be- fore mixture, then the temperature sinks ; but when the fluidity or the density of the new compound is less than that of the two bodies before mixture, the temperature rises; and the rise is pretty nearly propor- tional to the difference.— Thomson's Chem. i. 143. 5. Electricity. This will be particulary noticed in Chapter IV. 6. Combustion. This may be defined to be the disengagement of heat and light which accompanies chemical action. Modern discoveries have shown the insufficiency of former theories upon this subject. But in the present state of our knowledge, we are unable to substitute one that is wholly free from objection ; and we are left to the naked statement of the fact, that combustion is the general result of the actions of any substances which pos°css strong chemical attractions, or different electrical relations.— Ure's Chem. Dictionary. I shall notice the leading phenomena of combustion under the follow- ing heads, viz. 1. The temperature necessary to \mflame different bodies. 2. The nature of flame. 3. The heat given out by different combustibles in burning. 4. The causes which modify, promote or extinguish combustion. 1. The temperature necessary to inflame bodies, The temperature ne- cessary for inflammation, is very different in different bodies. Thus, if we heat phosphorus to 150° F. it takes fire ; but sulphur requires a heat of 500° for its inflammation. The successive combustibilities of bodies can be shown as follows : Exp. Into a long bottle with a narrow neck introduce a lighted taper, and let it burn till it is extinguished. Carefully stop the bottle and introduce another lighted taper. It will be extinguished before it 64 SOURCES OF CALORIC. reaches the bottom of the neck. Then introduce lighted sulphur. This will burn for some time ; and after its extinction, phosphorus will be as luminous as in the air, and if it be heated will burn with a pale yellow flame. The combustibilities of various gaseous bodies are to a certain extent as the masses of heated matter required to inflame them. Thus, an iron wire l-40th of an inch in diameter, heated to a cherry red, will not in- flame olefiant gas, but it will inflame hydrogen. A wire of l-8th of an inch, heated to the same degree, will inflame olefiant gas. But a wire l-500th of an inch, must be heated to whiteness to inflame hydrogen, though at a low red heat it will inflame bi-phosphuretted gas. 2. Nature of flame.—Flame is the rapid combustion of volatilized matter, or in other words, volatile combustible matter heated so highly as to become luminous. Davy has asserted that the flame of combus- tible bodies must be considered as the combustion of an explosive mix- ture of inflammable gas or vapour and air, and that this combustion takes place in the interior, as well as at the surface of contact. A simple experiment, however, proves that no combustion goes on in the interior of a flame. Exp. Place a piece of coin or metal upon an earthen plate; place a small piece of phosphorus upon the coin, and surround the latter with alcohol. The alcohol may be fired without setting fire to the phos- phorus, which remains unaffected ir- the interior of the flame, but as soon as the external air comesfcin contact with it, combustion instantly takes place. The interior of a flame consists of aqueous vapour, which may he exhibited by an ingenious apparatus contrived by Mr. Blackadder.— Edin. New Phil. Jour. i. 224. The light of a flame may be shown by the prism to consist of sev- eral colours. The flame of a candle consists of four portions, al- though these may be considerably modified by various circumstances. A blue portion which extends from the base to about the middle of the flame ;—an attenuated opaline brush over the whole exterior surface of the blue part of th§ flame ; a cone of yellowish white light, com- mencing on the inner surface, and at a short distance from the base of the blue portion ; and an interior cone of white light, the base of which is above the upper part of the blue portion.—Edin. New Phil. Jour. i. 228. Flame has electric polarity ; that of burning phosphorus being acid, is bent towards the positive pole, and that of a candle containing ig- nited carbon, toward the negative.—Brande, Phil. Trans. 18H,notiud in Ann. of Phil. iv. 441. The products of flame are usually water and carbon. The use of a wick is to raise the fluid by capillary attraction. It is not, however, an essential part of the oil or alcoholic lamp. Lamps without wicks have been constructed by Mr. Blackadder, and are supposed by him to possess many advantages,—Edin. New Phil. Jour. i. 52. The colour of flame depends upon the presence of various foreign substances, and an attention to it is of great utility in many analytical researches. Thus the flame of alcohol is tinged with a fine carmine red by nitrate of strontia, yellow by nitrate of baryta, greeu by nitrate of copper, &c. This is by some supposed to be owing to the reduction of the substances employed, to the metallic state. SOURCES OF CALORIC. 65 Flame is supposed to possess a very high temperature. By Sir H. Davy it was estimated at 7000° F. But there can be no doubt that the temperature varies with the combustible, and that it is influenced also by other circumstances ; and it perhaps never reaches the point first mentioned. One of the arguments advanced by Davy, viz. that a fine platina wire becomes white hot in a part of the flame of a spirit lamp where there is no visible light, has been weakened by the fact since discovered, that the mere contact of a jet of hydrogen with spongy platina causes the incandescence of the latter. The other argument of Davy appears also open to objection. 3. Heat given out by different combustibles in burning. On this sub- ject experiments have been made by Lavoisier, Crawford, Dalton, Rumford, and Davy. But the results of these experiments are so discordant that they can scarcely afford any correct guide. They ap- pear to agree, however, that of the numerous substances tried, hydro- gen gives out the most heat and carbonic oxide the least.—Ure's Chem. Dictionary. With respect to the heat given out by ordinary combustibles used as fuel, the experiments of Mr. Bull, of Philadelphia, are the most satis- factory. The following results are extracted from his table, in the Transactions of the American Philosophical Society, N. S. iii. 1. Amount of heat given out by various combustibles—a cord of shell- bark hickory being equal to 100. Shell-bark hickory White oak Hard maple White beech White pine Lehigh coal (a ton 2240 lbs.) Lackawaxen coal Schuylkill coal 100 81 60 65 42 99 99 103 4. Causes which modify, promote, or extinguish combustion. Although the progress of discovery has shown that the generalization proposed by Lavoisier, viz. that in all cases of combustion, oxygen combines with the burning body, is not of universal application ; it must be con- fessed that in most cases the presence of oxygen is essential to the pro- cess of combustion. In ordinary cases, the more complete the access of atmospheric air and the more perfect its contact with the combusti- ble, the more perfeet will be the combustion. It is upon this principle that most of the modern improvements in the construction of lamps and furnaces depend. Upon the same principle also the heat may be greatly increased, by causing a blast through a flame. For this purpose, a blow-pipe, to be blown either by the mouth or a pair of bellows, is used. The form of this instrument is not material; and several modifications of it have been proposed. The principle upon which it acts, is that a constant supply of air is brought to the inflammable matter, thus ren- dering the combustion more complete, and the consequent heat great- er. There are other forms of the blow-pipe, as that in which alcohol is employed, and the oxy-hydrogen blow-pipe, which will be noticed here- after. Flame being gaseous matter so highly heated as to become lumin- 6* 66 SOURCES OF CALORIC. ous, will be extinguished by a reduction of its temperature. This can be effected by bringing near it some metallic conductor. Let the smallest possible flame be made by a single thread of cotton immersed in oil, it will be found to yield a flame of about l-30thof an inch in diameter. Let a fine iron wireof l-180th of aninch, made intoa ring l-10th of an inch in diameter, be brought over the flame. Though at such a distance, it will instantly extinguish the flame, if it be cold ; but if it be held above the flame, so as to be slightly heated, the flame may be passed through without its being extinguished. That the ef- fect depends entirely on the power of the metal to abstract the heat of the flame, is shown by bringing a glass capillary ring of the same di- ameter and size over the flame. This being a much worse conductor of heat, will not, even when cold, extinguish it. If its size, however, be made greater, and its circumference smaller, it will act like the metallic wire, and requires to be heated to prevent it from extinguish- ing the flame.—Davy on Flame. Ure's Chem. Dictionary. It is upon this principle that Sir H. Davy constructed his Safety Lamp, one of the most valuable discoveries of the age. In this in- strument there is a succession of metallic orifices forming the wire gauze, which constitutes the cage of the lamp, and thus by cooling down the flame prevents it from communicating with the explosive mixture. Thesame principle has also been applied by the Chevalier Aldini, to the construction or a robe for the preservation of firemen against fire and flame. [See Silliman's Jour, xviii. 177, xx. 96.] M. Libri of Flo- rence ascribes the protection which Davy's lamp affords, to the repul- sion exerted by the metallic wire upon the flame.— Brewster's Edin. Jour. ix. 311. References. Graham on the Heat of Friction, Ann. of Phil, xxviii. 260. The article Combustion, in Ure's Chem. Dictionary, containing an account of the various theories on this subject; Also, Thomson's Chemistry. Davy on Flame. Rumford's Experiments on the combustion of Woods, See his Essays, and Thomson's Chemistry. Berzelius on the Blow-pipe. Linton on the colour of Flames, Emporium of Arts, v. 457. Porrett's observations on the flame of a Candle, Ann. of Phil. ix. 337. Sym on Flame, Ann. of Phil. viii. 321. Murray on the same subject, Ann. of Phil, x vi. 424. On 4he construction of Furnaces, and the management of Fuel, See Parkes' Essays; and also, Gray's Operative Chemist. General References on Heat. Count Rumford's Essays. Dalton's New System of Chemical Philosophy. Dr. Black's Lectures, "by Robinson. Berthollet's Chemical Statics. Scheele's Treatise on Air and Fire. Leslie's Experimental Inquiry into the Nature of Heat. Pictet's Essay on Fire. Biot's Precis Elementaire. Murray's System of Chemistry. Thomson's Outline of the Sciences of Heat and Electricity. Library of Useful Knowledge, article Heat. Lavoisier's Elements of Chemistry. Aikin's Dictionary of Chemistry and Mineralogy, article Caloric. Ar- nott's Physics, ii. Lardner on Heat. Depretz, Traite Elementaire de Physique. 1837. LIGHT. 67 CHAPTER III. LIGHT. When the sun rises above the horizon a mode of communication is established, which in spite of his great distance, acquaints us with his existence. This mode of communication is called Light. And bodies which can thus manifest their existence are said to be luminous, (of themselves,) as the sun, the stars, &c. Most bodies, however, as we shall hereafter see, become luminous when their temperature is sufficiently elevated, and they lose this property when they become cool. But even when they become opaque, if enlightened by a luminous body, they acquire the property of transmitting light in the same way as if they were luminous; and these bodies are said to be visible by re- flection. NATURE OF LIGHT. Concerning the nature of light, philosophers are divided into two classes. The one class consider it to consist of particles of matter ac- tually emanating from the luminous body ; the other conceive it to be transmitted by means of pulsations or vibrations excited in an elastic fluid, in the same way that sound is conveyed through the air. 1 he former opinion was adopted and maintained by Newton, and is now generally received, as best accounting for the phenomena which it ex- The consideration of the laws of light, so far as they relate to the phenomena of its movement, and its effects in producing vision, con- stitutes the science of Optics; and are the objects, therefore not of Chemistry, but of Natural Philosophy. I shall, however, briefly no- tice the physical properties of light, as they bear upon important ques- tions of chemical enquiry. „,„„™« -i The light of the sun moves with the velocity of 192,000 miles in a second of time, so that it passes through the whole distance from the sun to the earth in about eight minutes. Li°fit is transmitted through the air in straight 'lines, which ,are called rays of light, and it is by means of these that vision is effect- ed When a ray of light falls upon a polished surface, it is thrown off or reflected. And the angles of incidence and reflection are in this case always equal, whatever may be the obliquity of the incident ray. A ray of light passing obliquely from one medium to another, does not proceed in the same direction as before, but is refracted, or bent out of its course. If the new medium be denser than the old, the ray * Dr Brewster thus briefly contrasts the two systems. " In the Newtonian theorv light is supposed to consist of material particles emitted by luminous bodies'; and moving through space with a velocity of 192,000 miles in a second. In the undulatory theory, an exceedingly thin and elastic medium, called ether, is supposed to fill all space and to occupy the intervals ^between the particles of all material bodies."— Optics, 134. 68 LIGHT. of light is bent or reflected nearer to the perpendicular ; but in pass- ing out of a denser into a rarer medium, it is refracted from the per- pendicular, and there is a constant proportion between the sine of the angle of incidence and that of refraction. Transparent media, also, not only cause a change ,in the direction of a ray, but ^decompose it into its constituent parts, an effect which has been called disper- sion. When a ray of light, in passing through certain bodies, (Iceland spar for example,) exhibits a double image of any object viewed through them, it is called double refraction. In this case the light is divided into two pencils, the one following the law of ordinary refraction; the other being differently affected, constituting extraordinary refrac- tion. DECOMPOSITION OF LIGHT. Light is not a simple body, but is capable of being divided by the prism into seven primary rays or colours, viz. red, orange, yellow, green, blue, indigo, and violet. These rays differ in their refrangi- bility, the red being the least, and the violet the most refrangible. These facts can be shown by admitting a ray of light SD into a dark room and interposing the prism ABC, so that the ray passes oblique- ly through two surfaces and is refracted by both. On receiving the refracted ray upon a piece of white paper MN there appears instead of a spot of white light, an oblong coloured surface, composed of the seven different tints just enumerated, called the prismatic or solar spec- trum. According to Dr. Wollaston, the sceptrum consists of four co- lours only, viz. red, green, blue, and violet, and these occupy spaces in the proportion of 16,23,36,25. These different coloured rays being collected by a lens into a focus, again produce colourless light. Again, Dr. Brewster gives a new analysis of solar light indicating three pri- mary colours, forming coincident spectra of equal length.—Edin. Jour. of Science, N. S. v. 197. Sir W. Herschel found that the prismatic colours differ in their illu- minating powers. The orange possesses this property in a higher de- gree than the red; and the yellow rays illuminate bodies still more perfectly. The maximum of illumination lies in the brightest yellow or palest green. The green itself is nearly equally bright with the yel- low ; but from the full deep green, the illuminating power decreases LIGHT. 69 very sensibly. That of the blue is nearly equal to that of the red; the indigo has much less than the blue ; and the violet is very deficient.— Phil. Trans. 1800. The heating powers of the rays follow a different order. If the bulb of a very sensible thermometer be moved in succession through the differently coloured rays, it will be found to indicate the greatest heat in the red rays; next in the green, and so on, in a diminishing progression to the violet. But when it is removed entirely out of the confines of the red rays, but with its ball still in the line of the spec- trum, it rises even higher than in the red rays, and continues to rise, till removed half an inch beyond the extremity of the red rays. It was hence inferred by Herschel, that there exists in the solar beam a distinct kind of ray, which causes heat, but not light ; and that these rays being less refrangible than the luminous ones, deviate in a less degree from their original direction in passing through the prism. Though the truth of the statement, that the prismatic colours pos- sess different heating powers, has been confirmed by all succeeding ex- periments, there has been much difference with regard to the spot at which the heat is a maximum. The opinion of Herschel has however been fully confirmed by the recent observations of Mr. Seebeck.— Edin. Jour, of Science, i. 358. When the solar rays traverse a biconvex lens, they are collected to- gether in a focus, but the focus of the calorific rays is a short distance behind that of the luminous, showing that they are differently refract- ed.—Berzelius. Beyond the confines of the spectrum on the other side, viz. a little beyond the violet ray, the thermometer is not affected ; but in this place it is remarkable that there are also invisible rays of a different kind which produce all the chemical effeets of the rays of light, and with even greater energy. It is well known that if chloride of silver is exposed to the direct light of the sun, it is speedily changed from white to black. The rays separated by the prism possess this power of blackening chloride of silver in various degrees. The blue rays for example, effect a change in 15 seconds, which the red require 20 minutes to accomplish ; and, generally speaking, the power diminish- es as we recede from the violet extremity. But entirely out of the spectrum the effect is still produced. Hence it is inferred that there are certain rays which excite neither heat nor light; and which, from their peculiar agency, have been called chemical or de-oxidizing rays. It appears, therefore, that a ray of light contains three distinct sets of rays, viz. the illuminating, the heating, and the chemical or de-oxi- dizing. It may also be added, that the more refrangible rays of light, seem to possess the property of rendering steel or iron magnetic : a property discovered by Dr. Morichini of Rome, and confirmed by the experiments of Mrs. Somerville, who succeeded in magnetizing a sew- ing needle by less than two hours exposure to the violet ray. {Phil. Trans. 1826.) This effect.however, has been much questioned,and is wholly rejected by Priess and Moser.—Brewster's Edin. Jour. N. S, ii. 225. PHOSPHORESCENCE. There are many bodies in nature which possess the property of giv- ing out light without any sensible emission of heat; and these are 70 LIGHT. commonly known by the name oiphosphori. The leading divisions of these substances are: 1. Solar phosphori ; or those which require a previous exposure to solaror other light to become luminous. Such are Canton's, Baldwin's and the Bolognian phosphori, which will be described hereafter. To the same class belong several natural bodies which retain light and give it out unchanged. Thus, snow is a natural solar phosphorus. 2. Phosphori from heat; or those which become luminous, by heat alone. Thus powdered fluate of lime becomes luminous, when thrown on an iron plate raised to a temperature rather above that of boiling water, and one of its varieties known to mineralogists by the name of Chlorophane, gives out abundantly an emerald green light, by the mere heat of the hand. The yolk of an egg, when dried, becomes luminous on being heated ; and so also do spermaceti, wax and tallow, during liquefaction. To exhibit the last mentioned fact, it is only necessary to place a lump of tallow on a coal, heated below ignition, making the experiment in a dark room.—Brewster, on the phosphorescence of certain fluids, Edin. Jour, of Science, iv. 178. 3. Spontaneous phosphori ; or those animal and vegetable substances which emit light spontaneously at common temperatures, without the necessity of previous exposure to light. This property is possessed in a remarkable degree by fish and some other marine animals; and in these it makes its appearance before the commencement of putrefac- tion, and ceases when the latter is completely established. The lumi- nosity of sea water is ascribed to the presence of animalculse, which are naturally phosphorescent. Of vegetable matters which become lu- minous, the most remarkable is decayed wood. The chemical effects of light are very evident in the case of a mix- ture of chlorine and hydrogen, which explodes and produces muriatic acid. Again, chlorine and carbonic oxide have scarcely any tendency to unite, even at high temperatures, when light is excluded; but ex- osed to the solar rays, they enter into chemical union. Chlorine also as but little action on water unless exposed to light. To the same class of the chemical effects of light, may be referred the decomposition of nitric acid, and the decomposition or change of colour of the salts of gold and silver. The green colour of vegetables is also owing to the influence of solar light. [For additional facts on the subject of the chemical influence of light, see Phil. Mag. and Ann. vii. 462.] Photometers or measurers of Light. For measuring the intensities of light from various sources, an instrument has been constructed which is called the Photometer. That of Mr. Leslie is founded on theprinci- ple, that light in proportion to its absorption, produces heat. The de- gree of heat produced, and consequently of light absorbed, is measured by the expansion of a confined portion of air. It is merely a very deli- cate and small differential thermometer, enclosed in a thin and pellucid glass tube. One of the balls, however, is rendered opake, either by tinging the glass or covering it with a pigment; and hence this ball, when the instrument is suddenly exposed to light, becoming warmer than the clear bulb, indicates the effect by the depression of the fluid. —Leslie's Inquiry. Some objections to this instrument have been stated, for which the reader is referred to the Rev, Mr. Powell's Historical Sketch of Photo- I LIGHT. 71 metry, in the Ann. of Phil, xxvii. 371, where will also be found a no- tice of Rumford's Photometer, arid of that proposed by Dr. Brewster j —and to Mr Ritchie's paper on Leslie's Photometer, in the Edin. Jour, of Science, ii. 321. 339. and iii. 104. Brande's modification of Leslie's Photometer, is described in his Journal, viii. 220.* SOURCES OF LIGHT. The sources of light are, the Sun and Stars, Chemical action, Heat and Percussion. The first of these we have already noticed. Chemical action. Whenever combustion forms a part of the pheno- mena, light, as is well known, is given out; but light is also evolved in other instances, where nothing like combustion takes place. Thus, freshly prepared magnesia, added suddenly to highly concentrated sul- fihuric acid, exhibits a red light. When the vapour arising from a so- id, benzoic acid for instance, is condensed, light is evolved, and fused boracic acid, when passing to a solid form, exhibits the same phenome- non. So also some substances during "the process of crystallization give out scintillations of light, as sulphate of potassa and fluoride of sodium. Henry Rose states that if two or three drachms of the vitreous arsenious acid be put into a matrass along with an ounce and a half of muriatic acid and half an ounce of water, and the whole allowed to boil for ten or fifteen minutes, and then cooled as slowly as possible ; the crystallization is accompanied by a strong emission gof light, the formation of each little crystal being attended by a spark. If the ves- sel is then agitated, a great number of crystals suddenly shoot up, and an equal number of sparks occur at the same time. If a considerable quantity of arsenious acid, such as an ounce and a half, or more, is treated with a corresponding quantity of diluted muriatic acid, then, on shaking the vessel, if the right moment is seized, the emission of light from the shooting "of the crystals is so powerful that a dark room may be lighted up by it.— Lond. d> Edin. Phil. Mag. vii. 534. But, perhaps the most intense artificial light is that which is obtain- ed by directing the flame of the oxy-hydrogen blow-pipe upon a small spherule of lime, about a quarter of an inch in diameter. The lime from chalk is best adapted to this purpose, as it is most easy to give it the proper form. The light thus produced is so brilliant as scarcely to be borne by the naked eye, and its use is therefore suggested for illu- minating light houses. It acts also as solar light does on mixtures of chlorine and hydrogen, and on chloride of silver.—Drummond, Phil. Trans. 1826. Also Edin. New Phil. Jour. i. 182. Percussion. Light is also evolved by percussion and friction. Thus, two pieces of common bonnet cane, rubbed strongly against each other in the dark, emit a faint light. Two pieces of borax, quartz or sugar, have the same property in a more eminent degree. Some of the gases also, when highly compressed, give out light. * The indications of these photometers cannot be depended on when there is much difference in the colour of the lights. In such cases the best method of obtaining approximative results, is by observing the distance between each light at which any given object, as a printed page, ceases to be distinctly visi- ble. The illuminating power of the lights so compared is as the squares of their distances. 72 ELECTRICITY. Heat. When heat is applied to bodies, and continually increased, there is a certain temperature at which they become luminous; and the body is then said to be red hot or incandescent. The temperature at which solids in general begin to shine in jthe dark, is between 600° and 700° F.; but they do not appear luminous in broad daylight, till they are heated to about 1000° F. The colour of incandescent bodies varies with the intensity of heat. The first degree of luminousness is an obscure red. As the heat augments, the redness becomes more and more vivid, till at last it acquires a full red glow. Should the temperature still continue to increase, the character of the glow changes, and by degrees becomes white, shining with increasing brilliancy as the intensity of the heat augments. Liquids and gases likewise become incandescent when strongly heated; but a very high temperature is required to render a gas luminous, more than is suffi- cient for heating a solid body even to whiteness. The different kindsof flame, as of the fire, candles and gas light, are instances of incandes- cent gaseous matter. References. Biot's Traite Precis, ii. Brewster's Optics. Library of Useful Knowledge, Art. Double Refraction, and Polarization of Light. Thomson's Chemistry, i. Aikin's Chem. Diet Art. Phosphori. Lieut. In- galls on the Luminousness of the Ocean, Trans. Alb. Institute, i. Osann on some New Bodies, vmich ahsorb light strongly, Edin. New Phil. Jour. vi. 158. Mr. Powell on Solar Light and Heat, Ann. of Phil, xxiii. and xxiv. Also, remarks by the same author on Light and Heat from Ter- restrial Sources, Ann. of Phil. xxiv. 181, xxv. 201, 359, 401, and the same author's Report on Radiant Heat in the Transactions of the Brit- ish Association for 1832. Herschel s Treatise on Light. CHAPTER IV. ELECTRICITY. The term .Electricity, applied to the unknown cause of a peculiar kind of attraction, is derived from the Greek word electron, amber, be- cause the electric property was first noticed in this substance. In my remarks upon this subject, I shall briefly notice, 1st. the ge- neral or elementary facts of the science: 2d. the theory proposed to account for these facts: and 3d. the effects of accumulated electri- city. GENERAL FACTS OF ELECTRICITY. The general facts of electricity may be conveniently reduced to the following, viz: Excitation—Attraction—Repulsion—Distribution— Transferance and Induction. — Library of Useful Knowledge Art. Electricity. All these facts can be shown with the simple apparatus of a clean and dry glass tube, a dry silk handkerchief, and a few pith balls sus- pended by silken threads. But they are much more strikingly exhi- ELECTRICITY. 73 bited if we employ an electrical machine, an instrument which con- sists essentially of a circular plate of glass, or a glass cylinder, fixed upon an axle, and pressed by a cushion or rubber, which is generally besmeared with a soft compound, or an amalgam of mercury and tin with grease. At a short distance, is placed a metallic cylinder, sup- ported by glass feet, called the prime conductor, which at the end next the glass, has commonly a few projected teeth made of pointed wire. . If now the rubber is connected by a small chain or otherwise, with the table or floor, and the glass cylinder or plate made to revolve upon its axle, a crackling noise is heard, accompanied with a luminous ap- pearance, and on bringing the knuckle or a rounded metallic knob, near the prime conductor, a spark issues, accompanied with a snap or slight report, and with a pricking sensation when the knuckle is pre- sented. The glass is in this case said to be excited, and the substances thus susceptible of excitation are termed Electrics, in contradistinction to such as are not excited by a similar process, and which are termed Non-electrics. This distinction, however, is not founded in nature, for electricity may be excited in all solid bodies by friction, though for this purpose the friction is to be applied in different ways. Attraction and Repulsion. If, while the prime conductor is charged With electricity, a light body, as a pith ball, suspended by a silken thread, be brought near it, it is attracted by the conductor and adheres to it for a certain time ; after which it recedes, or is repelled from it. This change, however, does not take place in all bodies with equal ra- pidity ; some require a considerable time before they begin to recede; others, and especially metallic bodies, are repelled the instant after contact. The phenomena of attraction and repulsion can also be stri- kingly exhibited by several amusing experiments ; as the electrical bells, the electrical dance, &c. It appears therefore that when bodies are electrified by contact with excited glass,they are repelled by it. The same thing also takes place when they are electrified by excited resins. But when a body electri- fied by glass iiArought near a body electrified by the resins, instead of repelling each other, they are now attracted. From these facts the fol- lowing general laws have been deduced, viz : Bodies similarly electri- fied, repel each other ; bodies differently electrified, attract each other. It should be remarked also, that the production or excitation of one kind of electricity is always attended by the excitation of the other. Thus when glass is rubbed by silk or cloth, they assume opposite elec- trical states. This can be shown most strikinglyJsy insulating the rubber of an ordinary machine. Distribution and Transference. If a.pith ball or metallic globe, sus- Eended by silk, after having been in contact with a charged conductor, e removed, and the ball be brought near to another ball, suspended in a similar manner, they will approach each other and then again recede. This second ball, when brought near a third, will exhibit the same phenomena, although in a less striking manner, as the amount of elec- tricity becomes gradually diminished by these successive operations. It is evident, therefore, that the electricity communicated to the first ball,may be communicated to the second, and that from the second to the third, &c. in the same manner that heat is transferred from one body to another. If, however, we bring in contact with a ball thus electrified, the hand 74 ELECTRICITY. or a metallic body, we observe, in some instances, a spark, and it loses the property of attracting another unelectrified ball, in consequence of the electricity having passed into the body. But if a tube of glass be substituted for the hand or the metallic rod, the body touched retains the whole of its electricity. We therefore infer that some bodies, as glass, are incapable of conducting electricity, while others, such as the metals and the human body, readily convey that influence. Hence the division of bodies into conductors and non-conductors;—the latter being those which are electrics, and the former, non-electrics. The two qualites, of a capability of excitation and a power of conducting elec- tricity, appear to be incompatible with each other ; for the one is al- ways found to diminish in proportion as the other increases. The permanence of electricity in metallic bodies, which are suspended in the air by silk threads, shows that the air, as well as silk, is a non-con- ductor. Bodies which, in this way, are surrounded on all sides by non- conductors, are said to be insulated. The air, however, when it con- tains much moisture, becomes a conductor ;—and the conducting pow- ers of most bodies are influenced by changes of form and temperature, The metals'are the most perfect conductors; the resins, amber and gumlac, the most perfect non-conductors. The phenomena of the transference of electricity are modified by the form of bodies. Hence the different effects which are observed when conductors in the form of balls, or pointed ones, are charged with the fluid. In the former case the electricity is equally diffused over the surface of the ball, and it has a low intensity; in the latter, the intensity is greatly increased at the extremities, and this is accompa- nied with a powerful tendency in the fluid to escape.* Induction. It has been shown that when an excited electric is brought near to an insulated ball, the latter is attracted. This arises from the fact that the natural state of the body has been disturbed, and an electricity of an opposite kind has, as it is said, been induced, in that part of the ball nearest to the electric ; whereas that part of the ball which is most distant has the same electrical state as the electric. Thus when' an unexcited conductor AB, supported on an insulated * As the terms quantity and intensity or tension are often employed in treating of electricity and galvanism, it may be proper to explain them more particularly. The former implies the actual quantity of electric power in any body ; whereas1 intensity or tension, signifies the state of electricity indicated by the electrometer, and its power of flying off from surfaces and passing through a certain stratum of air or other ill conducting medium. Tension appears to depend upon the quan- tity of electricity accumulated on a given space; and hence the intensity of those substances is the greatest, which have the greatest excess or deficiency of electricity in proportion to their surface. Suppose a charged Leyden jar to give a spark when discharged, of one inch in length, another uncharged jar, com- municating with the former, would upon the same quantity of electricity being thrown in, reduce the length of the spark to half an inch : thus the quantity of electricity remaining the same, its intensity is diminished by one-half, by its dis- tribution over a larger surface. This accounts for the freedom with which electricity is given off by pointed conductors. For through the quantity of electricity accumulated on a sharp point, may be very small, it is still large when compared with the surface. The electric tension of the point is therefore very great, and hence, if positive, it gives off electricity to surrounding bodies, and if negative, receives it from them, with extreme velocity. ELECTRICITY. 75 c^Hft glass rod is brought near a con- C A_______B Sudor C charged with positive elec- tricity and similarly insulated ; the end A becomes negatively electri- fied while B has the same electric state as the conductor C. The elec- tricity thus developed by the conti- , . guity of an electrified body is said (**i C_D to be excited by induction ; and at- ---- traction is in all cases the result of this principle. But when the two conductors come into contact they soon become of the same electric condition and bodies attached to them repel each other. This principle can be more satisfactorily shown by the Leyden Jar, the inner side of which, by communication with the charged conduc- tor of a machine, is rendered positive, whereas the outer side is ren- dered negative by induction. When these two sides are brought into contact, by means of a discharger, the restoration of the equilibrium of electricity is attended with a snap or report. These explanations, however, will be better understood, from the following brief exposition of the THEORY OF ELECTRICITY. The phenomena of electricity are ascribed by some philosophers to the agency of one fluid, and by others to that of two distinct fluids. The latter opinion was originally proposed by Dufay, and is;almost ex- clusively adopted in France; the former was proposed by Franklin, and is generally received in England and America. It is somewhat singular that most of the phenomena can be equally well explained upon either hypothesis, and as has been remarked, " the selection de- pends more upon the taste than the judgment of the inquirer." The former of these theories is adopted in the present work. Electricity, according to the Franklinian hypothesis, is a subtle, highly elastic and imponderable fluid, which pervades all material bodies, and i» capable of moving through the pores or substances of various kinds of matter. It moves through bodies with various degrees of facility, through some, as the conductors or non-electrics, passing freely; through others, as the electrics or non-conductors, with great difficulty. The particles of this fluid repel one another, and attract the particles of all other matter, with a force varying inversely as the square of the distance. Bodies, on this hypothesis, are supposed to be in their natural state, with regard to electricity, when, the repulsion of the particles of the fluid is exactly balanced by the attraction of the matter for the same particles. In this state bodies are supposed to be saturated with the electric fluid. When they contain a quantity greater than this, they are said to be positively electrified, or over charged with the electric fluid; when the quantity is less than this, the body is said to be negatively electrified, or under charged. These different states are sometimes re- presented by the terms plus and minus. According to this theory, when a body is positively electrified, or has more than its natural quantity of electricity, the surplus, in con- sequence of the mutual repulsion of its particles, has a tendency to escape, and this continues until the body is reduced to its natural state. Again! when a body is negatively electrified, or contains less than its natural quantity of electricity, it has a tendency to attract the fluid 76 ELECTRICITY. from all sides, until the neutral state is again restored. When a glass globe or tube is excited by silk, the electricity leaves the silk and is accumulated on the glass; the silk, therefore, becomes negative and the glass positive. There was however, one defect in the original Franklinian theory, which was detected by iEpinus and confirmed by Cavendish, and which rendered it necessary to add another condition to it. The defect was, that while the repulsion which is observed between two positively elec- trified bodies was easily accounted for, the repulsion between bodies negatively electrified was left without explanation. By the theory, this could not be referred to the repulsion of the particles of electric fluid, for they were supposed to contain less than their natural amount. It was necessary, therefore, to annex the additional condi- tion, that particles of matter uncombined with electricity, exert a repul- sive action upon one another. Without this, as can easily be shown, we are not only unable to account for the repulsion existing between bodies negatively electrified, but also for the want of action between two neutral bodies. The repulsion of two negatively electrified bodies, therefore can only be considered as the result of the repulsion of the particles of matter. It will now be seen that the theory, as thus developed, satisfactorily explains the elementary facts which have been set forth, and will bear application to all the phenomena which electricity presents. Thus, by the operation of friction, the natural state of bodies is disturbed; and, as the result of this, there is an accumulation of electricity on one side and a diminution on the other, and the bodies are excited. The phenomena of attraction and repulsion, of distribution and transfer- ence, can also be readily accounted for. And the law of induction, moreover, is a consequence of the theory. For when a body over- charged with electricity is presented to a neutral body, the repulsive force existing among the particles of electricity in the former has a tendency to drive the electricity from the nearest part of the neutral body, if a conductor, to that which is the most remote. The nearest end of the body, therefore, becomes negatively electrified, while the remote end becomes positively electrified. Now if, instead of a positively electrified body, we bring a negatively electrified one near a neutral body, its attraction for the fluid in the second body will cause an ac- cumulation in the nearest end, and of course a deficiency in the remote end. Electroscopes and Electrometers. Forthepurpose of ascertaining when a body is electrified, and also the intensity or degree to which it is ex- cited, instruments have been constructed which are called Electro- scopes and Electrometers. Although the term electrometer is often in- discriminately applied to both those instruments, the latter denotes the intensity of electricity, the former merely indicates excitement and the electrical state by which it is produced. One of the simplest electroscopes is that of Hauy, which consists of a light metallic needle, terminated at each end by a light pith ball, which is covered with gold leaf, and supported horizontally by a cap at its centre, on a fine point. The attractive or repulsive power of any electrified body, presented to one of the balls, will be indicated by the movements of the needle. In some cases, however, it is more con- venient to employ a pair of similar balls, suspended from a brass ball, fixed to the end of a glass handle, by very fine silver wires, or by hem- ELECTRICITY. 77 pen threads, previously steeped in a solution of salt and afterwards Electrometers are constructed upon the principle, that the more high- ly an insulated conductor is charged with electricity, the more power- ful is the repulsion which it communicates to bodies which are brought near it Henley's Quadrant Electrometer, is in common use. Ben- nett's Gold Leaf Electrometer, is not only a more delicate instru- ment, but we are enabled by it to discriminate between positive and negative electricity. It consists essentially of a cylin- drical glass bottle, with its apertures closed by a brass plate, from the centre of which two slips of gold leaf are suspended. The brass plate, with its slips of gold leaf, are thus insulated, and the latter prevented from being moved by currents of air, by the glass with which they are surrounded. The approach of any electrified body, even though feebly excited, to the brass plate, is immediately detected by the divergence of the leaves. If the diver- gence be increased by the approach of flint glass exci- ted by silk, the electricity is said to be positive; if the divergence be diminished, they are said to be negatively electrified.—This instrument is, strictly speaking, an electroscope. Balance Electrometer. This instrument contrived by Mr. Harris, of Plymouth, [Eng] consists of a brass beam I, supported by a conductor CD, on a wooden frame AA ; E is a scale for holding weights and F its support; G and H are gilt cones made of light wood, G being suspended by a silver wire from I, and H insulated by the glass support KL. The instrument is prepared for use by placing G and E, in exact equi- poise. The cone G is suspended so that its base shall be opposite and parallel to the base of the cone H, as may be done by means of three adjusting screws in the frame AA and H is raised by help of a graduated brass slide N, until the bases of the cones are just in contact. The cone H is then depressed to any desired distance which may be varied at will during an experiment. The same cone is connected with the inner coating of a Ley- den jar, the outer coating of which communi- cates with the frame AA, and along DC I with the cone G; these cones may thus be made parts of a charged Leyden jar, and be oppositely excited, as indicated by the signs -f- and — . The attractive forces between their bases tends to draw down the cone G into,contact with H discharging the jar ; but before it can do so, it has to overcome the weight which may be in the scale E. By this ingenious contrivance any number of attractive forces are estimated by a common standard, namely, the number of grains which each is able to raise.— Turner. Mr. Harris has also invented another instrument which he calls the Unit Jar, the principal use of which is in charging other Leyden jars with known proportions of electricity. The most perfect electrometer for measuring small quantities of 7* 78 ELECTRICITY. electricity, is the apparatus described by Coulomb, and to which he has given the name of the Torsion Balance, for a description of which we must refer to elementary works on Natural Philosophy, or to the Library of Useful Knowledge, art. Electricity. EFFECTS OF ELECTRICITY. Independently of electrical attraction and repulsion, it does not ap- pear that the simple accumulation of electricity in any quantity In bod- ies, as long as it remains quiescent, produces the least sensible change in their properties. A person standing on an insulating stool, may be charged with any quantity of electricity from a machine, without be- ing perceptibly affected, until the equilibrium of the fluid is disturbed, by drawing sparks from his body or from the prime conductor with which he may be in communication. It is the passage of electricity therefore, which produces the effects now to be noticed. When this passage is uninterrupted, as is the case in rods of metal, no perceptible change in the mechanical properties of the body is pro- duced. But very considerable effects are produced, either if the body is so small as not to admit the whole quantity of electricity to pass with perfect freedom; or if the body, though large, is deficient in conduct- ing power. The effects of electricity may be conveniently arranged under the following heads, viz. 1. The mechanical effects, which can be illustrated by varioas experi- ments ; as by passing a charge of electricity through a capillary tube containing mercury, oil, water, alcohol, or ether ; or by passing it through a small plate of glass, apiece of pasteboard, a card, or through the leaves of a book. 2. The evolution of heat. The ignition and fusion of the metals by the electric discharge, are phenomena which have been long observ- ed. Thus a very fine wire may be fused by a powerful Leyden battery, or if the wire be sufficiently fine, by a single jar ; and so also alcohol, ether and spirits of turpentine may be inflamed by the spark from the prime conductor, as well as by the discharge from the jar. Gun-pow- der may be fired by partly interrupting the electric circuit, as by causing the wire or chain to pass through a vessel of water. Light as well as heat are emitted during the electrical discharge, at every point where the circuit is either interrupted or is occupied by bodies of inferior conducting powers. Thus a moderate charge from a Leyden jar will produce a bright spark when made to pass through water, and the spark is still more luminous in oil, alcohol, or ether, which are worse conductors than water ; on the contrary, 'in fluids of greater conducting power, there is greater difficulty of eliciting elec- tric light. 3. The chemical effects. These will be more strikingly shown when treating of Galvanism ; but they can also be exhibited by powerful electrical machines. Thus when a strong charge is sent through water, it is decomposed and resolved into oxygen and hydrogen which immediately assume a gaseous form. So also oxides of tin and mer- cury may be reduced to the metallic state. If a narrow glass tube containing chloride of silver be subjected to a series of electric sparks from an ordinary machine for five or ten min- ELECTRICITY. 79 utes, by means of two metallic wires fixed into the end of the tube, the silver will be reduced ; and if potassa be substituted for the chloride of silver, the potassium will be seen to take fire as it is produced. 4. Effects upon animals. When one hand communicates with the negative side of a charged jar, and the other hand is brought into contact with the positive side, a shock is observed more or less pow- erful according to the amount of electricity which has been accumu- lated. This shock is felt, especially at the joints, for the reason pro- bably that the fluid meets with greater resistance in passing from one bone to another. When a shock is passed through the muscles, a con- vulsive motion is produced, even in those cases where the nerves of a limb are completely paralyzed. But upon the nervous system, these effects are still more striking. Atmospheric Electricity. The atmosphere is generally in an electrical state. Thisfact can be shown by employing a metallic rod elevated to some height above the ground, and communicating at its lower end with an electroscope. Or, if we wish to collect electricity from the higher regions of the air, a kite may be raised, in the string of which a slender metallic wire should be interwoven, so as. to conduct the electricity. The electro- scope attached to this, will usually show the prevalence of the positive electricity, which increases as you ascend. This atmospheric electri- city, however, varies in quantity at different seasons of the year, and at different times of the day ; and alternates from the positive to the nega- tive. Upon the approach of thunder storms, these alternations succeed each other with great rapidity : strong sparks are given out by the conductor, and it becomes dangerous to prosecute the experiments with it in its uninsulated state.* Although the analogy between the electric spark and lightning, had been a subject of remark among the earliest experimenters upon elec- tricity, it remained for Dr. Franklin to prove it directly. He did this by the celebrated experiment of raising a kite during a storm, when he succeeded in obtaining sparks from a key attached to the lower end of the hempen string of the kite. This grand discovery excited the atten- tion of philosophers in every part of the world, and the experiment was every where repeated. It was, however, in many instances, attend- ed with risk, and proved fatal to Professor Richman, of St. Peters- burgh, in 1753. The most important practical application of thetheory of electricity, is the protection of* houses, ships, &c. from the effects of lightning. This is affected by means of a metallic conductor, which is called a Lightning-rod ; being a rod of iron or copper, about half an inch in diameter, the extremity of which projects some distance above the building, and is pointed with silver, platinum, or gold, the more com- * Water may be readily decomposed by atmospheric electricity in the man- ner proposed by M. Bonijol. The electricity is gathered by means of a very fine point fixed at the extremity of an insulated rod ; the latter is connected with the apparatus, in which the water is to be decomposed, by a metallic wire, of which the diameter does not exceed l-50th of an inch. In this way the de- composition of the water proceeds in a continuous and rapid manner, notwith- standing the electricity of the atmosphere is not very strong. Stormy weather is quite sufficient for the purpose.—Bib. Univ. 80 GALVANISM. pletely to preserve it from corrosion. No interruption should exist in the rod, and the lower end should be carried into the earth until it reaches water, or at least a moist stratum. References. Franklin's Works. Library of Useful Knowledge, art. Electricity. Priestley's History of Electricity. Cavallo's Philosophy. Biot's Traite de Physique, or Cambridge Course of Physics. Gay Lus- sac's instructions respecting Paratonnerres, or Conductors of Lightning, Ann. of Phil. xxiv. 427. Pouillet's Elemens de Physique, li. CuthberU son's Practical Electricity and Galvanism. GALVANISM. The science of Galvanism owes its name and origin to the experi- ments made by Galvani, Professor of Anatomy at Bologna, in Italy, in the year 1790. Being engaged in some researches on animal irritabil- ity, he observed that when a piece of zinc was placed in contact with the nerve of a frog, and a piece of silver or copper with the muscle, the animal was Violently convulsed. In this fact he conceived that he had found a strong confirmation of a theory which he had adopted, that the nervous fluid was somewhat analogous to electricity, and these con- vulsions were consequently ascribed by him to a discharge of this ner- vous or electrical energy from the muscles in consequence of the con- ducting power of the metals. And to this he gave the name of Ani- mal Electricity. The fallacy of the notions of Galvani was first pointed out by the celebrated Volta, of Pavia, who in repeating the experiments, soon es- tablished the fact that electricity is excited by the contact of the me- tals, and that the convulsions are owing to the current passing through the conducting muscle and nerve.* And as it is to him we are indebt- ed for the first true explanation of this curious fact, so did he first con- trive an apparatus for exciting electricity in this manner. The instrument constructed by Volta for this purpose was called the Pile; a description of which was first published in the Philosophical Transactions, for 1800. It consists of any number of pairs of zinc and copper, or zinc and silver, plates ZC, each pair being separa- ted from the adjoining ones by pieces of cloth K, nearly of the same size as the plates, and moistened in a saturated solution of salt. The relative position of the metals in each pair must be the same in the whole series; that is, if the copper is placed below the zinc in the first combination, the same order must be pre- served in all the others. The pile is contained in a proper frame, formed of glass pillars, fixed into a fiiece of thick wood, which both supports and insu- ates it. This form of the galvanic series, soon gave place to the more convenient one of the Trough, or batte- ry, invented by Mr. Cruickshank. This consists of * This fact may be illustrated by the following simple experiment. Place a piece of silver upon the tongue, and a piece of zinc under it; upon bringing their ends into contact we immediately perceive a saline taste and a peculiar sensation resembling a slight electric shock. Sometimes, also, when the sur- face of the metals 13 extensive, a flash of light appears to pass before the eyes ■ an effect which may be more certainly produced by placing one metal between the upper lip and the gums, and bringing their ends together as before GALVANISM. 81 a trough of baked wood AB, in which are placed at equal distances, any number of zinc and copper plates, previously soldered together, and so arranged that the same metal shall always be on the same side. Each pair is fixed in a groove cut in the sides and bottom of the box, the points of junction being made water-tight by cement. The appa- ratus thus constructed is always ready for use, and is brought into action by filling the cells left between the pairs of plates with some convenient solu- tion, which serves the same purpose as the moistened cloth in the pile of Volta. If then the zinc surfaces face towards Z and the copper towards C the pole or wire proceeding from C will be positive while _,.,,, „ - that from Z will be negative. But if these surfaces face in contrary directions the roles will be reversed. Several modifications in the construction of the battery were after- wards proposed. One of these was suggested by the Couronne des Tasses of Volta; in which the trough made either of baked wood or glazed earthen, is divided into partitions by the same material. A. more important improvement was suggested by Dr. Wollaston, who recommends that each cell should contain one zinc and two copper plates, so that both surfaces of the first metal are opposed to one of the second. By this arrangement the plates of copper communicate with each other, and the zinc between them with the copper of the ad- joining cell. An .increase of one half the power is obtained by this method. ,T ^ J After an extensive series of experiments, Mr. Faraday gives the pre- ference to the battery invented by Prof. Hare of Philadelphia, and de- scribed by him under the name of the Galvanic Deflagrator. In this in- strument, the copper plates, which pass round both surfaces of the zinc plates, are separated by thin veneers of wood, and the acid is poured on to, or off, the plates by a quarter revolution of an axis, to which both the trough containing the plates, and another trough to collect and hold the liquid, are fixed. Mr. Faraday adopted the following mode in con- Fig. 1. L , —I Fig. 2. Fig. 3. A V structing his battery. The zinc plates were cut from rolled metal and when soldered to the copper plates had the form delineated in fig. 1. These were then bent over.a gauge into the form fig. 2, and when pack- 82 GALVANISM. ed iii the wooden box constructed to receive them were arranged as in fig. 3, little plugs of cork being used to keep the zinc plates from touch- ing the copper plates, a single or double thickness of cartridge paper being interposed between the contiguous surfaces of copper to prevent them from coming in contact.—Phil. Trans, for 1835, Part II. The size and number of plates are subject to every variety. The largest battery ever made, is said to be the one of Mr. Children, the plates of which are six feet long and two feet eight inches broad. [See Ann. of Phil. vii. 11.] The great battery of the Royal Institution, with Which Sir Humphry Davy established the true nature of the fixed alkalies and alkaline earths, is composed of 2000 pairs of plates, each plate having 32 square inches of surface. These form a striking con- trast to the elementary battery of Dr. Wollaston, consisting of a sin- gle pair of plates, of very small dimensions, with which he succeeded in fusing and igniting a fine platina wire.—[Ann. of Phil, vi. 209.] All the effects of the battery may be exhibited by forty or fifty pairs of plates three inches square, arranged in the manner described by Mr. Faraday. The fluid generally used for rendering these batteries energetic, is one of the stronger acids, diluted with 20 or 30 times its weight of water. Mr. Children recommends a mixture of three parts of fuming nitrous acid, and one of sulphuric, diluted with 30 parts of water. According to Gay Lussac and Thenard, dilute nitric acid is the best where we wish to produce instantly the highest energy of the battery —while muriatic acid should be employed when the object is to obtain a prolonged action. For ordinary experiments Mr. Faraday prefers the following proportions viz. 200 parts of water, 4i of sulphuric acid and 4 of nitric acid. Whichever of these proportions we employ, it is of great importance that all the cells of the voltaic battery be excited with a liquid of the same strength. The Electric Column may be classed*among galvanic arrangements, It was originally contrived by M. De Luc, who formed it of disks of Dutch gilt paper, alternated with similar disks of laminated zinc. These were piled on each other in a dry state, (or at least in a state of dryness at ordinary temperatures, for paper cannot be made or preser- ved absolutely dry, except at a heat nearly sufficient to scorch it.) T