,^L»fi?%7.'WTr"r -—Ti*—- *: - r.- .,^jttBprfrr^'r~,r,'*,'*j;T,':'.' :■ ~ rt i,^.*.-/,V"-' ■;.*."'--■ r:.~ -V-".: llV.VA^-HK.r.-''---......'•- .,"' ARMED FORCES MEDICAL LIBRARY Washington, D. C. ____■_ ■- ■■..... Jt '" " ' " __ \ Boston^/ Me\ical Library Association, V-' 19 BOYLSTQN PLACE, /--—- Received.......... »\ fflntzulilt Sdiirarg Jtssariaii&tt. BOOMS IN SUMMER STEEET, BOSTON". REGULATIONS OF THE LIBRARY. ABTICLE III OP BY-LAWS. BOOKS. Sec. 1.—A member, upon application, or by written order to the Librarian, <■ take out one volume from the rooms, or he may remove both volumci of any w „.- . comprised in two volumes of duodecimo or smaller size, and retain the same two ^ weeks (subject to the provisions of Sees. 3 and 8,) at the expiration of which time, I an extension of one week only shall be given, if required; and the same person J shall not retake either of the -ime volumes until they shall have remained upon [J the shelves of the Libra? - one entire evening.. VI Sec. 2.—No person shall be allowed to exchange book* oftcner than twice in the (,. same day, except by tne consent of the Librarian. , j Sec. 3.—All new works may be limited in the time of their retention, during the > first six months after their purchase, which time shall be conspicuously marked on their covers; and all such volumes and periodicals shall be withheld from circula- r tion, as may be injured thereby. See. 4.—Any member who shall retain any book or books, longer thr.n the times specified in fees. 1 and 3, shall forfeit and pay, for every week so retained, the sum of ten cents; and a retention of one day over the stipulated time shall incur the same penalty. Sec. 5.—If any member shall lose or deface a volume, he shall replace the same, or present an equivalent in money. If it be one of a set which cannot be replaced, he may receive the remaining volumes at a fair appraisal, or make ample recompense, Sec. C—If any member shall refuse to pay the amount of any fine or fines which may be assessed, or refuse to 6ettle or account for any books'injured or lost by him, his right as a member of the Association shall be suspended until he complies with the requirements. Sec. 7.—Members shall not take out books on another's page, without a written order; and if any books so taken arc lost, the Librarian shall bo accountable therefor. Sec. 8.—All books must be returned on or before the first Monday in June, for the annual examination; and, on all books not thus returned, double the rates of fines enumerated in Section 4, shall be imposed. Sec. 9.—No person shall take from the rooms any books belonging to the Associa- tion without having the same recorded by the Librarian. • Booms open, from May to Oc er inclusive, at 8 o'clock, A. M. Prom November to April inclusive, at 7 o'clock, A. M. and close at 10 o'clock, P. M. (r~h<^^.---$2^1 BOOES. CONSTITUTION. —Art. xm. Sec. 1. Proprietors, upon application to the Librarian, may take one volume from the room and retain the same two weeks, (except those works described in Sec. 2,) and if desired, an extension of one week shall be given, provided no other member shall have left a written request for the volume in the meantime; but on the expiration of two weeks, if a renewal is not asked, or a., the close of three weeks, (the exten- sion being given,) the volume is not returned, the sum of twelve and a half cents shall be imposed, and rigidly exacted, for every week so retained, and the retention of a volume one day over the stipulated time shall incur the same penalty. Sec. 2. The Directors shall have power for the first six months after the purchase of a book to limit the time of its retention ; and the keeping of a book beyond the time marked upon its cover, shall incur the same penalty as is prescribed in Sec. 1. Sec. 3. No volume shall be reserved for any proprietor more than one evening, and no volume shall be returned or removed from the room during any general meeting of the proprietors. Sec. 5. If any proprietor shall lose or deface a volume, he shall replace the same, or present an equivalent in money. If it be one of a set, he shall receive the odd volumes at a fair appraisal, or make an ample recompense. Sec. 6. All books shall be returned one week previous to the annual examination. BY-LAWS. —Art. vii. Sec. 2. No member shall take away a book belonging to the institution, unless the same be recorded ; or remove a newspaper from the files. Sec. 3. Any member who defaces any book belonging to the Association, by marking upon it with pencil or otherwise, shall be considered amenable to Article 16th of the Constitution. I ^^---:------------—___________-d<^ Bmnntik jCilirnrif Jlssariatiuii, (KOOMS. CORNER OP BROMFIELD AND PROVINCE STREETS,) BOSTON. REGULATIONS OF THE LIBRARY. From Article XI. of By-Laws. BOOKS. Section 1. A member upon application to the Librarians, may take out one volume from the rooms, or he may remove both volumes of any work comprised in two volumes of duodecimo or smaller size, and retain the same two weeks, (except those works specified in Section 2,) at the expiration of which time an extension of one week shall be given if desired, after which time there shall be no extension, nor shall the same person re-take either of the same volumes until they shall have remained upon ' i the shelves of the Library one entire evening. I ! Sec. 2. The Directors shall have power to limit the time of retention for new books | I during the first six mouths after their purchase, which time shall be conspicuously ; L marked on their covers; and they shall also have power to withhold from circulation ' such volumes and periodicals as may be injured thereby. Sec. 3. If at the expiration of the'time specified in Sees. 1 and 2, for the retention of any book or books, the same are not returned, the sum of ten cents shall be imposed and demanded, for every week so retained, and a retention of one day over the stipu- lated time shall incur the same penalty. Sec. 4. If any member shall refuse to pay the amount of any fine or fines which may be assessed him, his right to remove books from the Library shall be suspended until he complies with the requirements. Sec 5. If any proprietor shall lose or deface a volume, he shall replace the same or present an equivalent in money. If it be one of a set, he shall receive the odd ! i volumes at a fair appraisal, or make an ample recompense. Sec 6 All books shall be returned one week previous to the annual examination. Sec. 7. Members shall not take out books on another's page without a written i order, and if any books so taken are lost, the Librarian shall be held accountable therefor. J Sec 8. No member shall have the liberty to transfer his right to take out books, i .. to a person not a member of the Association. \ \ Rooms open from. 1 o'clock *o 10 o'clock, P. M. \^^Jo^^ *»"*• Muter, 3 Conihill. ^^^~CjC^ C CONVERSATIONS CHYMISTRY, IK WHICH 27/£ ELEMENTS OF THAT SCIENCE ARE FAMILIARLY EXPLAINED AND ILLUSTRATED BF EXPERIMENTS AND PLATES, FROM THE LAST LONDON EDITION: THE SECOND AMERICAN ^/$ 3, 4> or more Bodies. 4. Produces a Change of Temperature. 5. The Properties that characterize Bodies in their feparate State, deftroyed by Combination. 6. The Force of Atti-aftion eftimated by that which is required by the Separation of the Conftituents. 7. Bodies have amongft themselves diffencut Degrees of Attraction. Of fimple elective and double elective Attraftions. Of quiefcent and divellent Forces. CONVERSATION XIII. - - 235 On Compound Bodies. Of the fimpleft Clafs of'Compounds. Of the various Combinations of Oxygen. Of the undecompounded Acids. Of the Claffification of Acids, ift Clafs, Acids of fimple and known Radicals. 2d Clafs, Acids of unknown Radicals, 3d Clafs, Acids of double Radicals. Of the Decompofition of Acids of the firft Clafs by combuftible Bodies. CONVERSATION XIV. - - 233 On the Combinations of Oxygen with Sulphur and with Phofphorus j 3r.d of the Sulphats and Phofphats. Of the Sulphuric Acid. Combuftion of Animal or Vegetable Bodies by this Acid. Method of preparing it. The Sulphurous Acid ob- tained in the form of Gas. May be obtained from Sulphuric Acid. May be reduced to Sulphur. Is abforbable by Water. Deftroys Vegetable Colours. Oxyd of Sulphur. Of Salts in general. Suluhats. Sulphat of Potafh, or Sal Polychreft. Cold produced by the melting of Salts. Sulphst of Soda, cr Glauber's Salt. Heat evolved during the Formation of Salts. CrvlUll^Btioii of Salts. Water of Cryftallization. Efflorefcence and Deliquefcence of Salts. Sulphat of Lime. Gypfum or PUifter of Paris, Sulphat of ?a the Muriatic and Oxygenated Mutiatk Acids; and on Muriats. Of the Muriatic Acid. Obtained from Muriats. Its gafeous From. Is abfjrbable by Water. Is fufceptible of a ftronger Degree of Oxygenation. Oxygenated Muriatic Acid. Its gafeous Form and other Properties. Combuftion of Bodies in this Gas. It diffolves Gold. Compofition of Aqua Regia. Oxygenated Muriatic Acid deftroys all Colours. Ufed for Bleaching and for Fumigations. Its offenfive fmell, &c. Muriats. Muriat of Soda, or common Salt. Muriat of Ammonia. Oxygenated Muriat of Potafh. De- tonates with Sulphur, Phofphorus, &c. Experiment of burning Phofphorus under Water by means of this Salt and of Sulphuric Acid. CONVERSATION XVII. - - 270 On the'Nature and Composition of Vegetables. Of Organized Bodies. Of the FundVions of Vegetables. Of the Elements of Vegetables. Of the Materials of Vegetables. Ana- lyfis of Vegetables. Of Sap. Mucilage, or Gum. Sugar. Man- na, and Honey. Gluten. . Vegetable Oils. Fixed Oils, Linfeed, Nut, and Olive Oils. Volatile Oils, forming Eifences and Per* fumes. Camphor, Refins and Varnifhes. Pitch, Tar, Copal, Maftic, Sec. Gum Refins. Myrrh, Affafeetida, &.c. Caoutchouc, or Gnm Elaftic. Extractive colouring Matter j its ufe in the Arts of Dyeing and Painting. Tanning ; its ufe in the Art of preparing Leather. Woody Fibre. Vegetable Acids. The Alkalies and Salts contained in Vegetables, CONVERSATION XVIII. - - 294 On the Decomposition of Vegetables. Of Fermentation in general. Of the Saccharine Fermentation, the Product of which is Sugar. Of the Vinous Fermentation, the Product of which is Wine. Alcohol, or Spirit of Wine. Analyfis of Wine by Diftillation. Of Brandy, Rum, Arack, Gin, &c. Tartrit of Potafh, or Cream of Tartar. Liqueurs. Chymical Properties of Alcohol. Its Combuftion. Of Ether. Of the Ace- tous Fermentation, the Product of which is Vinegar. Fermenta- tion of Bread. Of the Putrid Fermentation, which reduces Ve- getables to their Elements. Spontaneous SuccefBcn of thefe Fer- mentations. Of Vegetables faid to be petrified. Of Bitumens : Naphtha, Afphaltum, Jet, Coal, Succin, or Yellow Amber. Of Foflil Wood, Peat, and Turf. CONVERSATION XIX. - - 317 Hiftory of Vegetation. Connection between the Vegetable and Animal Kingdoms. Of Ma- nures. Of Agriculture. Inexhauftible Sources of Materials for the Purpofes of Agriculture. Of fowing Seed. Germination of the Seed. Function of the Leave-s of Plants. Effects of Light and Air on Vegetation. Effeils of Water on Vegetation. Effects of Vegetation on the, Atmofphere. Formation of Vegetable Mate- rials by the Organs of Plants. Vegetable Heat. Of the Organs of Plants. Of the Bark, confiding of Epidermis, Parenchyma, and Cortical Layers. Of Alburnum or Wood. Leaves, Flowers, and Seeds. Effeas of the Seafons on Vegetation. Vegetation of F.vergreens in Winter. j;j CONTENTS. CONVERSATION XX. - - 337 On the Compofition of Animals. Elements of Animals. Of the Three principal Materials of Animah, viz. Gelatine, Albumen, Fibrine. Of Animal Acids. Of Ani- mal Colours, Pruffian Blue, Carmine, and Ivory Black. CONVERSATION XXI. - - 349 On the Animal Economy. Of the,principal Animal Organs. Of Bones, Teeth, Horns, Liga- ments, and Cartilage. Of the Mufcles, conftituting the Organs of Motion. Of the Vafcular Syftem for the Conveyance of Fluids. Of the Glands for the Secretion of Fluids. Of the Nerves, con- ftituting the Organs of Senfation. Of the Cellular Subflance which connects the feveral Organs. Of the Skin. CONVERSATION XXII. 359 On Animalixation, Nutrition, and Refpiration. Digeftion. Solvent Power of the Gaftric Juice. Formation of Chyle. Its Affimilation, or Converfion into Blood. Of Refpiration. Mechanical Procefs of Refpiration. Chymical Procefs of Refpira. tion. Of the Circulation of the Blood. Of the Functions of the Arteries, the Veins and the Heart. Of the Lungs. Effects of Refpiration on the Blood. CONVERSATION XXIII. - - 37a On Animal Heat: and of va^ ious Animal ProduEls. Of the Analogy of Combuftion and Refpiration. Animal Heat evolved in the Lungs. Animal Heat evolved in the Circulation. Heat produced by Fever. Perfpiration. Heat produced by Exer- cife. Equal Temperature of Animals at all Seafons. Power of the Animal Body to refill the Effects of Heat. Cold produced by Perfpiration. Refpiration of Fifh, and of Birds. Effects of Re- fpiration on Mufcular Strength. Of feveral Animal Products,; viz. Milk, Butter, and Cheefe; Spermaceti j Ambergris; Wax; Lac ; Silk j Mufk; Civet} Carter. Of the putrid Fermentation. Conclufion. APPENDIX. Defcription and Manner of ujtng Cloud't Hydro-pneumatic 3i:-w-pipe. 391 Difquifition on Dyeing, - -«,.... 307 -» ■ on Tanning, - 4.07 1 1 uiu 111 on Currying, ---*.. 4^ CONVERSATIONS ON CHEMISTRY. ON SIMPLE BODIES. CONVERSATION I. Qn the General Principles of Chemistry- Mrs. B. HAVING now acquired some elementary notions of Natural Philosophy, I am going to propose to you another branch of science to which I am particularly anxious that you should devote a share of your attention. This is Chemistry, which is so closely connected with Natural Philosophy, that the study of the one must be incomplete without some knowledge of the other; for it is obvious that we can derive but a very imperfedt idea of bodies from the study of the general laws by which they are governed, if we remain totally ignorant of their intimate nature. s li Caroline. To confess the truth, Mrs. B. I am not disposed to form a very favourable idea of Chemistry, nor do I expect to derive much entertainment from it. I prefer those sciences that exhibit nature on a grand scale, to those which are confined to the mi- nutiae of petty details. Can the studies which we have lately pursued, the general properties of mat- ter, or the revolutions of the heavenly bodies, be compared to the mixing up of a few insignificant drugs? Mrs. B. I rather imagine that your want of taste for chemistry proceeds from the very limited idea you entertain of its object. You confine the che- mist's laboratory to the narrow precincts of the apo- thecary's shop, whilst it is subservient to an im- mense variety of other useful purposes. Besides, my dear, chemistry is by no means confined to works of art. Nature also has her laboratory, which is the universe, and there she is incessantly employ- ed in chemical operations. You are surprised, Ca- roline; but I assure you that the most wonderful and the most interesting phenomena of nature are al- most all of them produced by chemical powers. \V ithout entering therefore into the minute details of practical chemistry, a woman may obtain such a knowledge of the science, as will not only throw an interest on the common occurrences of life, but will enlarge the sphere of her ideas, and render the contemplation of Nature a source of delightful in- struclion. Caroline. If this is the case, I have certainly been much mistaken in the notion I had formed of che- mistry. I own that I thought it was chiefly con- fined to the knowledge and preparation of medi- cines. Mrs. B. That is only a branch of chemistry, which is called Pharmacy; and though the study of 15 it is certainly of great importance to the world at large, it properly belongs to professional men, and is therefore the last that I snould advise you to stu- Emily. But did not the chemists formerly employ themselves in search of the Philosopher's Stone, or the secret of making gold ? Mrs. B. These were a particular set of misguided philosophers, who dignified themselves with the name of Alchymists, to distinguish their pursuits from those of the common chemists, whose studies were confined to the knowledge of medicines. But, since that period, chemistry has undergone so complete a revolution, that, from an obscure and mysterious art, it is now become a regular and beau- tiful science, to which art is entirely subservient. It is true, however, that we are indebted to the alchy- mists for many very useful discoveries, wh ch sprung from their fruitless attempts to make gold, and which undoubtedly have proved of infinitely greater ad- vantage to mankind than all their chimerical pursuits. The modern chemists, far from directing their ambition to the imitation of one of the least useful productions of inanimate nature, aim at copying almost all her operations, and sometimes even form combinations, the model of which is not to be found in her own productions. They have little reason to regret their inability to make gold (which is often but a false representation of riches), whilst by then- innumerable inventions and discoveries, they have so greatly stimulated industry and facilitated labour, as prodigiously to increase the luxuries as well as the necessaries of life. Emily. But I do not understand by what means chemistry can facilitate labour; is not that rather the province of the mechanic ? 16 Mrs. B. There are many ways by which labour may be rendered more easy, independently of me- chanics ; but even the machine the most wonderful in its effects, the steam engine, cannot be under- stood without the assistance of chemistry. In agri- culture, a chemical knowledge of the nature of soils, and of vegetation, is highly useful; and in those arts which relate to the comforts and conve- niences of life, it would be endless to enumerate the advantages which result from the study of this science. Caroline. But, pray, tell us more precisely in what manner the discoveries- of chemists have pro- ved so beneficial to society. Mrs. B. That would be an unfair anticipation; for you would not comprehend the nature of such discoveries and useful applications, so well as you will do hereafter. Without a due regard to method, we cannot expect to make any progress in chemis- try. I wish to direct your observation chiefly to the chemical operation? of Nature; but those of Art are certainly of too high importance to pass unnoticed. We shall therefore allow them also some share of our attention. Emily. Well then, let us now set to work regu- t,arly, I am very anxious to begin. Mrs. B. The object of chemistry is to obtain a knowledge of the intimate nature of bodies and of their mutual action on each other. You find therefore, Caroline, that this is no narrow or con- fined science, which comprehends every thing ma- terial within our sphere. Carol';:e. On the contrary, it" must be inex- haustible ; and I am at a loss to conceive how any proficiency can be made in a science whose object? ire so numerous. It Mrs. B. If every Individual substance was form- ed of different materials, the study of chemistry would indeed be endless; but you must observe, that the various bodies in nature are composed of certain elementary principles, which are not very numerous. Caroline. Yes; I know that all bodies are com- posed of fire, air, earth, and water; I learnt that many years ago. Mrs. B. But you must now endeavour to forget it. I have already informed you what a great v change chemistry has undergone since it has become a regular science. Within these thirty years espe- cially, it has experienced an entire revolution, and it is now proved that neither fire, air, earth, nor water, can be called elementary bodies. For an elementary body is one that cannot be decomposed, that is to say, separated into other substances ; and fire, air, earth, and water, are all of them suscep- tible of decomposition. Emily. I thought that decomposing a body was dividing it into its minutest parts. And if so, I do not understand why an elementary substance is not capable of being decomposed, as well as any other. Mrs. B. You have misconceived the idea of Decomposition; it is very different from mere divi- sion: the latter simply reduces a body into parts, but the former separates it into the various ingredients, or materials, of which it is composed. If we were to take a loaf of bread, and separate the several in- gredients of which it is made, the flour, the yeast, the salt, and the water, it would be very different from cutting the loaf into pieces, or crumbling it into atoms. Emily. I understand you now very well. To decompose a body is to separate from each other the various elementary substances of which it con- sists. 82 18 Caroline. But flower, water, and the other ma- terials of bread, according to your definition, are not elementary substances ? Mrs. B. No my dear; I mentioned bread rather as a familiar comparison, to illustrate the idea, than as an example. The elementary substances of which a body is composed, are called the constituent parts of that bo- dy; in decomposing it, therefore, we separate its constituent parts. If on the contrary, we divide a body by chopping it to pieces, or even by grinding or pounding it to the finest powder, each of these small particles will still consist of a portion of the several constituent parts of the whole body: these we call the integrant parts; do you understand the difference ? Emily. Yes, I think, perfectly. We decompose a body into its constituent parts; and divide it into its integrant parts. . Mrs. B. Exactly so. If therefore a body con- sist of only one kind of substance, though we may divide it into its integrant parts, it is not possible to decompose it. Such bodies are therefore called simple or elementary, as they are the elements of which all other bodies are composed. Compound bodies are such as consist of more than one of these elementary principles. Caroline. But do not fire, air, earth, and water, consist, each of them, but of one kind of sub- stance ? Mrs. B. No, my dear; they are every one of them susceptible of being separated into various simple bodies. Instead of four, chemists now rec- kon upwards of forty elementary substances. These we shall first examine separately, and afterwards consider in their combinations with each other. 19 Their names are as follow: LIGHT, SILEX, ZINC, caloric, ALUM1NE, BISMUTH, OXYGEN, YTTRIA, ANTIMONY, NITROGEN, GLUCINA, ARSENIC, HYDROGEN, ZIECONIA, COBALT, SULPHUR, AGUSTINA, MANGANESE, PHOSPHORUS, (25 Metals.) TUNGSTEN, CARBONE, GOLD, MOLYBDENUM, (2 Alkalies.) PLATINA, URANIUM, POTASH, SILVER, TELLURIUM, SODA, MERCURY, TITANIUM, f 10 Earths.) COPPER, CHROME, LIME, IRON, OSMIUM, MAGNESIA, TIN, IRIDIUM, STRONTITES, LEAD, PALLADIUM, BARYTES, NICKEL, RHODIUM. Caroline. This is, indeed, a formidable list! Mrs. B. Not so much as you imagine; many of the names you are already acquainted with, and the others will soon become familiar to you. But, be- fore we proceed farther, it will be necessary to give you some idea of chemical attraction, a power on which the whole science depends. Chemical Attraction, or the Attraction of Compo- sition, consists in the peculiar tendency which bo- dies of a different nature have to unite with each other. It is by this force that all the compositions, and decompositions, are effected. Emily. What is the difference between chemical attraction, and the attraction of cohesion, or of ag- gregation, which you often mentioned to us in former conversations ? Mrs. B. The attraction of cohesion exists only between particles of the same nature, whether sim- ple or compound; thus it unites the particles of a piece of metal which is a simple substances and 20 likewise the particles of a loaf of bread which is e compound. The attraction of composition, on the contrary unites and maintains in a state of combi- nation particles of a dissimilar nature ; it is this" power that forms each of the compound particles of which bread consists; and it is by the attraction of cohesion that all these particles are connected into a single mass. Emily. The attraction of cohesion, then, is the power which unites the integrant particles of a bo- dy ; the attraction of composition that which com- bines the constituent particles. Is it not so? Mrs. B. Precisely: and observe that the at- traction of cohesion unites particles of a similar na- ture, without changing their original properties ; the result of such an union, therefore, is a body of the same kind as the particles of which it is formed; whilst the attraction> of composition, by combining particles of a dissimilar nature, produces new bo- dies, quite different from any of their constituent particles. If, for instance, I pour on the piece of copper, contained in this glass, some of this liquid (which is called nitric acid) for which it has a strong attraction, every particle of the copper will com- bine with a particle of acid, and together they will form a new body, totally different from either the copper or the acid. Do you observe the internal commotion that al- ready begins to take place ? It is produced by the combination of these two substances; and yet the acid has in this case to overcome, not only the re- sistance which the strong cohesion of the particles of copper oppose to its combination with them, but also the weight of the copper which makes it sink to the bottom of the glass, and prevents the acid from having such free access to it as it would if the metal were suspended in the liquid. 21 Emily. The acid seems, however, to overcome both these obstacles without difficulty, and appears to~be very rapidly dissolving the copper. Mrs. B. By this means it reduces the copper in- to more minute parts, than could possibly be done by any mechanical power. But as the acid can act only on the surface of the metal, it will be some time before the union of these two bodies will be completed. You may, however, already see how totally dif- ferent this compound is from either of its ingredi- ents. It is neither colourless like the acid, nor hard, heavy, and yellow, like the copper. If you tasted it, you would no longer perceive the sourness of the acid. It has at present the appearance of a blue li- quid ; but when the union is completed, and the water with which the acid is diluted is evaporated, it will assume the form of regular crystals, of a fine blue colour, and perfectly transparent. Of these I can shew you a specimen, as I have prepared some for that purpose. Caroline. How very beautiful they are, in co- lour, form and transparency ? Emily. Nothing can be more striking than this example of chemical attraction. Mrs. B. The term attraction has been lately in- troduced into chemistry as a substitute for the word affinity, to which some chemists have objected, be- cause it originated in the vague notion that chemical combinations depend upon a certain resemblance, or relationship, between particles that are disposed to unite ; and this idea is not only imperfect, but erro- neous, as it is generally particles of the most dissi- milar nature, that have the greatest tendency to combine. Caroline. Besides, there seems to be no advan- tage in using a variety of terms to express the same 22 meaning; on the contrary it creates confusion; and as we are well acquainted with the term attraction in natural philosophy, we had better adopt it in chemistry likewise. Mrs. B. If you have a clear idea of the mean- ing, I shall leave you at liberty to express it in the terms you prefer. For myself, I confess that I think the word attraction best suited to the general law that unites the integrant particles of bodies; and affinity better adapted to that which combines the constituent particles, as it may convey an idea of the preference which some bodies have for others, which the term attraction of composition does not so well express. Emily. So I think ; for though that preference may not result from any relationship or similitude, between the particles (as you say was once suppo- sed), yet, as it really exists, it ought to be express- ed Mrs. B. Well, let it be agreed that you may use the terms affinity, chemical attraclion, and attrac- tion of composition, indifferently, provided you recol- lect that they have all the same meaning. Emily. I do not conceive how bodies can be de- composed by chemical attraction. That this power should be the means of composing them, is very obvious; but how it can at the same time produce exactly the contrary effect, appears to me very sin- gular. Mrs. B. To decompose a body, is, you know to separate its constituent parts, which, as we have just observed, can never be done by mechanical means. Emily. No; because mechanical means separate only the integrant particles; they act merely against the attraction of cohesion. 23 Mrs. B. The decomposition of a body, there- fore, can only be performed by chemical powers. If you present to a body composed only of two principles, a third, which has a greater affinity for one of them than the two first have for each other, it will be decomposed, that is, its two principles will be separated by means of the third body. Let us call two ingredients, of which a body is compo- sed, A and B. If we present to it another ingre- dient C, which has a greater affinity for B, than that which unites A and B, it necessarily follows that B will quit A to combine with C. The new ingredient, therefore, has effeaed a decomposition of the original body A B; A, has been left alone, and a new compound, B C, has been formed. Emily. We might, I think, use the comparison of two friends, who were very happy in each other's society, till a third disunited them by the preference which one of them gave to the new-comer. Mrs. B. Very well, I shall now show you how this takes place in chemistry. Let us suppose that we wish to decompose the compound we have just formed by the combination of the two ingredients, copper and nitric acid : we may do this by presenting to it a piece, of iron, for which the acid has a stronger attraction than for copper; the acid will consequently quit the copper to combine with the iron, and the copper will be what the chemists call precipitated, that is to say, it will return to its separate state, and reappear in its simple form. In order to produce this effect, I shall dip the blade of this knife into the fluid, and, when I take it out, you will observe that instead of being wetted with a blueish liquid like that contained in the glass, it will be covered with a very thin pellicle of cop- per. 24 Caroline. So it is, really ! But then is it not the copper instead of the acid, that has combined with the iron blade ! Mrs. B. No; you are deceived by appearances: it is the acid which combines with the iron, and in so doing deposites the copper on the surface of the blade. Emily. But cannot three or more substances combine together, without any of them being pre- cipitated ? Mrs. B. That is sometimes the case; but in ge- neral, the stronger affinity destroys the weaker; and it seldom happens that the attraction of several sub- stances for each other is so equally balanced as to produce such complicated compounds. It is now time to conclude our conversation for this morning. But before we part, I must recom- mend you to fix in your memory the names of the simple bodies, against our next interview. CONVERSATION II. On Light and Heat. Caroline. We have learned by heart the names of all the simple bodies, which you have enumerated, and we are now ready to enter on the examination of each of them successively. You will begin I suppose, with LIGHT ? 25 Mrs. B. That will not detain us long: the na- ture of light, independent of heat, is so imperfectly known, that we have little more than conjectures respecting it. Emily. But is it possible to separate light from heat; I thought that they were only different de- grees of the same thing ? Mrs. B. They are certainly very intimately con- nected; yet it appears that they are distinct substan- ces, as they can, under certain circumstances, be in a great measure separated; the most striking in- stance of this was pointed out by Dr. Herschel. This philosopher discovered that heat was less re- frangible than light; for in separating the different coloured rays of ligtit by a prism (as we did some time ago), he found that the greatest heat was be- yond the spectrum, at a little distance from the red rays, which you may recollect are the least refran- gible. Emily. I should like to try that experiment,. Mrs. B. It is by no means an easy one: the heat of a ray of light, refracted by a prism, is so small that it requires a very delicate thermometer to distinguish the difference of the degree of heat within and without the spectrum. For in this ex- periment the heat is not totally separated from the light, each coloured ray retaining a certain portion of it, though the greatest part is not sufficiently re- fracted to fall within the spectrum. Emily. I suppose, then, that those coloured rays which are the least refrangible, retain the greatest quantity of heat ? Mrs. B. They do so. Caroline. Perhaps the different degrees of heat which the seven rays possess, may in some unknown manner occasion their variety of colour. I have heard that melted metals change colour accord- c 26 ing to the different degrees of heat to which they are exposed ; might not the colours of the spec- trum be produced by a cause of the same kind? Do let us try if we cannot ascertain this, Mrs. B ? I should like extremely to make some discovery in chemistry. Mrs. B. Had we not better learn first what is already known ? Surely you cannot seriously ima- gine that, before you have acquired a single clear idea on chemistry, you can have any chance of dis- covering secrets that have eluded the penetration of those who have spent their whole lives in the study of that science. Caroline. Not much, to be sure, in the regular course of events; but a lucky chance sometimes happens. Did not a child lead the way to the dis- covery of telescopes ? Mrs. B. There are certainly a few instances of this kind. But believe me, it is infinitely wiser to follow up a pursuit regularly, than to trust to chance for your success. Emily. But to return to our subject. Though I no longer doubt that light and heat can be separa- ted, Dr. Herschel's experiment does not appear to me to afford sufficient proof that they are essentially different; for light, which you call a simple body, may likewise be divided into the various coloured rays; is it not therefore possible that heat may only be a modification of light ? Airs. B. That is a supposition which, in the pre- sent state of natural philosophy, can neither be po- sitively affirmed nor denied: it is generally thought that light and heat are connected with each other as cause and effect, but which is the cause, and which the effect, it is extremely difficult "to deter- mine. But it would be useless to detain you any longer on this intricate subject. Let us now pass 27 «n to that of heat, with which we are much be**- ter acquainted. Caroline. Heat is not, I believe, amongst the number of the simple bodies ? Mrs. B. Yes, it is; but under another name— that of caloric, which is nothing more than the principle, or matter of heat.—We suppose caloric to be a very subtile fluid, originally derived from the sun, and composed of very minute particles, constantly in agitation, and moving in a manner si- milar to light, as long as they meet with no obsta- cle. But when these rays come in contact with the earth, and the various bodies belonging to it, part of them are reflected from their surfaces according to certain laws, and part enters into them. Caroline. These rays of heat, or caloric, pro- ceeding from the same source, and following the same direction, as the rays of light, bear a very strong resemblance to them. Mrs. B. So much so that it often requires great attention not to confound them. Emily. I think there is no danger of that, if we recollect one great distinction—light is visible, and caloric is not. Mrs. B. Very- right. Light affe&s the sense of Sight ',■ Caloric that of Feeling: the one produces Vision, the other the peculiar sensation of Heat. Caloric is found to exist in a variety of forms, and to be susceptible of certain modifications, all of which may be comprehended under the four fol- lowing heads: 3. FREE CALORIC 2. SPECIFIC HEAT- 3. LATENT HEAT. 4. CHEMICAL HEAT. The first, or free caloric, is also called heat of temperature ; it compf ehends all heat which 28 ss perceptible to the senses, and affects the thermo- meter. Emily. You mean such as the heat of the sun, of fire, of candles, of stoves; in short of every thing that burns ? Mrs. B. And likewise of things that do not burn, as for instance, the warmth of the body; in a word, all heat that is sensible, whatever may be its degree, or the source from which it is derived. Caroline. What then are the other modifications of caloric ? It must be a strange kind of heat that cannot be perceived by our senses ? Airs. B. None of the modifications of caloric should properly be called heat; for heat strictly speaking, is the sensation, produced by caloric, on animated bodies, and this word therefore should be confined to express the sensation. But custom has adapted it likewise to inanimate matter, and we say the heat of an oven, the heat of the suny without any reference to the sensation which they are capable of exciting. It "S7as in order to avoid the confusion which arose from thus confounding the cause and effect, that modern chemists adopted the new word Caloric, to express the principle which produces heat: but they do not yet limit the word heat (as they should do) to the expression of the sensation, since they still retain the habit of connecting this word with the three other modifications of caloric. Caroline. But you have not yet explained to us what these other modifications of caloric are. Mrs. B. Because you are not yet acquainted with the properties of free caloric, and you know we have agreed to proceed with regularity. One of the most remarkable properties of free caloric is its power of dilating bodies. This fluid is so extremely subtile, that it enters and pervades all 29 bodies whatever, forces itself between their parti- cles, and not only separates them, but, by its re- pulsive power, drives them asunder, frequently to a considerable distance from each other. It is thus that caloric dilates or expands a body so as to make it occupy a greater space than it did before. Emily. The effect of caloric on bodies therefore, is directly contrary to that of the attraction of cohe- sion; the one draws the particles together, the other drives them asunder. Mrs. B. Precisely. There is a kind of continual warfare between the attraction of aggregation jand the repulsive power of caloric; and from the action of these two opposite forces, result all the various forms of matter, or degrees of consistence, from the solid, to the liquid and aeriform state. And accor- dingly, we find that most bodies are capable of passing from one of these forms to the other, mere- ly in consequence of their receiving different quan- tities of caloric. Caroline. That is very curious; but I think I un- derstand the reason of it. If a great quantify of ca- loric is added to a solid body, it introduces itself between the particles in such a manner as to over- come in a considerable degree, the attraction of co- hesion ; and the body from a solid, is then convert- ed into a fluid. Mrs. B. This is the case whenever a body is melted ; but if you add caloric to a liquid, can you tell me what is the consequence ? Caroline. The caloric forces itself in greater a- bundance between the particles of the fluid, and drives them to such a distance from each other, that their attraction of aggregation is wholly destroyed; the liquid is then transformed into vapour. Mrs. B. Very well; and this is precisely the case with boiling water, when it is converted into steam or vapour. c2 so But each of these various states, solid, liquid, and aeriforui, ac'.nit 01 many different degrees of densi- ty, or consistence, still arising (partly at least) from the different quantities of caloric the bodies contain* Solids are of various degrees of density, from that of gold, to fhat of a thin jelly. Liquids, from the consistence of melted glue, or melted metals, to that of ether, which is the lightest of all liquids. The different elastic fluids (with which you are not ac- quainted) admit of no less variety in their degrees of density. Emily. But does not every individual body also admit of different degrees of consistence, without changing its state ? Airs. B. Undoubtedly; and this I can immedi- ately snow you by a*ery simple experiment. This piece of iron now exactly fits the frame or ring, made to receive it, but if heated red hot, it will no longer do so, for its dimensions will be so much in- creased by the caloric that has penetrated into it, that it will be much too large for the frame. The iron is now red hot; by applying it to the frame, we shall &ee how much k is dilated. Emily. Considerably so indeed! I knew that heat had this effect on bodies, but I did not imagine that it could be made so conspicuous. Airs. B. By means of this instrument (called a Pyrometer) we may estimate, in the most exact manner, the various dilatations of any solid body by heat. The body we are now going to submit to trial is this small iron bar; I fix it to this apparatus {Plate I. Fig. 1.) and then heat it by lighting the three lamps beneath it; when the bar dilates, it in- t ' Plate I. Fig. i. A yBar of metal, i z 3. Lam^s burning. E B. Wheel work. C. Index. Fir .a. A A. Glnss tubes *'ib bulbs. B B. Custca of wai~ r» which :vey are unmeried. iiW-. $i creases in length as well as thickness; and, as one end communicates with this wheel-work, whilst the other end is fixed and immoveable, no sooner does it begin to dilate than it presses against the wheel- work, and sets in motion the index, which points out the degrees of dilatation on the dial-plate. Emily. This is indeed a very curious instrument; but I do not understand the use of the wheels: would it not be more simple, and answer the pur- pose equally well, if the bar pressed against the in- dex, and put it in motion without the intervention of the wheels ? Mrs. B. The use of the wheels is merely to multiply the motion, and therefore render the effect of the caloric more obvious : for if the index moved no more than the bar increased in length, its motion would scarcely be perceptible : but by means of the wheels it moves in a much greater proportion, which therefore renders the variations much more conspi- cuous. By submitting different bodies to the test of the pyrometer, it is found that they are far from dila- ting in the same proportion. Different metals ex- » pand in different degrees, and other kinds of solid bodies vary still more in this respect. But this different susceptibility of dilatation is still more re- markable in fluids than in solid bodies, as I shall show you. I have here two glass tubes, terminated at one end by large bulbs. We shall fill the bulbs, the one with spirit of wine, the other with water. I have coloured both liquids, that the effect may be more conspicuous. The spirit of wine, you see, dilates merely by the warmth of my hand as I hold the bulb. Emily. It certainly dilates, for I see it is rising into the tube. But water, it seems, is not so easily affe£led by heat; for no apparent change is produ- ced on it by the warmth of the hand. 32 Mrs. B. True ; we shall now plunge the bulbs into hot water, {Plate I Fig. 2.), and you will see both liquids rise in the tubes; but the spirit of wine will begin to ascend first. Caroline. How rapidly it dilates! Now it has nearly reached the top of the tube, though the wa- ter has not yet began to rise. Emily. The water now begins to dilate. Are not these glass tubes, with liquids rising within them, very like thermometers ? Mrs. B A Thermometer is constructed exactly on the same principle, and these tubes require only a scale to answer the purpose of thermometers: but they would be rather awkward in their dimensions. The tubes and bulbs of thermometers, though of various sizes, are in general much smaller than these; the tube too is hermetically closed, and the air excluded from it. The fluid most generally used in thermometers is mercury, commonly called quicksilver, the dilatations and contractions of which correspond more exactly to the additions, and sub- tractions, of caloric, than those of any other fluid. Caroline. Yet I have often seen coloured spirit of wine used in thermometers. Mrs. B. The dilatations and contractions of that liquid are not quite so uniform as those of mercury; but in cases in which it is not requisite to ascertain the temperature with great precision, spirit of wine will answer the purpose equally well, and indeed in some respects better, as the expansion of the latter is greater and therefore more conspicuous. This fluid is used likewise in situations and experiments in which mercury would be frozen; for mercury becomes a solid body, like a piece of lead or any other metal, at a certain degree of cold: but no de- gree of cold has ever been known to freeze spirits of wine. 33 A thermometer therefore consists of a tube with a bulb, such as you see here, containing a fluid whose degrees of dilatation and contraction are in- dicated by a scale to which the tube is fixed. The degree which indicates the boiling point, simply means that, when the fluid is sufficiently dilated to rise to this point, the heat is such, that water ex- posed to the same temperature will boil. When, on the other hand, the fluid is so much condensed as to sink to the freezing point, we know that water will freeze at that temperature. The extreme points of the scales are not the same in all thermometers, nor are the degrees always divided in the same manner. In different countries philosophers have chosen to adopt different scales and divisions. The two thermometers most used are those of Fahren- heit, and of Reaumur; the first is generally pre- ferred by the English, the latter by the French. Emily. The variety of scale must be very incon- venient, and I should think liable to occasion con- fusion, when French and English experiments are compared. Mrs. B. This inconvenience is but very trifling, because the different graduations e£ the scales donot affect the principle upon which thermometers are constructed. When we know, for instance, that Fahrenheit's scale is divided into 212 degrees, in which 32° corresponds with the freezing point, and 212° with the point of boiling water; and that Reaumur's is divided only into 80 degrees, in which 0° denotes the freezing point, and 80° that of,boil- ing water, it is easy to compare the two scales to- gether, and reduce the one into the other. But, for greater convenience, thermometers are some- times constructed with both these scales, one on ei- ther side of the tube; so that the correspondence of the different degrees of the two scales, is thus instantly seen, tiere is one of these scales {Plate 34 II. Fig. 3.), by which you can at once perceive that each degree of Reaumur's corresponds to 21 of Fahrenheit's division. Emily. Are spirits of wine, and mercury, the only fluids used in the construction of thermome- ters. Mrs. B. I believe they are the only liquids now in use, though some others, such as linseed oil, would make tolerable thermometers; but for expe- riments in which a very quick and delicate test of the changes of temperature is required, air thermo- meters are sometimes employed. The bulb, in these, instead of containing a liquid, is filled only with common air, and its dilatations and contractions are made sensible, by a small drop of any coloured fluid, which is suspended within the tube, and moves up and down, according as the air within the bulb and tube expands or contracts. But air thermometers, however sensible to changes of tem- perature, are by no means accurate in their indica- tions. Emily. A thermometer, then, indicates the ex- act quantity of caloric contained either in the at- mosphere, or In any bwiy -wJtk which it is in con- tact? Airs. B. No: first, because there are other mo- difications of caloric which do not affect the ther- mometer; and, secondly, because the temperature of a body,' as indicated by the thermometer, is on- ly relative. When for instance, the thermometer remains stationary at the freezing point, we know that the atmosphere (or medium in which it is pla- ced, whatever it may be) is as cold as freezing wa- ter : and when it stands at the boiling point, we know that this medium is as hot as boiling water ; but we do not know the positive quantity of heat contained either in freezing or boiling water, any more than we know the real extremes of heat and /Jatrl/. Page 34 TJiKitzxroumTj.K Fir/. 3. J}oilino> point of Water „-;• Freezing point of Water 35 cold; and consequently, we cannot determine that of the body in which the thermometer is placed. Caroline. I do not quite understand this explana- tion. Mrs. B. Let us compare a thermometer to a well, in which the water rises to different heights, according as it is more or less supplied by the spring which feeds it: if the depth of this well be unfa- thomable, it must-be impossible to know the abso- lute quantity of water it contains ; yet we can with the greatest accuracy measure the number of feet the water has risen or fallen in the well at any time, and consequently know the precise quantity of its increase or diminution, without having the least knowledge of the whole quantity of water it con- tains. Caroline. Now I comprehend it very well: no- thing explains a thing so clearly as a comparison. Emily. But will thermometers bear any degree of heat ? Mrs. B. No ; for if the temperature be much above the highest degree marked on the scale of the thermometer, the mercury would burst the tube in an attempt to ascend. And at any rate, no ther- mometer can be applied to temperatures higher than the boiling point of the liquid used in its construc- tion. In furnaces, or whenever any very high tem- perature is to be measured, a pyrometer, invented by Wedgewood, is used for that purpose. It is made of a certain composition of baked clay, which has the peculiar property of contracting by heat, so that the degree of contraction of this substance indi- cates the temperature to which it has been exposed. Emily. But is it possible for a body to contract by heat ? I thought that heat dilated all bodies whatever. Mrs. B. That is, I believe, true. Yet heat frequently diminishes the bulk of a body by evapo- 36 rating some of its particles; thus, if you dry a wet sponge before the fire, the heat, though it must, according to the general law of nature, dilate the particles of the sponge, will very considerably con- tract its bulk by evaporating its moisturei Caroline. And how do you ascertain the degrees of contraction of this pyrometer ? Mrs. B. The dimensions of a piece of clay are measured by the bore of a graduated conical tube in which it is placed; the more it is contracted by the heat, the lower it descends into the narrow part of the tube. Let us now proceed to examine the other proper- ties of free caloric. Free caloric always tends to an equilibrium; that is to say, when two bodies are of different tempe- ratures, the warmer gradually parts with its heat to the colder, till they are both brought to the same temperature. Emily. Is cold then nothing but a negative qua- lity, simply implying the absence of heat ? Mrs. B. Not the total absence, but a diminu- tion of heat; for we know of no body in which some caloric may not be discovered. Caroline. But when I lay my hand on this mar- ble table. I feel it positively cold, and cannot con- ceive that there is any caloric in it. Mrs. B. The cold you experience consists in the loss of caloric that your hand sustains in an attempt to bring its temperature to an equilibrium with the marble. If you lay a piece of ice upon it, you will find that the contrary effect will take place ; the ice will be melted by the heat which it abstracts from the marble. Caroline. Is it not in this case the air of the room,'which being warmer than the marble, melts the ice ? Mrs. B, The air certainly acts on the surface 37 exposed to it, brut the table melts that part which is in contact with it. Caroline. But why does caloric tend to an equili- brium? It cannot be on the same principle as other fluids, since it has no weight? Mrs. B. Very true, Caroline, that is an excel- lent remark. The tendency cf caloric to an equili- brium is best explained by a supposed repulsive force of its particles, which having a constant tendency to fly from each other, diffuse themselves wherever there is a deficiency of that fluid, and thus gradually restore an equilibrium of temperature. But it is not only bodies which contain a greater proportion of caloric that part with it to those that contain less: in order to explain all the^>henomena of heat and cold, we must suppose that a mutual exchange of caloric takes place between all bodies, of whatever temperature, and that the rays of caloric, in passing from one body to another, are subject to' all the laws of reflection and refraction, the same as those of light. This theory was first suggested by Profes- sor Prevost, of Geneva, and is now, I believe, pret- ty generally adopted. Thus you may suppose all bodies whatever constantly radiating caloric: those that are of the same temperature give out and re- ceive equal quantities, so that no change of tempe- rature is produced in them; but when one body contains more free caloric than another, the ex- change is always in favour of the colder body, until an equilibrium is effected; this you found to be the case when the marble table cooled your hand, and again when it melted the ice. Caroline. This surprises me extremely: I thought, from what you first said, that the hotter bodies. alone emitted rays of caloric which were absorbed by the colder, for it seems jinfair that a hot body should receive any caloric from a cold one, even though it should return a greater quantity. D 38 Mrs. B. It may at first appear so, but it is no more extraordinary than that a candle should send forth rays of light to the sun, or that a stone in fal- ling should attract the earth, as you know it does from the law of gravitation. Caroline. Well, Mrs B. since you have all na- ture to oppose to me, I believe that I must give up the point. But I wish I could see these rays of ca- loric, I should then have greater faith in them. Mrs. B. Will you give no credit to any sense but that of sight? You may feel the rays of caloric which you receive from any body of a temperature higher than your own; the loss of the caloric you part with in return, it is true is not perceptible; for as you gain more than you lose, instead of suf- fering a diminution, you are really making an ac- quisition of caloric. It is therefore only when you are parting with it to a body of a lower temperature, that you are sensible of the sensation of cold, be- cause you then sustain an absolute loss of caloric. Emily. And in this case we cannot be sensible of the small quantity of heat we receive in exchange from the colder body, because it serves only to di- minish the loss. Mrs. B. Very well, indeed, Emily. Professor Pictet, of Geneva, has made some very interesting experiments to prove that caloric radiates from all bodies whatever, and that these rays may be reflect- ed, according to the laws of optics, in the same manner as light. I wish I could repeat these expe- riments before you, but the difficulty of procuring mirrors fit for the purpose puts it out of my power; you must therefore be satisfied with an account of them, illustrated by this diagram: {Plate III. Fig. 4.) Plate III. A A. and B B. Concave mirrors fixed on stands . C. Heat-d bul- let placed in the focus of the mirror A. D. The thermometer with its bulb placed in the focus of the mirror B. 1234. Rays of ca« 39 —He placed an iron bullet, about two inches in di- ameter, and heated to a degree not sufficient to ren- der it luminous7 in the focus of a large metallic mir- ror. The rays of heat which fell on this mirror were reflected, agreeably to the property of concave mirrors, in a parallel direction, so as to fall on a simi- lar mirror, which was placed opposite to the first, at the distance of about twelve feet; thence they converged to the focus of the second mirror, in which the bulb of a thermometer was placed, the consequence of which was, that the thermometer immediately rose several degrees. Emily. But would not the same effect have taken place, if the rays of caloric from the heated bullet had fallen directly on the thermometer, without the assistance of the mirrors ? Mrs. B. The effect would in that case have been so trifling, at the distance at which the bullet and the thermometer were from each other, as would probably have rendered it imperceptible. The mir- rors, you know, greatly increase the effect, by col- lecting a large quantity of rays into a focus; but their principal use was to prove that the calorific emanation was reflected in the same manner as light. Caroline. And the result I think was very con- clusive. Mrs. B. The experiment was afterwards re- peated with a wax taper instead of the bullet, with a view of separating the light from the caloric. For this purpose a transparent plate of glass was interpo- sed between the mirrors; for light you know passes with great facility through glass, whilst the trans- mission of caloric is considerably impeded by it. It was found however, in this experiment, that some of the calorific rays passed through the glass toge- loric radiating from the bullet and falling on the mirror A. 5678. The same rays reflected from the miror A to mirr iB, 9 10 U J2. The SAtrie rays reflected by the minor B to the thermometer. 40 ther with the light, as the thermometer rose a few degrees; but as soon as the glass was removed, and ^ free passage left to the caloric, it rose immediately j'ouble the number of degrees. Emily. This experiment as well as that of Dr. Iferschell's proves that light and heat may be separa- ted ; for in the latter experiment the separation was not perfect, any more than in that of Mr. Pictet. Caroline. I should like to repeat Mr. Pictet's ex- periments, with the difference of substituting a cold body instead of the hot one, to see whether cold would not be reflected as well as heat. Mrs. B. That experiment, was proposed to Mr. Pictet by an incredulous philosopher like yourself, and he immediately tried it by substituting a piece of ice in the place of the heated bullet. Caroline. Well, Mrs. B. and what was the result? Airs. B. The thermometer fell considerably. Caroline. And does not that prove that cold is not merely a negative quality, implying simply an inferior degree of heat? The cold must t>e positive, since it is capable of reflection. 'Airs. B. So it at first appeared ; but upon a lit- tle consideration it was found that it afforded only an additional proof of the reflection of heat: this I shall endeavour to explain to you. ,N We suppose that all bodies whatever radiate calo- ric; the thermometer used in these experiments therefore emits calorific rays in the saule manner as any other substance. When its temperature is in equilibrium with that of the surrounding bodies, it receives as much caloric as it parts with, and no change of temperature is produced. But when we introduce a body of a lower temperature, such as a piece of ice* which parts with less caloric than it re- ceives, the consequence is, that its temperature is raised, whilst that of the surrounding bodies is pro- portionally lowered; and as, from the effect of the 41 mirrors, a more considerable exchange of rays takes place between the ice and the thermometer, than between these and any of the surrounding bodies, die temperature of the thermometer must be more lowered than that of any other adjacent object. Caroline. I do not perfectly understand your ex- planation. Mrs. B. This experiment is exactly similar to that made with the heated bullet: for, if we consi- der the thermometer as the hot body (which it cer- tainly is in comparison to the ice), you may then ea- sily understand that it is by the loss of the calorific rays which the thermometer sends to the ice, and not by any cold rays received from it, that the fall of the mercury is occasioned; for the ice, far from emitting rays of cold, sends forth rays of caloric, which diminish the loss sustained, by the thermo- meter. Let us say, for instance, that the radiation of the thermometer towards the ice is equal to 20, and that of the ice towards the thermometer to 10; the ex- change in favour of the ice is as 20 is to 10, or the thermometer absolutely loses 10, whilst the ice gains 10. Caroline. But if the ice actually sends rays of ca- loric to the thermometer, must not the latter fall still lower when the ice is removed ? Alrs.B. No; for the air which will fill the space that the ice occupied, being of the same tempera- ture as the thermometer, will emit and receive an equal quantity of caloric, so that no alteration of temperature will be produced. Caroline. I must confess that you have explained this in so satisfactory a manner that I cannot help being convinced that cold has no real claim to the rank of a positive being So now we may proceed to the other modifications of caloric. D2 42 Airs. B. We have not yet concluded our obser- vations on free caloric. But I shall defer, till our next meeting, what I have further to £ay on this subject, as I believe it will afford ui ample conver- sation for another interview. ® CONVERSATION III. Continuation of the SubjeB. Mrs. B. In our last conversation, we began to examine the constant tendency of free caloric to restore an equilibrium of temperature. This property, when once well understood, affords the explanation of a great variety of facts which appeared formerly un- accountable. You must observe, in the first place, that the effect of this tendency is gradually to bring all bodies that are in contact, to the same tempera- ture. Thus, the fire which burns in the grate, com- municates its heat from one object to another, till every part of the room has an equal proportion of it. Emily. And yet this book is not so cold as the toble on which it lies, though both are at an equal distance from the fire, and actually in contatt with each other, so that, according to your theory, they should be exactly of the same temperature ? Caroline. And the hearth, which is much nearer the fire than the carpet, is certainly the colder of the two. Mrs, B. If you ascertain the temperature of 43 these several bodies by a thermometer (which is a much more accurate test than your feeling), you will find that it is exactly the same. Caroline. But if they are of the same tempera- ture, why should the one feel colder than the other? Mrs. B. The nearth and the table feel colder than the carpet or the book, because the latter are not such good conductors of heat as the former. Ca- loric finds a more easy passage through marble and wood, than through leather and worsted; the two former will therefore absorb heat more rapidly from your hand, and consequently give it a stronger sen- sation of cold than the two latter, although they are all of them really of the same temperature. Caroline. So, then, the sensation I feel on touch- ing a cold body, is in proportion to the rapidity with which my hand yields its heat to that body ? Mrs. B. Precisely; and, if you lay your hand successively on every object in the room, you will discover which are good, and which are bad con- ductors of heat, by the different degrees of cold you feel. But in order to ascertain this,pomt,. it is ne- cessary that the several substances should be of the same temperature, which will not be the case with those that are very near the fire, or those that are exposed to a current of cold air from a window or door. Emily. But what is the reason that some bodies are better conductors of heat than others ? Mrs. B. That is a point not well ascertain- ed. It is conjectured that a certain union or adhe- rence takes place between the caloric and the parti- cles of the body through which it passes. If this adherence be strong, the body detains the heat, and parts with it slowly and reluctantly; if slight, it pro- pagates it freely and rapidly. The conducting power of a body is therefore, inversely, 4vs its tendency %o unite with caloric. 44 Emily. That is to say, that the best conductors are those that have the least affinity for caloric. Mrs. B. Yes; but I object to the term affinity in this case, because as that word is used to express a chemical attraction (which can be destroyed only by decomposition), it cannot be applicable to the slight and transient union that takes place between free caloric and the bodies through which it passes; an union which is so weak, that it constantly yields to the tendency which caloric has to an equilibrium. Now you clearly understand, that the passage of ca- loric, through bodies that are good conductors, is much more rapid than through those that are bad conductors, and that the former both give and re- ceive it more quickly, and therefore, in a given time, more abundantly, than bad conductors, which makes them feel either hotter or colder, though they may be in fact, of the same temperature. Caroline. Yes, I understand it now; the table, and the book lying upon it, being really of the same temperature, would each receive in the same space of time, the same quantity of heat from my hand, were their con.iuihng powers equal; but as the ta- ble is the best conductor of the two, it will absorb the heat from my hand more rapidly, and conse- quently produce a stronger sensation of cold than the book. Mrs. B. Very well, my dear; and observe, like- wise, that if you were to heat the table and the book an equal number of degrees above the temperature of your body, the table which before felt the colder, would now feel the hotter of the two; for as in the first case it took the heat most rapidly from your hand, so it will now impart heat most rapidly to it. Thus the marble table, which seems to us colder than the mahogany one, will prove the hotter of the two to the ice; for if it takes heat more rapidly from our hands, which are warmer, it will give out 45 heat more rapidly to the ice, which is colder. Do you understand the reason of these apparently op- posite effects ? Emily. Perfectly. A body that is a good con- ductor of caloric, affords it a free passage ; so that it penetrates through that body more rapidly than through one which is a bad conductor; and, conse- quently, if it is colder than your hand, you lose more caloric, and if it is hotter, you gain more than with a bad conductor of the same temperature. Mrs. B. But you must observe that this is the case only when the conductors are either hotter or colder than your hand; for, if you heat different conductors to the temperature of your body, they will all feel equally warm, since the exchange of ra- diation between bodies of the same temperature is equal. Now, can you tell me why flannel clothing, which is a very bad conductor of heat, prevents our feeling cold? Caroline. It prevents the cold from penetrating. Mrs.. B. But you forget that cold is only a ne- gative quality. Caroline. True; it only prevents the heat of our bodies from escaping so rapidly as it would other- wise do. Mrs. B. Now you have explained it right: the flannel rather keeps in the heat, than keeps out the cold. Were the atmosphere of a higher tempera- ture than our bodies, it would be equally efficacious in preserving them of an«juniform temperature, as it would prevent the free access of the external heat, by the difficulty with which it conducts it. Emily. This, I think, is very clear. Heat, whe- ther external or internal, cannot easily penetrate flannel; therefore in cold weather it keeps us warm; and if the weather was hotter than our bodies, is would keep us cool. 46 Mrs. B. For the same reason, glass windows, which are very bad conductors of heat, keep a room warm in winter and cool in summer, provided the sun does not shine upon them. The most dense bodies are, generally speaking, the best conductors of heat. At the temperature of the atmosphere a piece of metal will feel much colder than a piece of wood, and the latter than a piece of woollen cloth: this again will feel colder than flannel; and down, which is one of the lightest, is at the same time, one of the warmest bodies. Caroline. This is, I suppose, the reason that the plumage of birds preserves them so effectually from the influence of cold in winter ? Mrs. B. Yes; but though feathers in general are an excellent preservative against cold, down is a kind of plumage peculiar to aquatic birds, and co- vers their chest, which is the part exposed to the water; for though the surface of the water is not of a lower temperature than the atmosphere, yet, as it is a better conductor of heat, it feels much colder, consequently the chest of the bird requires a warmer covering than any other part of its body. Most animal substances, especially those which Providence has assigned as a covering for animals, such as fur, wool, hair, skin, &c. are bad conduc- tors of heat, and are, on that account, such excel- lent preservatives against the inclemency of winter, that our warmest apparel is made of these materials. In fluids of different densities, the power of con- ducting heat varies no less remarkably; if you dip your hand into this vessel full of mercury, you will scarcely conceive that its temperature is not lower than that of the atmosphere. Caroline. Indeed I can hardly believe it, it feels so extremely cold —But we may easily ascertain its true temperature by the thermometer—It is really not colder than the air;—the apparent difference M then is produced merely by the difference of the conducting power in mercury and in air ? Mrs. B. Yes; hence you may judge how little the sense of feeling is to be relied on as a test of the temperature of bodies, and how necessary a thermometer is for that purpose. But I must not forget to tell you, that it has been doubted whether fluids have the power of conducting caloric in the same manner as solid bodies. Count Rumford a very few years since, attempted to prove, by a variety of experiments, that fluids, when at rest, were not at all endowed with this property. Caroline. How is that possible, since they are capable of imparting cold or heat to us; for if they did not conduct heat, they would neither take it from, nor give it to us ? , Mrs. B. Count Rumford did not mean to say that fluids do not -communicate their heat to-solid bodies; but only that heat does not pervade fluids, that is to say, is not transmitted from one particle of a fluid to another, in the same manner as in solid bodies. Emily. But when you heat a vessel of water over the fire, if the particles of water do not communi- cate heat to each other, how does the water become hot throughout ? Mrs. B. By constant agitation. Water as you have seen, expands by heat in the same manner as solid bodies; the heated particles of water there- fore, at the bottom of the vessel, become specifical- ly lighter than the rest of the liquid, and conse- quently ascend to the surface, where, parting with some of their heat to the colder atmosphere, they are condensed, and give way to a fresh succession o f heated particles ascending from the bottom, which having thrown off their heat at the surface, are in their turn displaced. Thus every particle is succes- sively heated at the bottom, and cooled at the sur- 4S face of the liquid; but as the fire communicates heat more rapidly than the atmosphere cools the succession of surfaces, the whole of the liquid in time becomes heated. Caroline. This accounts most ingeniously for the propagation of heat upwards. But suppose you were to heat the upper surface of a liquid, the particles being specifically lighter than those below, could not descend: how therefore woulpl the heat be com- municated downwards. Mrs. B. Count Rumford assures us, that if there was no agitation to force the heated surface down- wards, the heat would not descend. In proof of this, he succeeded in making the upper surface of' a vessel of water boil and evaporate, while a cake of ice remained frozen at the bottom. Caroline. That is very extraordinary indeed! Mrs. B. It appears so, because we are not ac- customed to heat liquids by their upper surface, but you will understand this theory better if I show you the internal motion that takes place in liquids when they experience a* change of temperature. The motion of the liquid itself is indeed invisible from the extreme minuteness of its particles; but if you mix with it any coloured dust, or powder, of nearly the same specific gravity, as the liquid, you may judge of the internal motion of the latter by that of the coloured dust it contains. Do you see the small pieces of amber moving about in the liquid contained in this phial ? • • Caroline. Yes, perfectly. Mrs. B. We shall now immerse the phial in a glass of hot water, and the motion of the liquid will be shown, by that.which it communicates to the amber. Emily. I see two currents, the one rising along the sides of the phial, the other descending in the centre; but I do not understand the reason of this. 49" Mrs. B. The hot water communicates its eald* ric, through the medium of the phial, to the par- ticles of the fluid nearest to the glass; these dilate and ascend laterally to the surface,' where, in part- ing with their heat, they are condensed, and in de- scending, form the central current. Caroline. This is indeed a very clear and satis- factory experiment; but how much slower the cur- rents now move than they did at first? Mrs. B. It is because the circulation of particles has nearly produced an equilibrium of temperature between the liquid in the glass and that in the phial. Caroline. But these communicate laterally, and I thought that heat in liquids could be propagated on- ly upwards? Mrs. B. You do not take notice that the heat is imparted from one liquid to the other, through the medium of the phial itself, the external surface of which receives the heat from the water in the glass, whilst its internal surface transmits it to the liquid it contains. Now take the phial out of the hot water, and observe the effects of its cooling. Emily. The currents are reversed^ the external current now descends, and the internal one rises___ I guess the reason of this change:—the phial being in contact with cold air instead of hot water, the external particles are cooled instead of being heat- ed; they therefore descend and force up the central particles, which being warmer are consequently lighter. Mrs. B. It is just so. Count Rumford infers from hence, that no alteration of temperature can take place in. a fluid, without an internal motion of its particles; and as this motion is produced only by the comparative levity of the heated, particles, heat cannot be propagated downwards. This theory explains the reason of the cold that is-found to prevail at the bottom of the lakes in 50 Switzerland, which are fed by rivers issuing from the snowy Alps. The water of these rivers being colder, and therefore more dense than that of the lakes, subsides to the bottom, where it cannot be affected by the warmer temperature of the surface; the motion of the waves may communicate this temperature to some little depth but it can descend no further than the agitation extends. Emily. But when^he atmosphere is colder than the lake, the colder surface of the water will de- scend for the very reason that the warmer will not? Mrs. B. Certainly ; and it is on this account that neHher a lake nor any body of water whatever, can be frozen until every particle of the water has risen to the surface to give off its caloric to the cold- er atmosphere; therefore the deeper a body of wa- ter is, the longer will be the time it requires to be frozen. Emily. But if the temperature of the whole bo- dy of water is brought down to the freezing point, why is only the surface frozen ? Mrs. B. The temperature of the whole body is lowered, but not to the freezing point. The dimi- nution of heat as you know, produces a contraction in the bulk of fluids, as well as of solids. This ef- fect, however does not take place in water below the temperature of forty degrees, which is eight de- grees above the freezing point. At that tempera- ture, therefore,, the internal motion, occasioned by the increased specific gravity of the condensed par- ticles, ceases; for when the water at the surface no longer condenses, it will no longer descend, and leave a fresh surface exposed to the atmosphere: this surface alone, therefore, will be further expo- sed to its severity, and will soon be brought down to the freezing point, when it becomes ice, which being a bad conductor of heat, preserves the water ^ ^ 31 beneath a long time from being affixed by the eX: ternal cold. Caroline. And the sea does not freeze, I suppose, because its depth ,s so great, that a frost never lasts long enough to bring down the temperature of such a great body of water to forty degrees ? m Airs. B. No, that is not the case; for salt water is an exception to this law, as it condenses even ma- ny degrees below the freezing point. When the caloric of fresh water therefore is imprisoned by the ice, the ocean still continues throwing off heat into the atmosphere, which is a most signal dispensation of 1 rovidence to moderate the intensity of the cold m winter. Emily. I admire this theory extremely;* but al- low me to ask you one more question relative to it. You said that when water was heated over the'fire the particles at the bottom of the vessel ascended as soon as heated/ in consequence of their specific le- vity : why does not the same.effect continue when the water boils, and is converted into steam ? and why does the steam rise from the surface instead c. the bottom of the liquid ? Mrs. B. The steam or vapour does ascend from the bottom, though it seems to arise from the sur- face of the liquid. We shall boil some water in this Florence flask; {Plate IV. Fig. 5.) you will then see through the glass, that the vapour rises in bub- ■t Plate IV. , Fig. 5. Boiling water in a flask over a patent lamp. a S\6' r Ether evaP°rated and WaCer fozen in the air pump. A. A phial of ether. B. Glass vessel containing water. C C. Ther- mometers, one in the ether, the other in the water. * This theory of the non-conducting power of fluids, notwith- standing all its plausibility, has been found, by a variety of subse quent experiments, to have been carried by Count Rumford rather too far; and it is now generally admitted that fluids are not »ntirelv destitute of condudibility, though they p.ooagate heat chhfly by mo- tion, in the manner just explained, and possess the conducting power but in a very imperfect degree. & ' 52 Dies from the bottom. We shall make it boil by means of a lamp, which is more convenient for this purpose than the chimney fire----■ Emily. I see some small bubbles ascend, and a ^reat many appear all over the inside, of the flask; does the water begin to boil already ? Mrs. B. No ; what you now see are bubbles of air, which were either enclosed in the water, or attached to the inner surface of the flask, and which, being rarefied by the heat, ascend in the water. • Emily. But the heat which rarefies the air enclo- sed in the water, must rarefy the water at the same time ; therefore, if it could remain stationary in the water when both were cold, I do not understand why it should not when both are equally heated ? Mrs. B. Air being much less dense than water, is more easily rarefied; the former therefore ex- pands to a great extent, whilst the latter continues to occupy nearly the same space; for water dilates comparatively but very little without changing its state and becoming vapour. Now that the water m the flask begins to boil, observe what large bubbles rise from the bottom of it. Emily. I see them perfectly; but I wonder that they have sufficient power to force themselves through the water. . Caroline. They must rise, you know, from their specific levity. -^~-' Mrs. B. You are right, Caroline; but vapour has not in all liquids (when brought to the degree of vaporisation) the power of overcoming the pres- sure of the less heated surface. Metals for instance, evaporate only from the surface; therefore no va- pour will ascend from them till the degree of heat which is necessary to form it has reached the sur- face; that is to say till the whole of the liquid..is brought to the boiling point. This is the case with all metals, mercury alone excepted. Emily. I have observed that steam, immediately issuing from the spout of a tea-kettle, is less visible than at a further distance from it; yet it must be more dense when it first evaporates than when it begins to diffuse itself in the air. Mrs. B. Your objection is a very natural one ; and in order to answer it, it will be necessary for me t® enter into some explanation respecting the nature of solution. Solution takes place whenever a Uody is melted in a fluid. In this operation the body is reduced to such a mioute state of division by the fluid, as to become invisible in it, and to par- take of its fluidity: but this happens without any decomposition, the body being only divided into its integrant particles by the fluid in which it is melted. Caroline. It is then a mode of destroying the at- traction of aggregation. Airs. B. Undoubtedly.—The two principal sol- vent fluids are water and caloric. You may have observed that if yotf melt salt in water, it totally disappears, and the water remains clear and transpa- rent as before; yet though the union of these two bodies appears so perfect, it is not produced by any chemical combination ; both the salt and the water remain unchanged; and if you were to separate them by evaporating the latter, you would find the salt in the same state as before. Emily. I suppose that water is a solvent for solid bodies, and caloric for liquids ? " Mrs. B. Liquids of course can only be convert- ed into vapour by caloric. But the solvent power of this agent is not at all confined to that class of bodies; a great variety of solid substances are dis- solved by heat: thus metals, which are insoluble in water, can be dissolved by intense heat, being first fused or converted into a liquid, and then rarefied into an invisible vapour. Many other bodies, such as salts, gums, &c. yield to either of these solvents. e 2 Ji'4 Caroline. And that, no doubt, is the reason why- hot water will melt them so much better than cold water ? Airs. B. It is so. Caloric may indeed be consi- dered as having, in every instance, some share in the solution of a body by water, since all water, however low its temperature may be, always con- tains more or less caloric. Emily. Then perhaps water owes its solvent pow- er merely to the caloric it contains ? Mrs. B. That probably would be carrying the speculation too far; I should rather think that wa- ter and caloric unite their efforts to dissolve a body, and that the difficulty or facility of effecting this, depend both on the degree of attraction of aggre- gation to be overcome, and on the arrangement of the particles which are more or less disposed to be divided and penetrated by the solvent. Emily. But have not all liquids the same solvent power as water ? Mrs. B. The solvent power of other liquids va- ries according to their nature, and that of the sub- stance submitted to their action. Most of these sol- vents, indeed, differ essentially from water, as they do not merely separate the integrant particles of the bodies which they dissolve, but attack their consti- tuent principles by the power of chemical attraction, thus producing a true decomposition. These more complicated operations, which may be distinguished by the name of chemical solutions, we must consider in another place, and confine our attention at pre- sent to the simple solutions by water and caloric. Caroline. But there are a variety of substances which, when dissolved in water, make it thick and muddy, and destroy its transparency. Mrs. B. In this case it is not a solution, but sinv ply a mixture. I shall show you the difference be- tween a solution and a mixture, by putting some 35 common salt into one glass of water, and some pow- der of chalk into another; both these substances are white, but their effect on the water will be very dif- ferent. Caroline. Very different indeed! The salt entire- ly disappears and leaves the water transparent, whilst the chalk changes it into an opake liquid like milk. Emily. And would lumps of chalk and salt pro- duce similar effects on water ? Mrs. B. Yes, but not so rapidly; salt is indeed soon melted though in a lump, but chalk which does not mix so readily with water, would require a much greater length of time; I therefore prefer- red showing you the experiment with both substan- ces reduced to powder, which does not in any res- pect alter their nature, but facilitates the operation merely by presenting a greater quantity of surface to the water. I must not forget to mention a very curious cir- cumstance respecting solutions, which is, that a fluid is not increased in bulk by holding a body in solu- tion. Caroline. That seems impossible; for two bodies cannot exist together in the same space. Mrs. B. That is true, my dear; but two bodies may, by condensation, occupy the same space which one of them filled before. It is supposed that there are pores or interstices, in which the salt lodges, be- tween the minute particles of the water. And these spaces are so small that the body to be dissolved must be divided into very minute particles in order to be contained in them; and it is this state of very great division that renders them invisible. Caroline. I can try this experiment immediately. —It is exactly so—the water in this glass, which I filled to the brim, is melting a considerable quantity of salt without overflowing. I shall try to add ,a :>g little more.—But now, you see, Mrs. B. the water runs over. Airs. B. Yes; but observe that the last quanti- ty of salt you put in remains solid at the bottom, and displaces the water; for it has already melted all the salt it is capable of holding in solution. This is called the point of saturation; and the water is now said to be saturated with salt. Emily. This happens, I suppose, when the inter- stices between the particles of the liquid are com- pletely filled ? Airs. B. Probably. But these remarks, you must observe do not apply to a mixture; for any substance which does not dissolve, increases the bulk of the li- quid. Emily. I think I now understand the solution of a solid body by water perfectly: but I have not so clear an idea of the solution of a liquid by caloric. Airs. B. It is precisely of the same nature ; but as caloric is an invisible fluid, its action as a solvent is not so obvious as that of water. Caloric dissolves water, and converts it into vapour by the same pro- cess as water dissolves salt; that is to say, the parti- cles of water are so minutely divided by the calorie as to become invisible. Thus, you are now enabled to understand why the vapour of boiling water, when it first issues from the spout of a kettle, is in- visible ; it is so, because it is then completely dissol- ved by caloric. But the air with which it comes in contact, being much colder than the vapour, the latter yields to it a quantity of its caloric The par- ticles of vapour being thus in a great measure de- prived of their solvent, gradually collect and become visible in the form of steam, which is water in a state of imperfect solution; and if you were further to deprive it of its caloric, it would return to its original liquid state. Caroline. That I understand very well; but in 51 what state is the steam, when it again becomes in- visible by being diffused in- the air? Mrs. B. It is carried off and again dissolved by the air. J - Emily. The air then has a solvent power, like water and caloric ? Mrs. B. Its solvent power proceeds chiefly, if not entirely, from the caloric contained in it, the atmosphere acting only as a vehicle. Sometimes the watery vapour diffused in the atmosphere is but imperfectly dissolved, as is the case in the formation of clouds and fogs; but if it gets into a region of air sufficiently warm, it becomes perfectly invisible. Emily. Does the air ever dissolve water, without its being previously converted into vapour by boil- ing? Mrs. B. Yes, it does. Water when heated to the boiling point, can no longer exist in the form of water, and must necessarily be converted into vapour, whatever may be the state and temperature of the surrounding medium; but the air (by means probably of the calorie it contains) can take up a certain portion of water at any temperature, and hold it in a state of solution Thus the atmosphere is continually carrying off moisture from the earth, until it is saturated with it. The tendency of free caloric to an equilibrium, together with its solvent power, are likewise con- nected with the phenomena of rain, of dew, &c. When a cloud of a certain temperature happens to pass through a colder region of the atmosphere, it parts with a portion of its heat to the surrounding air; the quantity of caloric therefore, which served to keep the cloud in a state of vapour, being dimi- nished, the watery particles approach each other, and form themselves into drops of water, which be- ing heavier than the atmosphere, descend to the earth. There are also other circumstances, and uar> 59 ticularly the variation in the weight of the atmos- phere, which may contribute to the formation of rain. This howeyer, is an intricate subject, into ^ which we cannot more fully enter at present. Emily. But in what manner do you account for the formation of dew ? Mrs. B. During the heat of the day the air is able to retain a greater quantity of vapour in a state of solution, than either in the morning or evening. As soon, therefore, as a diminution of heat takes place towards sun-set, a quantity of vapour is con- densed, and falls to the ground in form of dew. The morning dew, on the contrary, rises from the earth ; but when the sun has emitted a sufficient quantity of caloric to dissolve it, it becomes invisi- ble in the atmosphere. When once the dew, or any liquid whatever, is perfectly dissolved by the air, it occasions no humidity; it is only when in a state of imperfect solution, and floating in the form of wa- tery vapour in the atmosphere, that it produces dampness. Caroline. I have often observed, Mrs. B. that when I walk out in frosty weather, with a veil over my face, my breath freezes upon it. Pray what is the reason of that ? Mrs. B. It is because the cold air immediately seizes on the caloric of your breath, and reduces it, by robbing it of its solvent, to a denser fluid, which .'*, is the watery vapour that settles on your veil, and p there it continues parting with its ^caloric till it is U brought down to the temperature of the atmosphere, and assumes the form of ice. You may, perhaps, have observed that the breath j\ of animals, or rather the moisture contained in it, ..>j is visible during a frost, but not in warm weather.* It * Unless in very d »mp weather, when the atmosphere is already iji'.u rated with moisture. 59 In the latter case, the air is capable of retaining it in a state of solution, whilst in the former, the cold condenses it into visible vapour; and for the same reason, the steam arising from water that is warmer than the atmosphere, is visible. Have you never taken notice of the vapour rising from your hands after having dipped them into warm water ? Caroline. Often, especially in frosty weather. Mrs. B. When a bottle of wine is taken fresh from the cellar (in summer particularly), it will scon be covered with dew; and even the glasses into which the wine is poured will be moistened with a similar vapour. Let me hear if you can account for this ? Emily. The bottle is colder than the surround- ing air, therefore it must absorb caloric from it; and the moisture which that air held in solution must become visible, and form the dew which is de- posited on the bottle. Mrs. B. Very well, Emily. Now, Caroline, can you tell me why, in a warm room, or close carriage, the contrary effect takes place; that is to say, that the inside of the windows are covered with vapour? Caroline. I have heard that it proceeds from the^ breath of those within the carriage; and I suppose it is occasioned by the windows which, being cold- er than the breath, deprive it of part of its caloric, - and by this means convert it into watery vapour. Mrs. B. Very well, my dear: I am extremely glad to find that you both understand the subject so well. We have already observed that pressure is an ob- stacle to evaporation: there are liquids that contain so great a quantity of caloric, and whose particles consequently adhere so slightly together, that they may be converted into vapour without any elevation of temperature, merely by taking off the weight of the atmosphere. In such liquids, you perceive, it 60 is the pressure of the atmosphere alone that connect; their particles and keeps them fn a liquid state. Caroline. I do not well understand why the par- < tides of such fluids should be disunited and con- verted into vapour, without any addition of heat, in spite of the attraction of cohesion ? Mrs. B. It is because the quantity of caloric * which enters into the formation of these fluids is f.~ sufficient to overcome their attraction of cohesion. Ether is of this description; it will boil and be con- verted into vapour, without any application of heat, if the pressure of the atmosphere be taken off. Emily. I thought that ether would evaporate without either taking away the pressure of the at- mosphere, or applying heat, and that it was for that reason so necessary to keep it carefully corked up. Mrs. B. That is true; but in this case it will evaporate but very slowly. I am going to show you ; how suddenly the ether in this phial will be con- j verted into vapour, by means of the air-pump.— ,. .1 Observe with what rapidity the bubbles ascend, as I -.-' take off the pressure of the atmosphere. Caroline. It positively boils: how singular to see , a liquid boil without heat! Mrs. B. Now I shall place the phial of ether in this glass, which it nearly fits, so as to leave only a small space, which I fill with water ; and in this . \i state I put it again under the receiver. {Plate IV. Fig. 6.)*—You will observe, as I exhaust the air from it, that whilst the ether boils, the water freezes* 19 * Two pieces of tftin glass tubes, sealed at one end, might answer 1 this purpose better. The experiment, however, as here described, is :M difScuIr, and requires a very ni e apparatus. But if instead of phials ~Sm or tubes, two watch glasses be used, water may be frozen almost in- ^^ Ktantly in the ■same manner. The two glasses are placed over one 1 annther, with a few diops of water interp sed between them, and the * uppermost glass is filled with ether. After working the pump for a 1 minute or two, the glasses are found to adhere strongly together, and I ■a thin layer of ise is seen between them. 0 Caroline. It is indeed wonderful to see water freeze by means of a boiling fluid! Emily. There is another circumstance which I am unable to account for. How can the ether change to a state of vapour, without an addition of caloric; for it must contain more caloric in a state of vapour, than in a state of liquidity; and though you say that it is the pressure of the atmosphere which condenses it into a liquid, it must be, I sup- pose, by forcing out. part of the caloric that belongs to it when in an aeriform state f Mrs. B. You are right. Ether, in a liquid state, does not contain a sufficient quantity of caloric to become vapour. I have therefore, two difficulties to explain; first, from whence the ether obtains the calorie necessary to convert it into vapour when it is relieved from the pressure of the atmosphere; and, secondly, what is the reason that the water, in which the bottle of ether stands, is frozen ? Caroline. Now I think, I can answer both these questions. The ether obtains the addition of caloric required from the water in the glass; and the loss of caloric, which the latter sustains, is the occasion -of its freezing. Mrs. B. You are perfectly right; and if you look at the thermometer which I have placed in the water, whilst I am working the pump, you will see that every time bubbles of vapour are produced, the mercury descends; which proves that the heat bf - the water diminishes in proportion as the ether boils. Emily. This I understand now very well; but if the water freezes in consequence of yielding its ca- loric to the ether, the equilibrium of heat must in this case, be totally destroyed. Yet you have told us, that bodies of a different temperature are always communicating their heat to each other, till it be- comes every where equal; and besides, Ldo not see why the water though originally of the same tem- 62 perature as the ether, gives out caloric to it, till the water is frozen and the ether made to boil. Airs. B. I suspected that you would make these - objections; and in order to remove them, I enclosed . two thermometers in the air-pump ; one of which stands in the glass of water, the other in the phial , •,.' of ether; and you may^cee that the equilibrium of . ' . temperature is not destroyed ; for as the thermome- ter descends in the water, that in the ether sinks in •'*• the same manner; so that both thermometers indi- cate the same temperature^ though one of them is an a boiling, the other in a freezing liquid. Emily. The ether then becomes colder as it h ils? .This is so contrary to common experience, that I confess it astonishes me exceedingly. Caroline. It is, indeed, a most extraordinary cirT cumstance. But pray how do you account fur it? Mrs. B. I cannot satisfy your curiosity at pre- sent; for before we can attempt to explain this apr ;.? parent paradox, we must become acquainted with 5'^jj the subject of latent heat; and that, I think, we * must defer till our next interview. Caroline. I believe, Mrs. B that you are glad to put off the explanation; for it must be a very diffi- cult point to account for. Mrs. B. I hope, however, that I shall do it to your complete satisfaction. Emily. But before we part, give me leave to ask you one question. Would not water, as well as ether, boil with less heat, if the pressure of the at- ,.j mosphere were taken off? ;i Mrs. B. Undoubtedly. You must always recol- ' ledt that there are two forces to overcome, in order , ~; to make a liquid boil, or evaporate; the attraction of aggregation, and the weight of the atmosphere. On the summit of a high mountain (as Mr. De Sausr sure ascertained on Mount Blanc) less heat is requir red to make water boil than in the plain, where the Weight of the atmosphere is greater.—But I Can show you a very pretty experiment, which proves the effect of the pressure of the atmosphere in this res- pect. Observe, that this Florence fla.k is about half full of water, and the upper half of invisible vapour, the water being in the act of boiling—I take it from the lamp and cork it carefully—the water, you see, immediately ceases bailing.—1 shall now wrap a cold wet cloth round the upper part of tjie flask*_____^ Car-Jim. But look, Mrs B. the water begins to boil again, although the wet cloth must rob it more and more of its caloric! What can be the reason of that ? Airs. B. Let us examine its temperature. You see the thermometer immersed in it remains station- ary at 180 degrees, which is about 30 degrees below the boiling point. When I took the flask from the lamp, I observed to you that the upper part of it was filled with vapour; this being compelled to yield its caloric to the wet cloth, was again converted into water—What then filled the upper part of the flask? Emily. Nothing; for it Was too well corked for the air to gain admittance, and therefore the upper part of the flask must be a vacuum. Mrs. B. If the upper part of the flask be a va- cuum, the water below no longer sustains the pres- sure of the atmosphere, and will therefore boil at a much lower temperature. Thus, you see, though it had lost many degrees of heat, it began boiling again the instant the vacuum was formed above it. The boiling has now ceased: if it had been ether, instead of water, it would have continued boiling much longer; but water being a more dense fluid, * O. the whole flask mny be dipped in a bason of cold water. In order to show how much the wate* cools whilst it is boilinr, a ther-, mometer, graduated on the tube ilse f, may be introduced into the bottle through the cork. b-l "cquires a more considerable quantity of caloric to make it evaporate, even when the pressure of th*: atmosphere is removed. Emily. But if the pressure of the atmosphere keeps the particles of ether together, why does it evaporate when exposed to the air ? Nay, does not even water, the particles of which adhere so strong- ly together, slowly evaporate in the atmosphere? Airs. B. I have already told yon that air has the power of keeping a certain quantity of vapour in so- lution at any known temperature; and being con- stantly in a state of motion, and incessantly renew- ing itself on the surface of the liquid, it skims off,, and gradually dissolves, new quantities of vapour, Water also has the power of absorbing a certain quantity of air, so that their action on each other is reciprocal; the air thus enclosed,in water is that which you see evaporate in bubbles when water is heated previous to ks boiling. Emily. What proportion of vapour can air con- tain in a state of solution ? Mrs. B. I do not know whether it has been ex- actly ascertained by experiment; but at any rate this proportion must vary, both according to the tempe- rature and the weight of the atmosphere; for the lower the temperature, and the greater the pressure, the smaller must be the proportion of vapour that air can contain in a state of solution. But we have dwelt so long on the subject of free caloric, that we must reserve the other modifications of that fluid to our next meeting, when we shall endeavour tot proceed more rapidly. €5 CONVERSATION IV. Oh Specif c Heats Latent Heat> and Chemical Heat. Mrs. R + We are now to examine the three other modifi- cations of caloric. Caroline. I am very curious to know of what na- ture they can be; for I have no notion of any kind of heat that is not perceptible to the senses. Airs. R. In order to enable you to understand them, it will be necessary to enter into some pre- vious explanations. It has been discovered by modern chemists, that bodies of a different nature, heated to the same temperature, do not contain the same quantity of caloric. Caroline. How could that be ascertained ? Mrs. B. It was found that, in order to raise the temperature of different bodies the same number of degrees, different quantities of calorie were required for each of them. If, for instance, you place a pound of lead, a pound of chalk, and a pound of milk, in a hot oven, they will be gradually heated to the temperature of the oven; bat the lead will attain it first, the chalk next, and the milk last. Emily. As they were all of the same weight, and exposed to the same heat, I should have thought that they would have attained the temperature of the oven at the same time. *2 66 Caroline. And how is it that they do not.' Airs. B. It is supposed to be on account of the different capacity of these bodies for caloric. Caroline. What do you mean by the capacity of a body for caloric? Mrs. B. I mean a certain disposition of bodies to admit more or less caloric between their minute particles. Let us put as many marbles into this glass as it will contain, and pour some sand over them—ob- serve how the sand penetrates and lodges between them. We shall n6w fill another glass with pebbles- of various forms—you see that they arrange them- selves in a more compact manner than the marbles,- which, being- globular, can touch each other by a single point only. The pebbles, therefore, will not admit so-much sand between them; and consequent- ly one of these glasses will necessarily contain more and than the other, though both of them be equal- ly full. Caroline. This I understand perfectly. The mar- bles and the pebbles represent two bodies of differ- ent kinds, and the sand the caloric contained in diem; and it appears very plain, from this compari- son, that one body may admit of more caloric be- tween its particles than another. Mrs. B. If you understand this, you can nc • origer be surprised that bodies of a different, capa- city for caloric should require different proportions of that fluid to raise their temperatures equally. Emily. But I do not understand why the body that contains the most caloric should not be of the highest temperature; that is to say, feel hot in pro- portion to the quantity of caloric it contains ? Airs.. B. The caloric that is employed in filling* the capacity of a body, is not free caloric; but it is* imprisoned as it were in the body, and is therefore imperceptible: for we can feel only the. free racjia* 61 ting caloric which the body parts with, and not that which it retains. Caroline. It appears to be very extraordinary that heat should be confined in a body in such a manner as to be imperceptible. Mrs. B. If you lay your hand on a hot body, you feel only the caloric which leaves it, and enters your hand; for it is impossible that you should be sensible of that which remains in the body, The' thermometer, in the same manner, is affected only by the free caloric which a body transmits to it, and not at all by that which it does not part with. You see therefore, that the temperature of bodies can be raised only by free radiating caloric. Caroline. I begin to understand it; but I confess that the idea of insensible heat is so new and strange to me, that it requires some time to render it fami- liar. » Airs. B. Call it insensible caloric, and the diffi- culty will appear much less formidable. It is indeed a sort of contradiction to call it heat, when it is so situated as to be incapable of producing that sensa- tion. Emily. Yet is it not this modification of caloric which is called specific heat ? Mrs. B. It is so; but it certainly would have been more correct to have called it specific caloric. Emily. I do not understand how the term specif 'e applies to this modification of caloric ? Mrs. B. It expresses the relative quantity of ca- loric which different bodies of the same weight and temperature are capable of containing. This n edi- fication is also frequently called heat ^of capacity, a term perhaps preferable, as it explains better its own meaning. You now understand, I suppose, why the milk and chalk required a longer time than the lead to raise fcheir temperature to that of the oven ? 63' Emily. Yes: the milk and chalk having a great- er capacity for caloric than the lead, a greater pro- portion of that fluid became insensible in those bo- dies; and the more slowly, therefore their tempe- rature was raised. Mrs. B. You are quite right. And could we measure the heat communicated by the oven to these three bodies, we should find, that though they have all ultimately reached the same temperature,. yet they have absorbed different quantities of heat according to their respective capacities for caloric; that is to say* the milk most, the chalk next, and the lead least. Emily. But supposing that these three bodies* were made much hotter, would heat continue to be- come insensible in them, or is there any point be- yond which the capacity of bodies for caloric is so completely filled, that their heat of temperature can alone be increased? Mrs. B, No: there is no such point; for the capacity of bodies for caloric always increases or di- minishes in proportion to their temperature; so that whenever a body is exposed to an elevation of tem- perature, part of the caloric it receives is detained in an insensible state, in order to fill up its increa- sed capacity. Emily. The more dtense a body is, I suppose^, the less is its capacity for caloric? Mrs. B. That is the case with every individual body; its capacity is least when solid greater when melted and most considerable when converted into vapour. But this does not aLways hold good with? respect to bodies of different nature; iron, for in- stance, contains more specific heat than • ashes, though it is certainly much more dense. This seems to show that specific heat does not merely depend upon the interstices between the particUs; bur pro- bably, also upon some peculiar power of attraction1 69 for caloric. The word capacity therefore, which is generally used, is not perhaps strictly correct; but until we are better acquainted with the nature and cause of specific heat, we cannot adopt a more ap- propriate term. Emily. But Mrs. B. it would appear to me more proper to compare bodies by measure, rather than by weight, in order to estimate their specific heat. Why, for instance, should we not compare pints of milk, of chalk and of lead, rather than pounds of those substances; for equal weights may be compo- sed of very different quantities? Mrs. B. You are mistaken, my dear: equal weights must contain equal quantities of matter; and when we wish to know what is the relative quantity of caloric, which substances of various kinds are capable of containing, under the same tem- perature, we must compare equal weights, and not •qual bulks of those substances. Bodies of the same weight may undoubtedly be of very different dimen- sions; but that does not change the real quantity/ of matter. A pound of feathers does not contain one atom more than a pound of lead. Caroline. I have another difficulty to propose. It appears to me, that if the temperature of the three bodies in the oven did not rise equally, they would never reach the same degree ; the lead would always keep its advantage over the chalk, and milk* and would perhaps be boiling before the others had attained the temperature of the oven. I think you might as well say that, in the course of time, you and I should be of the same age? Mrs. B. Your comparison is not correct, my dear. As soon as the lead reached the temperature of the oven, it would remain stationary ; for it would then give out as much heat as it would receive. You should recollect that the exchange of radiating hear* between two bodies of equal temperature, is equal j, ?0 • It would be impossible, therefore, for the lead to accumulate heat after having attained the tempera- ture of the oven ; and that cf the chall* and milk therefore would u't'mately arrive at the same, stand- ard. Now I fear that this will not hold good with respect to our ages, and that, as long as I live, I shall never cease to keep my advantage over you. Emily. I think that I have found a comparison for specific heat, which is very applicable. Sup- pose that two .nen of equal weight and bulk, but who required different quantities of food to satisfy their appetites, sit down to dinner, both equally hungry; the one would consume a much greater quantity of provisions than the other, in order to1 be equally satisfied. Mrs. B. Yes, that is very fair; for the quanti- ty of food necessary to satisfy their respective appe- tites, varies in the same manner as the quantity of caloric requisite to raise equally the temperature of different bodies. Emily. The thermometer, then, affords no in- dication of the specific heat of bodies ? Airs. B. None at all: no more than satiety is a test of the quantity of food eaten. The thermometer, as I have repeatedly said, can be affected only by a free or radiating caloric, which alone raises the tem- perature of bodies. Emily. And is there no method of measuring the comparative quantities of caloric absorbed in the oven by the lead, the chalk, and the milk? Mrs. B. It may be done by cooling them to the same degree in an apparatus adapted to receive and measure the caloric which they give out. Thus, if you plunge them into three equal quantities of water, each at the same temperature, you will be able to judge of the relative quantity of catonc which the three bodies contained, by that, which, i.» cooling, £hey communicated to their respective portions ei 71 weter; for the same quantity of caloric which they each absorbed to raise their temperature, will aban- don them in lowering it; and on examining the three vessels of water, you will find the one in which you immersed the lead to be the least heated ; that which contained the chalk will be the next ; and that which contained the milk will be heated the most of ail. The celebrated Lavoisier has invented a machine to estimate, upon this principle, the spe- cific heat of bodies in a more perfect manner ; but I cannot explain it to you, till you are acquainted with the next modification of caloric, which is called latent heat. Caroline. And pray what kind of heat is that ? Mrs. B. It is so analogous to specific heat, that most chemists make no distinction bet \v< m\ them $ but Mr. Pictet, in his Essay on fire, has so ju- diciously discriminated them, that I think his view of the subject may contribute to render it clearer. We therefore call latent-heat (a name that was first used by Dr. Black) that portion of insensible caloric which is employed in changing the state of bodies ; that is to say, in converting solids into liquids, or liquids into vapour. The heat which performs these changes becomes fixed in the body which it has ; transformed, and, as it is perfectly concealed from our senses, it has obtained the name of latent heat. Caroline. I think it would be much more cor- rect to call this modification latent caloric, instead of latent heat, since it does not excite the sensation of heat. Mrs. B. That remark is equally applicable to both the modifications of specific and latent heat ; but we must not presume (unless amongst ourselves in order to explain the subject) to alter terms which are still used by much better chemists than ourselves. And, besides, you must not suppose that the nature .of heat is altered by being variously modified : for if 78 latent heat, and specific heat, do not excite the same sensations as free caloric, it is owing to their being in a state of confinement, which prevents them from acting upon our organs ; and, consequently, as soon as they are extricated fromthe body in which they are imprisoned, they return to their state of free caloric. Emily, But I do not yet clearly see in what re- spect latent heat differs from specific heat ; for they are both of them imprisoned and concealed in bodies ? Mrs. B. Specific heat is that which is employed in filling the capacity of a body for caloric, in the state in which this body actually exists ; while latent heat is that which is employed only in effecting a change of state, that is, in converting bodies from a solid to a liquid, or from a liquid to an aeriform state. But I think that, in a general point of view, both these modifications might be comprehended under the name of heat of capacity as in both cases the caloric is equally engaged in filling the capacities of bodies. I shall now show you an experiment which I hope will give you a clear idea of what is understood by latent heat. The snow which you see in this phial, has been cooled by certain chemical means (which I cannot well explain to you at present), to 5 degrees below the freezing point, as you will find indicated by the thermometer, which is placed in it. We shall ex- pose it to rhe heat of a lamp, and you will see the thermometer gradually rise, till it reaches the free- zing point----- Emily. But there the thermometer stops, Mrs. B. and yet the lamp burns just as well as before. Why is not its heat communicated to the thermo- meter ? Caroline. And the snow begins to melt, therefore it must be rising above the freezing point ? Mrs, B. The heat no longer affects the thermo- 73 meter, because it is wholly employed in converting the ice into water. As the ice melts, the caloric becomes latent in the new formed liquid, and there- fore cannot raise its temperature; and the thermome- ter will consequently remain stationary, till the who'e of the ice be melted. Caroline: Now it is all melted, and the thermo- meter begins to rise again. Mrs. B. Because the conversion of the ice into water being completed, the caloric no longer becomes latent; and therefore the heat which the water now receives raises its temperature, as you find the ther- mometer indicates. Emily. But I do not think that the thermometer rises so quickly in the water, as it did in the ice, previous to its beginning to melt, though the lamp burns equally well? Mrs. B. That is owing to the different specific heat of ice and water. The capacity of water for caloric being greater than that of ice, more heat is required to raise its temperature, and therefore the thermometer risesslower in the water than in the ice Emily. True; you said that a solid body always^ increased its capacity for heat by becoming fluid; and this is an instance of it. Mrs. B. But be careful not to confound this with latent heat. Emily. On the contrary, I think that this exam- ple distinguishes them extremely well; for though they both go into an insensible state, yet they differ in this respect, that the specific heat fills the capacity of the body in the state in which it exists, while la- tent heat changes that state, and is afterwards em- ployed in maintaining the body in its new form. Caroline. Now Mrs. B. the water begins to boil, and the thermometer is again stationary. Mrs. B. Well, Caroline, it is your turn to ex- plain the phenomenon. G 7* Caroline. It,is wonderfully curious. The caloric is now busy in changing the water into steam, in which it hides itself and becomes insensible. Ting is another example of latent heat, producing a change of form. At first it converted a solid body into a liquid, and now it turns the liquid into vapour! Airs. B. You see, my dear, how easily you have become acquainted with these modifications of in- sensible heat, which at first appeared so unintelligi- ble. If, now, we were to reverse these changes, and condense the vapour into water, and the water into ice, the latent heat would re-appear entirely, in the form of free caloric. Emily. Pray do let us ,see the effect of latent heat returning to its natural form. Mrs. B. For the purpose of shewing this, we need simply conduct the vapour through tins tube into this vessel of cold water, where it will part with its latent heat and return to its liquid form. Emily. How rapidly the steam heats the water! Mrs. B. That is because it does not merely im- part its free caloric to the water, but likewise its latent heat. This method of heating liquids has oeen turned to advantage, in several economical es- tablishments. At Leeds, for instance, there is a Jarge dye-house, in which a great number of coppers are kept boiling by means of a single one, which is situated without the building, and which alone is heated by fire. The steam of this last is conveyed through pipes into the bottom of each of the other coppers, and it appears extremely singular to see all these coppers boiling, though there is not a particle of, fire in the place. Caroline. That is an admirable-contrivance, and I wonder that it is not in common use. Mrs. B. The steam kitchens, which are get- ting into such general use, are upon the same principle. The steam is conveyed through a pipe in a similar manner, into the several vessels which con- 73 tain the provisions to be dressed, where it commu- nicates to them its latent caloric, and returns to the state of water. Count Rumford makes great use of this principle in many of his fire-places: his grand maxim is to avoid all unnecessary waste of caloric, for which purpose he confines the heat in such a manner, that not a 'particle of it 'shall unnecessarily escape; and while he economises the free caloric, he takes care also to turn the latent- heat to advan- tage. It is thus that he is enabled to produce a de- gree of heat superior to that which is obtained in common fire-places, though he employs but half the quantity of fuel. ,: Emily. When the advantages of such contrivan- ces are so clear and plain, I cannot understand why1 they are not universally used. Mrs. B. A long time is always required before innovations, however useful, can be reconciled with the prejudices of the vulgar. Emily. What a pity it is that there should be a prejudice against new inventions; how much more rapidly the world would improve, if such useful dis- coveries were immediately, and universally adopted? Mrs. B. I believe, my dear, that there are as many novelties attempted to be introduced, the adoption of which would be prejudicial to society, as there are of those which would be beneficial to it. The well informed, though by no means ex- empt from error, have an unquestionable advantage over the illiterate, in judging what is likely or not to prove serviceable; and therefore we find the for- mer more ready to adopt such discoveries as pro- mise to be really advantageous, than the latter, who, having no other test of the value of a novelty but time and experience, at first oppose its intro- duction. The well informed are, however, fre- quently disappointed in their most sanguine expec- tations, and the prejudices of the vulgar though they often retard the progress of knowledge, yet 76 sometimes, it must be admitted, prevent the propa- gation of error.—But we are deviating from our subject. We have converted steam into water, and are now to change water into ice, in order to render the latent heat sensible, as it escapes from the water on its becoming solid. For this purpose we must produce a degree of cold that will make water freeze. Caroline. That must be very difficult to accom- plish in this warm room. Mrs. B. Not so much so as you think. There are certain chemical mixtures which produce a ra- pid change from the solid to the fluid state, or the reverse, in the substances combined, in conse- quence of which change latent heat is either extri- cated or absorbed. Emily. I do not quite understand you. Mrs. B. This snow and salt, which you see me mix together, are melting rapidly; heat therefore must be absorbed by the mixture* and cold produced. Caroline. It feels even colder than ice, and yet the snow is melted. This is very extraordinary. Airs. B. The cause of the intense cold of the mixture is to be attributed to the change from a solid to a fluid state. The union of the snow and salt produces a new arrangement of their particles,. in consequence of which they become liquid, and the quantity of caloric required to effect this change is seized upon by the mixture whereever it can be obtained. This eagerness of the mixture for caloric, during its liquefaction, is such, that it converts part of its own free caloric into latent heat, and it is thus that its temperature is lowered. Emily. Whatever you put into this mixture there- fore, would freeze? Mrs. B. Yes; at least any fluid that is suscep- tible of freezing at that temperature; for the ex- change of radiant heat would always be in favour of the cold mixture, until an equilibrium of tempe- rature was established; therefore unless the body 77 immersed contained more free caloric than would become latent in the mixture during its conversion into a liquid, the former must ultimately give out its latent heat till it cools down to the temperature of the latter. I have prepared this mixture of salt and snow for the purpose of freezing the water from which you are desirous of seeing the latent heat es- cape. I have put a thermometer in the glass of wa- ter that is to be frozen, in order that you may ob- serve how it cools----- Caroline. The thermometer decends, but the heat which the water is now losing, is its free, not its la- tent heat? Mrs. B. Certainly; it does not part with its latent heat till it changes its state and is converted into ice. Emily. But here is a very extraordinary circum- stance! The thermometer is fallen below the free-- zing point, and yet the water is not frozen. Mrs. B. That is always the case previous .to the freezing of water when it is in a state of rest. Now it begins to congeal, and you may observe that the thermometer again rises to the freezing point. Caroline. It appears to me very strange that the thermometer should rise the very moment that the water freezes; for it seems to imply that the water was colder before it froze than when in the act of freezing, .. Airs. B. It is so; and after our long dissertation on this circumstance, I did not think that it would appear so surprising to you. Reflect a little, and I think you will discover the reason of it. Caroline. It must be, no doubt, the extrication of latent heat, at the instant the water freezes, that raise- the temperature. Airs. B. Certainly; and if you now examine the thermometer, you will find that its rise was but tem- porary, and lasted only during the disengagement G 2 78 of the latent heat; it has since fallen and will con- tinue to fall till the ice and mixture are of an equal temperature. Emily. And can you show us any experiments in which liquids, by being mixed, become solid, and disengage latent heat ? Mrs. B. I could show you several; but you are not yet sufficiently advanced to understand them well. I shall, however, try one which will afford you a striking instance of the fact. The fluid which you see in this phial consists of a quantity of a cer- tain salt called muriat of lime, dissolved in water. Now if I pour into it a few drops of this other fluid, called sulphuric acid, the whole or very nearly the whole, will be instantaneously converted into a solid mass. Emily. How white it turns! I feel the latent heat escaping, for the bottle is warm, and the fluid is changed to a solid white substance like chalk ! Caroline. This is indeed the most curious expe- riment we have seen yet. But pray what is that white vapour that ascends from the mixture ? Mrs. B. You are not yet enough of a chemist to understand that. But take care, Caroline, do not approach too near it, for it smells extremely strong. The mixture of spirit of wine and water affords another striking example of the extrication of latent heat. The particles of these liquids, by penetrating each other, change their arrangement, so as to be- come more dense, and (if I may use the expression), less fluid, in consequence of which they part with a quantity of latent heat. Sulphuric acid and water produce the same effect, and even in a much greater degree. We shall try both these experiments, and you will feel how much heat which was in a latent state, is set at li- bertv—Now each of you take hold of one of these glasses— •' 79 Caroline.^ I cannot hold mine; I am sure it is as hot as boiling water. Mrs. B. Your glass, which contains the sulphu- ric acid and water, is indeed, of as high a tempera- ture as boiling water ; but you do not find yours so hot, Emily? Emily. Not quite. But why are not these li- quids converted into solids by the extrication of their latent heat? Mrs. B. Because they part only with a portion of that heat, and therefore they suffer only a dimi- nution of their liquidity. Emily. Yet they appear as perfectly liquid as they did before they were mixed. Mrs. B. They are however considerably con- densed. I shall repeat the experiment in a gradu- ated tube, and you will see that the two liquids, when mixed, occupy less space than they did sepa- rately. This tube is graduated by cubit inches, and this little measure contains exactly one cubit inch; therefore, if I fill it twice, and pour its contents into the tube, they should fill it up to the second mark. Caroline. And so they do, exactly. Mrs. B. Because I put two measures of the same liquid into the tube; but we shall now try it with one of water and one of sulphuric acid; observe the difference— Emily. The two measures, this time, evidently take up less space, though the fluid does not appear to have suffered any change in its liquidity. Mrs. B. The two liquids, however, have un- dergone some degree of condensation from the new arrangement of their particles; they have penetra- ted each other, so as to form a closer substance, and have thus, as it were, squeezed out a portion of their latent heat. But this change of state is cer- tainly much less striking, and less complete, than when liquids are converted into solids. 80 The slakeing of lime is another curious instance of the extrication of latent heat. Have you never observed how quick-lime smokes when water is poured upon it, and how much heat it produces ? Caroline. Yes; but I do not understand what change of state takes place in the lime that occasions its giving out latent heat; for the quick-lime, which is solid, is (if I recollect right) reduced to powder by this operation, and is therefore rather expanded than condensed. Mrs. B. It is from the water, not the lime, that the latent heat is set free. The water incorporates with, and becomes solid in the lime; in consequence of which the heat, which kept it in a liquid state, is disengaged and escapes in a sensible form. Caroline. I always thought that the heat origina- ted in the lime. It seems very strange that water, and cold water too, should contain so much heat. Emily. After this extrication of caloric, the wa- ter must exist in a state of ice in the lime, since it parts with the heat which kept it liquid ? Airs. B. It cannot properly be called ice,- since ice implies a degree of cold, at least equal to the freezing point. Yet as water, in combining with lime, gives out more heat than in freezing, it must be in a state of still greater solidity in the lime, than it is in the form of ice; and you may have ob- served that it does not moisten or liquefy the lime in the smallest degree. Emily. But, Mrs. B. the smoke that rises is white; if it was only pure caloric which escaped, we might feel, but could not see it. Mrs. B. This white vapour is formed by some of the particles of lime, in a state of fine dust, which are carried off by the caloric. Emily In all changes of state, then, a body ei- ther absorbs or disengages latent heat ? Mrs. B. You cannot exactly say absorbs latent 61 heat, as the heat becomes latent only on being con- fined in the body; but you may say that bodies, in passing from a solid to a liquid form, or from the liquid state to that of vapour, absorb heat; and that when the reverse takes place heat is disengaged.* We have seen likewise, that a body may part with some of its latent heat without completely changing its form, as was the case with the mixtures of sul- phuric acid and water, and spirit of wine and wa- ter; but here you must observe, that the condensa- tion which forces out a portion of their latent heat, is occasioned by a new arrangement of the particles, produced by mixing the liquids, they therefore un- dergo a change of state, though no very sensible difference takes place in their form. Caroline. All solid bodies, I suppose, must have parted with the whole of their latent heat ? Mrs. B. We cannot precisely say that; for solid bodies are most of them susceptible of being brought to different degrees of density, during whith ope- ration a quantity of heat is disengaged; as it hap- pens in the hammering of metals, the boring of cannon, and in general whenever bodies are expo* sed to considerable'friction or violent pressure. It has been much disputed, however, to what mo- dification of heat caloric thus extricated belongs, though in general it is considered as latent heat; but it does not seem strictly entitled to that name, as its extrication produces no other change in the body than an increase of density. Emily. And may not the same objection be made to the heat extricated from the mixtures we have just witnessed? for the only alteration that is pro- duced by it is a greater density. Mrs. B. But I observed to you, that the densi- ty was produced by a new arrangement of the par- * This role, if not universal, admits of very few exceptions. 82 tides, owing to the mixing of two different sub- stances ; this cannot be the case, when heat is ex- tricated from solid bodies by mere mechanical force, such as hammering metals; no foreign particles are introduced, and except a closer union, no change of arrangement can take place. The caloric, thus extricated, seems therefore to have a still more du- bious title to the modification of latent heat, than that produced by mixtures. I know no other way of settling this difficulty than by calling them both heat of capacity, a title to which we have agreed that specific heat, and latent heat, have an equal claim. Emily. We can now, I think, account for the ether boiling, and the water freezing in vacuo, at the same temjperature. Mrs. B. Let me hear how you explain it ? Emily. The latent heat, which the water gave out in freezing, was immediately absorbed by the ether, during its conversion into vapour; and there- fore, from a latent state in one liquid, it passed in- to a latent state in the other. Mrs. B. But this only partly accounts for the experiment; it remains to be explained why the temperature of the ether, while in a state of ebul- lition, is brought down to the freezing temperature of the water. It is because the ether, during its evaporation, reduces its own temperature, in the same proportion as that of the water, by converting its free caloric into latent heat; so that, though one liquid boils, and fhe other freezes, their tempera- tures remain in a state of equilibrium. Having advanced so far on the subject of heat, I may now give you an account of the calorimeter, an instrument invented by Lavoisier, upon the prin- ciples just explained, for the purpose of estimating the specific heat of bodies. It consists of a vessel, the inner surface of which is lined with ice, so as to form a sort of hollow globe of ice, in the midst of 63 which the body, whose specific heat is to be ascer- tained, is placed. The ice absorbs caloric from this body, till it nas brought it down to the freezing point: this caloric converts into water a certain por- tion of the ice which runs out through an aperture at the bottom of the machine; and the quantity of ice changed to water is a test of the quantity of calo- ric which the body has given out in, descending from a certain temperature to the freezing point. .Caroline. In this apparatus, I suppose,.the milk, chalk, and lead, would melt different quantities of ice, in proportion to their different capacities for ca- loric ? Mrs. B. Certainly; and thence we are able to ascertain, with precision, their respective capacities for heat. But the calorimeter affords us no more idea of the absolute quantity of heat contained in a body, than the thermometer; for though by means of it we extricate both the free and confined calo- ric, yet we extricate them only to a certain degree, which is the freezing point: and we know not how much they contain of either below that point. Emily. According to this theory of latent heat, jt appears to me the weather should be warm when it freez<»s, and cold in a thaw: for latent heat is liberated from every substance that freezes, and such a large supply of heat must warm the atmos- phere; whilst, during a thaw, that very quantity of free heat must be taken from the atmosphere, and return to a latent state in the bodies which it thaws. Mrs. B. Your observation is very natural; but consider, that in a frost the atmosphere is so much colder than the earth, that all the caloric which it takes from the freezing bodies is insufficient to raise its temperature above the freezing point; otherwise the frost must cease. But if the quantity of latent Jbeat extricated does not destroy the frost, it serves to moderate the suddenness of the change of tern- 84 perature of the atmosphere, at the commencement both of a frost, and of a thaw. In the first instance, its extrication diminishes the severity of the cold; and, in the latter, its absorption moderates the warmth occasioned by a thaw: it even sometimes produces a discernible chill, at the breaking up of a frost. Caroline. But what are the general causes that produce those sudden changes in the weather, espe- cially from hot to cold, which we often experience? Mrs. B. This question would lead us into me- teorological discussions, to which I am by no means competent. One circumstance, however, we can easily understand. When the air has passed over cold countries, it will probably arrive here, at a tem- perature much below our own, and then it must ab- sorb heat from every object it meets with which will produce a general fall of temperature. But I think we have now sufficiently dwelt on the subject of iatent heat; we may therefore proceed to the last modification, which is chemical heat. In this state we consider caloric as one of the constitu- ent parts of bodies. Like any other substance, it is subject to the attraction of composition, and is thus capable of being chemically combined. Emily. In this case, then, it neither affefts the thermometer, nor the calorimeter, since principles united by the attraction of composition can be sepa- rated only by the decomposition of the body. Mrs. B. You are perfectly right. We may con- sider free caloric as moving constantly through the integrant particles of a body; specific and latent heat, as lodging between them, and being there detained by a mere mechanical union; but it is chemical heat alone that actually combines, in consequence of a true chemical affinity, with the constituent particles of bodies; and this union cannot be dissolved without a decomposition produced by superior attractions. 85 Caroline. But if this kind of heat is so perfectly concealed in the body, pray how is it known to exist? Mrs. B. By being freed from its imprisonment; for when the body in which it exists is decomposed, it then returns to the state of free caloric. This ca- loric, however, seldom shews itself entirely, as part of it generally enters into new combinations with some of the constituent parts of the decomposed body, and is thus again concealed under the form of latent heat. But it will be better to defer saying any thing further of this modification of heat at present. When we come to analyse compound bodies, and resolve them into their constituent parts, we shall have many opportunities of becoming better ac- quainted with it. Caroline. Caloric appears to me a most wonder- ful element: but I cannot reconcile myself to the idea of its being a substance •> for it seems to be constantly acting in opposition, both to the attrac- tion of aggregation and the laws of gravity; and yet you decidedly class it amongst the simple bodies. Mrs. B. You are not at all singular in the doubts you entertain, my dear, on this point; for although caloric is now generally believed to be a real sub- stance, yet there are certainly some strong circum-* stances which seem to militate against this doctrine. Caroline. But do you, Mrs. B. believe it to be a substance? Mrs. B. Yes, I do: but I am inclined to think, that its levity is, in all probability, only relative, like that of vapour which ascends through the hea-* vier medium, air. Caroline. If that be the case, it would not ascend in a vacuum. Mrs'. B. In an absolute vacuum, perhaps, it would not. But as the most complete vacuum we can obtain is never perfect, we may always imagine H 86 the existence of some unknown invisible fluid, which however light and subtile, may be heavier than calorie, and will gravitate in it. The fact has not, I believe, been yet determined by very decisive experiments; but it appears from some made by Professor Pictet, mentioned in his « Essay on Fire,' that heat has a tendency to ascend in the most com- plete vacuum which we are able to obtain. Emily. But if there exists such a subtle fluid as you imagine, do you not think that chemists would have discovered it by some of its properties ? Airs. B. It has been conjectured that light might be such a fluid; but I confess that I do not think it probable: for as it appears by Dr. Herschell's ex- periment that heat is less refrangible than light, I should be rather inclined to think it the heavier of the two. But, while you have so many well ascer- tained facts to learn, I shall not perplex you with conjectures. We have dwelt on the subject of ca- loric much longer than I intended, and I fear you will find it difficult to remember so long a lesson. At our next meeting we shall examine the nature of oxygen and nitrogen, two substances with which you must now be made acquainted. CONVERSATION V. On Oxygen and Nitrogen. Mrs. B. To-day we shall examine the chemical properties Of the ATMOSPHERE. 87 Caroline. I thought you said that we were to learn the natuVe of oxygen and nitrogen, which come next in our table of simple bodies ? Mrs. B. And so you shall: the atmosphere is composed of these two principles; we shall there- fore analyse it, and consider its component parts se- parately. Emily. I always thought that the atmosphere had been a very complicated fluid, composed of all the variety of exhalations from the earth. Mrs. B. In a general point of view, it may be said to consist of all the substances capable of ex- isting, in an aeriform state, at the common tempe- rature of our globe. But, laying aside these hete- rogenous and accidental substances (which rather float in the atmosphere than form any of its compo- nent parts), it consists of an elastic fluid called at- mospherical air, which is composed of two gasses, known by the names of oxygen gas and nitrogen or AZOTIC GAS. - Emily. Pray what is a gas ?" Airs. B. The name of gas is given to any aeri- form fluid, which consists of some substances che- mically combined with caloric, and capable of ex- isting constantly in an aeriform state, under the pressure, and at the temperature of the atmosphere. Every individual gas is therefore composed of two parts: 1st, the particular substance that is convert- ed into a gas, by caloric; this is called the basis of the gas, as it is from it that the gas derives all its specific and characteristic properties: and 2dly, the caloric, which, by its chemical combination with the basis, constitutes it a gas, or permanently elastic fluid. Emily. When you speak then of the simple sub- stances, oxygen and nitrogen, you mean to express, those substances which are the bases of the two gasses, independently of caloric ? Airs. B. Yes, in strict propriety; and they should 8« be called gasses, only when brought, by their com- bination with caloric, to an aeriform state. Caroline. Is not water, or any other substance, when evaporated by heat, called also a gas ? Airs B. No, my dear; vapour is, indeed, an elastic fluid, and bears so strong a resemblance to a gas, that there is some danger of confounding them; there are however, several points in which they es- sentially differ, and by which you may always dis- tinguish them. Vapour is nothing more than the solution, or me- chanical division, of any substance whatever in ca- loric. The calorie, in this case, becomes latent in the vapour; but its union with it is very slight, and ■as we have seen in a variety of instances, it is ne- cessary only to lower the temperature in order to separate them. But, to form a gas or permanently elastic fluid, a chemical combination must take place between the caloric and the substance, at the time of its being converted into a gaseous state; it is ne- cessary therefore, that there should be an affinity between them, and hence their combination cannot be destroyed by a mere change of temperature, or by any chemical agents, except such as have a stronger affinity, for either of the constituents of the gas, and by that means effect its decomposition. Caroline. Indeed, I ought not to have forgotten that caloric, in vapour, is only latent, and not che- mically combined. But pray, Mrs. B. what kinds of substances are oxygen and nitrogen, when not in a gaseous state ? Airs. B. We have never been able to obtain these substances in their pure simple state, because we cannot separate them entirely either from calo- ric or from the other bodies with which we find them united; it is therefore only by their effects in combining with other substances that we are ac- quainted with them. 89 Caroline. How much more satisfactory it would be if we could see them ! Emily. In what proportions are they combined in the atmosphere ? ' Airs. B. The oxygen gas constitutes about one- fourth, and the nitrogen gas three-fourths. When separated, they are found to possess qualities totally different from each other. Pure oxygen gas is es- sential both to respiration and combustion, while neither of these processes can be performed in ni- trogen gas. Caroline. But since nitrogen gas is unfit for res- piration, how does it happen that the three-fourths of this gas, which enter into the composition of the atmosphere, are not a great impediment to breathing? Mrs. B. We should breathe more freely than our lungs could bear, if we respired oxygen gas a- lone. The nitrogen is no impediment either to res- piration, or combustion; it appears to be merely passive in those functions; but it serves as it were, to dilute and weaken the oxygen which we breathe* as you would weaken the wine that you drink, by diluting it with water. Emijy. And by what means can the two gasses,. which compose the atmospheric air, be separated? Airs. B. There are many ways of analysing the atmosphere; the two gasses can be separated first by combustion. Emily. How is it possible that combustion should separate them ? Mrs. B. I must first tell you, that all bodies, ex- cepting the earths and alkalies, have so strong an affinity for oxygen, that they will, in certain cir- cu nstances, attract and absorb it from the atmos- phere; in this case the nitrogen gas remains alone, and we thus obtain it in its simple gaseous state. Caroline. I do not understand how a gas can be absorbed ? h 2 90 Airs. B. The gas is not absorbed, but decompo- sed; and it is oxygen only, that is to say, the basis of the gas, which is absorbed. Caroline. What then becomes of the caloric of the oxygen gas, when it is deprived of its basis? Mrs. B. We shall make this piece of dry wood absorb oxygen from the atmosphere, and you will see what becomes of the caloric. Caroline. You are joking, Mrs. B. you do not mean to decompose the atmosphere with a piece of stick ? Mrs. B. Not the whole body of the atmosphere, certainly; but if we can make this stick absorb any quantity of oxygen from it, will not a proportional quantity of atmospherical air be decomposed ? Caroline. Undoubtedly; but if wood has so strong an affinity for oxygen, as to attract it from the ca- loric with which it is combined in the atmosphere, why does it not decompose the atmosphere sponta- neously ? Mrs. B. Because the attraction of aggregation of the particles of the wood, is an obstacle to their combination with the oxygen: for you know that the oxygen must penetrate the wood in order to combine with its particles, and forcibly separate them in direct opposition to the attraction of aggregation. Emily. Just as caloric penetrates bodies? Mrs. B. Yes; but caloric being a much more subtile fluid than oxygen, can penetrate substances much more easily. Caroline. But if the attraction of cohesion between the particles of a body, counteracts its affinity for oxygen, I do not see how that body can decompose the atmosphere ? Mrs. B. That is now the difficulty which we have to remove with regard to the piece of wood.— Can you think of no method of diminishing the at- traction of cohesion ? 91 Caroline. Heating the wood, I should think, might answer the purpose; for the caloric would se- parate the particles, and make room for the oxygen. Mrs. B. Weil, we shall try your method; hold the stick close to the fire—closer still, that it may imbibe the caloric plentifully; otherwise the attrac- tion of cohesion between its particles will not be sufficiently overcome— Caroline. It has actually taken fire, and yet I did not let it touch the coals; but I held it so very close, that I suppose it caught fire merely from the inten- sity of the heat. Mrs. B. Or you might say, in other words, that the heat so far overcame the attraction of cohesion of the wood, that it was enabled to absorb oxygen very rapidly from the atmosphere. Emily. Does the wood absorb oxygen while it is burning ? Airs. B. Yes; and the heat and light are pro- duced by the caloric of the oxygen gas, which being set at liberty by the oxygen uniting with the wood* appears in its sensible form. Caroline. You astonish me ! Is it possible that the heat of a burning body should be produced by the atmosphere, and not by the body itself? Mrs. B. It is not precisely ascertained whether any portion of the caloric is furnished by thecom- bustible body; but there is no doubt that by far the most considerable part of it is disengaged from the oxyj produced ; but more time will be required, as you found to be the case with the piece of stick. Emily. But why is it not necessary to continue applying caloric throughout the process of combus- tion, in order to prevent the attraction of aggrega- tion from recovering its ground and impeding the absorption of the oxygen? Mrs. B. The caloric, which is gradually disen- gaged, by the decomposition of the oxygen gas, during combustion, keeps up the temperature of the burning body; so that when once combustion has begun, no further application of caloric is required. Caroline. Since I have learnt this wonderful the- ory of combustion, I cannot take my eyes from the fire; and I can scarcely conceive that the heat and light which I always supposed to proceed from the coals, are really produced by the atmosphere, and that the coals are only the instruments by which the decomposition of the oxygen gas is effected. Emily. When you blow the fire, you increase the combustion, I suppose, by supplying the coals with 3 greater quantity of oxygen gas ? Airs. B. Certainly; but of course no blowing will produce combustion, unless the temperature of the coals be first raised. A single spark, however, is sometimes sufficient to produce that effect; for as I said before, when once combustion has commen- ced, the caloric disengaged is sufficient to elevate the temperature of the rest of the body, provided that there be a free access of oxygen. There are, therefore, three things required in order to produce combustion; a combustible body, oxygen, and a tenv 91 perature at which the one will combine with the ©ther. Emily. You said that combustion was one me- thod of decomposing the atmosphere, and obtain- ing the nitrogen gas in its simple state; but how do you secure this gas, and prevent it from mixing with the rest of the atmosphere ? Mrs. B. It is necessary for this purpose to burn the body within a close vessel, which is easily done. —We shall introduce a small lighted taper {Plate V. Fig. 7.) under this glass receiver, Which stands in a bason over water, to prevent all communication with the external air. Caroline. How dim the light burns already !—It is now extinguished. Mrs. B. Can you tell us why it is-extinguished? Caroline. Let me consider—The receiver was full of atmospherical air; the taper, in burning with- in it, must have absorbed the oxygen contained in that air, and the caloric that was disengaged produ- ced the light of the taper. But when the whole of the oxygen was absorbed, the whole of its caloric was disengaged; consequently the taper ceased to burn, and the flame was extinguished. Mrs. B. Your explanation is perfectly correct. Emily. The two constituents of the oxygen gas being thus disposed of, what remains under the re- ceiver must'be pure nitrogen gas ? Mrs. B. There are some circumstances which prevent the nitrogen gas, thus obtained, from being perfectly pure; but we may easily try whether the oxygen has disappeared by putting another lighted taper under it.—You see how instantaneously the flame is extinguished for want of the oxygen; and were you to put an animal under the receiver, it would immediately be suffocated. But that is an experiment which I suppose your curiosity will not tempt you to try. 95 Emily. It must be very cruel indeed!—,But look, Mrs. B. the receiver is full of a thick white smoke. Is that nitrogen gas ? Mrs. B. No, my dear, pure nitrogen gas is per- fectly transparent, and invisible, like common air. This.cloudiness proceeds from a variety of exhala- tions, which arise from the burning taper, and the nature of which you cannot yet understand. Caroline. The water within the receiver has now risen a little above its level in the bason. What is the reason of this ? Airs. B. With a little refleaion, I dare say, you would have explained it yourself. The water rises in consequence of the oxygen gas within it having been destroyed or rather decomposed, by the com- bustion of the taper; and the water did not rise im- mediately because the heat of the taper whilst burn- ing, produced a dilatation of the air in the vessel, which counteracted this effect. Another means of decomposing the atmosphere is the oxygenation of certain metals. This process is very analogous to combustion; it is, indeed, only a more general term to express the combination of a body with oxygen. Caroline. In what respect, thea, does it differ from combustion? Mrs. B. The combination of oxygen in combus- tion is always accompanied by a disengagement of light and heat; whilst this circumstance is not a ne- cessary consequence of simple oxygenation. Caroline. But how can a body absorb oxygen without disengaging the caloric of the gas ? Mrs. B Oxygen does not always present itself in a gaseous state; it is a constituent part of a vast number of bodies, both solid and liquid, in which it exists in a much denser state than in the atmos- phere; and from these bodies it may be obtained without any disengagement of caloric. It may like- 96 wise, in some cases, be absorbed from the atmosphere without any sensible production of light and heat; for if the process be slow, the caloric is disengaged in such small quantities, and so gradually, that it is not capable of producing either light or heat. In this case, the absorption of oxygen is called oxyge- nation or oxydation, instead of combustion, as the dis- engagement of sensible light and heat is essential to the latter. Emily. I wonder that metals can unite with oxy- gen; for, as they are very dense, their attraction of aggregation must be very great, and I should have thought that oxygen could never have penetrated such bodies. Mrs. B. Their strong attraction for oxygen coun- terbalances this obstacle. Most metals, however, require to be made red hot before they are capable of attracting oxygen in any considerable quantity. By this process they lose most of their metallic pro- perties, and fall into a kind of powder, formerly called calx, but now much more properly termed an oxyd; thus we have oxyd of lead, oxyd of iron, &c. Caroline. The word oxyd, then, simply means a metal combined with oxygen ? Mrs. B. Yes; but the term is not confined to metals, though chiefly applied to them. Any body whatever, that has combined with a certain quanti- ty of oxygen, either by means of oxydation or com- bustion, is called an oxyd, and is said to be oxydated or oxygenated. This black powder is an oxyd of manganese, a metal which has so strong an attraction for oxygen, that it absorbs that substance from the atmosphere at any known temperature: it is therefore never found in its metallic form, but always in that of an oxyd, in which state, you see, it has very little of the appearance of a metal. It is now heavier than it was before oxydation, in consequence of the ad- 97 ditional weight of the oxygen with which it has combined. Caroline. I am very glad to hear that; for I con- fess I could not help having some doubts whether oxygen was really a substance, as it is not to be ob- tained in a simple and palpable state: but its weight is, I think, a decisive proof of its being really a bo- dy. Mrs. B. It is easy to estimate its weight, by se- parating it from the manganese, and finding how much the latter has lost. Emily. But if you can take the oxygen from the metal, shall we not then have it \n its palpable sim- ple state ? Airs. B. No; fori can only separate the oxy- gen from the manganese, by presenting to it some other body for which it has a greater affinity than for the manganese. Caloric possesses such a supe- rior affinity for oxygen, provided the temperature of the metal be sufficiently raised; if, therefore, I heat this oxyd of manganese to a certain degree, the caloric will combine with the oxygen, and car- ry it off in the form of gas. Emily. But you said just now, that manganese would attract oxygen from the atmosphere in which it is combined with caloric; how, therefore, can the oxygen have a superior affinity for calorie, since it abandons the latter to combine with the manga- nese ? Mrs. B. I give you credit for this objection, Emily; and the only answer I can make to it is, . that the mutual affinities of metals for oxygen and of oxygen for caloric, vary at different tempera- tures; a certain degree of heat will, therefore, dis- » pose a metal to combine with oxygen, vyhilst on the contrary, the former will be compelled to part with the latter when the temperature is further in- creased. I have put some oxyd of manganese into j 98 a retort, which is an earthen vessel with a bent neck, such as you see here {Plate V. Fig. 8.)—The retort containing the manganese you cannot see, as I have enclosed it in this furnace, where it is now red hot. But in order to make you sensible of the escape of the gas, which is itself invisible, I have connected the neck of the retort with this bent tube, the ex- tremity of which is immersed in this vessel of water {Plate V. Fig 9.)—Do you see the bubbles of air rise through the water? Caroline. Perfectly. This, then, is pure oxygen gas; what a pity it should be lost! Could you not preserve it? Mrs. B. We shall collect it in this receiver.— For this purpose, you observe, I first fill it with wa- ter, in order to exclude the atmospherical air; and then place it over the bubbles-that issue from the retort, so as to make them rise through the water to the upper part of the receiver. Emily. The bubbles of oxygen gas rise, I sup- pose, from their specific levity? Mrs. B. Yes; for though oxygen forms rather a heavy gas, it is light compared to water. You see how it gradually displaces the water from the recei- ver. It is now full of gas, and I may leave it inver- ted in water on this shelf, where I can keep the gas as long as I choose for future experiments. This apparatus (which is indispensable in all experiments in which gasses are concerned) is called a water-bath. Caroline. It is a very clever contrivance, indeed; it is equally simple and useful. How convenient the shelf is for the receiver to rest upon under wa- Platz V. Fig. 7. Combustion of a taper under a receiver. Fig. 8. A retort on a stand. Fig. 9. A furnace. B. Earthen retort in the furnace. C. Water bath. D. Receiver. E E. Tube conveying the gas from the retort through the water into the receiver. F. F. F. Shelf per- forated on which the receiver stands. Fig. 10. Combustion of iron wire in oxygen gas. Plate V. Paae ad. Tig. 8. Fig- 9. Preparation, of ojcvaen aas Drawn, by the Author. JSn&ravect for Jiun u« Humplirey.. JPAiladetpkia.. Tanner Sc. 99 ter, and the holes in it for the gas to pass into the receiver! I long to make some experiments with this apparatus. Mrs. B. I shall try your skill that way, when you have a little more experience. I am now going to show you an experiment, which proves, in a ve- ry striking manner, how essential oxygen is to com- bustion. You will see that iron itself will burn in this gas, tn the most rapid and brilliant manner. Emily. Really! I did not know that it was pos- sible to burn iron. Mrs. B. Iron is eminently combustible in pure oxygen gas, and what will surprise you still more, it can be set on fire without any very great rise of temperature. You see this spiral iron wire—I fasten it at one end to this cork, which is made to fit an opening at the top of the glass receiver {Plate V. Fig. 10.)-— Emily. I see the opening in the receiver; but it is carefully closed by a ground glass stopper. Mrs. B. That is in order to prevent the gas from escaping; but I shall take out the stopper, and put in the cork, to which the wire hangs.—Now I mean to burn this wire in the oxygen gas, but I must fix a small piece of lighted tinder to the extremity of it, in order to give the first impulse to combustion; for however powerful oxygen is in promoting com- bustion, you must recollect that it cannot take place without a certain elevation of temperature. I shall now introduce the wire into the receiver, by quick- ly changing the stoppers. Caroline. Is there no danger of the gas escaping while you change the stoppers ? Mrs. B. Oxygen gas is a little heavier than at- mospherical air, therefore it will not mix with it ve- ry rapidly; and if I do not leave the opening unco- vered we shall not lose any---- Caroline. Oh, what a brilliant and beautiful flame! 102 it is evidently diminished, and sometimes entirely consumed. Mrs. B. But what do you mean by the expres- sion consumed!' Y,ou cannot suppose that the smallest particle of any substance in nature can be actually destroyed. A compound body is decomposed by combustion; some of its constituent parts fly off in a gaseous form, while others remain in a concrete state; the former are called the volatile, the latter the fixedproducls of combustion. But if we collect rhe whole of them, we shall always find that they exceed the weight of the combustible body, by that of the oxygen which has combined with them du- ring combustion. Emily. In the combustion of a coal fire, then, I suppose that the ashes are what would be called the fixed product? and the smoke the volatile product? Mrs. B. Yet when the fire burns best, and the quantity of volatile products should be the greatest, there is no smoke; how can you account for that? Emily. Indeed I cannot; therefore I suppose that I was not right in my conjecture. Mrs. B. Not quite : ashes as you supposed, are a fixed product of combustion; but smoke, proper- ly speaking, is not one of the volatile products, as it consists of some minute undecomposed particles of the coals that are carried off by the caloric with- out being burnt, and are either deposited in the form of soot, or dispersed by the wind. Smoke therefore, ultimately becomes one of the fixed pro- ducts of combustion. And you may easily conceive that the stronger the fire is, the less smoke it pro- duces, because the fewer particles escape combus- tion. On this principle depends the invention of Argand's patent lamps; a current of air is made to pass through the cylindrical wick of the lamp, by which means it is so plentifully supplied with oxy- gen, that not a particle of oil escapes combustion, nor is an atom of smoke produced. 103 Emily. But what then are the volatile products of combustion ? Mrs. B Various new compounds, with which you are not yet acquainted, and which being con- verted by caloric, either into vapour, or gas, are invisible; but they can be collected, and we shall examine them, at some future period. Caroline. There are then other gasses, besides the oxygen and nitrogen gasses. Mrs. B. Yes, several: any substance that has a sufficient affinity for caloric to combine with it, and assume and maintain the form of an elastic fluid at the temperature of the atmosphere, is capable of being converted into a gas. We shall examine the several gasses in their respective places; but we must now confine our attention to those that compose the atmosphere. I shall show you another method of decomposing the atmosphere, which is very simple. In breath- ing we retain a portion of the oxygen, and expire the nitrogen gas; so that if we breathe in a closed vessel, for a certain length of time, the air within it will be deprived of its oxygen gas. Which of you will make the experiment ? Caroline. I should be very glad to try it. Mrs. B. Very well; breathe several times through this glass tube into the receiver with which it is connected, until you feel that your breath is ex- hausted— Caroline. I am quite out of breath already! Mrs. B. Now let us try the gas with a lighted taper— Emily. It is very pure nitrogen gas, for the ta- per is immediately extinguished. Mrs. B. That is not a proof of its being, pure, but only .of the absence of oxygen, as it is that prin- ciple alone that can produce combustion, every other gas being absolutely incapable of it. 104 Emily. In the methods which you have shown us, for decomposing rhe atmosphere, the oxygen always abandons the nitrogen; but is there no way of taking the nitrogen from the oxygen, so as to obtain the latter pure from the atmosphere ? Airs. B. You must observe, that whenever oxy- gen is taken from the atmosphere, it is by decom- posing the oxygen gas: we cannot do the same with the nitrogen gas, because nitrogen has a stronger affinity for caloric than for any other known prin- ciple: it appears impossible therefore to separate it from the atmosphere by the power of affinities. But if we cannot obtain the oxygen gas by this means, in its separate state, we have no difficulty (as you have seen) to procure it in its gaseous form, by ta- king it from those substances that have absorbed it from the atmosphere. This is done by combining the oxygen, at a high temperature, with caloric, as we did with the oxyd of manganese. Emily. Can atmospherical air be recomposed, by- mixing due proportions of oxygen and nitrogen gasses. Mrs. B. Yes: if about one-fourth of oxygen gas be mixed with three-fourths of nitrogen gas, atmospherical air is produced. Emily. The air then must be an oxyd of nitrogen i Mrs. B. No, my dear; for there must be a che- mical combination between oxygen and nitrogen in order to produce an oxyd; whilst in the atmosphere these two substances are separately combined with caloric, forming two distinct gasses, which are sim- ply mixed in the formation of the atmosphere.* I shall say nothing more of oxygen and nitrogen at present, as we shall continually have occasion to * This, at least, seems to be the prevailing opinion. Yet it has been questioned by some chemists, particularly of late, whether the union of oxygen and nitrogen in the atmosphere be not a true che- mical combination. 105 refer to them in our future conversations. They are both very abundant in nature; nitrogen is the most plentiful in the atmosphere, and exists also in all animal substances; oxygen forms a constituent part, both of the animal and vegetable kingdoms, from which it may be obtained by a variety of che- mical means. But it is now time to conclude our lesson. I am afraid you have learnt more to day than you will be able to remember. Caroline. I assure you that I have been too much interested in it, ever to forget it; as for nitrogen there seems to be but little to remember about it: it makes a very insignificant figure in comparison to oxygen, although it composes a much larger portion of the atmosphere. Mrs. B. It will not appear so insignificant when you are better acquainted with it; for though it seems to perform but a passive part in the atmos- phere, and has no very striking properties, when considered in its separate state, yet you will see by and by what a very important agent it becomes, when combined with other bodies. But no more of this at present; we must reserve it for its proper place. CONVERSATION VI. On Hydrogen. Caroline. The next simple body we come to is hydrogen. Pray what kind of a substance is that; is it also in- visible ? 10b' Mrs. B. Yes; we cannot obtain hydrogen in its pure concrete state. We are acquainted with it only in its gaseous form; as we are with oxygen and nitrogen. Caroline. But in its gaseous state it cannot be called a simple substance, since it is combined with caloric. Mrs. B. True, my dear; but as we do not know in nature of any substance which is not more or less combined with caloric, we are apt to say (rather in- correctly indeed) that a substance is in its pure state, when combined with caloric only. Hydrogen is derived from two Greek words, the meaning of which is to produce water. Emily. And how does hydrogen produce water ? Mrs. B. Water is composed of 85 parts, by weight, of oxygen, chemically combined with 15 parts of hydrogen gas, or (as it was formerly called) inflam- mable air. Caroline. Really! Is it possible that water should be a combination of two gasses, and that one of them should be inflammable air ? It must be a most extraordinary gas, that will produce both fire and water! Mrs. B. Hydrogen, I assure you, though a con- stituent part of water, is one of the most combus- tible substances in nature. Emily. But I thought you said that combustion could take place in no gas but oxygen ? Mrs. B. Do you recollect what the process of combustion consists in ? Emily. In the combination of a body with oxy- gen, with disengagement of light and heat. Mrs. B. Therefore, when I say that hydrogen is combustible, I mean that it has an affinity for oxygen; but like all other combustible substances, it cannot burn unless supplied with oxygen, and heated to a proper temperature. 107 Caroline. But I cannot conceive how, by mixing hfteen parts of it, with eighty-five parts of oxygen gas, the two gasses can be converted into water ? Mrs. B. The simply mixing these proportions ot oxygen and hydrogen gasses, will not produce water; because the great quantity of caloric to which they owe their gaseous form would prevent their bases from coming into contact, and entering into chemical combination; besides, water is a much denser fluid than gas, and therefore it is necessary, in order to reduce these gasses to a liquid, to dimi- nish the quantity of caloric. Can you think of any means of accomplishing this ? Caroline. By putting a colder body in contaft with the gasses, which would take some of their ca- loric from them. Mrs. B. That would lower the temperature of the gas; but could not affect the caloric that is che- mically combined with the basis. Caroline. True; I forgot, that in order to sepa- rate caloric from a body with which it is chemically combined, a decomposition must take place; but I cannot imagine how this is effected. Mrs. B. A decomposition can be effected only by superior attractions which produce new combi- nations. At a certain temperature, oxygen will a- bandon its caloric, to combine with hydrogen; if, therefore we raise it to that temperature, the oxygen will combine with the hydrogen, and set its own caloric at liberty; and it is thus that the combustion of hydrogen gas produces water. Caroline. You love to deal in paradoxes to-day, Mrs. B.—Fire then produces water ! Mrs. B. The combustion of hydrogen gas cer- tainly does; but you do not seem to have remem- bered the theory of combustion so well as you thought you would. Can you tell me what happens in the combustion of hydrogen gas ? 108 Caroline. The hydrogen gas combines with the basis of the oxygen gas, and the caloric of the lat- ter is disengaged.—Yes, I think, I understand it now/ the caloric of the oxygen gas being set at li- berty, and the basis of the two gasses coming in contact, they combine, and condense into a liquid. Emily. But does all the caloric, produced by the combustion of hydrogen gas, proceed from the oxy- gen gas ? Mrs. B. That is a doubtful point; but I rather believe that in this, as probably in every other in- stance of combustion, some portion of heat and light is disengaged by the combustible itself. Emily. Water then, I suppose, when it evapo- rates and incorporates with the atmosphere, is de- composed and converted into hydrogen and oxygen gasses ? Mrs. B. No my dear; there you are quite mis- taken : the decomposition of water is totally differ- ent from its evaporation; for in the latter case (as you should recollect) water is only in a state of very minute division; and is merely suspended in the at- mosphere, without any chemical combination, and without any separation of its constituent parts. As long as these remain combined, they form water, whether in a state of liquidity, or in that of an elas- tic fluid, as vapour, or under the solid form of ice. In our experiments on latent heat, you may re- collect that we caused water successively to pass through these three forms, merely by an increase or diminution of caloric, without employing any power of attraction, or effecting any decomposition. Caroline. But are there no means of decomposing water ? Mrs. B. Yes, several: charcoal, and metals, when heated red hot, will attract the oxygen from water in the same manner, as they will from the .atmosphere; but in this process there is no disen- 109 gagement of caloric, as that which the oxygen a- bandons, instead of becoming sensible, combines immediately with the hydrogen, which it converts Into gas, and carries off in that form. Caroline. So, then, the quantity of calorie that was employed in maintaining the combined substan- ces in a liquid form, is just sufficient to convert the hydrogen singly, into a gas. Mrs.B. That is a very ingenious inference; but I doubt whether it is strictly accurate, as the hot body (whether charcoal or metal) by means of which the water is decomposed, supplies, in cooling, a por- tion of the caloric which enters into the formation of the gas. Emily. Water, then, may be resolved into a solid substance and a gas; the oxygen being condensed into a solid, by the loss of caloric, and the hydro- gen expanded into a gas, by the acquisition of it. Mrs. B. Very well; but remember that the basis of the oxygen gas, or what you call solid oxy- gen, can never be obtained alone; it can be sepa- rated from the hydrogen only by combining it with some other body for which it has a greater affinity. Caroline. Hydrogen, I see, is like nitrogen, a poor dependant friend of oxygen, which is conti- nually forsaken for greater favourites. Mrs. B. The connection, or friendship, as you ^ choose to call it, is much more intimate between oxygen and hydrogen, in the state of water, than between oxygen and nitrogen, in the atmosphere : for in the first case, there is a chemical union and condensation of the two substances; 'in the latter they are simply mixed together in their gaseous state. You will find, however, that, in some cases, nitrogen is quite as intimately connected with oxygen, as hy- drogen is.—But this is foreign to our present subject. Emily. Water, then, is an oxyd, though the a" fnospherical air is not ? no Airs. B. It is not commonly called an oxyd, though according to our definition, it may, no doubt, be referred to that class of bodies. Caroline. I should like extremely to see water decomposed. Mrs. B. I can easily gratify your curiosity by a much more easy process than the oxydation of char- coal or metal; the decomposition of wr.ter by these latter means, take up a great deal of time, and is attended with much trouble; for it is necessary that the charcoal or metal should be made red hot in a furnace, that the water should pass over them in a state of vapour, that the gas formed should be col- lected over the water-bath, &c. In short it is a ve- ry complicated affair. But the same effect may be produced with the greatest facility, by adding some sulphuric acid (a substance \\ ith the nature of which you are not yet acquainted), to the water which the metal is to decompose. The acid disposes the me- tal to combine with the oxygen of the water so rea- dily and abundantly, that no heat is required to hasten the process. Of this I am going to show you an instance.—I put into this bottle the water that is to be decomposed, the metal that is to effect that decomposition by combining with the oxygen, and the acid which is to facilitate the combination of the metal and the oxygen. You will see with what violence these will act on each other. Caroline. But what metal is it that you employ for this purpose ? Airs. B. It is iron; and it is used in the state of filings, as these present a greater surface to the acid than a solid piece of metal. For, as it is the surface of the metal which is acted upon by the acid, and is disposed to receive the oxygen produced by the decomposition of the water, it necessarily fol- lows that the greater is the surface, the more con- siderable is the effect. The bubbles which are now rising are hydrogen gas— ■/?*■$■'*;"'■. „ Ill Caroline. How disagreeably it smells ! |*: Airs. B. It is indeed unpleasant, but net inV wholesome. We shall not, however, suffer any more to escape, as it will be wanted for experiments. I shall therefore collect it in a glass receiver, by ma- king it pass through this bent tube, which will con- dua it into the water-bath. {Plate VI. Fig. 11.) Emily. How very rapidly the gas escapes! it is perfectly transparent, and without any colour what- ever.—Now the receiver is full—■- Airs. B. We shall therefore remove it and sub- stitute another in its place. But you must observe, that when the receiver is full, it is necessary to keep it inverted with the mouth under water, otherwise the gas would escape. And in order that it may not be in the way, I introduce within the bath, un- der the water, a saucer, into which I slide the re- ceiver, so that it can be taken out of the bath and conveyed any where, the water in the saucer being equally effectual in preventing its escape as that in the bath. {Plate VI. Fig. 12.) Emily. I am quite surprised to see what a large quantity of hydrogen gas can be produced by such / a small quantity of water, especially as oxygen is the principal constituent of water. Mrs. B. In weight it is: but not in volume. For though the proportion, by weight, is nearly six parts of oxygen to one of hydrogen, yet the proportion of the volume of the gasses, is about one paw: of oxygen, to two of hydrogen ; so much heavier is the former than the latter. Caroline. But why is the vessel in which the wa- PlATE VI. Fig. ir. Apparatus for preparing and collecting hydrogen gas. Fig. \%. Receiver full of hydrogen gas inverted over water. Fig.1%, Slow combustion of hydrogen gas. Fig. 14. Apparatus for illus- trating th? formation of water by the combustion of hydrogen gas. Fig. 15. Anparatus for proJucing harmonic sounds by the combus- tion of hydrogen gas. H2 ter is decomposed so hot ? As the water changes from a liquid to a gaseous form, cold should be pro- duced instead of heat. Mrs. B. No ; for if one of the constituents of water is converted into a gas, the other becomes so- lid in combining with the metal; and the caloric which the oxygen loses by being thus rendered so- lid, is just sufficient to transform the hydrogen into a gas. Emily. In this case, neither heat nor cold would be produced; for the caloric disengaged from the oxygen, being immediately combined with the hy- drogen, cannot become sensible ? Airs. B. That is very true; but the sensible heat which is disengaged in this operation is not owing to the decomposition of the water, but to an extrication of latent heat produced by the mixture of water and sulphuric acid, as you saw in a former experiment. If I now set the hydrogen gas, which is contain- ed in this receiver, at liberty all at once, and kindle it as soon as it comes in contact with the atmos- phere, by presenting it to.a candle, it will so sud- denly and rapidly decompose the oxygen gas, by combining with its basis, that an explosion, or a de- tonation as (chemists commonly call it), will be pro- duced. For this purpose, I need only take up the receiver, and quickly present its open mouth to the candle----so..... Caroline. It produced only a sort of hissing noise, with a vivid flash of light. I had expected a much greater report. Mrs. B. . And so it would have been, had the gasses been closely confined at the moment they were made to explode. If for instance, we were to put in this bottle a mixture of hydrogen gas and atmospheric air; and if, after corking the bottle, we should kindle the mixture by a very small orifice, 113 from the sudden dilatation of the gasses at the mo- ment of their combination, the bottle must either fly to pieces, or the cork be blown out with consi- derable violence. Caroline. But in the experiment which we have just seen, if you did not kindle the hydrogen gas, would it not equally combine with the oxygen ? Mrs. B. Certainly not; have I not just explain- ed to you the necessity of the oxygen and hydrogen gasses being burnt together, in order to combine chemically and produce water ? Caroline. That is true; but I thought this was a different combination, for I see no water produced. Mrs. B The water produced by this detonation was so small in quantity, and in such a state of mi- nute division, as to be invisible. But water certain- ly was produced ; for oxygen is incapaW#*of combi- ning with hydrogen in any other proportions than those that form water ; therefore water must always be the result of their combinatipn. If, instead of bringing the hydrogen gas into sud- den contact with the atmosphere (as we did just now) so as to make the whole of it explode the moment it is kindled, we allow but a very small surface of gas to burn in contact with the atmosphere, the combustion goes- on quietly and gradually at the point of contact, without any detonation, because the surfaces brought together are too small for the immediate union of gasses. The experiment is a very easy one. This phial with a narrow neck, {Plate VI. Fig. 13.), is full of hydrogen gas, and is carefully corked. If 1 take out the cork, without moving the phial, and quickly approach the candle to the orifice, you will see how different the result will be— Emily How prettily it burns, with a blue flame! The flame is gradually sinking within the phial— K 2 114 s,now it has entirely disappeared. But does not this •combustion likewise produce water ? Mrs. B. Undoubtedly. In order to make the formation of water sensible to you, I shall procure a fresh supply of hydrogen gas, by putting into this bottle {Plate VI.. Fig. 14.) iron filings, water, and sulphuric acid, materials similar to those which we have just used for the same purpose. I shall then cork up the bottle, leaving only a small orifice in the cork, with a piece of glass tube fixed to it, through which the gas will issue in a continued rapid stream. Caroline. I hear already the hissing of the gas through the tube, and I can feel a strong current against my hand. Airs. B. This current I am going to kindle with the candle—'see how vividly it burns— Emily. . It burns like a candle with a long flame. —But why does this combustion last so much longer than in the former experiment ? Mrs. B. The combustion goes on uninterrupt- edly as long as the new gas continues to be produ- ced. Now if I invert this receiver over the flame, you will soon perceive its internal surface covered with a very fine dew, which is pure water— Caroline. Yes, indeed; the glass is now quite dim with moisture ! How glad I am that we can see the water produced by this combustion. Emily. It is exactly what I was anxious to see; for I confess I was a little incredulous. Mrs. B. If I had not held the glass-bell over the flame, the water wpuld have escaped in the state of vapour, as it did in the former experiment. We have here, of course, obtained but a very small quantity of water; but the difficulty of procuring a proper apparatus, with sufficient quantities of gas- ses, prevents my shewing it to you on a larger scale. The composition of water was discovered about the same period, both by Mr. Cavendish, in this 115 country, and by the celebrated French chemist La- voisier. The latter invented a very perfect and in- genious apparatus to perform with great accuracy, and upon a large scale, the formation of water "by the combination of oxygen and hydrogen gasses. Two tubes, conveying due proportions, the one of oxygen, the other of hydrogen gas, are inserted at opposite sides of a large globe of glass, previously exhausted of air; the two streams of gas are kindled within the globe, by the electric spark, at the point where they come in contact; they burn together, that is to say, the hydrogen gas combines with the basis of the oxygen gas, the caloric of which is set at liberty; and a quantity of water is produced, ex- actly equal in weight to that of the two gasses in- troduced into the globe. Caroline. And what was the greatest quantity of water ever formed in this apparatus ? Airs. B. Several ounces; indeed, very near a pound, if I recollect right; but the operation lasted many days. Emily. This experiment must have convinced all the world of the truth of the discovery. Pray, if improper proportions of the gasses were mixed and set fire to, what would be the result ? Mrs. B. Water would equally be formed, but there would be a residue of either one or other of the gasses, because, as I have already told you, hy- drogen and oxygen will combine only in the propor- tions requisite for the formation of water. There is another curious effect produced by the combustion of hydrogen gas, which I shall shew you, though I must acquaint you first, that I can- not well explain the cause of it. For this purpose, I must put some more materials into our apparatus, in order to obtain a stream of hydrogen gas, just as we have done before. The process is already going on, and the gas is rushing through the tube—I shall now kindle it with the taper. 116 Emily. It burns exactly as it did before-----What is the curious effect which you were mentioning ? Airs. B. Instead of the receiver, by means of which we have just seen the drops of water form, we shall invert over the flame this piece of tube, which is about two feet in length, and one inch in diameter {Plate VI. Fig. 15.); but you must observe that it is open at both ends. Emily. What a strange noise it makes! some- thing like the jEolian harp, but not so sweet. Caroline. It is very singular, indeed ; but I think rather too powerful to be pleasing. And is not this sound accounted for ? Mrs. B. That the percussion of glass, by a ra- pid stream of gas, should produce a sound, is not extraordinary; but the sound here is so peculiar, that no other gas has a similar effect. Perhaps it is owing to a brisk vibratory motion of the glass occa- sioned by the successive formation and condensation of small drops of water on the sides of the glass tube, aud the air rushing in to replace the vacuum formed.* Caroline. How very much this flame resembles the burning of a candle. Mrs. B. The burning of a candle is produced by much the same means. A great deal of hydro- gen is contained in candles, whether of tallow or wax. This hydrogen being"converted into gas by the heat of the candle, combines with the oxygen of the atmosphere, and flame and water result from this combination. So that, in fact, the flame of a candle is nothing but the combustion of hydrogen gas. An elevation of temperature, such as is pro- duced by a lighted match or taper, is required to give the first impulse to the combustion; but after- * This ingenirus explanation was first suggested by Dr. Delarivr- See Journals of the Royal Institution, vol. i. p. 259. 117 wards it goes on of itself, because the candle finds a supply of caloric in the. successive quantities of chemical heat which become sensible by the combina- tion of the two gasses. But there are other acces- sory circumstances connected with the combustion of candles and lamps, which I cannot explain to you till you are acquainted with carbone, which is one of their constituent parts. In general, however, when- ever you see flame, you may infer that it is owing to the formation and burning of hydrogen gas; for flame is the peculiar mode of burning of hydrogen gas, which, with only one or two apparent excep- tions, does not belong to any other combustible. Emily. You astonish me! I understood that flame was the caloric abandoned by the basis of the oxy- gen gas, in all combustions whatever ? Mrs. B. Your error proceeded from your vague and incorrect idea of flame ; you have confounded it with light and caloric in general. Flame always implies caloric, since it is produced by the combus- tion of hydrogen gas; but all caloric does not im- ply flame. Many bodies burn with intense heat without producing flame. Coals, for instance, burn - with flame until all the hydrogen which they con- tain is evaporated; but when they afterwards be- come red hot, much more caloric is disengaged than when they produce flame. Caroline. But the iron wire, which you burnt in oxygen gas, appeared to me to omit flame; yet as it was a simple metal, it could contain no hydrogen? Mrs. B. It produced a sparkling dazzling blaze of light, but no real flame. Emily. And what is the cause of the regular shape of the flame of a candle ? Mrs. B. The regular stream of hydrogen gas which exhales from its combustible matter. Caroline. But the hydrogen gas must from its great levity, ascend into the upper regions of the 118 atmosphere; why therefore does not the flame con tinuerto accompany it ? Mrs. B. The combustion of the hydrogen gas is completed at the point where the flame termi- nates ; it then ceases to be hydrogen gas, as it is converted by its combustion into watery vapour; but in a state of such minute division as to be invisible. Caroline. I do not understand what is the use of the wick of a candle; since the hydrogen gas burns so well without it ? Mrs. B. The combustible matter of the candle must be decomposed in order to emit the hydrogen gas, and the wick is instrumental in effecting this decomposition. Its combustion first melts the com- bustible matter, and ....... Caroline. But in lamps the combustible matter is already fluid, and yet they also require wicks ? Mrs. B. I was going to add that, afterwards, the burning wick (by the power of capillary attrac- tion) gradually draws up the fluid to the point where combustion takes place; for you must have obser- ved, that the wick does not burn quite to the bot- tom. Caroline. Yes; but I do not understand why it does not. Mrs. B. Because the air has not so free an ac- cess to that part of the wick which is immediately in contact with the candle, as to the part just above, so that the heat there is not sufficient to produce its decomposition; the combustion therefore begins a little above this point.—But we dwell too long on a subject which you cannot yet thoroughly under- stand.—J have another experiment to shew you with hydrogen gas, which I"think will entertain you. Have you ever blown bubbles with soap and water? Emily. Yes, often, when I was a child; and I used to make them float in the air by blowing them upwards. ^\^ "7T'™7^ / 'i ' / \ \ 1 'ft'- ) IX ^ "s. 1*1 c \. •§> ^•^ k; III ' ^ 119 Airs. B. We shall fill some such bubbles with hydrogen gas, instead of atmospheric air, and you will see with what ease and rapidity they will ascend, without the assistance of blowing, from the light- ness of the gas.—Will you mix some soap and wa- ter whilst I fill this bladder with the gas contained in the receiver which stands on the shelf in the wa- ter-bath. Caroline. What is the use of the brass stopper and turn-cock at the top of the receiver ? Airs. B. It is to afford a passage to the gas when required. There is, you see, a similar stop-cock fastened to this bladder, which is made to fit that on the receiver. I screw them one on the other, and now turn the two cocks, to open a communica- tion between the receiver and the bladder; then, by sliding the receiver off the shelf, and gently sink- ing it into the bath, the water rises in the receiver and forces the gas into the bladder. {Plate VII. 2% 16.) Caroline. Yes, I see the bladder swell as the wa- ter rises in the receiver. Mrs. B. I think that we have already a suffici- ent quantity in the bladder for our purpose; we must be careful to stop both the cocks before we separate the bladder from the receiver, lest the gas should escape.—Now I must fix a pipe to the stop- per of the bladder, and, by dipping its mouth into the soap and water, take up a few drops:—then I again turn the cock, and squeeze the bladder in or- der to force the gas into the soap and water at the mouth of the pipe. {Plate VII. Fig. 17.) Emily. There is a bubble—but it bursts before it leaves the mouth of the pipe. Mrs. B. We must have patience and try again; Plate VII. tFig. 16. Apparatus for transferring gasses from a receiver ihto a Wadder. Fig. 17. Apparatus for blowing soap bubbles. 120 it is not so easy to blow bubbles by means of a blad- der, as simply with the breath. Caroline. Perhaps there is not soap enough in the water; I should have had warm water, it would have dissolved the soap better. Emily. Does not some of the gas escape between the bladder and the pipe ? Mrs. B. No, they are perfectly air-tight; we shall succeed presently, I dare say. Caroline. Now a bubble ascends; it moves with the rapidity of a balloon. How beautifully it refracts the light! Emily. It has burst against the ceiling—you suc- ceed now wonderfully; but why do they all ascend and burst against the ceiling ? Airs. B. Hydrogen gas is so much lighter than atmospherical air, that it ascends rapidly with its very light envelope, which is burst by the force with which it strikes the ceiling. Air balloons are filled with this gas, and if d*ey carried no other weight than their covering, would ascend as rapidly as these bubbles. Caroline. Yet their covering must be much hea- vier than that of these bubbles ? Mrs. B. Not in proportion to the quantity of gas they contain. I do not know whether you have ever been present at the filling of a large balloon. The apparatus for that purpose is very simple. It consists of a number of vessels, either jars or bar- rels, in which the materials for the formation of the gas are mixed, each of these being furnished with a tube, and communicating with a long flexible pipe, which conveys the gas into the balloon. Emily. But the fire balloons which were first invented, and have been since abandoned, on ac- count of their being so dangerous, were construct- ed, I suppose, on a different principle. ' Mrs. B. They were filled simply with atmos- 121 pherical air, considerably rarefied, and the necessi- ty of having a fire underneath the balloon, in order to preserve the rarefaction of the air within it, was the circumstance productive of so much danger. If you are not yet tired of experiments, I have another to shew you. It consists in filling soap bubbles with a mixture of hydrogen and oxygen gasses, in the proportions that form water; and af- terwards setting fire to them. Emily. They will detonate, I suppose ? Mrs. B. Yes, they will. As you have seen the method of transferring the gas from the receiver into the bladder it is not necessary to repeat it. I have, therefore, provided a bladder which contains a due proportion of oxygen and hydrogen gasses, and we have only to blow bubbles with it. Caroline. Here is a fine large bubble rising— shall I set fire to it with the candle ? Airs. B. If you please .... Caroline. Heavens, what an explosion!—It was like the report of a gun: I confess it frightened me much, I never should have imagined it could be so loud. Emily, And the flash was as vivid as lightning. Mrs. B. The combination of the two gasses takes place during that instant of time that you see the flash, and hear the detonation. Emily. This has a strong resemblance to thun- der and lightning. Airs. B. These phenomena, however, are most probably of an electrical nature. Yet various me- teorological effects may be attributed to accidental detonations of hydrogen gas in the atmosphere; for nature abounds with hydrogen; it constitutes a ve- ry considerable portion of the whole mass of water belonging to our globe, and from that source, al- most every other body obtains it. It enters into the composition of all animal substances, and of a L 122 great number of minerals; but it is most abundant in vegetables. From this immense variety of bo- dies, it is often spontaneously disengaged ; its great levity makes it rise into the superior regions of the atmosphere, and when, either by an electric spark, or any casual elevation of temperature, it takes fire, it may produce such meteors or luminous appearan- ces as are occasionally seen in the atmosphere. Of this kind are probably those broad flashes which we often see on a summer evening, without hearing any detonation. Emily. Every flash I suppose, must produce a quantity of water ? Caroline. And this water, naturally, descends in the form of rain ? Mrs. B. That probably is often the case, though it is not a necessary consequence; for the water may be dissolved by the atmosphere, as it descends to- wards the lower regions, and remain there in the form of clouds.—But pray do not question me too closely on this subject, for the phenomena of the at- mosphere are not yet well understood; and even with the little that is known I am but imperfectly acquainted. CONVERSATION VII. On Sulphur and Phosphorus. Mrs. B. Sulphur is the next simple substance that comes under our consideration. It differs in one essential point from the preceding, as it exists in a solid form at the temperature of -the atmosphere. 123 Caroline. I am glad that we have at last a solid body to examine; one that we can see and touch. Pray, is it not with sulphur that the points of match- es are covered to make them easily kindle ? Mrs. B. Yes, it is ; and you therefore already know that sulphur is a very combustible substance. It is seldom discovered in nature in a pure unmixed state ; so great is its affinity for other substances, that it is almost constantly found combined with some of them. It is most commonly united with metals, under various forms, and is separated from them by a very simple process. It exists likewise in many mineral waters, and some vegetables yield it in various proportions, especially those of the cruciform tribe. It is also found in animal matter; in short, it may be discovered in greater or less quantity, in the mineral, vegetable, and animal king- doms. Emily. I have heard of flowers of sulphur, are they the produce of any plant ? Mrs. B. By no means: they consist of nothing more than common sulphur reduced to a very fine powder by a process called sublimation.—You see some of it in this phial; it is exactly the same sub- stance as this lump of sulphur, only its colour is a paler yellow, owing to its state of very minute di- vision. Emily. Pray what is sublimation ? Mrs. B. It is the evaporation, or, more proper- ly speaking, the volatilization of solid substances, which, in cooling, condense again in a concrete form. The process, in this instance, must be per- formed in a closed vessel, both to prevent combus- tion, which would take place if the access of air was not carefully precluded, and likewise in order to collect the substance after rhe operation. As it is rather a slow process, we shall not try the expe- riment now; but you will understand it perfectly if 124 i show you the apparatus used for the purpose.— {Plate VIII. Fig. 18.) Some lumps of sulphur are put into a receiver of this kind, which is called a cucurbit. Its shape, you see somewhat resembles that of a pear, and it is open at the top so as to a- dapt itself exactly to a kind of conical receiver of this sort called the head. The cucurbit, thus co- vered with its head, is placed over a sand-bath; this is nothing more than a vessel full of sand, which is kept heated by a furnace, such as you see here, so as to preserve the apparatus in a moderate and uni- form temperature. The sulphur then soon begins to melt, and immediately after this, a thick white smoke rises, which is gradually deposited within the head, or upper part of the apparatus, where it con- denses against the sides, somewhat in the form of a vegetation, whence it has obtained the name of flowers of sulphur. This apparatus^ which is called an alembic, is highly useful in all kinds of distilla- tions, as you will-see when we come to treat of those operations. Alembics are not commonly made of glass, like this, which is applicable only to distilla- tions upon a very small scale. Those used in ma- nufactures are generally made of copper, and are, of course, considerably larger. The principal con- struction, however, is always the same, although their shape admits of some variation. Caroline. What is the use of that neck, or tube, which bends down from the upper piece of the ap- paratus ? Mrs. B. It is of no use in sublimations ; but in distillations (the general object of which is to eva- Plate viii. Fig. 18. A. Alembic. B. Sand-bath. C. Furnace. Fig. 19, Eudiometer. Fig. 20. A. Retort containing water. B. Lamp to heat the water. C. C. Porcelain tube containing Carbone. D.Furr nace through which the tube passes. E. Receiver for the gas pro- duced. F. Water-bath. Plate Till. Tig. 18. Sublimation of Sulphur Drai.it bv 1/uAut/urt Tngravetf for .lame.- HumphreysT^iiladtlokia. 125 porate, by heat, in closed vessels, the volatile parts of a compound body, and to condense them again into a liquid), it serves to carry off the condensed fluid, which otherwise would fall back into the cu- curbit. But this is rather foreign to our present subject. Let us return to the sulphur. You now perfectly understand, I suppose, what is meant by sublimation ? Emily. I believe I do. Sublimation appears to consist in destroying, by means of heat, the attrac- tion of aggregation of the particles of a solid body, which are thus volatilized; and as soon as they lose the caloric which produced that effect, they are de- posited in the form of a fine powder. Caroline. It seems to me to be somewhat similar to the transformation of water into vapour, which returns to its liquid state when deprived of caloric. Emily. There is this difference, however, that the su\phur does not return to its former state, since, instead of lumps, it changes to a fine powder: Mrs. B. Chemically speaking, it is exactly the same substance, whether in the form of lump or powder. For if this powder be melted again by heat, it will in cooling, be restored to the same so- lid state in which it was before its sublimation Caroline. But if there be no real change produ- ced by the sublimation of the sulphur, what is the use of that operation ? Mrs. B. It divides the sulphur into very mi- nute parts, and thus disposes it to enter more rea- dily into combination with other bodies. It is used also as a means of purification. Caroline. Sublimation appears to me like the be- ginning of combustion, for the completion of which one circumstance only is wanting, the absorption of oxygen. Mrs. B. But that circumstance is every thing. No essential alteration is produced in sulphur by sub- l 2 126 limation ; whilst in combustion it combines with the oxygen and forms a new compound totally different in every respect from sulphur in its pure state.—We shall now burn some sulphur, and you will see how very different the result will be. For this purpose I put a small quantity of flowers of sulphur into this cup, and place it in a dish, into which I have poured a little water ; I now set fire to the sulphur with the point of this hot wire; for its combustion will not begin unless its temperature be considera- bly raised.—You see that it burns with a faint blueish flame ; and as I invert over it this receiver, white fumes arise from the sulphur and fill the vessel.— You will soon perceive that the water is rising with- in the receiver, a little above its level in the plate. —Well, Emily, can you account for this ? Emily. I suppose that the sulphur has absorbed the oxygen from the atmospherical air within the receiver ; and that we shall find some oxygenated sulphur in the cup. As for the white smoke, I am quite at a loss to guess what it may be. Mrs.B. Your first conjecture is very right; but you are quite mistaken in the last; for nothing will be left in the cup. The white vapour is the oxyge- nated sulphur, which assumes the form of an elas- tic fluid of a pungent and offensive smell, and is a powerful acid. Here you see a chemical combina- tion of oxygen and sulphurl producing a true gas, which would continue such under the pressure and at the temperature of the atmosphere, if it did not unite with the water in the plate, to which it imparts its acid taste and all its acid properties.—You see, now, with what curious effects the combustion of sulphur is attended. Caroline. This is something quite new; and I confess that I do not perfectly understand why the sulphur turns acid. Airs. B^ It is because it unites with oxygen. 127 which is the general acidifying principle. And, in- deed, the word oxygen, is derived from two Greek words signifying to produce an acid. Caroline. Why then is not water, which contains such a quantity of oxygen, acid ? Mrs. B. Because hydrogen, which is the other constituent of water, is not susceptible of acidifica- tion. I believe it will be necessary, before we pro- ceed further, to say a few words of the general na- ture of acids, though it is rather a deviation from our plan of examining the simple bodies separately, before we consider them in a state of combination. Acids may be considered as a peculiar class of burnt bodies, which, during their combustion, or combination with oxygen, have acquired very cha- racteristic properties. They are chiefly discernible by their sour taste, and by turning red most of the blue vegetable colours. These two properties are common to the whole class of acids; but each of them is distinguished by other peculiar qualities. Every acid consists of some particular substance (which constitutes its basis, and is different in each), and of oxygen, which is common to them all. Emily. But I do not clearly see the difference between acids and oxyds ? Mrs. B. Acids were, in fact, oxyds, which, by the addition of a sufficient quantity of oxygen, have been converted into acids. For acidification, you must observe, always implies previous oxydation, as a body must have combined with the quantity of oxygen requisite to constitute it an oxyd, before it can combine with the greater quantity that is neces- sary to render it an acid. Caroline. Are all oxyds capable of being con- verted into acids ? Mrs. B. Very far from it; it is only certain substances which will enter into that peculiar kind of union with oxygen that produces acid?, and the 128 number of these is proportionally very small; but all burnt bodies may be considered as belonging ei- ther to the class of oxyds, or to that of acids. At a future period, we shall enter more at large into this subject. At present, I have but one circum- stance further to point out to your observation res- pecting acids: it is, that most of them are suscepti- ble of two degrees of acidification, according to the different quantities of oxygen with which their ba- sis combines. Emily. And how are these two degrees of acidi- fication distinguished ? Mrs. B. By the peculiar properties that result from them. The acid we have just made is the first or weakest degree of acidification, and is called sulphurous acid; if it were fully saturated with oxy- gen, it would be called sulphuric acid. You must therefore remember, that in this, as in all- acids, the first degree of acidification is expressed by the ter- mination in ous; the stronger, by the termination in ic. Caroline. And how is the sulphuric acid made? Mrs. B. By burning sulphur in pure oxygen gas, and thus rendering its combustion much more complete. I have provided some oxygen gas for this purpose; it is in that bottle, but we must first decant the gas into the glass receiver which stands on the shelf in the bath, and is full of water. Caroline. Pray, let me try to do it, Mrs. B? Mrs. B. It requires some little dexterity—hold the bottle completely under water, and do not turn the mouth upwards, till it is immediately under the aperture in the shelf, through which the gas is to pass into the receiver, and then turn it up gradual- ly..—Very well, you have only let a few bubbles es- cape, and that must be expected at a first trial.— Now I shall put this piece of sulphur into the re- ceiver, through the opening at the top, and intro- 129 duce along with it a small piece of lighted tinder to set fire to it. This requires being done very quick- ly, lest the atmospherical air should get in, and mix with the pure oxygen gas. Emily. How beautifully it burns! Caroline. But it is already buried in the thick vapour. This I suppose is sulphuric acid ? Emily. Are these acids always in a gaseous state? Airs. B. Sulphurous acid, as we have already ob- served, is a permanent gas, and can be obtained in a liquid form only by condensing it in water. In its pure state, the sulphurous acid is invisible, ,^nd it appears in the form of a white smoke, only from its combining with the moisture. But the vapour of sulphuric acid, which you have just seen to rise du- ring the combustion, is not a gas, but only a vapour, which condenses into liquid sulphuric acid, merely by losing its caloric. And this condensation is much hastened and promoted by receiving the vapour into cold water; which may afterwards be separated from the acid by evaporation. Before we quit the subject of sulphur, I must tell you that it is susceptible of combining with a great variety of substances, and especially with hydrogen, with which you are already acquainted. Hydrogen gas can dissolve a small portion of it. Emily. What; can a gas dissolve a solid substance? Airs. B. Yes; a solid substance may be so mi- nutely divided by heat, as to become soluble in a gas; and there are several instances of it. But you must observe that, in this case, a chemical solution, that is to say, a combination of the sulphur with the hydrogen gas, is produced. In order to effect this, the sulphur must be strongly heated in contact with the gas; the heat reduces the sulphur to such a state of extreme division, and diffuses it so thoroughly through the gas, that they combine and incorporate together. And as a proof that there must be a 130 chemical union between the sulphur and the.gas, it is sufficient to remark, that they are not separated when the sulphur loses the caloric by which it was volatilized. Besides, it is evident, from the pecu- liar fetid smell of this gas, that it is a new compound totally different from either of its constituents; it is called sulphurated hydrogen gas, and is contained in great abundance in sulphurous mineral waters. Caroline. Are not the Harrogate waters of this nature ? Mrs. B. Yes; they are naturally impregnated with sulphurated hydrogen gas, and there are many other springs of the same kind; which shews that this gas must often be formed in the bowels of the earth by spontaneous processes of nature. Caroline. And could not such waters be made artificially by impregnating common water with this gas? Mrs. B. Yes; they can be so well imitated as perfectly to resemble the Harrogate waters. Sulphur combines likewise with phosphorus, and with the alkalies, and alkaline earths, substances with which you are yet unacquainted. We cannot, therefore, enter into these combinations at present. In our next lesson we shall treat of phosphorus. Emily. May we not begin that subject to-day; this lesson has been so short ? Mrs. B. I have no objection, if you are not tired. What do you say, Caroline ? Caroline. I am as desirous as Emily of prolong- ing the lesson to-day, especially as we are to enter on a new subject; for I confess that sulphur has not appeared to me so interesting as the other simpl e bodies. Airs. B. Perhaps you may find phosphorus more entertaining. You must not, however, be discou- raged when you meet with some parts of a study less amusing than others; it would answer no good 131 purpose to seleft the most pleasing parts, since, if we did not proceed with some method, in order to acquire a general idea of the whole, we could scarce- ly expect to take interest in any particular subjefts. PHOSPHORUS. Phosphorus is a simple substance that was for- merly unknown. It was first discovered by Brandt, a chemist of Hamburgh, whilst employed in re- searches after the philosopher's stone ; but the me- thod of obtaining it remained a secret till it was a second time discovered both by Kunckel and Boyle, in the year 1680. You see a specimen of phospho- rus in this phial; it is generally moulded into small sticks of a yellowish colour, as you find it here. Caroline. I do not understand in what the disco- very consisted ; there may be a secret method of making a composition, but a simple body cannot be made, it can only be found. Mrs. B. But a body may exist in nature so close- ly combined with other substances, as to elude the observation of chemists, or render it extremely dif- ficult to obtain it in its simple state. This is the case with phosphorus, which is always so intimately combined with other substances, that its existence remained unnoticed till Brandt discovered the means of obtaining it free from all combinations. It is found in all animal substances, and is now chiefly extracted from bones, by a chemical process. It exists also in some plants, that bear a strong analo- gy to animal matter in their chemical composition. Emily. But is it never found in its simple state? Mrs. B. Never, and this is the reason of its ha- ving remained so long undiscovered. 132 Emily. It is possible, then, that in course of time other new simple bodies may be discovered ? Mrs. B. Undoubtedly ; and we may also learn that some of those, which we now class among the simple bodies, may, in fact, be compound; indeed, you will soon find that discoveries of this kind are by no means unfrequent. Phosphorus is eminently combustible ; it melts and takes fire at the temperature of 100°, and ab- sorbs in its combustion nearly once and a half its own weight of oxygen ? Caroline. What ! will a pound of phosphorus consume a pound and a half of oxygen ? Mrs. B. So it appears from accurate experiments. I can show you with what violence it combines with oxygen, by burning some of it in that gas. We must manage the experiment in the same manner as we did the combustion of sulphur.—You see I am obliged to cut this little bit of phosphorus under water, otherwise there would be danger of its taking fire by the heat of my fingers.—I now put it into the receiver, and kindle it by means of a hot wire. Emily. What a blaze ! I can hardly look at it. I never saw any thing so brilliant. Does it not hurt your eyes, Caroline ? Caroline. Yes ; but still I cannot help looking at it. A prodigious quantity of oxygen must indeed be absorbed, when so much light and caloric are disengaged ! Mrs. B. In the combustion of a pound of phos- phorus, a sufficient quantity of caloric is set free to melt upwards of a hundred pounds of ice; this has been computed by direct experiments with the ca- lorimeter. Emily. And is the result of this combustion, like that of sulphur, an acid ? Mrs. B. Yes; phosphoric acid. And had we duly proportioned the phosphorus and the oxygen, 133 they would have been completely converted into phosphoric acid, weighing together, in this, new state, exactly the sum of their weights separately. The water would have ascended into the receiver, on account of the vacuum formed, and would have filled it entirely. In this case, as in the combustion of sulphur, the acid vapour formed is absorbed and condensed in the water of the receiver. But when this combustion is performed without any water or moisture being present, the acid then appears in the form of concrete whitish flakes, which are, however, extremely ready to melt upon the least admission of moisture. Emily. Does phosphorus, in burning in atmosphe- rical air, produce like sulphur, a weaker sort of the same acid? Airs. B. No; for it burns in atmospherical air nearly at the same temperature, as in pure oxygen gas; and it is, in both cases, so strongly disposed to combine with the oxygen, that the combustion is perfect, and the product similar; only in atmosphe- rical air being less rapidly supplied with oxygen, the process is performed in a slower manner. Caroline. But is there no method of acidifying phosphorus in a slighter manner; so as to form phos- phorus acid? Mrs. B. Yes, there is. When simply exposed to the atmosphere, phosphorus undergoes a kind of slow combustion at any temperature above zero. Emily. But is not the process in this case rather an oxydation than a combustion ? For if the oxygen is too slowly absorbed for a sensible quantity of light and heat to be disengaged, it is not a true combus- tion. Mis. B. The case is not as you suppose; a faint light is emitted which is very discernible in the dark; but the heat evolved is not sufficiently strong to be sensible; a whitish vapour arises from this combus- M 134 tion, which uniting with water, condenses into li- quid phosphorus acid. Caroline. Js it not very singular that phosphorus should burn at so low a temperature in atmospheri- cal air, whilst it does not burn in pure oxygen*with- out the application of heat ? Mrs. B. So it at first appears. But this circum- stance seems to be owing to the nitrogen gas of the atmosphere. This gas dissolves small particles of phosphorus, which being thus minutely divided and diffused in the atmospherical air, combines with the oxygen, and undergoes this slow combustion. But the same effect does not take place in oxygen gas, because it is not capable of dissolving phosphorus; it is therefore necessary, in this case, that heat should be applied to effect that division of particles, which, in the former instance, is produced by the nitrogen. Emily- I have seen letters written with phospho- rus, which are invisible by day-light, but may be read in the dark by their own light. They look as if they were written with fire; yet they do not seem to burn. Mrs. B. But they do really burn; for it is by their slow combustion that the light is emitted; and phosphorus acid is the result of this combustion. Phosphorus is sometimes used as a.test to estimate the purity of atmospherical air. For this purpose, it is burnt in a graduated tube called an eudiometer (Plate Vlii. Fig 19.), and from the quantity of air which the phosphorus absorbs, the proportion of oxy- gen in the air examined, is deduced; for the phos- phorus will absorb all the oxygen, and the nitrogen alone will remain. Emily. And the more oxygen is contained in the atmosphere, the purer I suppose it is esteemed? Mrs. B. Certainly.. Phosphorus, when melted, combines with a great variety of substances. With 135 sulphur it forms a compound so- extremely combus- tible, that it immediately takes fire on coming in con^ tact with the air. It is with this composition that the phosphoric matches are prepared, which kindle as soon as they are taken out of their case and are exposed to the air. Emily. I have a box of these curious matches; but I have observed, that in very cold weather, they will not take fire without being previously rub- bed. Mrs. B. By rubbing them you raise their tem- perature; for you know, friction is one of the means of extricating heat. Emily. Will phosphorus combine with hydrogen gas, as sulphur does? v Mrs. B. Yes; and the compound gas which re- sults from this combination has a smell still more fetid than the sulphurated hydrogen? it resembles that of garlic. The phosphorated hydrogen gas has this remarkable peculiarity, that it takes fire spontaneously in the atmosphere at any temperature. It is thus that are produced those transient flames, or flashes of light, called by the'vulgar Will-of-the-Wisp, or more pro- perly Ignes-Fatui, which are often seen in church yards, and places where the putrefaction of animal matter exhales phosphorus and hydrogen gas; Caroline. Country people, who are so much fright- ened by those appearances, would soon be reconci- led to them, if they knew from what a simple cause they proceed. Mrs. B. There are other combinations of phos» phorus that have also very singular properties, par- ticularly that which results from its union with lime. ^ Emily. Is there any name to distinguish the com- bination of two simple substances, like phosphorus and lime, neither of which are oxygen, and which 136 therefore cannot produce neither an oxyd nor an acid? *" Airs. B. The names of such combinations are composed from those of their ingredients, merely by a slight change in their termination. Thus we call the combination of sulphur with lime asulphurety and that of phosphorus, a phosphoret of lime. This latter compound, I was going to say, has the singular property of decomposing water, merely by being thrown into it. It effects this by absorbing the oxy- gen of water, in consequence of which bubbles of hydrogen gas ascend, holding in solution a small quantity of phosphorus. Emily. These bubbles then are phosphorated hydro- gen gas? Airs. B. Yes; and they produce the singular ap- pearance of a flash of fire issuing from water, as the bubbles kindle and detonate on the surface of the water, at the instant that they come in contact with the atmosphere. Caroline. Is not this effect nearly similar to that produced by the combination of phosphorus and sul- phur, or, more properly speaking, the phosphoret of sulphur ? Airs. B. Yes; but the phenomenon appears more extraordinary in this case, from the presence of wa- ter and from the gaseous form of the combustible compound. Besides the experiment surprises by its great simplicity. You only throw a piece of phos- phoret of lime into a glass of water, and bubbles of fire will immediately issue from it. Caroline. Cannot we try the experiment? Airs. B. Very easily: but we must do it in the open air; for the smell of the phosphorated hydro- gen gas is so extremely fetid, that it would be in- tolerable in the house. But before we leave the room, we may produce, by another process, some bubbles of the same gas, which are much less offensive. 137 There is in this little glass retort a solution of pot- ash in water; I add to it a small piece of phospho- rus. We must now heat the retort over the lamp, after having engaged its neck under water—you see it begins to boil; in a few minutes bubbles will ap- pear, which take fire and detonate as they issue from the water. Caroline. There is one—and another. How cu- rious it is!—But I do not understand how this is produced ?■ Airs. Bi It is the consequence of a display of affinities too complicated, I fear, to be made per- fectly intelligible to you at present. In a few words, the reciprocal action of the pot- ash, phosphorus, caloric, and water, are such that some of the water is decomposed, and the hydrogen thereby formed carries off some minute particles of phosphorus* with which it forms phosphorated hy- drogen gas, a compound which spontaneously takes fire at almost any temperature. Emily. What is that circular rjng of smoke which slowly rises from each bubble after its detonation? Airs. B. It consists of water and phosphoric acid in vapour, which are produced by the combustion •. of the hydrogen and phosphorus. CONVERSATION V1IL On Carbone. Caroline; • To-day, Mrs. B.—I believe we are to learn th& aature and properties of carbone. This substance M2- 138 is quite new to me; I never heard it mentioned be- fore. Mrt. B. Not so new as you imagine ; for car- bone is nothing more than charcoal in a state of perfect purity. Caroline. But charcoal is made by art, Mrs. B. and a body consisting of one simple substance can- not be fabricated ? Mrs. B. You again confound the idea of ma- king a simple body, with that of separating it from a compound. The chemical processes by which a simple body is obtained in a state of purity, consist in unmaking the compound in which it is contained, in order to separate from it the simple substance in question. The method by which charcoal is usually obtained, is, indeed, commonly called making it; but, upon examination, you will find this process to consist simply in separating it from other substances with which it is found combined in nature. Carbone forms a considerable part of the solid matter of all organized bodies; but it is most abun- dant in the vegetable creation, and it is chiefly ob- tained from wood. When the oil and water (which are other constituents of vegetable matter) are eva- porated, the black, porous, brittle substance that remains, is charcoal. Caroline. But if heat be applied to the wood in order to evaporate the oil and water, will not the temperature of the charcoal be raised so as to make it burn; and if it combines with oxygen, can we any longer call it pure ? Mrs. B. I was going to say, that in this opera- tion, the air must be excluded. Caroline. How then can the vapour of the oil and water fly off? Mrs. B. In order to produce charcoal in its purest state (which is, even then, but a less imperfect sort of carbone), the operation should be performed in 139 an earthen retort. Heat being applied to the body of the retort, the evaporable parts of the wood wilL escape through its neck, into which no air can pe- netrate as long as the heated vapour continues to fill it. And if it be wished to collect these volatile products of the wood, this can easily be done by introducing the neck of the retort into the water- bath apparatus, with which you are acquainted. But the preparation of common charcoal, such as is nsed in kitchens and manufactures, is performed on a much larger scale, and by an easier and less ex- pensive process. Emily. I have seen the process of making com- mon charcoal. The wood is ranged on the ground in a pile of a pyramidical form, with a fire under- neath ; the whole is then covered with clay, a few holes only being left for the circulation of air. Mrs. B. These holes are closed as soon as the wood is fairly lighted, so that the combustion is checked, or at least continues but in a very imper- fect manner ; but the heat produced by it is suffici- ent to force out and volatilize, through the earthy cover, most part of the oily and watery principles of the wood, although it cannot reduce it to ashes. Emily. Is pure carbone as black as charcoal ? Mrs. B. The more charcoal is purified, that is to say, the nearer it approaches to the state of sim- ple carbone, the deeper its black colour appears; but the utmost efforts of chemical art, are not able to bring it to its perfect elementary state; for in that state it is both colourless and transparent, and as different in appearance from charcoal as any sub- stance can possibly be. This ring which I wear on my finger, owes its brilliancy to a small piece of carbone. Caroline. Surely you are jesting, Mrs. B. ? Emily. I thought that your ring was diamond ? Mrs. B. It is so. But diamond is nothing more than carbone in its purest and most perfect state. 140 Emily. That is astonishing ! Is it possible to see two things apparently mor^e different than diamond and charcoal ? Caroline. It is, indeed, curious to think that we adorn ourselves with jewels of charcoal! Mrs. B. When you are better acquainted with the nature of crystallization, in which state bodies are generally the purest, you will more readily con- ceive the possibility of carbone assuming the trans- parency and brilliancy of diamond. There are many other substances, consisting chief- ly of carbone, that are remarkably white. Cotton, for instance, is almost wholly carbone. Caroline. That, I own, I could never have ima- gined!—But pray, Mrs.B. since it is known of what substance diamond and cotton are composed, why should they not be manufactured, or imitated, by some chemical process, which would render them much cheaper and more plentiful than the present mode of obtaining them ? Mrs. B. You might as' well my dear propose that we should make flowers and fruit, nay perhaps even animals, by a chemical process; for it is known of what these bodies consist, since every thing which we are acquainted with in nature, is formed from the various simple substances that we have enumerated. But, you must not suppose that a knowledge of the component parts of a body will in every case enable us to imitate it. It is much less difficult to decompose bodies* and discover of what materials they are made, than it is to recom- pose them. The first of these processes is called analysis, the last synthesis. When we are able to ascertain the nature of a substance by both these methods, so that the result of one confirms that cf the other, we obtain the most complete knowledge of it that we are capable of acquiring. This is the case with water, with the atmosphere, with most of*: 141 the oxyds, acids, and neutral salts, and with many other compounds But the more complicated com- binations of nature, even in the mineral kingdom, are in general beyond our reach, and any attempt to imitate organized bodies must ever prove fruit- less; their formation is a secret that rests in the bo- som of the Creator. You see, therefore, how vain- it would be to attempt the formation of cotton by chemical means. But, surely, we have no reason to regret our inability in this instance, when nature has so clearly pointed a method of obtaining it in perfection and abundance. Caroline. I did not imagine that the principle of life could be imitated by the aid of chemistry; but it did not appear to me ridiculous to suppose that chemists might attain a perfect imitation - of inani- mate nature. Mrs. B. They have succeeded in this point in a variety of instances; but, as you justly observe, the principle of life, or even the minute and intimate organization of the vegetable kingdom, are secrets that have almost entirely eluded the researches of philosophers ? nor do I imagine that human art will ever be capable of investigating them with complete success. Emily. But diamond, since it consists merely of one simple unorganized substance, might be, one would think, perfectly imitable by art ? Mrs. B. It is sometimes as much beyond our power to obtain a simple body in a state of perfect purity, as it is to imitate a complicated combina- tion ; for the operations by which nature decompo- ses bodies are frequently as inimitable as those which she uses for their combination. This is the case with carbone; all the efforts of chemists to separate it entirely from other substances, have been fruit- less, and in the purest state in which it can be ob- tained by art, it still retains a portion of oxygen, 1*2 and probably of some other foreign ingredients. It is in the diamond alone, as I have observed before, that carbone is supposed to exist in its perfect form; we are ignorant of the means which nature employs to bring it to that state; it may probably be the work of ages, to purify, arrange, and unite the particles of carbone in the form of diamond. And with re*- gard to our artificial carbone, which we call char- coal, ..we must consider it as an oxyd of carbone; since, whatever may be the means employed for obtaining it, it always retains a small portion of oxygen. Here is some charcoal in the purest state we can procure it: you see that it is a very black, brittle, light, porous substance, entirely destitute of either taste or smell. Heat, without air, produ- ces no alteration in it, as it is. not volatile; but on the contrary, it invariably remains at the bottom of the vessel after all the other parts of the vegetable are evaporated. Emily. Carbone is, no doubt, combustible, since you say that charcoal would absorb oxygen if air was admitted during its preparation ? Caroline. Unquestionably. Besides, you know, Emily, how much it is used in cooking. But pray what is the reason that charcoal burns without smoke, whilst a wood fire smokes so much ? Mrs. B. Because, in the conversion of wood in- to charcoal, the volatile particles of the former have been evaporated. Caroline. Yet I have frequently seen charcoal burn with flame ; therefore it must, in that case, contain some hydrogen. Mrs. B. Very true ; but you must recollect that charcoal, especially that which is used for common purposes, is very far from being pure. It generally retains, as we have seen, not only a small quantity of oxygen, but also some remains of the various other component parts of vegetables, and hydrogen 143 particularly, which accounts for the flame in ques- tion. Caroline. But what becomes of the carbone itself during its combustion ? Mrs. B. It gradually combines with the oxygen of the atmosphere, in the same way as sulphur and phosphorus, and, like those substances, it is con- verted into a peculiar acid, which flies off in a ga- seous form. There is this difference, however, that the acid is not, in this instance, as in the two cases just mentioned, a mere condensable vapour, but a permanent.elastic fluid, which always remains in the state of gas, under any pressure and at any temperature. The nature of this acid was first as- certained by Dr. Black, of Edinburgh; and, before the introduction of the new nomenclature, it was called fixed air. It is now distinguished by the more appropriate name of carbonic acid gas. Emily. Carbone, then, can be volatilized by burning, though, by heat alone, no such effect is produced ? Mrs. B. Yes ; but then it is no longer simple carbone, but an acid of which carbone forms the basis. In this state, carbone retains no more ap- pearance of solidity or corporeal form, than the ba- sis of any other gas. And you may, I think, from this instance, derive a more clear idea of the basis of the oxygen, hydrogen, and nitrogen gasses, the existence of which, as real bodies, you seemed to doubt, because they were not to be obtained simply in a solid form. Emily That is true; we may.conceive the basis of the oxygen, and of the other gasses, to be solid, heavy substances, like carbone; but so much ex- panded by caloric, as to become invisible. Caroline. But does not the carbonic acid gas par- take of the blackness of charcoal ? Mrs.B. Not in the least. Blackness, you know. 144 does not appear to be essential to carbone, and it is pure carbone, and not charcoal, that we must con- sider as the basis of carbonic acid. We shall make some carbonic acid, and, in order to hasten the pro- cess, we shall burn the carbone in oxygen gas. Emily. But how can you make carbonic acid, unless you can burn diamond; since that alone is pure carbone ? Mrs. B. Charcoal will answer the purpose still better; for the carbone being, in that state, already combined with some portion of oxygen,' it will re- quire less of that principle to complete its oxygena- tion. Caroline. But is it possible to burn diamond ? Mrs. B. Yes, it is; and, in order to effect this combustion, nothing more is required than to apply a sufficient degree of heat by means of the blow- pipe, and of a stream of oxygen gas. Indeed it is by burning diamond that its chemical nature has been ascertained. It is long since it has been known, as a combustible substance, but it is within these few years only that the product of its combustion has been proved to be pure carbonic acid. This discovery is due to Mr. Tennant. But still more recent experiments have shown, that diamond re- quires a greater proportion of oxygen than charcoal to be converted into carbonic acid. It appears that 15 parts of diamond require 85 parts of oxygen to form 100 parts of carbonic acid ; whilst 28 parts of charcoal take up only 72 parts of oxygen to produce 100 parts of carbonic acid; from which it is natu- rally inferred that carbone, in the state of charcoal, is already combined with a portion of oxygen. Now let us try to make some carbonic acid—Will you, Emily, decant some oxygen gas from this large jar into the receiver in which we are to burn the carbone; and I shall introduce this small piece of charcoal, with a little lighted tinder, which will be necessary to give the first impulse to the combustion. 145 Emily. I cannot conceive how so small a piece of tinder, and that but just lighted, can raise the tem- perature of the carbone sufficiently to set fire to it; for it can produce scarcely any sensible heat, and it hardly touches the carbone. Mrs. B. The tinder thus kindled has only heat enough to begin its own combustion, which, how- ever, soon becomes, so rapid in the oxygen gas, as to raise the temperature of the charcoal sufficiently for this to burn likewise, as you see is now the case./ Emily. I am surprised that the combustion of carbone is not more brilliant; it does not disengage near so much light or caloric as phosphorus, or sul- phur. Yet, since it combines with so much oxy- gen, why is not a proportional quantity of light and heat disengaged from the decomposition of the oxy- gen gas ? Mrs. B. It is not surprising that less light and heat shourtfbe disengaged in this than in almost any other combustion, since the oxygen, instead of en- tering into a solid or a liquid combination, as it does in the phosphoric and sulphuric acids, is employed in forming another elastic fluid. Emily. True; and, on second consideration, it appears, on the contrary, surprising that the oxygen should, in its combination with carbone, retain a sufficient portion of caloric to maintain both sub- Stances in a gaseous state. Caroline. We may then judge of the degree of solidity in which oxygen is combined in a burnt bo- dy, by the quantity of caloric liberated during its combustion ? Airs. B. Yes; provided that you take into the account the quantity of oxygen absorbed by the combustible body, and observe the proportion which the caloric bears to it. Caroline. But why should the water, after the combustion of carbone, rise in the receiver since the gas within it retains an .aeriform state ? Mrs: B. -Because carbonic acid gas is more dense, and consequently occupies less space than oxygen gas; the water therefore rises to fill the vacuum formed by the diminution of volume of the gas. Caroline. That is very clear: and the condensa- tion of the new gas depends, I suppose, on the quan- tity of caloric that has been disengaged. Mrs. B. The gas must be decreased in volume, from that circumstance, in a certain proportion; but its density is still further increased by the addition of the carbone. But besides this condensation, there is in our experiment another cause of the diminu- tion of volume, which is, that carbonic acid gas, by standing over water, is gradually absorbed by it, an effect which is promoted by shaking the receiver. Emily. The charcoal is now extinguished, though at is not nearly.consumed; it has such an extraordi- nary avidity for oxygen, I suppose, that the recei- ver did not contain enough to satisfy the whole. Mrs. B. That is certainly the case; for if the combustion was performed in the. exact proportions of 28 parts of carbone to 72 of oxygen, both these ingredients would disappear, and 100 parts of car- bonic acid would be produced. Caroline. Carbonic acid must be a very strong acid, since it contains so great a proportion of oxy^ gen ? Airs. B. That is a very natural inference ; yet it is erroneous. For the carbonic is the weakest of all the acids. The strength of an acid seems to de- pend upon the nature of its basis and its mode of combination, as well as upon the proportion of thei acidifying principle. The same quantity of oxygen that will convert some bodies into strong acids, will pnly be sufficient simply to oxydate others. Caroline. Since this acid is so weak, I think che- 147 mists should have called it the carbonoiis, instead of the carbonic acid. Emily. But, I suppose, the carbonous acid is still . weaker, and is formed by burning carbone in at- mospherical air., Mrs. B. No, my dear. Carbone does not ap- pear to be susceptible of more than one degree of acidification, whether burnt in oxygen gas, or at- mospherical air. There is therefore no carbonous Scid. It has indeed been lately discovered; that carbone may be converted into a gas, by uniting with a smal- ler proportion of oxygen; but as this gas does jiot possess any acid properties, it is no more than an' oxyd; and in order to distinguish it from charcoal, which contains a still smaller proportion of oxygen, it is called gaseous oxyd of carbone. Caroline. Pray is not carbonic acid a very whole- some gas to breathe, as it contains so much oxygen ? Mrs. B. On-the contrary, it is extremely perni- cious. Oxygen, when in a state of combination with other substances, loses, in almost every in- stance, its respirable properties, and the salubrious ^effects which it has on the animal economy when in its uncombined state. Carbonic acid is not only un- fit for respiration, but extremely deleterious if taken into the lungs. Emily. You know, Caroline, how very unwhole- some the fumes of burning charcoal are reckoned. Caroline. Yes; but to confess the truth, I did not consider that a charcoal fire produced carbonic acid gas.—Pray, can this gas be condensed into a liquid ? Mrs. B. No: for, as I told you before, it is a permanent elastic fluid. But water can absorb a certain quantity of this gas, and can even be im- pregnated with it, in a very strong degree, by the assistance of agitation and pressure, as I am going 14b ;o show you. 'I shall decant some carbonic *tid gas into this bottle, which I fill first with water, in or- der-to exclude the atmospherical air; the gas is then introduced through the water, which you see it dis- places, for it will not mix with it in any quantity nnless strongly-agitated, or allowed to stand over it for some time. The bottle is now about half full of carbonic acid gas, and the other half is still occupied by the water. By corking the bottle, and then vio- lently shaking it, in this way, I can mix the gas and water together.—Now will you taste it ? Emily. It has a distinct acid taste, Caroline. Yes, it is sensibly sour, and appears full of little bubbles. Mrs. B. It possesses likewise all the other pro- perties of acids, but of course in a less degree than the pure carbonic acid gas, as it is so much diluted by water. This is a kind of artificial Ueltzer water. By ana- lysing that which is produced by nature, it was found to contain scarcely any thing more than common water impregnated with a certain proportion of car- bonic acid gas. We are, therefore, able to imitate it, by mixing those proportions of water, and car- bonic acid. Here, my dear, is an instance, in which, by a chemical process, we can exactly copy the ope- rations of nature; for the artificial Seltzer waters can be made in every respect similar tp those of na- ture : in one point, indeed, the former have an ad- vantage, since they may be prepared stronger, or weaker, as occasion requires. Caroline. I thought I had tasted such water be- fore. But what renders it so brisk and sparkling? Airs. B. This sparkling, or effervescence, as it is called, is always occasioned by the action of an elastic fluid escaping from a liquid; in the artificial Seltzer water it is produced by the carbonic acid, which being lighter than th,e water in which it was 149 strongly condensed, flies off with great rapidity the instant the bottle is uncorked; this makes it neces- sary to drink it immediately. The bubbling that took place in this bottle was but trifling, as the wa- ter was but very slightly impregnated with carbonic acid. It requires a particular apparatus to prepare the gaseous artificial mineral waters. Emily. If, then, a bottle "of Seltzer water re- mains for any length of time uncorked, I suppose it returns to the state of common water ? Mrs. B. The whole of the carbonic acid gas, or very nearly so, will soon disappear; but there is likewise in Seltzer water a very small quantity of soda, and of a few other saline or earthy ingredients, which will remain in the water, though it should be kept uncorked for any length of time. Caroline. I have often heard of people drinking soda water, pray what sort of water is that ? Mrs. B. It is a kind of artificial Seltzer water, holding in solution, besidestthe gaseous acid, a par- ticular saline substance, called soda; which imparts to the water certain medicinal qualities. Caroline. But how can these waters be so whole- some, since carbonic acid is so pernicious ? Airs. B. A gas we may conceive though very prejudicial to breathe, may be beneficial to the sto- mach.—But it would be of no use to attempt ex- plaining this more fully at present. Caroline. Are waters never impregnated with other gasses ? Mrs. B. Yes ; there are several kinds of gaseous waters. I forgot to tell you that waters have for some years past been prepared, impregnated both with oxygen and hydrogen gasses. ' These are not an imitation of nature, but are altogether obtained by artificial means. They have been lately used medicinally, particularly abroad, where, I under- stand,, they have acquired some reputation^ n 2. 150 Emily. • If I recollect right, Mrs. B. you told us that carbone was capable of decomposing water; the affinity between oxygen and carbone must there- fore be greater than between oxygen and hydrogen? Mrs. B. Yes; but this is not the case unless their temperature be raised to a certain degree. It ,is only when carbone is red hot, that it is capable of separating the oxygen from the hydrogen. Thus, if a small quantity of water be thrown on a red hot fire, it will increase, rather than extinguish the combustion; for the coals or wood (both of which contain a great quantity of carbone) decompose the water, and thus supply the fire both with oxygen and hydrogen gasses. If, on the contrary, a large mass of water be thrown over the fire, the diminu- tion of heat thus produced is such that the combus- tible matter loses the power of decomposing the wa- ter, and the fire is extinguished. , EmUy. I have heard that fire engines sometimes do more harm than good, and that they actually in- crease the fire when they cannot throw water enough to extinguish it. It must be owing no doubt, to the decomposition of the water by the carbone during the conflagration. Airs. B. Certainly.—The apparatus which you see here {Plate VIII. Fig. 20.) may be used to ex- emplify what we have just said. It consists in a kind of open furnace, through which a porcelain tube,, containing charcoal, passes. To one end of the tube is adapted a glass retort with water in it; and the other end communicates with a receiver placed on the water bath.—A lamp being applied to the retort, and the water made to boil, the vapour is gradually conveyed through the red hot charcoal, by which it is decomposed; and the hydrogen gas which results from this decomposition is collected in the receiver. But the hydrogen thus obtained is far from being pure; it retains in solution a mi- 151 nute portion of carbone, and contains also a quan- tity of carbonic acid. This renders it heavier than pure hydrogen gas, and gives it some peculiar pro- perties : it is distinguished by the name of carbotiar ted hydrogen gas. Caroline. And whence does it obtain the carbo? nic acid that is mixed with it ? Emily. I believe I can answer that question, Ca- roline.—From the union of the oxygen (proceeding from the decomposed water) with the carbone, which, you know, makes carbonic acid. Caroline. True ; I should have recollected that.— The product of the decomposition of water by red hot charcoal, therefore, is carbonated hydrogen gas and carbonic acid gas. Mrs. B. You are perfectly right now. Carbone is frequently found combined with hy- drogen in a state of solidity, especially in coals, which owe their combustible nature to these two principles. Emily. Is it the hydrogen, then, that produces the flame of coals ? Mrs. B. It is so; and when all the hydrogen is consumed, the carbone continues to burn without flame. But again the hydrogen gas produced by the combustion of coals is not pure; for, during the combustion, particles of carbone are successive- ly volatilized with the hydrogen, with which they form what is called a hydro-carbonate, which is the essential combustion. Carbone is a very bad conductor of heat; for this reason, it is employed (in conjunction with other ingredients) for coating furnaces and other chemical apparatus. Emily. Pray what is the use of coating furnaces ? Mrs. B. In most cases, in which a furnace is used, it is necessary to produce and preserve a great degree of heat, for which purpose every possible means are used to prevent the hea,t from escaping 152 by communicating with other bodies, and this ob- ject is. attained by coating over the inside of the fur- nace with a kind of plaster, composed of materials that are bad conductors of heat. - Carbone combined with a small quantity of iron* forms- a compound called plumbago, or black lead, of which pencils are made. This substance, agree* ably to the nomenclature, is a carburet of iron. Caroline. Why, then, is it called, black, lead ? Mrs. B. I really cannot say; but it is certainly a most improper name for it, as there is not a parti- cle of lead in the composition. There is another carburet of iron in which the iron though united only to an extremely small proportion of carbone, acquires very remarkable properties; this is steel. Caroline. Really; and yet steel, is much harder than iron ? Airs. B. But carbone is not ductile* like iron, and therefore may render the steel more brittle, and prevent its bending so easily. Whether it is that the carbone by introducing itself into the pores of the iron, and by filling them, makes the metal both harder and heavier; or whether this change depends upon some chemical cause, I cannot pretend to de<- cide. But there is a subsequent operation, by which the hardness of steel is very much increased, which simply consists in heating the steel till it is red hot, and then plunging it into cold water. Carbone besides the combination just mentioned, enters into the composition of a vast number of na- tural productions, such, for instance, as all the va- rious kinds of oils, which result from the combina- tion qf carbone, hydrogen, and caloric, in various proportions. Emily/ I thought that carbone, hydrogen, and caloric, formed carbonated hydrogen gas ? Mrs. B. That is the case when a small portion of carbonic acid gas is held in solution by hydrogen 153 fas. Different proportions of the same principles, together with the circumstances of their union, pro- duce very different combinations; of this you will see innumerable examples. Besides we are not now talking of gasses, but of carbone and hydrogen, combined only with a quantity of caloric sufficient to bring them to the consistency of oil or fat. Caroline. But oil and fat are not of the same consistence ? Mrs. B. Fat is only congealed oil; or oil, melt- ed fat. The one requires a little more heat to main- tain it in a fluid state, than the other. Have yon never observed the fat of meat turned to oil by the caloric it has imbibed from the fire ? ^ Emily. Yet oils in general, as salad oil, and lamp Oil, do not turn to fat when cold ? Mrs. B. Not at the common temperature of the atmosphere, because they retain too much caloric to congeal at that temperature j but if exposed to a sufficient degree of cold, their latent heat is extri- cated, and they become solid fat substances. Have you never seen salad oil frozen in winter ? Emily. Yes; but it appears to me in that state very different from animal fat. Mrs. B. The essential constituent parts of either vegetable or animal oils are the same, carbone and hydrogen; their variety arises from the different proportions of these substances, and from other ac- cessary ingredients that may be mixed with them. The oil of a whale, and the oil of roses, are, in their essential constituent parts, the same; but the one is impregnated with the offensive particles of animal matter, the other with the delicate perfume of a flower. The difference of fixed oils, and volatile or essential oils, consist also in the various proportions of car- bone and hydrogen. Fixed oils are those which will not evaporate without being decomposed; this 15* is the case with all the common oils, which contain a greater proportion of carbone than the essential oils. The essential oils (which comprehend the whole class of essences and perfumes) are lighter; they contain more equal proportions of carbone and hydrogen, and are volatilized or evaporated without being decomposed. Emily. When you say that one kind of oil will evaporate, and the other be decomposed, you mean, I suppose, bjr the application of heat ? Mrs. B. Not necessarily; for there are oils that- will evaporate slowly at the common temperature of the atrftosphere; but for a more rapid volatilization, or for their decomposition, the assistance of, heat is required. Caroline. I shall now remember, I think, that fat and oil are really the same substances, consisting l»oth of carbone and hydrogen; that in fixedoils the carbone preponderates, and heat produces a de- composition ; while, in essential oils, the proportion of hydrogen is greater, and heat produces volatili- zation only. Emily. I suppose the reason why oil burns so well in lamps, is because its two constituents are so combustible ? Mrs. B. Certainly; the combustion of oil is just the same as that of a candle; if tallow, it'is only oil in a concrete state; if wax, or spermaceti, its chief chemical ingredients are still hydrogen and carbone. Emily. I wonder, then", there should be so great a difference between tallow and wax ? Mrs. B. I must again repeat that the same sub- stances, in different proportions, produce results that have sometimes scarcely any resemblance to each other. But this is rather a general remark that 1 wish to impress upon your minds, than one which is applicable to the present case; for tallow and waSi are far from being very dissimilar; the chief 155 difference consists in the wax being a purer com- ^)ound of carbone and hydrogen than the tallow, which retains more of the gross particlesof animal matter. The combustion of a candle, and that of a lamp, both produce, water and carbonic acid gas. Can you tell me how these are formed ? Emily. Let me think.....Both the candle and lamp burn by means of fixed oil—thi's is decompo- sed as the combustion goes on; and the constituent -parts of the oil being thus separated, the carbone unites to a portion of oxygen from the atmosphere to form carbonic acid gas, whilst the hydrogen com- bines with another portion of oxygen, and forms with it water.—The products therefore, of the com- bustion of oils, are water and carbonic acid gas. Caroline. But we see neither water nor carbonic .acid produced by the combustion of a candle ? Mrs. B. The carbonic acid gas, you know, is invisible, and the water being in a state of vapour, is so likewise. Emily is perfectly correcVin her ex- planation, and I am very much pleased with it. All the vegetable acids consist of various propor- tions of. carbone and hydrqgen, acidified by oxy-' gen. Gums, sugar, and starch, are likewise com- posed of these ingredients; but as the oxygen which they contain is not sufficient to convert them into acids, they are classed with the oxyds, and called vegetable oxyds. Emily. I am very much delighted with all these new ideas; but, at the same time, I cannot help being apprehensive that I may forget many of them. Airs. B. I would advise you to take notes, or, what would answer better still, to write down, after every lesson, as much of it as you can recollect. And, in order to give you a little assistance, I shall' lend you the heads or index, which I occasionally -consult for the sake of preserving some method and arrangement in those conversations. Unless yon 15C follow some such^plan, you cannot expect to retain nearly all that you learn, how great soever be the impression it may make on you at first. Emily. I will certainly follow your advice.—Hi- therto I have found that I recollected pretty well what you have taught us; but the history of carbone is a more extensive subject than any of the simple bodies we have yet examined. Mrs. B. I have little more to say on carbone at present, but hereafter you will see that it performs a considerable part in most chemical operations. Caroline. That is, I suppose, owing to its enter- ing into the composition of so great a variety of substances ? Mrs. B. Certainly j it is the basis, you have seen, of all vegetable matter; and you will find that it is very essential to the process of animalization. But in the mineral kingdom also, particularly in its form of carbonic acid, we shall often discover it combi- ned with a great variety of substances. In chemical operations, carbone is particularly useful, from it£ very great attraction for oxygen, as it will absorb this substance from many oxygenated or burnt bodies, and thus_ deoxygenate, or unburn, them, and restore them to their original combusti- ble state. Caroline. I do not understand how a body can be unburnt, and restored to its original state. This piece of tinder, for instance, that has been burnt, if by any means the oxygen was extracted from it, would not be restored to its former state of linen; for its texture is destroyed by burning, and that must be the case with all organized or manufactured substances, as you observed in a former conversation. Mrs. B. A compound body is decomposed by combustion, in a way which generally precludes the possibility of restoring* it to its former state; the oxygen, for instance, does not become fixed in the 157 tinder, but it combines with its volatile parts, and flies off in the shape of gas, or watery vapour. You see therefore, how vain it would be to attempt the' recomposition of such bodies. But, with regard to simple bodies, or at least bodies whose constituents are not disturbed by the process of oxygenation or deoxygenation, it is often possible to restore them, after combustion, to their original state.—The me- tals, for instance, undergo no other alteration by combustion than a combination with oxygen; there- fore, when the oxygen is taken from them, they return to their pure metallic state. But I shall'say nothing further of this at present, as the metals will furnish ample subject for another morning; and they are the class of simple bodies that come next •under our consideration. 0 CONVERSATION IX. On Metals. Airs. B. The metals, which we are now to examine, are bodies of a very different nature from those which we have hitherto considered. They do not, like the elements of gasses, elude the immediate observation of our senses: for they are the most brilliant, the most ponderous, and the most palpable substances in nature. Caroline. I doubt, however, whether the metals will appear to us so interesting, and give us so much o 158 entertainment as those mysterious elements which conceal themselves from our view. Besides, they cannot afford so much novelty; they are bodies with which we are already so well acquainted. Airs. B. But the acquaintance, you will soon perceive, is but very superficial; and I trust that you will find both novelty, and entertainment in consid- ering the metals in a chemical point of view. To treat of this subject fully, would require a whole course of lectures; for metals form of themselves a most important branch of practical chemistry. We must, therefore, confine ourselves to a general view of them. These bodies are seldom found naturally in their metallic form; they are generally more or less oxygenated or combined with sulphur, earths, or acids, and are often blended with each other. They are found buried in the bowels of the earth in most parts of the globe, but chiefly in mountainous dis- tricts, where the surface.of the globe has suffered from earthquakes, volcanoes, and other convulsions of nature. They are there spread in strata or beds, called veins, and these veins are composed of a cer- tain quantity of metal, combined with various earthy jubstances, with which they form minerals of differ- ent nature and appearance, which are called ores. Caroline. I am now amongst old acquaintance, for my father has a lead mine in Yorkshire, and I have heard a great deal about veins of ore, and of the roasting and smelting of the lead; but, I confess, that I do not understand in what these operations consist. Mrs. B. Roasting is the process by which the volatile parts of the ore are evaporated; smelting, that by which the pure metal is afterwards separa- ted from the earthy remains of the ore. This is done by throwing the whole into a furnace, and mixing with it certain substances, that will combine with the earthy parts, and other foreign ingredients .of the ore; the metal being the heaviest, falls to 159 the bottom, and runs out by proper openings, in its pure metallic state. Emily. You told us in a preceding lesson that me- tals had a strong affinity for oxygen. Do they not, therefore, combine with oxygen, when strongly heated in the furnace, and run out in the state of oxyds? Mrs. B. No; for the scorise, or oxyd, which soon forms on the surface of the fused metal, when it is oxydable, prevent the air from having any fur- their influence on the mass; so that neither combus- tion nor oxygenation can take place. Caroline. Are all the metals combustible? Mrs. B. Yes, without exception; but their at- traction for oxygen varies extremely: there are some that will combine with it only at a very high tempe- rature, or by the assistance of acids; whilst there are others that oxydate of themselves very rapidly, even at the lowest temperature, as manganese, which scarcely ever exists in its metallic state, as it imme- diately absorbs oxygen on being exposed to the air* and crumbles to an oxyd in the course of a few- hours. Emily. Is it not from that oxyd that you extract- ed the oxygen gas ? Mrs. B. It is; so that, you see, this metal at- tracts oxygen at a low temperature, and parts with it when strongly heated. Emily. Is there any other metal that oxydates at the temperature of the atmosphere ? Mrs. B. They all do, more or less, excepting gold, silver, and platina. Copper, lead, and iron, oxydate slowly in the air, and cover themselves with a sort of rust, a pro- cess which depends on the gradual conversion of the surface into an oxyd. This rusty surface preserves the interior metal from oxydation, as it prevents the air from coming in contact with it. Strictly speak- 160 ing, however, the word rust applies only to the oxyd, which forms on the surface of iron, when exposed to air and moisture, which oxyd appears to be united with a small portion of carbonic acid. Emily. When metals oxydate from the atmos- phere without an elevation of temperature, some ught and heat, I suppose, must be disengaged, though not in sufficient quantities to be sensible. Mrs. B. Undoubtedly; and, indeed, it is not surprising that in this case the light and heat should not be sensible, when you consider how extremely slow, and, indeed, how imperfectly, most metals oxydate by mere exposure to the atmosphere. For the quantity of oxygen with which metals are capa- ble of combining, generally depends upon their temperature ; and the absorption stops at various points of oxydation, according to the degree to which their temperature is raised. Emily. That seems very natural; for the greater* the quantity of caloric introduced into a metal, the further its particles are separated from one another, and the more easily, therefore, can they attract the oxygen and combine with it. Mrs. B. Certainly; and besides, in proportion as the resistance diminishes on one hand, the affini- ty increases on the other. When the metal oxyge- nates with sufficient rapidity for light and heat to become sensible, combustion actually takes place. But this happens only at very high temperatures, « and the product is nevertheless an oxyd; for though, ;1 as I have just said, metals will combine with differ- ,-jj ent proportions of oxygen, yet, with the exception " ^j of only five of them, they are not susceptible of acidification. Metals change colour during the different degrees ■< of oxydation which they undergo. Lead, when heated in contact with the atmosphere, first becomes grey; if its temperature be then raised, it turns 161 yellow, and a still stronger heat changes it to red: . Iron becomes successively a green, brown, and white oxyd. Copper changes from brown to blue, and lastly green. Emily. Pray, is the white lead with which houses are painted prepared by oxydating lead ? Airs. B. Yes; almost all the metallic oxyds are used as paints. Red lead is another oxyd of that metal. The various sorts of ochres chiefly consist of iron more or less oxydated. And it is a remark- able circumstance, that if you burn metals rapidly, the light or flame they emit during combustion partakes of the colours which the oxyd successively assumes. Caroline. How is that accounted for, Mrs. B. ? For light, you know, does not proceed from the burning body, but from the decomposition of the oxygen gas -? I hope you have a satisfactory answer to give me, for I am under some apprehensions for my favourite theory of combustion ; and for the world I would not have it overthrown. Mrs. B. Do not be alarmed, my dear; I do not think it was ever supposed to be in danger from this circumstance. The correspondence of the colour" of the light with that of the oxyd which emits it, is, in all probability, owing to some particles of the metal which are volatilized and carried off by the- caloric. Caroline. It is then a sort of metallic gas. Emily. Why is it reckoned, so unwholesome to breathe the air of a place in which metals are melt- ing? Mrs. B. For this double reason, that most me- tals in melting oxydate more or less at their surface, and thereby diminish the purity of the air; but, more especially because the particles of the oxyd that are volatilized by the heat, and breathed with the air of the room, are very noxious. This is par- o 2- 162 ticularly the case with lead and arsenic. Besides the large furnaces that are required for these fu- sions, contribute also materially to alter the salubri- ty of the air in those places where the process is car- ried on. I must shew you some instances of the combus- tion of metals; it would require the heat of a fur- nace to make them burn in the common air, but if we supply them with a stream of oxygen gas, we may easily accomplish it. Caroline. But it will still, I suppose, be necessa- ry in some degree to raise their temperature; for the oxygen will not be able to penetrate such dense substances, unless the caloric forces a passage for it. Mrs. B. This, as you shall see, is very easily done, particularly if the experiment be tried upon a small scale.—I begin by lighting this piece of char- coal with the candle, and then increase the rapidity. of its combustion by blowing upon it with a blow- pipe. {Plate IX. Fig. 21.) Emily. That I do not understand ; for it is not every kind of air, but merely oxygen gas, that pro- duces combustion. Now you said that in breathing we inspired, but did not expire, oxygen gas. Why,. therefore, should the air which you breathe through the blow-pipe, promote the combustion of the char- coal ? Mrs. B. Because the air, which has but once passed through the lungs, is yet but little altered, a small portion only of its oxygen being destroyed; so that a great deal more is gained by increasing the rapidity of the current, by means of the blow-pipe, than is lost in consequence of the air passing once through the lungs, as you shall see— PlATI IX. Fig. 21. Igniting charcoal with a taper and blow-pipe. Fig. 22. Combustion of m°tals by means of a blow-pipe conveying a stream of oxygen gas from a. gas-holder.. «u- IX. _PagrJ&A Appamtus for the combustion of metals by meows of oxygen gas. Tig. 22. JJrum. t-fHu-Aurtvir Jingrnwd /br James Humphreys. Uuladel/tAia. 163 Emily. Yes, indeed, it makes the charcoal burn much brighter. Mrs. B. W hilst it is red hot, I shall drop some iron filings on it, and supply them with a current of oxygen gas, by means of this apparatus {Plate IX. Fig. 22.) which consists simply of a closed tin cylindrical vessel, full of oxygen gas, with two aper- tures and stop-cocks, by one of which a stream of water is thrown into the vessel through a long fun- nel, whilst by the other the gas is forced out through a blow-pipe adapted to it, as the water gains admit- tance.—Now that I pour water into the funnel, you may hear the gas issuing from the blow-pipe—I bring the charcoal close to the current, and drop the filings upon it— Caroline. They emit much the same vivid light as the combustion of the iron wire in oxygen gas. Mrs. B. The process is, in fact, the same; there is only some difference in the mode of conducting it. Let us burn some tin in the same manner—you see that it is equally combustible—Let us now try some copper— Caroline. This bums with a greenish flame ; it is I suppose, owing to the colour of the oxyd ? Emily. Pray, shall we not also burn some gold? Mrs. B. That is not in our power, at least in this way. Gold, silver, and platina, are incapable of being oxydated by the greatest heat that we can produce by the common method. It is from this circumstance that they have been called perfect me- tals. Even these, however, have an affinity for oxy- gen ; but their oxydation or combustion can only be performed by means of electricity. The spark given out by the Galvanic Pile produces in the point of contact a greater degree of heat than any other process; and it is at this very high temperature on- ly that the affinity of these metals for oxygen will enable them to act on each other. 164 I am sorry that I cannot shew you the combustion of the perfect metals by this process, but it requires' a considerable Galvanic Battery. You will, howe- ver see these experiments performed in the most perfect manner, when you attend the chemical lec- tures of the Royal Institution. Caroline. I think you said that the oxyds of me- tals could be restored to their metallic state ? Airs. B. Yes; this is called reviving a metaL Metals are in general capable of being revived by charcoal, when heated red hot, charcoal having, at that temperature, a greater attraction for oxygen than the metals. You need only therefore, decom- pose, or unburn the oxyd, by depriving it of its oxygen, and the metal will be restored to its pure. state. Emily. But will the carbone, by this operation, be burnt, and be converted into carbonic acid ? Mrs. B. Certainly. There are other combusti- ble substances to which metals at a high tempera- ture will part with their oxygen. They will also yield it to each other, according to their several de- grees of attraction for it; and if the oxygen goes into a more dense state in the metal which it enters,, than it existed in that which it quits, a proportional disengagement of caloric will take place. Caroline. And cannot the oxyds of gold, silver,. and platina, which are formed by means of the elec- tric fluid, be restored to their metallic state ? Mrs. B. Yes, they may; but the intervention of a combustible body is not required; heat alone? will fake the oxygen from them, convert it into a. gas, and revive the metal. Emily. You said that rust was an oxyd of iron;. how is it, then, that water, or merely dampness,. produces it, which, you know, it very frequently. does on steel grates, or any iron instruments. Mrs. B. In that case the metal decomposes the 165 water, or dampness (which is nothing but water in a state of vapour), and obtains the oxygen'fromSt. Caroline. I thought that it was necessary to bring metals to a very high temperature to enable them to decompose water. Mrs. B. It is so, if it is required that the process should be performed rapidly, and if any considera- ble quantity is to be decomposed. Rust you know is sometimes months in forming, and then it is only the surface of the metal that is oxydated. Emily. Metals, then, that do not rust, are inca- pable of spontaneous oxydation, either by air or wa- ter? Mrs. B. Yes ; and this is the case with the per- fect metals, which on that account, preserve their metallic lustre so well. Caroline. When metals are oxydated by means of water, is there no sensible disengagement of light and heat ? Mrs. B. "No; because the oxygen exists already in a dense state in water; and the portion of caloric that it parts with combines with the hydrogen to convert it into a gas. Emily. Are all metals capable of decomposing water, provided their temperature be sufficiently raised ? Mrs. B. No; a certain degree of attraction is requisite, besides the assistance of heat. Water you recollect, is composed of oxygen and hydrogen; and unless the affinity of the metal for oxygen be stronger than that of hydrogen, it is in vain that we raise its temperature, for it cannot take the oxy- gen from the hydrogen. Iron, zinc, tin, and anti- mony, have a stronger affinity for oxygen than hv- drogen has, therefore these four metals are capable of decomposing water. But hydrogen having" an advantage over all the other metals with respect to its affinity for oxygen, it not only withholds its oxy- 166 gen from them, but is even capable in certain cir- cumstances, of taking the oxygen from the oxyd of these metals. Emily. I confess that I do not quite understand why hydrogen can take oxygen from those metals that do not decompose water. Caroline. Now I think I do perfectly. Lead for instance will not decompose water, because it has not so strong an attraction for oxygen, as hydrogen has. Well, then, suppose the lead to be in a state of oxyd ; hydrogen will take the oxygen from the lead, and unite with it to form water, because hy- drogen has a stronger attraction for oxygen, than oxygen has for lead; and it is the same with all the other metals-which do not decompose water. Emily. I understand your explanation, Caroline, very well; and I imagine that it is because lead can- not decompose water that it is so much employed* for pipes for conveying that fluid. Mrs. B. Certainly; lead is, on that account, particularly appropriate to such purposes; whilst, on the contrary, this metal, if it was oxydable by wa- ter, would impart to it very noxious qualities, as all oxyds of lead are more or less pernicious. But, with regard to the oxydation of metals, there is a mode of effecting it more powerful than either of the former, which is by means of acids. These, you know, contain a much greater proportion of oxygen than either air or water; and will, most of them, easily yield it to metals. Have you never ob- served, that if you drop vinegar, lemon, or any acid, on the blade of a knife, or on a pair of scissars, it will immediately produce a spot of rust. Caroline. Yes, often ; and I am very careful now to wipe off the acid immediately to prevent the rust from forming. Emily. Metals have, then, three ways of obtain- ing oxygen ; from the atmosphere, from water, and from acids. 167 Mrs. B. The two first you have already wit- nessed, and I shall now show you how metals take the oxygen from an acid. This bottle contains nitric acid; I shall pour some of it over this piece of cop- per-leaf ..... Caroline. Oh, what a disagreeable smell! Emily. And what is it that produces the effer- vescence and that thick yellow vapour ? Mrs. B. It is the acid, which being abandoned by the greatest part of its oxygen, is converted into a weaker acid, which escapes in the form of gas. Caroline. And whence proceeds this heat ? Mrs. B. Indeed, Caroline, I think you might .now be able to answer that question yourself. Caroline. Perhaps it is that the oxygen enters in- to the metal in a more solid state than it existed in the acid, in consequence of which caloric is disen- gaged. Mrs. B. You have found it out, you see, with- out much difficulty. Emily. The effervescence is over ; therefore -I suppose that the metal is now oxydated. Mrs. B. Yes. But there is another important connection between metals and acids, with which I must make you acquainted Petals when in the state of oxyds, are capable of being dissolved by acids. In this operation they enter into a chemical combination with the acid, and form an entirely new compound. Caroline. But what difference is there between the oxydation and the dissolution of a metal by an acid? Mrs. B. In the first case, the metal merely combines with a portion of oxygen taken from the acid, which is thus partly deoxygenated, as in the instance you have just seen; in the second case, the metal after being previously oxydated, is actually dissolved in the acid, and enters into a chemical . combination with it, without producing any further 168 decomposition or effervescence.—This complete combination of an oxyd and an acid forms a pecu- liar and important class of compound salts. Emily. The difference between an oxyd and a compound salt, therefore, is very obvious: the one consists of a metal and oxygen; the other of an oxyd and acid. Mrs. B. Very well: and you will be careful to remember that the metals are incapable of entering into this combination with acids, unless they are previously oxydated; therefore, whenever you bring a metal in contact with an acid, it will be first oxy- dated and afterwards dissolved, provided that there be a sufficient quantity of acid for both operations. There are some metals, however, whose solution is more easily accomplished, by diluting the acid in water; and the metal will, in this case, be oxyda- ted, not by the acid, but by the water, which it will decompose. But in proportion as the oxygen of the water oxydates the surface of the metal, the acid combines with it, washes it off, and leaves a fresh surface for the oxygen to act upon: then other coats of oxyd are successively formed, and rapidly dissolved by the acid, which continues combining with the new-formed surfaces of the oxyd till the whole of the metal is dissolved. During this process the hydrogen gas of the water is disengaged, and ; flies off with effervescence. 1 Emily. Was not this the manner in which the sul- h phuric acid assisted the iron filings in decomposing '-H water. "'* Mrs. B. Exactly; and it is thus that several me- tals, which are incapable alone of decomposing wa- j ter, are enabled to do it by the assistance of an acid, which, by continually washing off the covering of 1 oxyd, as it is formed, prepares a fresh surface of 7 metal to act upon the water. Caroline. The acid here seems to a£t a part not 169 very different from that of a scrubbing-brush—But pray would not this be a good method of cleaning grates and metallic utensils? Mrs. B. You forget that acids have the power of oxydating metals, as well as that of dissolving their oxyds; so that by cleaning a grate in this way, you would create more rust than you could destroy. Caroline. True; how thoughtless I was to forget that! Let us watch the dissolution of the copper in the nitric acid; for I am very impatient to see the salt that is to result from it. The mixture is now of a beautiful blue colour; but there is no appearance of the formation of a salt; it seems to be a tedious operation. Mrs. B. The crystallization of the salt requires some length of time to be completed; if, however, you are so impatient, I can easily shew you a me- tallic salt already formed. Caroline. But that would not satisfy my curiosity half so well as one of our own manufacturing. Mrs. B. It is one of our own preparing that I mean to shew you. When we decomposed water a few days since, by the oxydation of iron filings, through the assistance of sulphuric acid, in what did the process consist ? Caroline. In proportion as the water yielded its oxygen to the iron, the acid combined with the new-formed oxyd, and the hydrogen escaped alone. Mrs. B. Very well: the result, therefore, was a compound salt, formed by the combination of sul- phuric acid with oxyd of iron. It still remains in the vessel in which the experiment was performed. Fetch it, and we shall examine it. Emily. .What a variety of processes the decom- position of water, by a metal and an acid, implies ! 1st, The decomposition of the water; 2dly, the oxydation of the metal; and 3dly, the formation of a compound salt. v 170 Caroline. Here it is, Mrs. B.—What beautiful green crystals ! But we do not perceive any crystals in the solution of copper in nitrous acid ? Mrs. B. Because the salt is now suspended in the water which the nitrous acid contains, and will remain so till it is deposited in consequence of rest and cooling. Emily. I am surprised that a body so opaque as iron can be converted into such transparent crystals. - Mrs. B. It is the union with the acid that pro- duces the transparency; for if the pure metal was melted, and afterwards permitted to cool and crys- tallize, it would be found just as opaque as before. Emily. I do not understand the exact meaning of crystallization ? Mrs. B. You recollect that when a solid body is dissolved either by water or caloric, it is not de- composed ; but that its integrant parts are only sus- pended in the solvent. When the solution is made in water, the integrant particles of the body will, on the water being evaporated, again unite into a solid mass, by the force of their mutual attraction. But when the body is dissolved by caloric alone, no- - thing more is necessary, in order to make its parti- cles reunite, than to reduce its temperature. And, in general, if the solvent, whether water or caloric, be slowly separated by evaporation or by cooling, and care taken that the particles be not agitated du- ring their reunion, they will arrange themselves in regular masses, each individual substance assuming a peculiar form or arrangement; and that is what is called crystallization. Emily. Crystallization, therefore, is simply the reunion of the particles of a solid body that has been dissolved in a fluid. Mrs. B. That is a very good definition of it. But I must not forget to observe, that heat and wa- ter may unite their solvent powers; and in this case, 171 crystallization may be hastened by cooling, as well as by evaporating the liquid. Caroline. But if the body dissolved is of a vola- tile nature, will it not evaporate with the fluid ? Mrs. B. A crystallizable body, held in solution only by water, is scarcely ever so volatile as the fluid itself, and care must be taken to manage the heat, so that it may be sufficient to evaporate the water only. I should not omit to mention that bodies, in crys- tallizing from their watery solution, always retain a small portion of water, which remains confined in the crystal in a solid form, and does not reappear, unless the body loses its crystalline state. This is called the water of crystallization. It is also necessary that you should here more par- ticularly remark the difference, to which we have formerly alluded, between the simple solution of bodies either in water or in caloric, and the'solution of metals in acids; in the first case, the body is merely divided by the solvent into its minutest parts. In the latter, a similar effect is, indeed, produced; but it is by means of a chemical combination be- tween the metal and the acid, in which both lose their characteristic properties. The first is a me- chanical operation, the second a chemical process. We may, therefore, distinguish them by calling the first a simple solution, and the other a chemical solution. Do you understand this difference ? Emily. Yes; simple solution can affect only the attraction of aggregation. But chemical solution im- plies also an attraction of composition, that is to say, an actual combination between the solvent and the body dissolved. Mrs. B. You have expressed your idea very well indeed. But you must observe, also, that whilst a body may be separated from its solution in water or caloric, simply by cooling or by evaporation, an acid 172 can be taken from a metal with which it is combi- ned, only by stronger affinities, which produce a decomposition. Emily. I think that you have rendered the dif- ference between these two kinds of solution so ob- vious, that we can never confound them. Mrs. B. Notwithstanding, however, the real difference which there appears to be between these two operations, they are frequently confounded. Indeed, several modern chemical writers, of great eminence, have even thought proper to generalize the idea of solution, and to suppress entirely the distinction introduced by the great Lavoisier, which I have taken so much pains to explain, and which I confess appears to me to render the subject much clearer. Emily. Are the perfect metals susceptible of be- ing dissolved and converted into compound salts by acids ? Mrs. B. Gold is acted upon by only one acid, the oxygenated muriatic, a very remarkable acid, which, when in its most concentrated state, dissolves gold or any other metal, by burning them rapidly. Gold can, it is true, be dissolved likewise by a mixture of two acids, commonly called aqua regia* but this mixed solvent derives that property from containing the peculiar acid which I have just men- tioned. Platina is also acted upon by this acid only; but silver is dissolved by several of them— Caroline. I think you said that some of the me- tals might be so strongly oxydated as to become acid ? Airs. B. There are five metals, arsenic, molyb- dena, chrome, tungsten, and columbium,* which * Columbium, which has not long since been discovered by^ Mr. Hatchett, was inadvertently omitted in the enumeration of the simple bodies &iven in the first conversation. 173 ire susceptible of combining'with a sufficient quan- tity of oxygen to be converted into acids. Caroline. Acids are connected with metals in such a variety of ways, that I am afraid- of some confu- sion in remembering them.—In the first place, acids will yield their oxygen to metals. Secondly, they will combine with them in their state of oxyds, to form compound salts; and lastly, several of the me- tals are themselves susceptible of acidification. Mrs. B. Very well; but though metals have so great an affinity for acids, it is not with that class of bodies alone that they will combine. They are most of them in their simple state, capable of uni- ting with sulphur, with phosphorus, with carbone, " and with each other; these combinations, according to the nomenclature which was explained to you on a former occasion, are called sulphurets^ phosphorets> carburets, &c. The metallic phosphorets offer nothing very re- markable. The sulphurets form the peculiar kind of mineral called pyrites, from which certain kinds of mineral waters, as those of Harrogate, derive their chief chemical properties. In this combina- tion, the sulphur, together with the iron, have so strong an attraction for oxygen, that they obtain it both from the air and from water, and by conden- sing it in a solid form, produce the heat which raises the temperature of the water in such a re- markable degree. Emily. But if pyrites obtain oxygen from water, that water must suffer a decomposition, and hydro- gen gas be evolved ? Mrs. B. That is actually the case in the hot springs alluded to, which give out an extremely fe- tid gas, composed of hydrogen impregnated with sulphur. Caroline, If I recollect right, steel and plumba* p 2 174 go, which you mentioned in the last lesson, are both carburets of iron ? Mrs. B. Yes; and they are the only carburets of much consequence. A curious combination of metals has lately very much attracted the attention of the scientific world: I mean the stones that fall from the atmosphere. They all consist principally of native or pure iron which is never formed in that state in the bowels of the earth ; and contain also a small quantity of nickel and chrome, a combination likewise new in the mineral kingdom. These circumstances have led many scientific per- sons to believe that those substances have fallen from the moon or some other planet, while others are of opinion either that they are formed in the atmos- phere, or are projected into it by some unknown volcano on the surface of our globe. Caroline. I have heard much of these stones, but I believe many people are of opinion, that they are formed on the earth, and laugh at their pretended celestial origin. Mrs. B. The fact of their falling is'so well as- certained, that I think no person who has at all in- vestigated the subject, can now entertain any doubt of it. Specimens of these stones have been disco- vered in all parts of the world, and to each of them has some tradition or story of its fall been found connected. And as the analysis of all the specimens affords precisely the same results, we have thus a very strong proof that they all proceed from the same source. It is to Mr. Howard that philosophers are indebted, for having first analysed these stones, - and directed their attention to this interesting sub- ject. But we must not suffer this digression to take up too much of our time. The combinations of metals with each other are 175 called alloys; thus brass is an alloy of copper and zinc ; bronze, of copper and tin, &c. Emily. And is not pewter also a combination of metal ? Mrs. B. It is. The pewter made in this coun- try, is mostly composed of tin, with a very small proportion of zinc and lead.* Caroline. Block-tin is a kind of pewter, I believe ? Mrs. B. No ; it is iron plated with tin, which renders it more durable, as tin will not so easily rust.f Tin alone, however, would be too soft a metal to be worked for common use, and all the tin vessels or utensils are in fad made of plates of iron thinly coated with tin, which prevents the iron from rusting. Caroline. Say rather oxydating, Mrs. B.—Rust is a word that should be exploded in chemistry. Mrs. B. Take care, however, not to introduce the word oxydate instead of rust, in general con- versation ; for either you will not be understood, or you will be laughed at for your conceit. Caroline. I confess that my attention is, at pre- sent, so much engaged by chemistry, that it some- times leads me into ridiculous observations. Every thing in nature I refer to chemistry, and have often been laughed at for my continual allusions to it. Mrs. B. You must be more cautious and dis- creet in this respect, my dear, otherwise your en- thusiasm, although proceeding from a sincere ad- miration of the science, will be attributed to pedan- try. Metals differ very much in their affinity for each other; some will not unite at all, others readily , i-H * The metal bismuth also enters into all good pewter, as it in- creases the brightness and hardness of the composition. Am, Ed. y Striclly speaking, the metal as refined from its ore is block-tin. Tin plates are prepared by dipping sheet iron in melted tin. The iron is previously immersed in an acid liquor, Am, Ed, 176 combine together, and on this property of metals the art of soldering depends. Emily. What is soldering ? Mrs. B. It is joining two pieces of metal toge- ther, by beating them, with a thin plate of a more fusible metal interposed between them. Thus tin is a solder for lead; brass, gold, or silver, are sol- der for iron, &c. Caroline. And is not plating metals something of the same nature ? Mrs. B. In the operation of plating, two me- tals are united, one being covered with the other, but without the intervention of a third ; iron or tin may thus be covered with gold or silver. Emily. Mercury appears to me of a very'differ- ent nature from the other metals. Mrs. B. One of its greatest peculiarities is that it retains a fluid state at the temperature of the at- mosphere. All metals are fusible at different de- grees of heat, and they have likewise each the pro- perty of freezing or becoming solid at a certain fixed temperature. Mercury congeals only at 72° below the freezing point. Emily. That is to say, that in order to freeze, it requires a temperature 72° colder than that at which water freezes. Mrs. B. Exactly so. Caroline. But is the temperature of the atmos- phere ever so low as that ? Mrs. B. Scarcely ever, at least in any inhabited part of the globe; therefore mercury is never found solid in nature, but it may be congealed by artificial cold; I mean such intense cold as can be produced by some chemical mixtures. Caroline. And can mercury be made to boil and evaporate ? Mrs. B. Yes, like any other liquid ; only it re- quires a much greater degree of heat. At the tem- 177 perature of 600°, it begins to boil and evaporate like water. Mercury combines with gold, silver, tin, and with several other metals; and, if mixed with any of them in a sufficient proportion, it penetrates the solid metal, softens it, loses its own fluidity, and forms an amalgam, which is the name given to the combination of any metal with mercury, forming a substance more or less solid, according as the mer- cury or the other metal predominates. Emily. In the list of metals there are some whose names I have never before heard mentioned. Mrs. B. There are several that have been re- cently discovered, whose properties are yet but lit- tle known, as for instance, titanium which was dis- covered by the Revd. Mr. Gregor, in the tin mines of Cornwall; columbium, which has lately been discovered by Mr. Hatchett; and osmium, iridium, palladium, and rhodium, all of which Dr. Woolaston and Mr. Tennant found mixed with crude platina. Caroline. Arsenic has been mentioned amongst the metals ; I had no notion that it belonged to that elass of bodies, for I had never seen it but as a pow- der, and never thought of it but as a most deadly poison. Mrs. B. In its pure metallic state, I believe, it is not so poisonous; but it has so great an affinity for oxygen, that it absorbs it from the atmosphere at its natural temperature; you have seen it, there- fore, only in its state of oxyd, when, from its com- bination with oxygen, it has acquired its very poi- sonous properties. Caroline. Is it possible that oxygen can impart poisonous qualities ? That valuable substance which produces light, and fire, and which all bodies in na- ture are so eager to obtain ! Mrs. B. Most of the metallic oxyds are poison- ous, and derive this property from their union with 178 oxygen. The white lead, so much used in paint, owes its pernicious effects to oxygen. In general oxygen, in a concrete state, appears to be particu- larly destructive in its effects on flesh or any animal matter; and those oxyds are most caustic that have an acrid burning taste,, which proceeds from the metal having but a slight affinity for oxygen, and therefore easily yielding it to the flesh which it cor- rodes and destroys. Emily. What is the meaning of the word caustic^ which you have just used ? Airs. B. It expresses that property which some bodies possess, of disorganizing and destroying ani- maf matter, by operating a kind of combustion, or at least a chemical decomposition. You must often have heard of caustics used to burn warts, or other animal excrescences; most of these bodies owe their destructive power to the oxygen with which they are combined. The common caustic,* called lunar caustic, is a compound formed by the union of nitric acid and silver; and it is supposed to owe its caustic qualities to the oxygen contained in the nitric acid. Caroline. But, pray, are not acids still more caus- tic than oxyds, as they contain a greater proportion of oxygen ? Mrs. B. Some of the acids are; but the caustic property of a body depends not only upon the quan- tity of oxygen which it contains, but also upon its slight affinity for that principle, and the consequent facility with which it yields it. Emily. Is not this destructive property of oxygen accounted for ? Mrs. B. It proceeds probably from the strong attraction of oxygen for hydrogen; for if the one * The author means to say, " the caustic in common use." The " common caustic," of the shops, is pot-ash deprived of all its car- bonic acid. Am. Editor. 179 rapidly absorbs the other from the animal fibre, a disorganization of the substance must ensue. Emily. Caustics are then very properly said to burn the flesh, since the combination of oxygen and hydrogen is an actual combustion. Caroline. Now, I think, this effect would be more properly termed an oxydation, as there is no disen- gagement of light and heat. Mrs. B. But there really is a sensation of heat produced by the action of caustics; and the caloric that is disengaged must, I think, partly, if not whol- ly, proceed from the oxygen which the caustic yields to the flesh. Caroline. Yet the oxygen of a caustic is not in a gaseous state, and can therefore have no caloric to part with ? Mrs. B. In whatever state oxygen exists, we may suppose that, like every other body in nature, it retains some portion of caloric; and if, in com- bining with the hydrogen of the flesh, it becomes more dense than it previously was in the caustic, it must part with caloric whilst this change is taking place. I believe I have once before observed that we may, in a great measure, judge of the compara- tive degree of solidity which oxygen assumes in a body, by the quantity of caloric liberated during its combination ; and when we find, that, in its pas- sage from one body to another, heat is evolved, we may be certain that it exists in a more solid state in the latter. Emily. But if oxygen is so caustic, why does not that contained in the atmosphere burn us ? Mrs. B. Because it is in a gaseous state, and has a greater attraction for its caloric than for the hy- drogen of our bodies. Besides, should the air be slightly caustic, we are in a great measure sheltered from its effects by the skin; you all know how much a wound, however trifling, smarts on being exposed to it. 180 Caroline. It is a curious idea, however, that we should live in a slow fire. But, if the air was caus- tic, would it not have an acrid taste ? Mrs. B. It possibly may have such a taste ; though in so slight a degree, that custom has ren- dered it insensible. Caroline. And why is not water caustic ? Wheri I dip my hand into water, though cold, it ought to burn me from the caustic nature of its oxygen. Mrs. B. Your hand does not decompose the Water; the oxygen in that state is much better sup- plied with hydrogen than it would be by animal matter, and if its causticity depend on its affinity for that principle, it will be very far from quitting Its state of water to act upon your hand. You must not forget that oxyds are caustic in proportion as the oxygen adheres slightly to them. Emily. Since the oxyd of arsenic is poisonous, its acid, I suppose, is fully as much so? Mrs. B. Yes; it is one of the strongest poisons in nature. Emily. There is a poison called verdigris, which forms on brass and copper when not kept very clean; and this, I have heard, is an objection to these me- tals being made into kitchen utensils. Is this poi- son likewise occasioned by oxygen? Mrs. B. It is produced by the intervention of oxygen; for verdigris is a compound salt formed by the union of vinegar and copper; it is of a beautiful green colour, and much used in painting. Emily. But, I believe, verdigris is often formed on copper when no vinegar has been in contact with it. Mrs. B. Not real verdigris, but compound salts, somewhat resembling it, may be produced by the action of any acid on copper. There is a beautiful green salt produced by the combination of cobalt with muriatic acid, which has- 181 the singular property of forming what is called sym. pathetic ink. Charaders written with this solution are invisible when cold, but when a gentle heat is applied, they assume a fine blueish green colour. Caroline. I think one might draw very curious, landscapes with the assistance of this ink; I would first make a water colour drawing of a winter scene, in which the trees shall be leafless and the grass scarcely green; I would then trace all the verdure with the invisible ink, and whenever I chose to cre- ate spring, I should hold it before the fire, and its warmth would cover the landscape with a rich ver- dure. Airs. B. That will be a very amusing experiment, and I advise you by all means to try it.—I must now, however, take my leave of you; we have had a ve- ry long lecture, and I hope you will be able to re- member it. Do not forget to write down all you can recollect of this conversation, for the subject is of great importance, though it may not appear at first very entertaining. @ CONVERSATION X. On Alkalies. Mrs. B. After having taken a general view of combusti- ble bodies, we now come to the alkalies, and the earths, which compose the class of incombustibles; that is to say, of such bodies as do not combine with oxygen at any known temperature. 182 Caroline. I am afraid that the incombustible sub- stances will not be near so interesting as the others; for I have found nothing in chemistry that has plea- sed me so much as the theory of combustion. Mrs. B. Do not however depreciate the incom- bustible bodies before you are acquainted with them; you will find they also possess properties highly im- portant and interesting. Some of the earths bear so strong a resemblance in their properties to the alkalies, that it is a diffi- cult point to know under which head to place them. The celebrated French chemist, Fourcroy, has class- ed two of them (Barytes and Strontites) with the alkalies; but, as lime and magnesia have almost an equal title to that rank, I think it better not to se- parate them, and therefore have adopted the com- mon method of classing them with the earths, and of distinguishing them by the name of alkaline earths. We shall first take a review of the alkalies, of which there are three species: potash, soda, and ammonia. The two first are called fixed alkalies, because they exist in a solid form at the tempera- ture of the atmosphere, and require a great heat to be volatilized. The third, ammonia, has been dis- tinguished by the name of volatile alkali, because its natural form is that of gas. Caroline. Ammonia ? I do not recoiled that name in the list of simple bodies. Mrs. B. The reason why you do not find it there is, that it is a compound; and if I introduce it to your acquaintance now, it is on account of its close connection with the two other alkalies, which it resembles essentially in its nature and properties. Indeed it is not long since ammonia has resigned its place among the simple bodies, as it was not, till lately,supposed to be a compound; nor is it impro- bable that potash and soda may some day undergo the same fate, as they are strongly suspe&ed of be- ing compounds also. 183 The general properties of alkalies are, an acrid burning taste, a pungent smell, and a caustic action on the skin and flesh. ' Caroline. How can they be caustic, Mrs. B. since they do not contain oxygen ? Mrs. B. Whatever substance has an affinity for any one of the constituents of animal matter, suffi- ciently powerful to decompose it, is entitled to the appellation of caustic. The alkalies, in their pure state, have a very strong attraction for water, for hydrogen, and for carbone, which, you know, are the constituent principles of oil, and it is chiefly by absorbing these substances from animal matter, that they effect its decomposition : for, when diluted with a sufficient quantity of water, or combined with any oily substance, they lose their causticity. But, to return to the general properties of alka- lies—they change the colour of syrup of violets, and other blue vegetable infusions, to green ; and have, in general, a very great tendency to unite with acids, although the respective qualities of these two class- es of bodies form a remarkable contrast. We shall examine the result of the combination of acids and alkalies more particularly when we have completed our general view of the simple bodies. It will be sufficient at present to inform you, that whenever acids are brought in contact with alkalies, or alkaline earths, they unite with a remarkable ea- gerness, and form compounds perfectly different from either of their constituents; these compounds are called neutral or compound salts. Caroline. Are they of the same kind as the salts formed by the combination of a metal and an acid ? Mrs. B. Yes; they are analogous in their na- ture, although different in many of their properties. A methodical nomenclature, similar to that of the acids, has been adopted for the compound salts. Each individual salt derives its name from its consti- 184 Cuents, so that every name implies a knowledge of the composition of the salt. The three alkalies, the alkaline earths, and the metals, are called salifiable bases or radicals, and the acids, salifying principles. The name of each salt is composed both of that of the acid and the salifiable base ; and it terminates in at or it, according to the degree of oxygenation of the acid. Thus, for in- stance, all those salts which are formed by the com- bination of the sulphuric acid with any of the salifi- able bases, are called sulphats, and the name of the radical is added for the specific distinction of the salt; if it be potash, it will compose a sulphat of pot- ash : if ammonia, sulphat of ammonia, &c. Emily. The crystals which we obtained from the combination of iron and sulphuric acid, were there- fore sulphat of iron F Mrs. B. Precisely; and those which we prepa- red by dissolving copper in nitric acid, nitrat of cop- per, and -so on. But this is not all; if the salt be formed by that class of acids which ends in ous (which you know, indicates a less degree of oxyge- nation), the termination of the name of the salt will be in it, as sulphit of potash, sulphit of ammonia, &c. Emily. There must be an immense number of compound salts, since there is so great a variety of idifiable radicals, as well as of salifying principles. Mrs..B. Their real number cannot be ascertain- ed, since it increases every day as the science advan- ces. But, before we proceed farther in the investi- gation of the compound salts, it is necessary that we should examine the nature of the ingredients from which they are composed. Let us therefore return to the'alkalies. The dry white powder which you see in this phial is pure caustic potash ; it is very difficult to preserve it in this state, as it attracts with extreme avidity the moisture from the atmos- 185 phere, and if the air were not perfectly excluded, it would in a very short time be actually melted. Emily. It is then, I suppose, always found in a liquid state ? Mrs. B. No; it exists in nature in a great varie- ty of forms and combinations, but is never found in Its pure separate state; it is combined with carbonic acid, with which it exists in every part of the ve- getable kingdom, and is most commonly obtained from the ashes of vegetables, which compose the substance that remains after all the other parts have been volatilized by combustion.* Caroline. But you once said, that after the vola- tile parts of a vegetable were evaporated, the sub- stance that remained was charcoal ? Mrs. B. What, my dear ? Do you still confound the processes of simple volatilization and combus- tion ? In order to procure charcoal we evaporate such parts as can be reduced to vapour by heat alone; but when we burn the vegetable, we volatilize the carbone also, by converting it into carbonic acid gas. Caroline. That is true; I hope I shall make no more mistakes in my favourite theory of combustion. Mrs. B. Potash derives its name from the pots in which the vegetables from which it was obtained used formerly to be burnt; the alkali remained mix- ed with the ashes at the bottom, and was tMence called potash. Caroline. There is some good sense in this name as it will always remind us of the .operation, and of the general source from which this alkali is derived. * Profe;sor Davy of the Royal Institution in London has, since the publication of this work, discovered the bases of pot tsh and of soda. It appea-s from the accounts given, tha. he has obtained them sepa- rately, and they look like metals, both in their solid and fluid form. They also combine with metals, preserving their metallic appearance! With oxygen they recompose potash and soda. Am. Editor, 0^2 ifcb Emily. The ashes of a wood lire, then, are pot- ash, since they are vegetable ashes ? Airs. B. They always contain more or less pot- ash, but are very far from consisting of that substance alone, as they are a mixture of various earths and salts which remain after the combustion of vegeta- bles, and from which it is not easy to separate the alkali in its pure form. The process by which pot- ash is obtained, even in the imperfect state in which it is used in the arts, is much more complicated than simple combustion. It was once deemed im- possible to separate it entirely from all foreign sub- stances, and it is only in chemical laboratories that it is to be met with in the state of purity in which you find it in this phial. Wood ashes are, howe- ver, valuable for the alkali which they contain, and are used for some purposes without any further pre- paration. Purified in a certain degree, they make what is commonly called pearl ash, which is of great efficacy in taking out grease, in washing linen, &c. for potash combines readily with oil or fat, with which it forms a compound well known to you un- der the name of soap. Caroline. Really ! Then I should think it would be better to wash all linen with pearl ash than with soap, as, in the latter case, the alkali, being already combined with oil, must be less efficacious in ex- tracting grease. Mrs. B. Its effect would be too powerful on fine linen, and would injure its texture; pearl-ash is therefore only used for that which is of a strong coarse kind. For the same reason you cannot wash your hands with plain potash; but, when mixed with oil in the form of* soap, it is soft as well as cleansing, and is therefore much better adapted to the purpose. Caustic potash, as we already observed, acts on the skin, and animal fibre, in virtue of its attraction 187 for water and oil, and converts all animal matter in- to a kind of saponaceous jelly. Emily. Are vegetables the only source from which potash can be derived ? Mrs. B. No: for though far most abundant in vegetables, it is by no means confined to that class of bodies, being found also on the surface of the earth mixed with various minerals, especially with earths and stones, whence it is supposed to be con- veyed into vegetables by the roots of the plant. It is also met with, though in very small quantities, in some animal substances. The most common state of potash is that of carbonat; I suppose you under- stand what that is ? Emily. I believe so ; though I do not recoiled that you ever mentioned the word before. If I am not mistaken, it must be a compound salt formed by the union of carbonic acid with potash. Mrs. B. Very true; you see how admirably the nomenclature of modern chemistry is adapted to assist the memory; when you hear the name of a compound, you necessarily learn what are its con- stituents ; and when you are acquainted with the constituents, you can immediately name the com- pound that they form. Caroline. Pray, how were bodies arranged and distinguished before this nomenclature was introdu- ced ? Mrs. B. Chemistry was then a much more diffi- cult study; for every substance has an arbitrary name, which it derived either from the person who discovered it, as Glauber's salts for instance, or from some other circumstance relative to it, though quite unconneded with its real nature, as potash. These names have been retained for some of the simple bodies; for as this class is not numerous, and therefore can easily be remembered, it has not been thought necessary to change them. 188 Emily. Yet I think it would have rendered the new nomenclature more complete to have methodized the names of the elementary as well as of the com- pound bodies, though it could not have been done in the same manner. But the names of the simple substances might have indicated their nature, or at least, some of their principal properties ; and if, like the acids and compound salts, all the simple bo- dies had a similar termination, they would have been immediately known as such. So complete and regular a nomenclature would I think, have given a clearer and more comprehensive view of chemistry, than the present, which is a medley of the old and new terms. Mrs. B. But you are not aware of the difficulty of introducing into science an entire set of new terms; it obliges all the teachers and professors to go to school again ; and if some of the old names, that are least exceptionable, were not left as an in- trodudion to the new ones, few people would have had industry and perseverance enough to submit to the study of a completely new language; and the inferior classes of artists, who can only ad from ha- bit and routine, would, at least, for a time, have felt material inconvenience from a total change of their habitual terms. From these considerations, Lavoisier and his colleagues, who invented the new nomenclature, thought it most prudent to leave a few links of the old chain, in order to conned it with the new one. Besides, you may easily con- ceive the inconvenience which might arise from giving a regular nomenclature to substances, the simple nature of which is always uncertain ; for the new names might, perhaps, have proved to have been founded in error. And, indeed, cautious as the inventors of the modern chemical language have been, it has already been found necessary to modify it in many respects. In those few cases, however. 189 in which new names have been adopted to designate simple bodies, these names have been so contrived as to indicate one of the chief properties of the bo- dy in "question ; this is the case with oxygen, which, as I explained to you, signifies to produce acids; and hydrogen, to produce water. But to return to the alkalies.—We shall now try to melt some of this caustic potash in a little water, as a circumstance occurs during its solution very worthy of observation.—Do you feel the heat that is produced ? Caroline. Yes, I do ; but is not this diredly con- trary to our theory of caloric, according to which heat is disengaged when fluids become solid, and cold produced when solids are melted ? Mrs. B. The latter is really the case in all solu- tions ; and if the solution of caustic alkalies seems to make an exception to the rule, it does not, I be- lieve, form any solid objedion to the theory. The matter may be explained thus: When water first comes in contad with the potash, it produces an ef- fed similar to the slakeing of lime, that is, the wa- ter is solidified in combining with the potash, and thus loses its latent heat; this is the heat that you now feel, and which is, therefore, produced not by the melting of the solid, but by the solidification of the fluid. But when there is more water than the potash can absorb and solidify, the latter then yields to the solvent power of the water ; and if we do not perceive the cold produced by its melting, it is be- cause it is counterbalanced by the heat previously disengaged.* * If, however, this defence of the general theory be true, it ought to be found, tm accurate examination, that a certain quantity of heat ultimately disappears : or should this explanation be rejected, the phenomenon might be accounted for by supposing that a solution of alkali in water has less capacity for heat than either water of alkali in their separate state. 190 A very remarkable property of potash is the for- mation of glass by its fusion with silicious earth. You are not yet acquainted with this last substance further than its being in the list of simple bodies. It is sufficient, for the present, that you should know that sand and flint are chiefly composed of it: alone, it is infusible; but mixed with potash, it melts when exposed to the heat of a furnace, com- bines with the ilkali, and runs into glass. Caroline. Who would ever have supposed that the same substance that converts transparent oil in- to such an opaque body as soap, should transform that opaque substance, sand, into transparent glass! Mrs. B. The transparency, or opacity of bo- dies, does not, I conceive, depend so much upon their intimate nature, as upon the arrangement of their particles; we cannot have a more striking in- stance of this, than that which is afforded by the different states of carbone, which, though it com- monly appears in the form of a black opaque body, sometimes assumes the most dazzling transparent form in nature, that of diamond, which, you recoi- led, is nothing but carbone, and which, in all pro- bability, derives its beautiful transparency from the peculiar arrangement of its particles during their crystallization. Emily. I never should have supposed that the formation of glass was so simple a process as you describe it. Mrs. B. It is by no means an easy operation to make perfed glass; for if the sand, or flint, from which the silicious earth is obtained be mixed with any metallic particles, or other substance which cannot be vitrified, the glass will be discoloured, or defaced by opaque specks. Caroline. That I suppose is the reason why ob- jeds so often appear irregular and shapeless through a common glass window. 191 Mrs. B. This species of imperfedion proceeds, I believe, from another cause. It is extremely dif- ficult to prevent the lower part of the vessels in which the materials of glass are fused, from con- taining a more dense vitreous matter than the upper, on account of the heavier ingredients falling to the bottom. When this happens, it occasions the ap- pearance of veins or waves in the glass, from the difference of density in its several parts, which pro- duces an irregular refradion of the rays of light that pass through it. Another species of imperfedion sometimes arises from the fusion not being continued for a length of time sufficient to combine the two ingredients com- pletely, or from the due proportions of potash and silex (which are as two to one), not being carefully observed ; the glass, in those cases, will be liable to alteration from the adion of the air, of salts, and especially of acids, which will effed its decomposi- tion by combining with the potash and forming compound salts. Emily. What an extremely useful substance pot- ash is! Mrs. B. Besides the great importance of potash in the manufadures of glass and soap, it is of very considerable utility in many of the other arts, and in its combinations with several acids, particularly the nitric, with which it forms saltpetre. Caroline. Then saltpetre must be a nitrat of pot- ash F But we are not yet acquainted with the nitric acid ? Mrs. B. We shall, therefore, defer entering in- to the particulars of these combinations, till we come to a general review of the compound salts. In or- der to avoid confusion, it will be better at present to confine ourselves to the alkalies. Emily. Cannot you shew us the change of colour which you said the alkalies produced on blue vege- table infusions? 192 Mrs. B. Yes; very easily. I shall dip a piece bf white paper into this syrup of violets, which, you see, is of a deep blue, and dyes the paper of the same colour—As soon as it is dry, we shall dip it into a solution of potash, which, though itself co- lourless, will turn the paper green— Caroline. So it has indeed! And do the other al- kalies produce a similar effed? Mrs. B. Exadly the same.—We may now pro- ceed to Soda, which, however important, will de- tain us but a very short time; as in all its general properties it very strongly resembles potash; indeed, so great is their similitude, that they have been long confounded, and they can now scarcely be distin- guished except by. the difference of the salts which they form with acids. The great source of this alkali is the sea, where, combined with a peculiar acid, it forms the salt with which the waters of the ocean are so strongly im- pregnated. Emily. Is not that the common table salt ? Mrs. B. The very same; but again we must post- pone entering into the particulars of this interesting combination, till we treat of the neutral salts. Soda may be obtained from common salt; but the easiest and most usual method of procuring it, is by the combustion of marine plants, an operation perfed- ly analogous to that by which potash is obtained from vegetables. Emily. From what does soda derive its name? Mrs. B. From a plant called by us soda, and by the Arabs kali; which affords it in great abundance. Kali has, indeed, given its name to the alkalies in general. Caroline, Does soda form glass and soap, in the same manner as potash ? Mrs. B Yes; it does; it is of equal importance in the arts, and is even preferred to potash for some 193 purposes; but you will not be able to distinguish their properties, till we examine the compound salts which they form with acids; we must therefore leave soda for the present, and proceed to AMMO- NIA, or the VOLATILE ALKALI. Emily. I long to hear something of this alkali; is it not of the same nature as hartshorn ? Airs. B. Yes, it is, as you will see by and by. This alkali is seldom found in nature in its pure state; it is most commonly extraded from a com- pound salt called sal ammoniac, which was formerly imported from Ammonia, a region of Lybia', from which both the salt and the alkali, derive their names. The crystals contained in this bottle are specimens of this salt, which consists of a combination of am- monia and muriatic acid. Caroline. Then it should be called muriat of am- monia ; for though I am ignorant what muriatic acid is, yet I know that its combination with ammonia cannot but be so called ; and I am surprised to see sal ammoniac inscribed on the label. Mrs. B. That is the name by which it has been so long known, that the. modern chemists have not yet succeeded in banishing it altogether; and it is still sold under that name by druggists; though by scientific chemists it is more properly called muriat of ammonia. Emily. By what means can the ammonia be se- parated from the muriatic acid ? Mrs. B. By a display of chemical attradions; but this operation is too complicated for you to un- derstand, till you are better acquainted with the agency of affinities. * Emily. And when extraded from the salt, what kind of substance is ammonia ? ■Mrs. B. Its natural form at the temperature of the atmosphere, when free from combination, is that of gas; and in this state it is called ammoniacal R 191 gas. But it mixes very readily with water, and can be thus obtained in a liquid form. Caroline. You said that ammonia was a com- pound ; pray, of what principles is it composed ? Mrs. B. It was discovered a few years since, by Berthollet, a celebrated French chemist, that it Con- sisted of about one part of hydrogen to four parts of nitrogen. Having heated ammoniacal gas under a receiver, by causing the eledrical spark to pass repeatedly through it, he found that it increased considerably in bulk, lost all its alkaline properties, and was adually converted into hydrogen and nitro- gen gasses. Emily. Ammoniacal gas must, I suppose, be ve- ry heavy, since it expands so much when decompo- sed ? Mrs. B. Compared with hydrogen gas, it cer- tainly is; but it is considerably lighter than oxygen gas, and only about half the weight of atmospheri- cal air. It possesses most of the properties of the fixed alkalies; but cannot be of so much use in the arts on account of its volatile nature. It is, there- fore, never employed in the manufadure of glass, but it forms soap with oils equally as well as potash and soda; it resembles them likewise in its strong attradion for water; for which reason it can be col- leded in a receiver over mercury only. Caroline. I do not understand this ? Mrs. B. Do you recoiled the method which we used to colled gasses in a glass receiver over water? Caroline. Perfedly. Mrs. B. Ammoniacal gas has so strong a ten- dency to unite with water, that, instead of passing through that fluid, it would be instantaneously ab- sorbed by it. We can therefore neither Use water for that purpose, nor any other liquid of which wa- ter is a component part; so that, in order to colled this gas, we are obliged to have recourse to mercu- 195 ry (a liquid which has no action upon it), and we use a mercurial bath,, instead of a water bath, as we did on former occasions. Water impregnated with this gas, is nothing more than the fluid which you mentioned at the beginning of the conversation —hartshorn ;. it is the ammoniacal gas escaping from the water which gives it so powerful a smell. Emily.- But there is no appearance of efferves- cence in hartshorn ? Mrs. B. Because the particles of gas that rise from the water are too subtle and minute for their effed to be visible. Water diminishes in density by being impregna- ted with ammoniacal gas ; and this augmentation of bulk increases its capacity for caloric. Emily. In making hartshorn, then,, or impreg- nating water with ammonia, heat must be absorbed, and cold produced ? Mrs. B. That effed would take place if it was not counteraded by another circumstance; the gas is liquefied by incorporating with the water, and gives out its latent heat. The condensation of the gas more than counterbalances the expansion of the water; therefore, upon the whole, heat is produced. —But if you dissolve ammoniacal gas with ice or snow, cold is produced.—Can you account for that? Emily. The gas, in being condensed into a li- quid, must give out heat; and, on the other hand, the snow or ice, in being rarefied into a liquid, must absorb heat; so that, between the opposite effeds, I should have supposed the original temperature would have been preserved. Mrs. B. But you have forgotten to take into the account the rarefadion of the water (or melted ice) by the impregnation of the gas; and this is the cause of the cold which is ultimately produced. Caroline. Is the sal volatile (the smell of which so 196 strongly resembles hartshorn) likewise a preparation of ammonia ? Mrs. B. It is carbonat of ammonia dissolved in water; and which, in its concrete state, is common- ly called salts of hartshorn. Ammonia is caustic like the fixed alkalies, as you may judge by the pun- gent effeds of hartshorn, which cannot be taken in- ternally, or applied to delicate external parts, with- out being plentifully diluted with water—Oil and acids are very excellent antidotes for alkaline poi- sons ; can you guess why ? Caroline. Perhaps, because the oil combines with the alkali, and forms soap, and thus destroys its caustic properties; and the acid converts it into a compound salt, which I suppose, is not so pernicious is caustic alkali. Airs. B. Precisely so. Ammoniacal gas, if it be mixed with atmospheri^ eul air, and a burning taper repeatedly plunged into it, will burn with a large flame of a peculiar yellow colour. Emily. ..I thought that all the alkalies were in- combustible ? Caroline. Besides, you say that flame is produced by the combustion of hydrogen only ? Airs. B. And is not hydrogen gas one of the constituents of ammoniacal gas ? Therefore, though generally speaking, the alkalies are incombustible, yet one of the constituents of ammonia is eminent- ly combustible. Emily. I own I had forgotten that ammonia was a compound. But pray tell me, can ammonia be procured from this Lybian salt only. Mrs. B. So far from it, that it is contained in, and may be extraded from, all animal substances whatever. Hydrogen and nitrogen are two of the chief constituents of animal matter; it is therefore not surprising that they should occasionally meet 197 and combine in those proportions that compose arh- monia. But this alkali is more frequently generated by the spontaneous decomposition of animal sub- stances ; the hydrogen and nitrogen gasses that arise from putrified bodies combine, and form the vola- tile alkali. Muriat of ammonia, instead of being exclusively brought from Lybia, as it originally was, is now chiefly prepared in Europe, by chemical processes. Ammonia, although principally extraded from this salt, can also be produced by a great variety of other substances. The horns of cattle, especially those of the deer, yield it in abundance, and it is from this circumstance that a solution of ammonia in wa-», ter has been called hartshorn. It may likewise be procured from wool, flesh, and bones ; in a word, any animal substance whatever yields it by decom- position. We shall now lay aside the alkalies, however im- portant the subjed may be, till we treat of their combination with acids. The next time, we meet we shall examine the earths, which will complete our review of the class of simple bodies, after which we shall proceed to their several combinations. CONVERSATION XI. On Earths. Mrs. B. The earths, which we are to-day to examine are ten in number: r 2 198 SILEX, STRONTITES, ALUM1NE, YTTR1A, BARYTES, GLUCINA, LIME, ZIRCONIA, MAGNESIA, GARGONIA. The five last are of very late discovery; their properties are but imperfedly known ; and as they have not yet been applied to use, it will be unneces- sary to enter into any particulars respeding them ; we shall confine our remarks, therefore, to the six first. The earths in general are, like the alkalies, incombustible substances. Caroline. Yet I have seen turf burnt in the coun- try, and it makes an excellent fire; the earth be- comes red hot, and produces a very great quantity of heat. Mrs. B. It is not the earth that burns, my dear, but the roots, grass, and other remnants of vegeta- bles that are intermixed with it. The caloric, which is produced by the combustion of these substances, makes the earth red hot, and this being a bad con- dudor of heat, retains its caloric a long time; but were you to examine it when cooled, you would find that it had not absorbed one particle of oxygen, nor suffered any alteration from the fire. Earth is, however, from the circumstance just mentioned, aa excellent refledor of heat, and owes its utility when mixed with fuel, solely to that property. It is in this point of view that Count Rumford has recom- mended balls of incombustible substances to be ar- ranged in fire places, and mixed with the coals, by which means the caloric disengaged by the combus- tion of the latter, is more perfedly refleded into the room, and an expense of fuel is saved. Earth, you know, was supposed to be one of the four elements; but now that a variety of earths have been discovered and clearly discriminated, no single one can be exclusively called an element; and 199 as none of them have been decomposed, they have all an equal title to the rank of simple bodies, which are the only elements that we now acknowledge.— It is from these earths, either in their simple state, or mixed together and combined with other mine- rals, that the solid part of our globe is formed.* Emily. When I think of the great variety of soils, I am astonished that there are not a great number of earths to form them. Airs. B. You might, indeed, almost confine that number to four; for barytes, strohtites, and the others of late discovery, ad but so ysmall a part in this great theatre, that they cannot be reckoned as essential to the general formation of the globe. And you must not confine your idea of earths to the for- mation of soil; for rock, marble, chalk, slate, sand, flint, and all kinds of stones, from the precious jew- els to the commonest pebbles; in a wcrd all the im- mense variety of mineral produds, may be referred to some of these earths, either in a simple state, or combined the one with the other, or blended with other ingredients. Caroline. Precious stones composed of earths That seems very difficult to conceive. Emily. Is it more extraordinary than that the most precious of all jewels, diamond, should be com- posed of carbone? But diamond forms an exception, Mrs. B—; for, though a stone, it is not composed of earth. Mrs. B. I did not specify the exception, as I knew you were so well acquainted with it. Besides, I would call diamond a mineral rather than a stone, * From his late experiments it is Professor Davy's opinion that the different earths consist of bases of a peculiar metallic nature, having a very strong affinity foi oxy.en, by uniting with which they form those earths respectively. He believes he has already made visible by the assistance of Galvanism the basis of the one called barytes. Am, Editor. 200 as the latter term always implies the presence of some earth. Caroline. I cannot conceive how such coarse ma- terials can be converted into such beautiful produc- tions. Airs. B. We are very far from understanding all the secret resources of nature; but I do not think the spontaneous formation of the crystals, which we call precious stones, one of the most difficult phe- nomena to comprehend. By the slow and regular work of ages, perhaps of hundreds of ages, these earths may be gradually dis- solved by water, and as gradually deposited by their solvent in the slow and undisturbed process of crystal- lization. The regular arrangement of their particles, during their reunion in a solid mass, gives them that brilliancy, transparency, and beauty, for which they are so much admired: and renders them in appear- ance so totally different from their rude and primi- tive ingredients. Caroline. But how does it happen that they are spontaneously dissolved, and afterwards crystallized? Mrs. B. The scarcity of many kinds of crystals, as rubies, emeralds, topazes, &c. shows that their for- mation is not an operation very easily carried on in nature. But cannot you imagine that when water, holding in solution some particles of earth, filters through the crevices of hills or mountains, and at length dribbles into some cavern, each successive drop may be slowly evaporated, leaving behind it the particle of earth which it held in solution ? You know that crystallization is more regular and perfed, in proportion as the evaporation of the solvent is slow and uniform; Nature, therefore, who knows no limit of time, has, in all works of this kind, an infinite advantage over any artist who attempts to imitate such productions. Emily. I can now conceive that the arrangement 201 of the particles of earth, during crystallization, may be such as to occasion transparency, by admitting a free passage to the rays of light; but I cannot un- derstand why crystallized earths should assume such beautiful colours as most of them do. Sapphire, for instance, is of a celestial blue; ruby, a deep redj topaz, a brilliant yellow? Mrs. B. Nothing is more simple than to suppose that the arrangement of their particles is such, as to transmit some of the coloured rays of light, and to refled others, in which case the stone must appear of the colour of the rays which it refleds. But, be- sides, it frequently happens, that the colour of a stone is owing to a mixture of some metallic matter. Caroline. Pray, are the different kinds of pre- cious stones each composed of one individual earth, or are they formed of a combination of several earths ? Mrs. B. A great variety of materials enters into the composition of most of them; not only several earths, but sometimes salts and metals. The Earths, however, in their simple state, frequently form ve- ry beautiful crystals; and, indeed, it is in that state only that they can be obtained perfedly pure. Emily. Is not the Derbyshire spar produced by the crystallization of earths, in the way you have just explained ? I have been in some of the subter- raneous caverns where.it is found, which are such as you have described. Airs. B. Yes; but this spar is a very imperfed specimen of crystallization ; it consists of a great variety of ingredients confusedly blended together, as you may judge by its opacity, and by the various colours and appearances which it exhibits. But, in examining the earths in their most per- fed and agreeable form, we must not lose sight of that state in which they are most commonly found, and which, if less pleasing to the eye, is far more 202 interesting by its utility. Before we proceed fur- ther, however, I should observe, that although the earths are considered as simple substances (as che- mists have not succeeded in decomposing them), yet there is considerable reason to suppose that they, as well as the alkalies, are compound bodies. From the circumstance of their being incombustible, it has been conjectured with some plausibility, that they may possibly be bodies that have already been burnt, and which, being saturated with oxygen, will not combine with any additional quantity of that principle. Caroline. But if they have been burnt, they must contain oxygen, which would easily be discovered ? Mrs. B. Not if their attradion for it be so strong that they will yield it to no other substance ; for, during its state of combination, the properties of oxygen may be so altered, as to be concealed entire- ly from our observation; and it is possible that this may be fhe case with the earths. Let us suppose them, for instance, to have been originally some peculiar metals, whose affinity for oxygen was so great, that they attraded it from every substance, and consequently would yield it to none; such me- tals must ever exist in the state of oxyds ; and, as we should not have known them under their metal- lic form, we could not consider them as metals, but should distinguish them by some specific name, as we have done with regard to the earths. Caroline. That, indeed, seems very probable ; for metals, when oxydated, become to all appear- ance a kind of earthy substance. Emily. But have the earths any of the properties of the metallic oxyds ? Mrs. B. Their strongest feature of resemblance is their property of combining with the acids to form compound salts. You must not, however, consider the idea of 203 earths being burnt bodies, as any thing more than mere conjecture; for whatever may be their consti- tuents, until we succeed in decomposing them, we cannot consider them in any other light than as simple bodies. Emily. Pray which of the earths are endued with alkaline properties ? Mrs. B. AH of them, more or less; but there are four, barytes, magnesia, lime, and strontites, which are called alkaline earths, because they possess those qualities in so great a degree, as to entitle them, in most respeds, to the rank of alkalies. They combine and form compound salts with acids in the same way as alkalies; they are, like them, suscep- tible of a considerable degree of causticity, and are similarly aded upon by chemical tests.—The other earths, silex and alumine, with one or two others of late discovery, are in some degree more earthy, that is to say, they possess more completely the pro- perties common to all the earths, which are, insipi- dity, dryness, unalterableness in the fire, infusibi- lity, &c. Caroline. Yet, did you not tell us that silex, or silicious earths, when mixed with an alkali, was fu- sible, and ran into glass ? Mrs. B. Yes, my dear; but the charaderistic properties of earths, which I have mentioned, are to be considered as belonging to them in a state of purity only; a state in which they are very seldom 40 be met with in nature.—Besides these general properties, each earth has its own specific charac- ters, by which it is distinguished from any other sub- stance. Let us therefore review them separately. Silex, or silica, abounds in flint, sand, sand- stone, agate, jasper, Sec. it forms the basis of ma- ny precious stones, and particularly of those that strike fire with steel. It is rough to the touch, scratches and wears away metal; it is aded upon by 204 no acid but the fluoric, and is not soluble in water by any known process -/jbut nature certainly dissolves it by means with which we are unacquainted, and thus produces a variety of silicious crystals, and a- mongst these rock crystal, which is the purest speci- men of this earth. Silex appears to have been in- tended by Providence to form the solid basis of the globe, to serve as a foundation for the original moun- tains, and give them that hardness and durability which has enabled them to resist the various revolu- tions which the surface of the earth has successively undergone. From these mountains silicious rocks have, during the course of ages, been gradually de- tached by torrents of water, and brought down in fragments; these, in the violence and rapidity of their descent, are sometimes crumbled to sand, and in this state form the beds of rivers and of the sea, chiefly composed of silicious materials. Sometimes the fragments are broken without being pulverized by their fall, and assume the form of pebbles, which gradually become rounded and polished. Emily. Pray what is the true colour of silex, which forms such a variety of different coloured substances ? Sand is brown, flint is nearly black, and precious stones are of all colours ? Mrs. B. Pure silex, such as is found only in the chemist's laboratory, is perfedly white, and the various colours which it assumes, in the different substances you have just mentioned, proceed .from the different ingredients with which it is mixed in them. Caroline. I wonder that silex is not more valua- ble, since it forms the basis of so many precious stones. Mrs. B. You must not forget that the value we set upon precious stones, depends in a great measure upon the scarcity with which nature affords them; for, were those prddudions either common, or per- 205 fedly imitable by art, they would no longer, not- withstanding their beauty, be so highly esteemed. But the real value of silicious earth, in many of the most useful arts, is very extensive. Mixed with clay, it forms the basis of all the various kinds of earthen ware, from the most common utensils to the most refined ornaments. Emily. And we must not forget its importance in the formation of glass with potash. Mrs. B. Nor should we omit to mention, like- wise, many other important uses of silex, such as being the chief ingredient of some of the most du- rable cements, of mortars, &c. I said before, that silicious earth combined with no acid but the fluoric: it is for this reason that glass is liable to be attacked by that acid only, which, from its strong affinity for silex, forces that substance from its combination with the potash, and thus destroys the glass. We will now hasten to proceed to the other earths, for I am rather apprehensive of your grow- ing weary of this part of our subject. Caroline. The history of earths is not quite so en- tertaining as that of the other simple substances. Mrs. B. Perhaps not; but it is absolutely indis- pensable that you should know something of them; for they form the basis of so many interesting and important compounds, that their total omission would throw great obscurity on our general outline of chymical science. We shall, however, review them in as cursory a manner as the subject will ad- mit of. Alumine derives its name from a compound salt called alum, of which it forms the basis. Caroline. But it ought to be just the contrary, Mrs. B The simple body should give, instead of ta* Jcing its name from the compound. S 206 Airs. B. Very true, my dear ; but as the com- pound salt was known long before its basis was dis- covered, it was natural enough when that earth was at length separated from the acid, that it should de- rive its name from the compound from which it was obtained. However, to remove your scruples, we will call the salt according to the new nomencla- ture, sulphat of Alumine. From this combination, alumine may be obtained in its pure state ; it is then soft to the touch, makes a paste with water, and hardens in the fire. In nature, it is found chiefly in clay, which contains a considerable proportion of this earth ; it is very abundant in fuller's earth, Slate, and a variety of other mineral produdions.— There is indeed scarcely any mineral substance more useful to mankind than alumine. In the state of clay, it forms large strata of the earth, gives con- sistency to the soil of vallies, and of all low and damp spots, such as swamps and marshes. The beds of lakes, ponds, and springs, are almost entirely of clay; instead of allowing of the filtration of water, as sand does, it forms an impenetrable bottom, and by this means water is accumulated in the caverns of the earth, producing those reservoirs whence springs issue, and spout out at the surface. Emily. I always thought that these subterraneous reservoirs of water were bedded by some hard stone, or rock, which the water could not penetrate. Mrs. B. That is not the case; for in the course of time water would penetrate, or wear away silex, or any other kind of stone, while it is effedually stopped by clay, or alumine. The solid compact soils, such as are fit for corn, owe their consistence in a great measure to alumine; this earth is therefore used to improve sandy or chalky soils, which do not retain a sufficient quan- tity of water for the purpose of vegetation. 207 Alumine is the most essential ingredient in all potteries. It enters into the composition of brick," as well as that of the finest china ; the addition of silex and water hardens it, renders it susceptible of a degree of vitrification, and makes it perfedly fit for its various purposes. Caroline. I can scarcely conceive that brick and china should be made of the same materials. Airs. B. Brick consists almost entirely of baked clay; but a certain proportion of silex is essential to the formation of earthen or stone ware. In com- mon potteries sand is used for that purpose ; a more pure silex is, I believe, necessary for the composi- tion of porcelain, as well as a finer kind of clay; and these materials are, no doubt, more carefully prepared, and curiously wrought, in the one case than in the other. Porcelain owes its bpautiful semi-transparency to a commencement of vitrifica- tion. Emily. But the commonest earthen ware, though not transparent, is covered with a kind of glazing. Mrs. B. That precaution is equally necessary for use as for beauty, as the ware wouid be liable to be spoiled and corroded by a variety of substances, if not covered with a coating of this kind. In porce- lain it consists of enamel, which is a fine white o- paque glass, formed of metallic oxyds, sand, salts, and such other materialsas are susceptible of vitrifica- tion. The glazing of common earthen ware is made chiefly of oxyd of lead, or sometimes merely of salt, which, when thinly spread over earthen vessels, will, at a certain heat, run into opaque glass. Caroline. And of what nature are the colours which are used for painting china ? Mrs. B. They are all composed of metallic oxyds, so that these colours, instead of receiving injury from the "application of fire, are strengthened and developed by its action, which causes them to un- dergo different degrees of oxydation. 203 Alumine and silex are not only often combined by art, but they have in nature a very strong tendency to unite, and are found combined, in different pro- portions, in various gems and other minerals. In- deed, many of the precious stones, such as ruby, oriental sapphire, amethyst, &c. consist chiefly of ' Alumine. We may now proceed to the alkaline earths. I shall say but a few words on Barytes, as it is hardly ever used, except in chymical laboratories. It is re- markable for its great weight, and its strong alkaline properties, such as destroying animal substances, turning green some blue vegetable colours, and shewing a powerful attradion for acids; this last pro- perty it possesses to such a degree, particularly with regard to the sulphuric acid, that it will always de- tect its presence in any substance or combination whatever, by immediately uniting with it and form- ing a sulphat of barytes. This renders it a very va- luable chymical test. It is found pretty abundantly in nature in the state of carbonat, from which the pure earth can be easily separated. The next earth we have to consider is Lime.—- This is a substance of too great and general im- portance to be passed over so slightly as the last. Lime is strongly alkaline. In nature it is not meC with in its simple state, as its affinity for water and carbonic acid is so great, that it is always found combined with these substances, with which it forms the common lime-stone ; but it is separated in the kiln from these ingredients, which are volatilized whenever a sufficient degree of heat is applied. Emily. Pure lime then is nothing but lime-stone, which has been deprived in the kiln, of its water, and carbonic acid ? Airs. B. Precisely; in this state it is called quick- lime, and is so caustic, that it is capable of decom- posing the dead bodies of animals very rapidly, 209 without their undergoing the process of putrefac- tion.—I have here some quick-lime, which is kept carefully corked up in a bottle to prevent the access of air; for were it at all exposed to the atmosphere, it would absorb both moisture and carbonic acid gas from it, and be soon slaked. Here is also some lime-stone—we shall pour a little water on each, and observe the effeds that result from it. Caroline. How quick the lime hisses ! It is be- come excessively hot !—It swells, and now it bursts and crumbles to powder, while the water on the lime-stone appears to produce no kind of alteration. Airs. B. Because the lime-stone is already satu- rated with water, whilst the quick-lime, which has been deprived of it in the kiln, combines with it with very great avidity, and produces this prodigious disengagement of heat, the cause of which I for? merly explained to you ; do you recoiled it ? Emily. Yes; you said that the heat did not pro- ceed from the lime, but from the water which was solidified, and thus parted with its heat of liquidity. Airs. B. V-ery well. If we continue to add suc- cessive quantities of water to the lime after being slaked and crumbled as you see, it will then gradu- ally be diffused in the water, till it will at length be dissolved in it, and entirely disappear; but for this purpose it requires no less than 700 times its weight of water. This solution is called lime-water. Caroline. How very small, then, is the proportion of lime dissolved. Airs. B. Barytes is still of more difficult solu- tion ; it dissolves only in 900 times its weight of water : but it is much more soluble in the state of crystals. The liquid contained in this bottle is lime- water ; it is often used as a medicine, chiefly, I be- lieve, for the purpose of combining with, and neu- tralizing the super-abundant acid which it meets with in the stomach. s2 210 Emily. I am surprised that it is so perfedly clear ; i.t does not at all partake of the whiteness of lime. Airs. B. Have you forgotten that, in solutions, the solid body is so minutely subdivided by the flu- id, as to become invisible, and therefore will not in the least degree impair the transparency of the sol- vent. I said that the attradion of lime for carbonic acid was so strong, that it would absorb it from the at- mosphere. v We may see this effect by exposing a glass of limevwater to the air ; the lime will then separate from the water, combine with the carbonic acid, and re-appear on the surface in the form of a white film, which is carbonat of lime, commonly called chalk. Caroline. Chalk is, then, a compound salt? I ne- ver should have supposed that those immense beds of chalk that we see in many parts of the country, were a salt. Now, the white film begins to appear on the surface of the water •, but it is far from re- sembling hard solid chalk. Mrs. B. That is owing to its state of extreme di- vision; in a little time it will colled into a more compad mass, and subside at the bottom of the glass. If you breathe into lime-water, the carbonic acid, which is mixed with the air that you expire, will produce the same effed. It is an experiment easily made—I shall pour some lime-water into this glass tube, and, by breathing repeatedly into it, you will soon perceive a precipitation of chalk— Emily. I see already a small white cloud formed. Mrs. B. It is composed of minute particles af chalk; at present it floats in the water, but it will soon subside. Carbonat of lime, or chalk, you see, is insoluble in water, since the lime which was dissolved re-ap- pears when converted into chalk ; but you must take notice of a very singular circumstance which is, 211 that chalk is soluble in water impregnated with car- bonic acid. Caroline. It is very curious, indeed, that carbonic acid gas should render lime soluble in one instance, and insoluble in the other! Mrs. B. I have "here a bottle of Seltzer water, which, you know, is strongly impregnated with car- bonic acid—let us pour a little of it into a glass of lime-water. You see that it immediately forms a precipitation of carbonat of lime ! Emily. Yes, a white cloud appears. Airs. B. I shall now pour an additional quantity of the Seltzer water into the lime-water— Emily. How singular ! The cloud is re-dissolved, and the liquid is again transparent. Mrs. B. All the mystery depends upon this cir- cumstance, that carbonat of lime is soluble in car- bonic acid, whilst it is insoluble in water; the first quantity of carbonic acid, therefore, which I intro- duced into the lime-water, was employed in forming the carbonat of lime, which remained visible, until an additional quantity of carbonic acid dissolved it. Thus, you see, when the lime and carbonic acid are in proper proportions to form chalk, the white cloud appears, but when the acid predominates, the chalk is no sooner formed than it is dissolved. Caroline. That is now the case ; but let us try whether a further addition of lime-water will again precipitate the chalk. Emily. It does, indeed ! the cloud re-appears, be- cause, I suppose, there is now no more of the car- bonic acid than is necessary to form chalk ; and, in order to dissolve the chalk, a superabundance of acid is required. Mrs B. We have, I think, carried this experi- ment far enough; every repetition would but exhi- bit the same appearances. 212 Lime combines with most of the acids, to which the carbonic (as being the weakest) readily yields it; but these combinations we shall have an opportunity of noticing more particularly hereafter. It unites with phosphorus, and with sulphur, in'their simple state; in short, of all the earths, lime is that which nature employs most frequently and most abundant- ly, in its innumerable combinations. It is the basis of all calcareous earths and stones ; we find it like- wise in the animal and the vegetable creations. Emily. And in the arts is not lime of very great utility ? Airs. B. Scarcely any substance more so; you know that it is a most essential requisite in build- ing, as it constitutes the basis of all cements, such as mortars, stucco, plaster, &c. Lime is also of infinite importance in agriculture; it lightens and warms soils that are too cold, and compad, in consequence of too great a proportion of clay. But it would be endless to enumerate the va- rious purposes for which it is employed; and you know enough of it to form some idea of its impor- tance : we shall, therefore, now proceed to the third alkaline earth, Magnesia. Caroline. I am already pretty well acquainted with that earth, it is a medicine. Mrs. B. It is in the state of carbonat that mag- nesia is usually employed medicinally ; it then dif- fers but little in appearance from its simple form, which is that of a very fine light white powder. It dissolves in 2000 times its weight of water, but forms with acids extremely soluble salts. It has not so great an attradion for acids as lime, and conse- quently yields them to the latter. It is found in a great variety of mineral combinations, such as slate, mica, amianthus, and more particularly in a certain lime-stone, which has lately been discovered by Mr. Tennant to contain it in very great quantities.. It 213- does not attrad and solidify water, like lime; but, when mixed with water, and exposed to the atmos- phere, it slowly absorbs carbonic acid from the lat- ter, and thus loses its causticity. Its chief use in medicine is, like that of lime, derived from its rea- diness to combine with, and neutralize, the acid which it meets with in the stomach. Emily. Yet, you said that it was taken ip the state of carbonat, in which case it is already combined with an acid ? Mrs. B. Yes; but the carbonic is the last of all the acids in the order of affinities; it will therefore yield the magnesia to any of the others. It is, how- ever, frequently taken in its caustic state as a reme- dy for flatulence. Combined with sulphuric acid, magnesia forms another and more powerful medi- cine, commonly called Epsom salt. Caroline. And properly, sulphat of magnesia, I sup- pose ; Pray why was it ever called Epsom salt ? Mrs. B. Because there is a spring in the neigh- bourhood of Epsom, which contains this salt in great abundance. The last alkaline earth which we have to men- tion is Strontian, or Strontites, discovered by Dr. Hope, a few years ago. It so strongly resem- bles barytes in its properties, and is so sparingly found in nature, and of so little use in the arts, that it will not be necessary to enter into any particulars respeding it. One of the most remarkable charac- teristic properties of strontites, is, that its salts, when dissolved in spirit of wine, tinge the flame of a deep red, or blood colour. We shall here conclude this ledure ; and at our next meeting, you will be introduced to a subjed, totally different from any of the preceding. CONVERSATIONS ON CHYMISTRY. ^ VOLUME II. ON COMPOUND BODIES. CONVERSATIONS ON CHYMISTRY. OiV COMPOUND BODIES. CONVERSATION XII. ON THE ATTRACTION OF COMPOSITION. HAVING completed our examination of the sim- * pie or elementary bodies, we are now to proceed to those of a compound nature; but before we enter on this entensive subjed, it will be necessary to make you acquainted with the principal laws by which chymical combinations are governed. You recollect, I hope, what we have formerly said of the nature of the attradion of composition, or chymfol attradion, or affinity, as it is also called ? T 218 Emily. Yes, I think perfedly; it is the attradion ttiat subsists between bodies of a different nature, which occasions them to combine and form a com- pound, when they come in contad. Airs. B. Very well; your definition comprehends the first law of chymical attradion, which is, that it takes place only between bodies of a different nature ; as, for instance, between an acid and an alkali; be- tween oxygen and a metal, &c. Caroline. That we understand of course ; for the attradion between particles of a similar nature is that of aggregation, or cohesion, which is inde- pendent of any chymical power. Mrs. B. The second law of chymical attradion is, that // takes place only between the most minute par- ticles of bodies; therefore, the more you divide the particles of the bodies to be combined, the more readily they ad upon each other. Caroline. That is again a circumstance which we might have supposed ; for the finer the particles of the two substances are, the more easily and perfed- ly they will come in contad with each other, which must greatly facilitate their union. It was for this purpose, you said, that you used iron filings in pre- ference to wires or pieces of iron, for the decompo- sition of water. Mrs. B. It was once supposed that no mecha- nical power could divide bodies into particles suf- ficiently minute for them to ad upon each other; and that, in order to produce the extreme division requisite for a chymical adion, one, if not both of the bodies, should be in a fluid state. There are, however, a few instances, in which two solid bodies very finely pulverized, exert a chymical adion on one another; but such exceptions to the general rule are very rare indeed. 219 Emily. In all the combinations that we have hi- therto seen, one of the constituents has, I believe, been either liquid or aeriform. In combustions, for instance, the oxygen is taken from the atmosphere, in which it existed in the state of gas; and when- ever we have seen acids combine with metals or with alkalies, they were either in a liquid or an aeriform state. Mrs. B. The third law of chymical attraction is, that it can take place between two, three, four or even a greater number of bodies.—Can you recoiled any examples of these double, triple, and quadru- ple combinations ? Caroline. Oxyds and acids are bodies composed of two constituents ; compound salts of three : but I recoiled no instance of the combination of four principles, unless it be amongst the earths in the formation of stones. Mrs. B. Such examples very frequently occur amongst the earths ; but you might have quoted, as instances of quadruple compounds, all those that result from the combinations of acids with ammo- nia, or volatile alkali. Caroline. True. As ammonia is itself a com- pound, its union with the acids, which are also composed of two principles, must form a quadruple combination. Mrs. B. You will soon become acquainted with a great variety of these complicated compounds. The fourth law of chymical attradion is, that a change of temperature always takes place at the moment of combination. This is occasioned by the change of capacity for heat, which takes place in bodies, when passing from a simple to a combined state. Do you recoiled any instance of this, Emily ? Emily. Yes ; when lime, or any of the alkalies, or alkaline earths, combine with, and solidify wa- ter, the whole of its ^heat of liquidity is set at liberty. 220 Airs. B. I had rather that you had chosen any other instance, as the union of water with the al- kalies and alkaline earths is not, stridly speaking, a chymical combination ; for the water remains in the state of water, though condensed and solidified in the alkali ; and can be separated from it and re- stored to its fluid state, merely by the restitution of its heat of liquidity. I am going to show you a very striking instance of the change of temperature arising from the com- bination of different bodies.—I shall pour some ni- trous acid on this small quantity of oil of turpen- tine—the oil will instantly combine with the oxy- gen of the acid, and produce a considerable change of temperature. Caroline. What a blaze ! The temperature of the oil and the acid must be elevated, indeed, to produce such a violent combustion. Mrs. B. There is, however, a peculiarity in this combustion, which is, that the oxygen, instead of being derived from the atmosphere alone, is prin- cipally supplied by the acid itself. Emilv. And are not all combustions instances of the change of temperature produced by the chymi- jcal combination of two bodies? Airs. B. Undoubtedly; when oxygen loses its gaseous form in order to combine with a solid body, it becomes condensed, and the caloric evolved pro- duces the elevation of temperature. The specific gravity of bodies is at the same time altered by chymical combination; for in consequence of a change of capacity for heat, a change of density must be produced. Caroline. That was the case with the sulphuric acid and water, which by being mixed together, gave out a great deal of heat, and proportionally increased in density. 221 Airs. B. I do not think the instance to which yon refer is quite in point; for there does not appear to be what we have called a true * chymical com- bination between sulphuric acid and water, since they are only mixed together, and undergo no other change than-a loss of caloric, so that they may be separated again from each other merely by evapo- rating the water. Yet you have truly observed in this instance that the particles of the two fluids so far penetrate each other, as to form a more com- pact substance, in consequence of which a quantity of latent heat is forced out, and there is an increase of specific gravity. The 5th law of chymical attradion is, that the properties which characlerise bodies when separate, arc altered or destroyed by their combination. Caroline. Certainly; what, for instance, can be so different from watei as the hydrogen and oxygen gasses ? Emily. Or what more unlike sulphat of iron, than iron or sulphuric acid? Caroline. But of all metamorphoses, that of sand and pot ash into glass; is the most striking! Airs. B. Every chymical combination is an illus- tration of this rule. But let us proceed— The 6th law is, that the force of chymical affinity,, between the constituents of a body, is estimated by that which is required for their separation. This force is by no means proportional to the facility with which bodies unite; for manganese, for instance, which, you know, has so great an -attradion for oxygen,, that it is never found in a metallic state, yields it more easily than any other metal. Caroline. And likewise lime, which has a great attradion for carbonic acid, yields it to any of the' other acids, and even to heat alone. Emily. But, Mrs. B. you speak of estimating the force of attradion between bodies, by the force re- t2 r n 7 quired to separate them; how can you measure these forces ? Mrs. B. They cannot be precisely measured, but they are comparatively ascertained by experiment, and can be represented by numbers which express the relative degrees of attradion. The 7th law is, that bodies have amongst themselves different degrees oi attraction. Upon this law (which you may have discovered yourselves long since), the whole science of chymistry depends; for it is by means of the various degrees of affinity which bodies have for each other, that all the chymical composi- tions and decompositions are effeded. Thus if you pour sulphuric acid on soap, it will combine with the alkali to the exclusion of the oil, and form a sulphat of potash. Every chymical fad or expe- riment is an instance of the same kind; and when- ever the decomposition of a body is performed by the addition of any single new substance, it is said to be effected by simple eleElive attractions. But it often happens that no simple substance will decom- pose a body, and, that, in order to effed this, you must offer to the compound a body which is itself composed of two, or sometimes three principles, which would not, each separately, perform the de- composition. In this case there are two new com- pounds formed in consequence of a reciprocal de- composition and recomposition. All instances of this kind are called double eleclive attractions. Caroline. I confess I do not understand this clearly. Airs. Pi. You will easily comprehend it by the assistance of this diagram, in which the reciprocal forces of attradion are represented by numbers: 22-) Original Cr.mpziou. Sulphat of Sod*. —>------.A.. f Soda 8 Sulphunc Acid "1 4? Result Nitrat « of Soda 7 Divellent Attractions 6 ? r ■? Result y Sulphat o f Lime Nitric Acid 4. Lime Original Compound Nitrat of Lime. We here suppose that we are to decompose sul- phat of soda; that is, to separate the acid from the alkah: if, for this purpose we add some lime, in order to make it combine with the acid, we shall fail in our attempt, because the soda and the sul- phuric acid attract each other by a force which is (by way of supposition) represented by the number 8; while the lime tends to unite with this acid by an affinity equal only to the number 6. It is plain, therefore, that the sulphat of soda will not be de- composed, since a force equal to 8 cannot be over- come by a force equal only to 6. Caroline. So far, this appears very clear. Mrs. B. If, on the other hand, we endeavour to decompose this salt by nitric acid, which tends to combine with soda, we shall be equally unsuccess- ful, as nitric acid tends to unite with the alkali by a force equal only to 7. In neither of these cases of simple eledive attrac- tion, therefore, can we accomplish our purpose. 224 But let us previously combine together the lime and nitric acid, so as to form a nitrat of lime, a compound salt, the constituents of which are united by a pow- er equal to 4. If then we present this compound to the sulphat of soda, a decomposition will ensue, because the sum of the forces which tend to pre- serve the two salts in their adual state, is not equal to that of the forces which tend to decompose them, and to form new combinations. The nitric acid, therefore, will combine withvthe soda, and the sul- phuric acid with the lime. Caroline. I understand you now very well. This double effed takes place because the numbers 8 and 4, which represent the degrees of attradion of the constituents of the two original salts, make a sum less than the numbers 7 and 6, which represent the degrees of attradion of the two new compounds that will in consequence be formed. Airs. B. Precisely so. Caroline. But what is the meaning of quiescent and divel/ent forces, which are written in the diagram? Mrs. B. Quiescent forces are those which tend to preserve compounds in a state of rest, or such as they adually are : divellent forces are those which tend to destroy that state of combination, and to form new compounds. These are the principal circumstances relative to the dodrine of chymical attradions, which have been laid down as rules by modern chymists ; a few others might be mentioned respedingthe same the- ory, but of less importance, and such as would take us too far from our plan. I should, however, not omit to mention that Mr. Berthollet, a celebrated French chymist, has shewn, that whenever in chy- mical operations there is a display of contrary at- tradions, the combinations which take place depend not only upon the affinities, but -dso. in some de-. gree, on the proportions of the substances concerned. 225 CONVERSATION XUI. On Compound Bodies. Mrs. B. . Having now given you some idea of the laws by which chymical attractions are governed, we may proceed to the examination of bodies that are form- ed in consequence of these attradions. The first class of compounds that present them- selves to our notice, in our gradual ascent to the most complicated combinations, are bodies com- posed of only two principles. The sulphurets, phoft- phorets, carburets, &c. are of this description; bi^ the most numerous and important of these com-" {pounds are the combinations of oxygen with the va- rious simple substances with which it has a tendency to unite. Of these you have already acquired some knowledge, and I hope you will not be at a loss to tell me the general names by which the combina- tions of oxygen with other substances are distin- guished ? Emily. I believe you told us that all the combi- nations of oxygen produced either oxyds or acids ? Airs. B. Very right; and with what simple bo- dies will oxygen combine, Caroline ? Caroline. With all the elementary substances, ex- cepting the earths and alkalies. Airs. B. Very well, my dear; we may now, therefore, come to the oxyds and acids. Of the metallic oxyds, you have already some general no- 226 tions. This subjed, though highly interesting in its details, is not of sufficient importance to our con- cise view of chymistry, to be particularly treated of; but it is absolutely necessary that you should be bet- ter acquainted with the acids, and likewise with their combinations with the alkalies, which form the triple compound called Neutral Salts. Yqu have, I believe, a clear idea of the nomen- clature by which the base (or radical) of the acid, and the various degrees of acidification, are ex- pressed ? Emily. Yes, I think so; the acid is distinguished by the name of its base, and its degree of acidity by the termination of that name in ous or ic ,- thus sul- phurs/ acid is that formed by the smallest propor- tion of oxygen combined with sulphur; sulphur/V acid is that which results from the combination of sulphur with the greatest quantity of oxygen. Mrs. B. A still greater latitude may, in many cues, be allowed to the proportions of oxygen than can be combined with acidifiable radicals; for seve- ral of these radicals are susceptible of uniting with a quantity of oxygen so small as to be insufficient to give them the properties of acids; in these cases therefore, they are converted into oxyds. Such is sulphur, which by exposure to the atmosphere with a degree of heat inadequate to produce inflamma- tion, absorbs a small proportion of oxygen, which colours it red or brown. This therefore is the first degree of oxygenation of sulphur ; the 2d converts it into sulphurs/ acid; the 3d into the sulphur/V acid; and, 4-thJy, if it was found capable of com- bining with a still larger proportion of oxygen, it would then be termed super-oxygenated sulphuric acid. Emily. Are these various degrees of oxygenation common to all the acids ? 227 Airs. B. No; they vary much in this resped; some are susceptible of only one degree of oxygena- tion; others, of two, or three; there are but very few that will admit of more. Caroline. The modern nomenclature must be of immense advantage in pointing out so easily the na- ture of the acids, and their various degrees of ox- ygenation. Mrs. B. Certainly. But great as are the advan- tages of the new nomenclature in this resped, it is not possible to apply it in its full extent to all the acids, because the radicals or bases of some of them are still unknown. Caroline. If you are acquainted with the acid, I cannot understand how its basis can remain un- known; _you have only to separate the oxygen from it by eledive attradions, and the basis must remain alone ? Mrs. B. This is not always so easily accomplish- ed as you imagine; for there are some acids which no chymist has hitherto been able to decompose by any means whatever. It appears that the bases of these undecompounded acids have so strong an at- tradion for oxygen, that they will yield it to no other substance; and in that case, you know, all the efforts of the chymists are vain. Emily. But if these acids have never been decom- posed, should they not be classed with the simple bodies; for you have repeatedly told us that the simple bodies are rather such as chymists are una- ble to decompose, than such as are really supposed to consist of only one principle? Mrs. B. Analogy affords us so strong a proof of the compound nature of the undecompounded acids, that I never could reconcile myself to classing them with the simple bodies, though this division has been adopted by several chymical writers. It is 228 certainly the most stridly regular; but, as a syste- matical arrangement is of use only to assist the memory in retaining fads, we may, I think be al- lowed to deviate from it when there is danger of producing confusion by following it too closely:— and this, I believe, would be the case, if you were taught to consider the undecompounded acids as elementary bodies. Emily. \ am sure you would not deviate from the methodical arrangement without good reason. But pray Vhat are the names of these undecompound- ed acids ? Airs. B. There are three of that description: The Aiuriatic acid. The Boracic acid. The' Fluoric acid. Since these acids cannot derive their names from their radicals, they are called after the compound substances from which they are extraded. Caroline. We have heard of a great variety of acids; pray how many are there in all ? Mrs. B. I believe there are reckoned at present thirtyfour, and their number is constantly increas- ing, as the science improves; but the most impor- tant, and those to which we shall almost entirely confine our attention, are but few. I shall, how- ever, give you a general view of the whole; and then we shall more particularly examine those that are the most essential. This class of bodies was formerly divided into mi- neral, vegetable, and animal acids, according to the substances from which they were extraded. Caroline. That I should think must have been an excellent arrangement; why was it altered ! Mrs. B Because in many cases it produced con- fusion. In which class, for instance, would you place carbonic acid ? 229 * Caroline. Now I see the difficulty. I should be at a loss where to place it, as you have told us that it exists in the animal, vegetable, and mineral king- doms. Emily. There would be the same objedion with resped to phosphoric acid, which, though obtained chiefly from bones, can also, you said, be found in small quantities in stones, and likewise in some plants. Mrs. B. You see, therefore, the propriety of changing this mode of classification. These objec- tions do not exist in the present nomenclature; for the composition and nature of each individual acid is in some degree pointed out, instead of the class of bodies from which-iris extraded; and, with regard to the more-general; dj vision of acids, they are classed under these four heads: 1st. Acids of known and simple bases, which are formed by the union of these bases with oxygen. They are the following: ,..,.-. The Sulphuric Carbonic Nitric Phosphoric ! Acids of known Arsenical <* and simple bases. Tungstenic Molybdenic 2dly. Those of unknown bases: The Aiuriatic ~) Boracic > Acids of unknown bases. Fluoric j These two classes comprehend the most anciently known and most .important acids. The sulphuric, ni- tric, and muriatic, were formerly, and are still fre- quently, called mineral acids. 3dly. Acids that have double or binary radicals and which consequently consist of triple combinations. v 2^0 These are the vegetable acids whose common radical is a compound of hydrogen and carbone. Caroline. But if the basis of all the vegetable acids be the same, it should form but one acid; it may in- deed combine with different proportions of oxygen, but the nature of the acid must be the same? Mrs. B. The only difference that exists in the ba- sis of vegetable acids, is the various proportions of hydrogen and carbone from which it is composed. But this is enough to produce a number of acids ap- parently very dissimilar. That they do not, how- ever, differ essentially, is proved by their suscepti- bility of being converted into each other, by the ad- dition or subtradion of a portion of hydrogen or of carbone. The names of these acids are. The Acetic Oxalic Tartarous Citric Malic Acids of double bases, Gallic ► being of vegetable Mucous origin. Benzoic Succinic Camphoric Suberic The 4-th class of acids consists of those which have triple radicals, and are therefore of a still more com- pound nature. This class comprehends the animal acids, which are: The LaBic Prussic Formic Bombic Sebacic Zoonic Lithic f Acids of triple bases, or animal acids. 231 I have given this summary account or enumera- tion of the acids, as you may find it more satisfado- ry to have at once an outline, or general notion of the extent of the subjed; but we shall now confine our- selves to the two first classes, which require our more immediate attention; and defer the remarks which we shall have to make on the others, till we treat of the chymistry of the animal and vegetable kingdoms. The acids of simple and known radicals are all ca- pable of being decomposed by combustible bodies, to which they yield their oxygen. If, for instance, I pour a drop of sulphuric acid on this piece of iron, it will produce a spot of rust; you know what that is? Caroline. Yes, it is an oxyd, formed by the oxy- gen of the acid combining with the iron. Mrs. B. In this case you see the sulnhiirdenoe-i sites the oxygen by winch it was acidified on the metal. And again, if we pour some acid on a com- pound combustible substance, (we shall try it on this piece of wood), it will combine with one or more of the constituents of that substance, and occasion a de- composition. Emily. It has changed the colour of the wood to black. How is that? Mrs. B. The oxygen deposited by the acid has burnt it; you know that wood in burning becomes black before it is reduced to ashes. Whether it de- rives the oxygen which burns it from the atmosphere, or from any other source the chymical effed on the wood is the same. In the case of real combustion, wood becomes black because it is reduced to the state of charcoal by the evaporation of its other constitu- ents. But can you tell me the reason why wood turns black when burnt by the application of an acid? Caroline. First, tell me what are the ingredients of wood ? ^ Airs. B. Hydrogen and carbone are the chief con- stituents of wood, as of all other vegetable substan- ces. 232 Caroline. Well, then, I suppose that the oxygen of the acid combines with the hydrogen of the wood, to form water; and that the carbone of the wood, re- maining alone, appears of its' usual black colour. Mrs. B. Very well, indeed, my dear; that is cer- tainly the most plausible explanation. E ily. Would not this be a good method of ma- king charcoal ? Airs. B. It would "be an extremely expensive, and 1 believe, very imperfed method? for the adion of the acid on the wood, and the heat produced by it, are far from sufficient to deprive the wood of all its evaporable parts. Caroline. What is the reason that vinegar, lemon, and the acids of fruits, do not produce this effed on . woodi _/_. ... . "Mrs. B. They are vegetable acids whose bases are. composed of hydrogen and carbone; the oxygen, therefore, will not be disposed to quit this radical, where it is aleady united with hydrogen. Tlje strong- est of these may, perhaps, yield a little of their oxy- gen to the wood, and produce a stain upon it; but the carbone will not be sufficiently uncovered to as- sume its black colour. Indeed, the several mineral acids themselves possess this power of charring wood in very different degrees. Emily. Cannot vegetable acids be decomposed by any combustibles? Mrs. B. No; because their radical is composed of two substances which have a greater attradion for ox- ygen than any known body. Caroline. And are those strong acids which burn and decompose wood, capable of producing similar effeds on the skin and flesh of animals ? Airs' B. Yes; all the mineral acids, and one of \ them more especially, possess powerful caustic qua- lities. They adually corrotie and destroy the skin and flesh; but they do not produce upon these ex- 233 adly the same alteration as they do on wood, proba- bly because there is a great proportion of nitrogen and other substances in animal matter, which pre- vents the separation of carbone from being so con- spicuous. CONVERSATION XIV. Of the Sulphuric and Phosphoric Acids : or the combina- tions of Oxygen with Sulphur and Phosphorus; and of the Sulphats and Phosphats. Mrs. B. In addition to the general survey which we have taken of acids, I think you will find it interesting to examine individually a few of the most important of them, and likewise some of their principal combinati- ons with the alkalbs, alkaline earths, and metals. The first of the acids, in point of importance, is the sulphuric, formerly called oil of vitriol. Caroline. I have known it a long time by that name, but had no idea that it was the same fluid as sulphuric acid. What resemblance or connedion can there be between oil of vitriol and this acid? Mrs. B. Vitriol is the common name for sulphat of iron, a salt which is formed by the combination'of sulphuric acid and iron; the sulphuric acid was for- merly obtained by distillation from this salt, and it very naturally received its name from the substance which afforded it. Carsline. But it is still usually called oil of vitric'? u2 234 Airs. B. Yes; a sufficient length of time has not yet elapsed, since the invention of the new nomen- clature, for it to be generally disseminated; but as it is adopted by all scientific chymists, there is every reason to suppose that it will gradually become uni- versal. When I received this bottle from the chy- mist's, the name written on the label was oil of vitriol; but, as I knew you were very pundilious in regard to the nomenclature, I changed it, and substituted the modern name. Emily. This acid has neither colour nor smell, but it appears much thicker than water. Mrs. B. It is twice as heavy as water, and has, you see, an oily consistence. Caroline. And it is probably from this circumstance that it has been called an oil, for it can have no real claim to that name, as it does not contain either hydrogen or carbone, which are the essential consti- tuents of oil. Airs. B. Certainly ; and therefore it would be the more absurd to retain a name which owed its origin to such mistaken analogy. Sulphuric acid, in its purest state, would be a con- crete substance, but its attradion for water is such, that it is impossible to preserve it in that state ; it is, therefore, always seen in a liquid form, such as you here find it. One of the most striking properties of sulphuric acid is that of evolving a considerable quan- tity of heat when mixed with water; this I have al- ready shewn you. Ejnily. Yes, I recoiled it; but what was the de- gree of heat produced by that mixture ? Mrs B. The thermometer may be raised by it to 300, which is considerably above the degree of boil- ing water. Caroline. Then water might be made to boil m that mixture ? (10 X Airs. B. Nothing mpre easy, provided that you employ sufficient quantities of acid and of water, and in the due proportions. The greatest heat is pro- duced by a mixture of one part of water to four of the acid : we shall make a mixture of these propor- tions, and immerse this thin glass tube, which is full of water, into it. Caroline. -The vessel feels extremely hot, but the water does not boil yet. Mrs. B. You must allow some time for the heat to penetrate the tube, and raise the temperature of the water to the boiling point— Caroline. Now it boils—and with increasing vio- lence. Mrs. B. But it will not continue boiling long; for the mixture gives out heat only while the parti- cles of the water and the acid are mutually pene- trating each other: as soon as the new arrangement of those particles is effeded, the mixture will gradu- ally cool, and the water return to its former tempe- rature. You have seen tho manner in which sulphuric acid decomposes all combustible substances, whether animal, vegetable, or mineral, and burns them by means of its oxygen ? Caroline. I have very unintentionally repeated the experiment on my gown, by letting a drop of the acid fall upon it, and it has made a stain, which, I sup- pose, will never wash out. Mrs. B. No, certainly ; for, before you can put it into water, the spot will become a hole, as the acid has literally burnt the muslin. Caroline. So it has indeed ! Well, I will fasten the stopper and put the bottle away, for it is a dangerous substance.—Oh, now I have done worse still, fof J have spilt some on my hand ! Mrs. B. ft is then burned, as well as your gown, for you know that oxygen destroys animal as well as 236 vegetable matter; and, as far as the decomposition of the skin of your finger is effeded, there is no remedy; but, by washing it immediately in water, you will dilute the acid, and prevent any farther in- jury. Caroline. It feels extremely hot, I assure you. Airs. B. You have now learned, by experience, how cautiously this acid must be used. You will soon become acquainted with another acid, the ni- tric, which though it produces less heat on the skin, destroys it still quicker, and makes upon it an inde- lible stain. You should never handle any substances of this kind, without previously dipping your fingers in water, which will weaken their caustic effeds.— But since you will not repeat the experiment, I must put in the stopper, for the acid attrads the moisture from the atmosphere, which would destroy its strength and purity. Emily. Pray how can sulphuric acid be extraded from sulphat of iron by distillation ? Airs. B The process of distillation, you know, consists in separating substances from one another by means of their different degrees of volatility, and by the introdudion of a new chymical agent, caloric. Thus, if sulphat of iron be exposed in a retort to a proper degree of heat, it will be decomposed, and the sulphuric acid will be volatilized Emily. But now that the process of forming acids by the combustion of their radicals is known, why should not this method be used for making sulphu- ric acid ? Airs. B. This is adually done in most manufac- tures ; but the usual method of preparing sulphuric acid does not consist in burning the sulphur in oxy- gen gas, (as we formerly did by way of experiment), but in beating it together with another substance, nitre, which yields oxygen in sufficient abundance to render the combustion in common air rapid and complete, 237 Caroline. This substance, then, answers the same purpose as oxygen gas ? Mrs. B. Exadly. In manufadures the combustion is performed in a leaden chamber, with water at the bottom, to receive the vapour, and assist its conden- sation. The combustion is, however, never so per- fed, but that a quantity of sulphurous acid is formed at the same time ; for you recoiled that the sulphu- rous acid differs from the sulphuric only by containing less oxygen. From its own powerful properties, and from the various combinations into which it enters, sulphuric acid is oi great importance in many of the arts. It is us^d also in medicine in a state of great dilu- tion ; for were it taken internally, in a concentrated state, it would prove a most dangerous poison. Caroline. I am sure it would burn the throat and stomach. Mrs. B. Can you think of any thing that would prove an antidote to this poison? Caroline. A large draught of water to dilute it. Jylrs. B. That would certainly weaken the power of the acid, but it would increase the heat to an in- tolerable degree. Do you recoiled nothing that would destroy its deleterious properties more effed- ually ? Emily. An alkali might, by combining with it; but, then, a pure alkali is itself a poison, on account of its causticity. Airs. B. ■ There is no necessity that' the alkali should be caustic. Soap, in which it is combined with oil: or magnesia, either in the state of carbo- nat, or mixed with water, would prove the best an- tidotes. Emily. In those cases, then, I suppose, the potash and the magnesia would quit their combinations to form salts with the sulphuric acid ? <■ 238 Mrs. B. Precisely. We may now make a few observations on the sulphuroax acid, which we have f >und to be the produd of sulphur slowly and imperfedly burnt.— This acid is distinguished by its,pungent smell, and its gaseous form.- Caroline. Its aeriform state is, I suppose, owing to the smaller proportion of oxygen, which renders it lighter than sulphur/V acid ! Mrs. B. Probably; for by adding oxvgen to the weaker acid, it may be converted into the stronger kind. But this change of statf may also be conned- ed with a change of affinity with regard to caloric. Emily. And may sulphurous acid be obtained from sulphuric acid by a diminution of oxygen ? Mrs. B. Yes; it can be done by bringing any combustible substance in contad with the acid. This decomposition is most easily performed by some of the metals ; these absorb a portion of the oxygen from the sulphuric acid, which is thus converted in- to the sulphurous, and flies off in its gaseous form. Caroline_ And cannot the sulphurous acid itself be decomposed and reduced to sulphur ? Mrs. B. Yes; if this gas be heated in contad with charcoal, the oxygen of the acid will combine with it, and the pure sulphur be regenerated. Sulphurous acid is readily absorbed by water ; and in this liquid state it is found particularly useful in bleaching linen and woollen cloths, and is much used in manufadures for those purposes. I can shew you its effed in destroying colours, by taking out any iron mould, or vegetable stain—I think I see a spot on your gown, Emily, on which we may try the ex- periment. Emily. It is the stain of mulberries; but I shall be almost afraid of exposing my gown to the experi- ment, after seeing the effed which the sulphuric acid produced on that of Caroline— 239 Airs. B. There is no such danger from the sul- phurous ; but the experiment must be made with great caution ; for, during the formation of sulphu- rous acid by combustion, there is always some sul- phuric produced. Caroline. But where is your sulphurous acid ? Mrs. B. We may easily prepare some ourselves, simply by burning a match; we must first wet the stain with a little water, and now hold it in this way, at a little distance, over the lighted match : the va- pour that arises from it is sulphurous acid, and the stain, you see, gradually disappears. Emily. I have frequently taken out stains by this means, without understanding the nature of the pro- cess. But why is it necessary to wet the stain before , it is exposed to the acid fumes ? Airs. B. The moisture attrads and absorbs the sulphurous acid; and it serves likewise to dilute any particles of sulphuric acid which might injure the linen. Sulphur is susceptible of a third combination with oxygen, in which the proportion of the latter is too small to render the sulphur acid. It acquires this slight oxygenation by mere exposure to the atmos- phere, without any elevation of temperature : in this case, the sulphur does not change its natural form, but is only discoloured, being changed to red or brown; and in this state it is an oxyd of sulphur. » Before we take leave of the sulphuric acid, we shall say a few words of its principal combinations. It unites with all the alkalies, alkaline earths, and metals, to form compound salts. Caroline. Pray, give me leave to interrupt you for a moment: you haye never mentioned any other salts than the compound or neutral salts ; is there no other kind ? Mrs. B. The term salt has been used, from time immemorial, as a kind of general name, for any sub- 240 stance that has savour, odour, is soluble in water, and crystallizable, whether it be of an acid, an alka- line, or compound nature; but the compound salts alone retain that appellation in modern chymistry. The most important of the salts, formed by the combinations of the sulphuric acid, are, first, sulphat of potash, formerly called sal polychrest; this is a very bitter salt, much used in medicine ; it is found in the ashes of most vegetables, but it may be prepared ar- tificially by the immediate combination of sulphuric acid and potash. This salt is easily soluble in boiling water. Solubility is, indeed, a property, common to all salts ; and they always produce cold in melting. Emily. That must be owing to the caloric which they absorb in passing from a solid to a fluid form. Airs. B. That is, certainly, the most probable ex- planation. . Sulphat of soda, commonly called Glauber's salt, is another medicinal salt, which is still more bitter than the preceding. We must prepare some of these com- pounds, that you may observe the phenomena which take place during their formation. We need only pour some sulphuric acid over the soda which I put into this glass. Caroline. What an amazing heat is disengaged. I thought you said that cold was produced by the melt- ing of salts! Airs. B. But you must observe that we are now making not melting a salt. Heat is disengaged during the formation of compound salts, because the acid goes into a more dense state in the salt than that in which it existed before. A faint light is also emitted, which may sometimes be perceived in the dark. Emily. If the oxygen, in combining with the alka- li, disengages light and heat, an adual combustion takes place. 241 N Airs. B. Not so fast, my dear; recoiled that the alkalies are incombustible substances, and incapable of combining with oxygen singly. They are not aded on by this principle, unless it presents itself in a state of union with another body; and, therefore, the combination of an acid with an alkali cannot be called combustion. Caroline. Will this sulphat of soda become solid ? Mrs^B. We have not, I suppose, mixed the acid and the alkali in the exad proportions that are re- quired for the formation of the salt, otherwise the mix- ture would have been almost immediately changed to a solid mass; but, in order to obtain it in crystals, as you see it in this bottle, it would be necessary first to dilute it with water, and afterwards evaporate the water, during which operation the salt would gradu- ally crystallize. Caroline. But of what use is the addition of water, if it is afterwards to be evaporated ? Mrs. B. When suspended in water, the acid and the alkali are more at liberty to ad on each other, their union is more complete, and the salt assumes the regular form of crystals during the slow evapo- ration or its solvent. Sulphat of soda liquefies by heat, and effloresces in the air. Emily. Pray what is the meaning of the word efflo- resces F I do not recoiled your having mentioned it before. Mrs. B. A salt is said to effloresce wh?n it loses its water of crystallization on being exposed to the atmosphere, and is thus gradually converted into a dry powder: you may observe that these crystals of sulphat of soda are far from possessing the transpa- rency which belongs to their crystalline*state ; they are covered with a white powder, occasioned by their having h-cu exposed to the atmosphere, which has deprived their surface of its lustre, by absorbing its water of crystallization. Salts are, in general, either x 242 efflorescent, or deliquescent; this latter property is pre- cisely the reverse of the former ; that is to say, deli- quescent salts absorb water from the atmosphere, and are moistened and gradually melted by it. Muriat of lime is an instance of great deliquescence. Emily. But are there no salts that have the same degree of attradion for water as the atmosphere, and that will consequently not be affeded by it ? Mrs. B. Yes ; there are many such salts; as, for instance, common salt, sulphat of magnesia, and a variety of others. Sulphat of lime is very frequently met with in nature, and constitutes the well known substance calied gyp- sum, or plaster of Paris. Sulphat of magnesia, commonly called Epsom salt, is another very bitter medicine, which is obtained from sea-water and from several springs, or may be prepa- red by the dired combination of its ingredients. We have formerly mentioned sulphat of alumine as constituting the common alum ; it is found in nature chiefly in the neighbourhood of voleanos, and is par- ticularly useful in the arts, from its strong astringent qualities. It is chiefly employed by dyers and calico- printers to fix colours ; and is used also in the ma- nufadure of leather. Sulphuric acid combines also with the metals. Caroline. One of these combinations, sulphat of i- rsn, we are already well acquainted with. Mrs. B. That is the most important metallic salt formed J?y sulphuric acid, and the only one that we shall here notice. It is of great use in the arts; and in medicine, it affords a very valuable tonic : it is of this salt that most of those preparations called steel medicines are composed. Caroline. But does any carbone enter into these compositions to form steel. Airs. B. Not an atom; they are, therefore, very improperly called steel; but it is the vulgar appella- 243 tion, and medical men themselves often comply with the general custom. Sulphat of iron may be prepared, as you have seen, by dissolving iron in sulphuric acid ; but it is gene- rally obtained from the natural produdion called P\- rites, which, being a sulphuret of iron, requires on- ly exposure to the atmosphere to be oxydated, in or- der to form the salt ; this, therefore, is much the most easy way of procuring it on a large scale. Emily. I am surprised to find that both acids and compound salts are generally obtained from their va- rious combinations, rather than from the immediate union of their ingredients. Mrs. B. Were the simple bodies always at hand, their combination would naturally be the most con- venient method of forming compounds; but you must consider that, in most instances, there is great difficulty and expense in obtaining the simple ingre- dients from their combinations; it is, therefore, often more expedient to procure compounds from the de- composition of other compounds. But to return to the sulphat of iron.—There is a certain vegetable acid called Gallic acid, which has the remarkable proper- ty of precipitating this salt black.—I shall pour a few drops of the gallic acid into this solution of sul- phat of iron— Caroline. It is become as black as ink ! Mrs. B. And it is ink in rea'ry. Common wri- ting ink is a precipitate of sulphat of iron by gallic acid ; the black colour is owing to the formation of gallat of iron, which being insoluble, remains sus- pended in- the fluid. This acid has also the property of altering the co- lour of iron in its metallic state. You may fre- quently see its effeds on the blade of a knife that has been used to cut certain kinds of fruits. Caroline. True; and that is perhaps the reason that a silver knife is preferred to cut fruits; the gal- 244 ;»c acid, I suppose, does not ad upon silver.—Is this acid found in all fruits ? ' Mrs. B It is contained, mpre or less, in the rind of most fruits and roots, especially the radish, which, if scraped with a steel or iron knife, has its bright red colour changed to a deep purple, the knife being at the same time blackened. But the vegetable sub- "stance in which the gallic acid most abounds is nut- gall, a kind of excrescence that grows on oaks, and from which the acid is commonly obtained for its various purposes. Airs. B. We now come tb the phosphoric and phosphorous acids. In treating of phosphorus, you have seen how these acids may be obtained from it by combustion ? Emily. Yes; but I should be much surprised if it was the usual method of obtaining them, since it is -.o very difficult to procure phosphorus in its pure state. Mrs. B. You are right, my dear ; the phosphoric acid, for general purposes, is extraded from bones, in which it is contained in the state of phosphat of lime ; from this salt the phosphoric acid is separated by means of the sulphuric, which combines with the lime. In its pure state, phosphoric acid is either liquid or solid, according to its degree of concentra- tion. Amongst the salts formed by this acid, phosphat of HmeAS the only one that affords much interest; and • this, we have already observed, constitutes the basis of all bones. It is also found in very small quan- *iues in some vegetables. i 245 CONVERSATION XV. Of the nitric and carbonic acids ; or the combinations of oxygen with nitrogen and carbone ,• and of the nitrats and carbonats. Mrs. B. I am almost afraid of introducing the subjed of the nitric acid, as I am sure that I shall be blamed by Caroline, for not having made her acquainted with it before. Caroline. Why so, Mrs. B— ? Airs. B. Because you have long known its radical, which is nitrogen or azote ; and, in treating of that element, I did not even hint that it was the basis of an acid. Caroline. Indeed, that appears to me a great omis- sion ; for you have made us acquainted with all the other acids, in treating of their radicals. Emily I would advise you not to be too hasty in your censure, Caroline; for I dare say that Mrs. B. had some very good reason for not mentioning this acid sooner. Airs. B. I do not know whether you will think the reason sufficiently good to acquit me; but the omission, I assure you, did not proceed from negli- gence. You may recoiled that nitrogen was one of the first simple bodies which we examined ; you were then ignorant of the theory of combustion, which I believe was, for the first time, mentioned in that, lesson; and therefore it would have been in; x 2 246 vain, at that time, to have attempted to explain the nature and formation of acids. Caroline. 1 wonder, however, that it never occur- red to us to inquire whether nitrogen could be aci- dified;" for, as we knew it was classed amongst the combustible bopies, it was natural to suppose that it might produce an acid. Mrs. B. That is not a necessary consequence; for it might combine with oxygen only m the degree re- quisite to form an oxyd. But you will find that ni- trogen is susceptible of various degrees of oxygena- tion, some of which convert it merely into an oxyd, and others give it all the acid properties. The acids, resulting from the combination of oxy- gen with nitrogen, are called the nitrous and ni- tric acids. We will begin with the nitric, in which nitrogen is in the highest state of oxygena- tion This acid naturally exists in the form of gas; but it is so extremely soluble in water, and has so great an affinity for it, that one grain of water will absorb and condense ten grains of acid gas, and form the limpid fluid which you see in this bottle. Caroline. What a strong offensive smell it has ! Airs. B. This acid contains a greater abundance of oxygen than any other, but it retains it with very little force. Emily. Then it must be a powerful caustic, both from the facility with which it parts with its oxygen, and the quantity which it affords ? Mrs. B Very well, Emily; both cause and effed are exadly such as you describe : nitric acid-burns and destroys all kinds of organized matter. It even sets fire to some of the most combustible substances. We shall pour a little of it over this piece of dry warm charcoal—you see it inflames it immediately; it would do the same with oil of turpentine, phos- phorus, and several other very combustible bodies. This shews you how easily this acid is decomposed by 247 combustible bodies, since these effeds must depend upon the absorption of its oxygen. Nitric acid has been used in the arts from time im- .memorial, but it is not more than twenty five years that its chymical nature has been ascertained. The celebrated Mr. Cavendish discovered that it consisted of about 10 parts of nitrogen, and 25 of oxygen.* These principles, in their gaseous state, combine at a high temperature; and this may be effeded by repeatedly passing the eledrical spark through a mixture of the two gasses. Emily. The nitrogen and oxygen gasses, that com- pose the atmosphere, do not combine, I suppose, be- cause their temperature is not sufficiently elevated ? Caroline. But in a thunder storm, when the light- ning repeatedly passes through them, may it not produce nitric acid; we should be in a strange situ- ation if a violent storm should at once convert the atmosphere into nitric acid. Airs. B. There is no danger of it my dear ; the lightning can affed but a very small portion of the at- mosphere, and though it were occasionally to produce a little nitric acid, yet this never could happen to such an extent as to be perceivable. Emily. But how could the nitric acid be known, and used, before the method of combining its con- stituents was discovered ? Airs. B. Before that period the nitric acid was obtained, and it is indeed still extraded for the com- mon purposes of art, from the compound salt which it forms with potash, commonly called nitre. Caroline. Why is it called so ? Pray, Mrs B let these old unmeaning names be entirely given up, by us at least ; and let us call this salt nitrat of potash. Airs B. With all my heart; but it is necessary that I should, at leust, mention the old names, and * The proportions stated by Mr. Davy, in'his Chymical Research- es, are as i to a. 389. ■ 248 more especially those that are yet in common use, otherwise, when you meet with them, you would not be able to understand their meaning. Emily. And how is the acid obtained from this salt ? Airs. B. By the intervention of sulphuric acid, which combines with the potash, and sets the nitric acid at liberty. This I can easily shew you, by mix- ing some nitrat of potash and sulphuric acid in this retort, and heating it over a lamp ; the nitric acid will come over in the form of vapour, which we shall colled in a glass bell. This acid diluted in water is commonly called aquafortis, if Caroline will allow me to mention that name. Caroline. I have often heard that aqua fortis will dissolve almost all metals; it is no doubt because it yields its oxygen so easily. Mrs. B. Yes; and from this powerful solvent property, it derived the name of aqua fortis, or strong water. Do you not recoiled that we oxydated, and afterwards dissolved some copper in this acid ? Emily. If I remember right, the nitrat of copper was the first instance you gave us of a compound salt. Caroline. Can the nitric acid be completely decom- posed and converted into nitrogen and oxygen. Emily. That cannot be the case, Caroline, since the acid can be decomposed only by the combination of its constituents with other bodies. Mrs. B. True; but caloric is sufficient for this purpose. By making the acid pass through a red hot porcehiu tube, it is decomposed; the nitrogen and oxygen regain the caloric which they had lost in combining, and are thus both restored to their gase- ous state. The nitric acid may also be partly decomposed, and is by this means converted into nitrous acid. 249 Caroline. This conversion must be easily effeded, as the oxygen is so slightly combined with the ni- trogen. Airs. B. The partial decomposition of nitric acid is readily effeded by most metals; but it is sufficient to expose the nitric acid to a very strong light to ' make it give out oxygen gas, and bfe thus converted into nitrous acid. Of this acid there are various de- grees, according to the proportions of oxygen which it contains ; the strongest and that into which the nitric acid is first converted, is of a yellow colour, as you see it in this bottle. Caroline. Hjw it fumes when the stopper is taken out. Airs. B. The acid exists naturally in a gaseous state, and is here so strongly concentrated in water that it is constantly escaping. Here is another bottle of nitrous acid, which, you see is of an orange red colour ; this acid is weaker, the nitrogen being combined with a smaller quantity of oxygen ; and with a still less proportion of oxygen it is an olive green colour, as it appears in this third bottle. In short, the weaker the acid, the deeper is its colour. Nitrous acid ads still more powerfully on some inflammable substances than the nitric. Emily. I am surprised at that, as, it contains less oxygen. Airs B. But, on the other hand, it parts with its oxygen much more readily : you may recoiled that we once inflamed oil with this acid. The next combinations of nitrogen and oxygen form only oxyds of nitrogen, the first of which is commonly called nitrous air : or more properly nitric ox\d i:.:s. This may be obtained from nitric acid, by exposing the latter to the adion of met.ds, as in dissolving them it does not yield the whole of its ox- ygen, but retains a portion of this principle sufficient 250 to convert it into this peculiar gas, a specimen of which I have prepared, and preserved within this inverted glass bell. , Emily. It is a perfedly invisible elastic fluid. Airs. B. Yes; and it may be kept any length of time in this manner over water, as it is not, like the nitric and nitrous acid;, absorbable by it. It is ra- ther heavier than atmospherical air, and is incapable of supporting either combustion cr respiration. I am going to incline the glass gently on one side, so as to let some of the gas escape-— Emily. How very curious!—It produces orange fumes like the nitrous acid ! that is the more extraor- dinary, as the gas within the glass is perfedly invisible. Airs. B. It would give me much pleasure if you could make out the reason of this curious change without requiring any further explanation. Caroline. It seems, by the colour and smell, as if it were converted into nitrous acid gas : yet that can- not be, unless it combines with more oxygen; and how can it obtain oxygen the very minute it escapes from the glass ? Emily From the atmosphere, no doubt. Is it not so, Mrs. B. ? Mrs. B. You have guessed it; as soon as it comes in contad with the atmosphere it absorbs from it the additional quantity of oxygen necessary to convert it into nitrous acid gas—And, if I now remove the bottle entirely from the water, so as to bring at once the whoie of the gas into contad with the atmos- phere, this conversion will appear still more striking. ( Emily. Look, Caroline, the whole capacity of the bottle is instantly tinged of an orange colour ! Airs. B. Thus you see.it is the most easy process imaginable to convert nitrous, oxyd gas into nitrous ■. acid gas. The property of attrading oxygen from the atmosphere, without any elevation of tempera- ture, has occasioned this gaseous oxyd being used as - 251 a test for ascertaining the degree of purity of the at- mosphere. I am going to show you how it is applied to this purpose—You see this gradua-ed glass tube, which is closed at one end ; {Plate Vill Fir. 19.)__ I first fill it with water, and then introduce a certain measure of nitrous gas, which, not being absorbable by water, passes through it, and occupies the upper part of the tube. I must now add rather above two thirds of oxygen gas, which will just be sufficient to convert the nitric oxyd gas, into nitrous acid gas. Caroline. So it has !—I saw it turn of an orange colour; but it immediately afterwards disappeared entirely, and the water, you see, has risen, and al- most filled the tube. Mrs. B. That is because the acid gas is absorb ■ able by water, and in proportion as the gas impreg- nates the water, the latter rises in the tube. When the oxygen gas is very pure, and th? required pro- portion of nitric oxyd gas very exad, the whole is absorbed by the water; but if any other gas be mixed with the oxygen, instead of combining with the nitric oxyd, it will remain and occupy the upper part of the tube ; or, if the gasses be not in the due proportion, there will be a residue of that which predominates.—Before we leave this subjed, I must not forget to remark, that nitric acid may be formed by dissolving nitric oxyd gas in nitric acid. This 'so- lution may be effeded simply by making bubbles of nitric oxyd gas pass through nitric acid. Emily. That is to say, that nitrogen, at its highest degree of oxygenation, being mixed with nitrogen at its lowest degree of oxygenation, will produce a kind of intermediate substance, which is nitric acid, Mrs. B. You have stated the fad with great pre- cision.—There are various other methods of prepa- ring nitrous oxyd, and of obtaining it from com- pound bodies ; but it is not necessary to enter into these particulars. It remains for me only to men- 252 tion another curious modification of oxygenated nitrogen, which has been distinguished by the name of gaseous oxyd of nitrogen. It is but lately that this gas has been accurately examined, and its properties have been chiefly investigated by Mr. Davy. It has obtained also the name of exhilirating gas, from the very singular property which that gentleman has discovered in it, of elevating the animal spirits, when inhaled into the lungs, to a degree sometimes resem- bling delirium or intoxication. Caroline. It is respirable, then ? Mrs. B It can scarcely be called respirable, as it would not support life for any length of time ; but it may be breathed for a few moments without any other effeds, than the singular exhiliration of spirits I have just mentioned. It affeds different people, however, in a very different manner. Some become violent, even outrageous: others experience a lan- guor, attended with faiiitness; but most agree in opinion, that the sensations it excites are extremely pleasant. Caroline. I think I should like to try it—how do you breathe it ? Mrs. B. By colleding the gas in a bladder, to which a short tube with a stop cock is adapted ; this is applied to the mouth with ohe hand, whilst the nostrils are kept closed with the other, that the com- mon air may have no access. You then alternately inspire, and expire the gas, till you perceive its ef- feds. But I cannot consent to your making the ex- periment ; for the nerves are sometimes unpleasantly affVded by it, and I would not run any risk of that kind. ~~ Emily. I should like, at least, to see somebody breathe it; but pray by what means is this curious gas obtained ? Mrs] B. It is procured from nitrat of ammonia, an artificial salt, which yields this gas on the application 253 of a gentle heat—I have put some of the salt into a retort, and by the aid of a lamp the gas will be extri- cated— Caroline. Bubbles of air begin to escape through the neck of the retort into the water apparatus; will you not colled them ? Mrs. B. The gas that first comes over is never preserved, as it consists of little more than the com- mon air which was in the retort; besides, there is always in this experiment a quantity of watery va- pour which must come away before the nitrous ox- yd appears. Emily. Watery vapour! Whence does that pro- ceed ? there is no water in nitrat of ammonia ! Mrs. B. You must recoiled that there is in every salt a quantity of water of crystallization, which may be evaporated by heat alone. But, besides this, wa- ter is adually generated in this experiment, as you will see presently. But first tell me, what are the constituent parts of nitrat of ammonia ? Emily. Ammonia, and nitric acid : this salt, there- fore, contains three different elements, nitrogen and hydrogen, which produce the ammonia ; and oxygen, which, with nitrogen, forms the acid. Mrs. B. Well, then, in this process the ammonia is decomposed ; the hydrogen quits the nitrogen to combine with some of the oxygen of the nitric acid, and forms with it the watery vapour which is now co- ming over. When that is effeded, what will you ex- ped to find ? Emily. Nitrous acid instead of nitric acid, and ni- trogen instead of ammonia. Mrs. B. Exadly so ; and the nitrous acid, and ni- trogen combine, and form the gaseous oxyd of nitro- gen, in which the proportion of oxygen is 37 parts to 63 of nitrogen. You may have observed, that for a little while no bubbles of air have come over, and we have perceived only a stream of vapour condensing as it issued into the water.—Now bubbles of air again make their ap- y 25* pearance, and I imagine that by this time all the wa- tery vapour is come away, and that we may begin to colled the gas. We may try whether it is pure by filling a phial with it, arfd plunging a taper into it— yes, it will do now, for the taper burns brighter than in the common air, and with a greenish flame. Caroline. But how is that ? I thought no gas would support combustion but oxygen. Mrs. B. Or any gas that contains oxygen, and is ready to yield it, which is the case with this in a con- siderable degree; it is not, therefore, surprising that it should accelerate the combustion of the taper. You see that the" gas is now produced in great abundance ; we shall colled a large quantity of it, and I dare say we shall find some of the family who will be curious to make the experiment of respiring it. Whilst this process is going on, we may take a general survey of the most important combinations of the nitric and nitrous acids with the alkalies. The first of these is nitrat of potash, commonly cal- led nitre or saltpetre. ♦> Caroline. Is not that the salt with which gunpow- der is made Mrs. B.' Yes. Gunpowder is a mixture of five parts of nitre to one of sulphur, and one of char- coal.—Nitre from its great proportion of oxygen, and from the facility with which it yields it, is the basis of most detonating compositions. Emily. But what is the cause of the violent deto- nation of gunpowder when set fire to ? Airs. B. Detonation may proceed from two causes; the sudden formation or destrudion of an elastic flu- id. In the first case, when either a solid or liquid is instantaneously converted into an elastic fluid, the prodigious and sudden expansion of the body strikes thp air with great violence, and this concussion pro- duces the sound called detonation. Caroline. That I comprehend very well; but how can a similar effed be produced by the destrudion of a gas ? 255 Airs. B. A gas can be destroyed only by conden- sing it to a liquid or solid state ; when this takes place suddenly, the gas", in assuming a new and more compad form produces a vacuum into which the surrounding air rushes with great impetuosity; and it is by that rapid and violent motion that the sound is produced. In all detonations, therefore, gasses are either suddenly formed, or destroyed. In that of gunpowder, can you tell me which of these two circumstances takes place ? Emily: As gunpowder is a solid, it must, of course, produce the gasses in its detonation ; but how, I can- not tell. Airs. B. The constituents of gunpowder, when heated to a certain degree, enter into a number of new combinations, and are instantaneously converted into a variety of gasses, the sudden expansion of which gives rise to the detonation. Caroline. And in what instance does the destruc- tion or condensation of gasses produce detonation. Mrs. B. I can give you one with which you are well acquainted; the sudden combination of the ox- ygen and hydrogen gasses. Caroline. True; I recoiled perfedly that hydrogen detonates with oxygen when the two gasses are con- verted into water. Airs. B. But let us return to the nitrat of potash. This salt is decomposed when exposed to heat, and mixed with any combustible body, such as carbone, sulphur, or metals, these substances oxydating rapid- ly at the expense of the nitrat. I must shew you an instance of this.—1 expose to the fire some of the salt in a small iron ladle, and, when it is sufficiently heat- ed, add to it some powdered charcoal;. this will at- trad the oxygen from the salt, and be converted in- to carbonic acid— *- Emily. But what occasions that crackling noise, and those vivid flashes that accompany it ? Mrs. B. The rapidity with which the carbonic a- cid gas is formed, occasions a succession of small de- 256 tonations, which, together with the emission of flame, is called deflagration. Nitrat of Ammonia we have already noticed, on ac- count of the gaseous oxyd of nitrogen which is ob- tained from it. Nitrat of Silver is the lunar caustic, so remarkable for its property of destroying animal fibre, for which purpose it is often used by surgeons.—We have said so much on a former occasion, on the mode in which caustics ad on animal matter, that I shall not detain you any longer on this subjed. We now come to the carbonic acid, which we have already had many opportunities of noticing. You recoiled: that this acid may be formed by the combustion of carbone, whether in its imperfed state of charcoal, or in its purest form of diamond. And it is not necessary, for this purpose, to burn the car- bone in pure oxygen gas, as we did in a preceding ledure; for you need only light a piece of charcoal and suspend it under the receiver on the water bath. The charcoal will soon be extinguished, and the air in the receiver will be found mixed with carbonic a- cid,'the process, however is much more expeditious if the combustion be performed in pure oxygen gas. Caroline. But how can you separate the carbonic acid, obtained in this manner, from the air with which it is mixed. Airs. B. The readiest mode is to introduce under the receiver, a quantity of caustic lime, or caustic alkali, which soon attracts the whole of the carbonic acid to form a carbonat.—The alkali is found increa- sed in weight, and the volume of the air is diminished by a quantity equal to that of the carbonic acid which was mixed with it. Emily. Pray is there no method of obtaining pure carbone from carbonic acid ? 257 Airs. B. For a long time it was supposed that car- bonic acid was not decomposable; but Mr. Tennant discovered, a few years ago, that this acid may be decomposd by burning phosphorus in a closed ves- sel with carbonat of soda or carbonat of lime : the phosphorus absorbs the oxygen from the carbonat, whilst the carbone is separated in the form of a black powder. - Caroline Cannot we make that experiment ? Mrs. B. Not easily ; it requires being performed with extreme nicety, in order to obtain any sensible quantity of carbone, and the experiment is much too delicate for me to attempt it. But there can be no doubt of the accuracy of Mr. Tennant's results'; and all chymists now agree, that 100 parts of carbonic acid gas consist of about 28 parts of carbone to 72 of oxy- gen gas. Carbonic acid gas is found very abundantly in na- ture ; it is supposed to form about a hundredth part of the atmosphere, and is constantly produced by the respiration of animals; it exists in a great variety of combinations, and is exhaled from many natural de- compositions. It is contained in a state of great pu- rity in certain caves, such as the Grotto del Cane, near Naples. Emily. I recoiled having read' an account of that grotto, and of the cruel experiments made on the poor dogS) to gratify the curiosity of strangers. But T understood that the vapour exhaled by this cave was caUed fixed air. Mrs. B. That is the name by which carbonic acid was known before its chymical composition Was dis- covered.—This gas is more destrudive of life than any other ; and if the poor animals that are submit- ted to its effeds, are not plunged into cold water as soon as they become senseless, they do not recover. It extinguishes flame instantaneously. I have collec- ted some in this glass, which I will pour over the candle. v 2 258 Caroline. This is extremely singular—it seems to extinguish it as it were by enchantment, as the gas is invisible. I never should have imagined that a gas could have been poured like a liquid. Mrs. B. It can be done with carbonic acid only, as no other gas is sufficiently heavy to be susceptible of being poured out in the atmospherical air, without mixing with it. Emily. Pray by what means did you obtain this gas? Mrs. B. I procured it from marble. Carbonic acid gas has so strong an attradion for all the alka- lies and alkaline earths, that these are always found in nature in the state of carbonats. Combined with lime, this acid forms chalk, which may be consider- ed as the basis of all kinds of marble, and calca- reous stones. From these substances carbonic acid is easily separated, as it adheres so slightly to its com- binations, that the carbonats are all decomposable by any of the other acids. I can easily shew you how I obtained this gas ; I poured some diluted sulphuric acid over pulverized marble in this bottle (the same which we used the other day to prepare hydrogen gas), and the gas escaped through the tube connected with it; the operation still continues, as you may easily - ; perceive— -1 Emily. Yes, it does; there is a great fermentation in the glass vessel. What singular commotion is ex- t cited by the sulphuric acid taking possession of the , lime, and driving out the carbonic acid ? Caroline. But did the carbonic acid exist in a gas- eous state in the marble ! Airs. B. Of course not; the acid, when in a state .': of combination, is capable of existing in a solid form. Caroline. Whence, then, does it obtain the calo- ric necessary to convert it into a gas ? Mrs. B. It may be supplied in this case from the mixture of sulphuric acid and water, which produces an evolution of heat, even greater than is required for the purpose; since, as you may perceive by ouching the glass vessel, a considerable quantity of 259 the caloric disengaged becomes sensible. But a sup- ply of caloric may be obtained also from a diminu- tion of capacity for heat, occasioned by the new combination which takes place; and, indeed, this must be the case when other acids are employed for the disengagement of carbonic acid gas, which do not, like the sulphuric, produce heat on being mix- ed with water. Carbonic acid may likewise be dis- engaged from its combinations by heat alone, which restores it to its gaseous state. Caroline. It appears to me very extraordinary that the same gas, which is produced by the burning of wood and coals, should exist also in stones, marble, and chalk, which are incombustible substances. Airs. B. I will not answer that objedion, Caro- line, because I think I can put you in a way of doing it yourself. Is carbonic acid combustible ? Caroline. Why, no—because it is a body that has been already burnt, it is carbone only, and not the acid, that is combustible. Mrs. B. Well and what inference do you draw from this ? Caroline. That carbonic acid cannot render the bodies in which it is contained combustible; but that simple carbone does, and that it is in this ele- mentary state that it exists in wood, coals, and a great variety of other combustible bodies.—Indeed, Mrs. B. you are very ungenerous ; you are not satis- fied with convincing me that my objedions are fri- volous, but you oblige me to prove them so myself. Airs. B. You must confess, however, that I make ample amends for the detedion of error, when I enable you to discover the truth. You understand, now, I hope, that carbonic acid is equally produced by the decomposition of chalk, or by the combustion of charcoal. These processes are certainly of a very different nature; in the first case the acid is already formed, and requires nothing more than heat to re- store it to its gaseous state; whilst, in the latter, the acid is adually formed by the process of combustion. 260 Caroline. I understand it now perfediy. But I have just been thinking of another difficulty,' which I hope you will excuse my not being able to remove myself. How does the immense quantity of calca- reous earth, which is spread all over the globe, ob- tain the carbonic acid which is combined with it ? Mrs. B. This question is, indeed, not very easy to answer ; but I conceive that the general carbonic zation of calcareous matter may have been the effed of a general combustion, occasioned by some revolu- tion of our globe, and producing an immense supply of carbonic acid, with which the calcareous matter became impregnated ; or that this may have been ef- feded by a gradual absorption of carbonic acid from the atmosphere.—But this subjed would lead us to discussions which we cannot indulge in, without de- viating too much from our subjed. Emily. How does it happen that we do not per- ceive the pernicious effeds of the carbonic acid that is floating in the atmosphere ? Airs. B. Because of the state of very great dilu- tion in which it exists there. But can you tell me, Emily, what are the sources which keep the atmos- phere constantly supplied with this acid ? Emily. I suppose the combustion of wood, coals, and other substances, that contain carbone. Mrs. B. And also the bre ith of animals. Caroline. The breath- of animals ! I thought you said that this gas was not at all respirable, but, on the contrary, extremely poisonous. Mrs. B. So it is; but although animals cannot breathe in carbonic acid gas, yet, in the process of respiration, they have the power of forming this gas in their lungs; so that the air which we expire, or rejed from the lungs, always contains a certain pro- portion of carbonic acid, which is much greater than that which is' commonly found in the atmosphere. Caroline. But what is it that renders carbonic acid such a deadly poison ? 261 Mrs. B. The manner in which this gas destroys life, seems to be merely by preventing the access of respirable air ; for carbonic acid gas, unless very much diluted with common air, does not penetrate ■into the lungs, as^ the windpipe actually contrads, and refuses it admittance.—But we must dismiss this subjed at present, as we shall have an opportunity of treating of respiration much more fully, when we come to the chymical fundions of animals. Emily. Is carbonic acid as destrudive to the life of vegetables, as it is to that of animals ? Mrs. B. If a vegetable be completely immersed in it, I believe it generally proves fatal to it; but mix- ed in certain proportions with atmospherical air, it is on the contrary, very favourable to vegetation. You remember, I suppose, our mentioning the mineral waters, both natural and artificial, which contain carbonic acid gas ? Caroline. You mean the Seltzer water ? Mrs. B. That is one of those which are the most used ; there are, however, a variety of others into which carbonic acid enters as an ingredient; all these waters are usually distinguished by the name of aci- dulous or gaseous mineral waters. The class of salts called carbonats is the most nume- rous in nature ; we must pass over them in a very cursory manner, as the subjed is far too extensive for us to enter on in detail. The state of carbonat i s the natural state of a vast number of minerals, and particularly of the alkalies and alkaline earths, as they have so great an attradion for the carbonic acid, that they are almost always found combined with it; and you may recoiled that it is only by se- parating them from this acid, that they acquire that causticity and those striking qualities which I have formerly described. All marbles, chalks, shells, calcareous spars, and lime-stones of every descrip- tion, are neutral salts, in which lime, their common basis, has lost all its charaderistic properties. 262 Emily. But if all these various substances are form- ed by the union of lime with carbonic acid, whence arises their diversity of form and appearance ? Mrs. B. Both from the different proportions of their component parts, and from a variety of foreign ingredients which may be occasionally mixed with them : the veins and colours of marble, for instance, proceed from a mixture of metallic substances ; silex and alumine also frequently enter into these combi- nations. The various carbonats therefore, that I have enumerated, cannot be considered as pure una- dulterated neutral salts, although .they certainly be- long to that class of bodies. CONVERSATION XVI. On the muriatic and oxygenated muriatic acids ; and on muriats. Mrs. B. WE come now to tile undecompounded acids.— The muriatic, formerly called the marine acid, is , the only one that requires our particular attention. The basis of this acid, as I have told yon before, is unknown, all attempts to decompose it having hi- therto proved fruitless ; it is, therefore, by analogy only, 'that we suppose it to consist of a certain sub- stance or radical, combined with oxygen. Caroline. It can then never be formed by the com- bination of simple bodies, but must always be drawn from its compounds. Emily. Unless the acid should be icund in nature uncombined with other substances. 263 Airs. B. I believe that is never the case. Its principal combinations are with soda, lime, and mag- nesia. Muriat of soda, is the common sea salt, and from this substance the acid is usually disengaged by means or the sulphuric acid. The natural state of the muriatic acid, is that of an invisible permanent gas, at the common temperature of the atmosphere; but it has an extremely strong attradion for water, and assumes the form of a whitish cloud, whenever it meets with any moisture to combine with. This acid is remarkable for its peculiar and very pungent smell, and possesses, in a powerful degree, most of the acid properties. Here is a bottle containing mu- riatic acid in a liquid state— Caroline. And how is it liquified ? Mrs. B. By impregnating water with it; its strong attradion for water makes it very easy to ob- tain it in a liquid form. Now, if I open the phial, you may observe a kind of vapour rising from it, which is muriatic acid gas, of itself invisible, but made apparent by combining with the moisture of the atmosphere. Emily. Have you not any of the pure muriatic acid gas ? Mrs. B. This jar is full of that acid in its gaseous state—it is inverted over mercury instead of water, because, being absorbable by water, this gas cannot be confined by it.—I shall now raise the jar a little on one side, and suffer some of the gas to escape.— You see that it immediately becomes visible in the form of a cloud. Emily. It must be, no doubt, from its uniting with the moisture of the atmosphere, that it is converted into this dewy vapour. Mrs. B. Certainly; and for the same reason, that is to say, its extreme eagerness to unite with water, this gas will cause snow to melt as rapidly as an intense fire. Emily. Since this acid cannot be decomposed, I suppose that it is not susceptible of different degrees of oxygenation ? 264 Airs. B. You are mistaken in your conclusion; for though we cannot deoxygenate this acid, yet we may add oxygen to it. Caroline. Why then is not the least degeee of ox- ygenation of the acid, called the muriatous, and the higher degree the muriatic acid ? Mrs. B. Because, instead of becoming, like other acids, more dense, and more acid by an addition of oxygen, it is rendered on the contrary more vola- tile, more pungent, but less acid, and less absorba- ble by water. These circumstances, therefore, seem to indicate the propriety of making an exception to the nomenclature. The highest degree of oxygena- tion of this acid has been distinguished by the addi- tional epithet of oxygenated, or, for the sake of brevi- ty, oxy, so that it is called the oxygenated, or oxy-mu- riatic acid. This likewise exists in a gaseous form, at the temperature of the atmosphere ; it is also suscep- tible of being absorbed by water, and can be con- gealed, or solidified, by a certain degree of cold. Emily. And how do you obtain the oxy-muriatic acid ? Mrs. B. By distilling liquid muriatic acid over ox- yd of manganese, which supplies the acid with the additional oxygen. One part of the acid being put into a retort, with too parts of the oxyd of manga- nese, and the heat of a lamp applied, the gas is soon disengaged, and may be received over water, as it it but sparingly absorbed by it. I have colleded some in this jar— Caroline. It is not invisible, like the generality of gasses; for it is of a yellowish colour. Mrs. B. The muriatic acid extinguishes flame, whilst, on the contrary, the oxy-muriatic makes the flame larger, and gives it a dark red colour. Can you account for this difference in the two acids F Emily, Yes, I think so ; the muriatic acid cannot be decomposed, and therefore will not supply the flame with the oxygen necessary for its support; but when this acid is farther oxygenated it will part with 265 hs additional quantity of oxygen, and in this way support combustion. Airs. B. That is exadly the case ; indeed the ox- ygen, added to the muriatic acid, adheres so slightly to it, that it is separated by mere exposure to the sun!s rays. This acid is decomposed also by com- bustible bodies, many of which it burns, and adual- ly inflames, without any previous'increase of tempe- rature. Caroline. That is extraordinary, indeed! I hope you mean to indulge us with some of these experi- ments ? i Mrs. B. I have prepared several glass jars of oxy- muriatic acid gas, for that purpose. In the first we shall introduce some Dutch gold leaf.—Do you ob- serve that it takes fire 2 Emily. Yes, indeed it does—how wonderful it is} it became immediately red hot, but was soon smo- thered in a thick vapour. Caroline. Good heavens 1 what a disagreeable smell. Mrs. B. We shall try the same experiment with phosphorus in another jar of this acid.—You had better keep your handkerchief to your nose when I o- pen k—now let us drop into it this little piece of phosphorus— Caroline. It burns really : and almost as brilliantly as in oxygen gas ! But what is most extraordinary, these combustions take place without the metal or phosphorus being previously lighted, or even in the least heated. Airs. B. All these curious effeds are owing to the very great facility with which this acid yields oxygen to such bodies as are strongly disposed to combine with it. It appears extraordinary indeed to see bo- dies, and metals in particular, melted down and in- flamed, by a gas, without any increase of tempera- ture, either of the gas or of the" combustible. The phenomenon, 'however, is, ycu see, well account- ed for. -66 Emily. Why did you burn a piece of Dutch gold leaf "rather than a piece of any other metal ? Airs. B. Because, in the first place, it is a compo- sition of metals consisting chiefly of copper, which burns readily; and I use a thin metallic leaf in pre- ference to a lump of metal, because it offers to the adion of the gas but a small quantity of matter un- der a large surface.—Filings, or shavings, would an- swer the purpose nearly as well; but a lump of me- tal, though the surface would oxydate with great ra- pidity, would not take fire. Pure gold is not infla- med by oxy-muriatic acid gas, but it is rapidly oxyda- ted, and dissolved by it; indeed, this acid is the on. ly one that will dissolve gold. Emily. This, I suppose, is what is commonly cal- led aqua region which you know, is the only thing that will ad upon gold. Mrs. B. That is not exadly the case either ; for aqua regia is composed of a mixture of muriatic and nitric acid.—But, in fad, the result of this mixture is nothing more than oxy-muriatic acid, as the muri- atic acid oxygenates itself at the expense of the nitric ; this mixture, therefore, thoilgh it bears the name of nitro muriatic acid, ads on gold merely in virtue of the oxy-muriatic acid which it contains. Sulphur, volatile oils, and many other substances, will burn in the same manner in oxy-muriatic acid , gas; but I have not prepared a sufficient quantity of it, to shew you the combustion of all these bodies. Caroline. Yet there are several jars of the gas re- maining." Mrs. B. We must reserve these for other experi- ments. The oxy muriatic acid does not, like other acids, redden the blue vegetable colours; but it to- . tally destroys any colour, and turns all vegetables , perfedly white. Let us colled some vegetable sub- • stances to put into this glass which is full of gas. Emily. Here is a sprig of myrtle— Caroline. And here some coloured paper— Mrs. B. We shall also put in this piece of coque- lieot ribbon, and a rose— 267 , Emily. Their colours begin to fade immediately I But how does the-gas produce this effed ? Airs. B. The oxygen combines with the colour- ing matter of these substances, and destroys it; that is to say, destroys the property which these colours had of refleding only one kind of rays, and renders them capable of refleding them all, which, you know, will make them appear white. Old prints may be cleaned by this acid, for the paper will be whitened without injuring the impression, as printer's ink is made of materials (oil and lamp black) which are not aded upon by acids. This property of the oxy-muriatic acid has lately been employed in manufadories in a variety of bleaching processes; but for these purposes the gas must be dissolved in water, as the acid is thus ren- dered much milder and less powerful in its effeds ; for, in a gaseous state, it would destroy the texture, as well as the colour, of the substance submitted to its adion. Caroline. Look at the things which we put into the gas; they have now entirely lost their colour ! Mrs. B. The effed of the acid is almost comple- ted ; and, if we were to examine the quantity that remains, we should find it consist chiefly of muriatic acid. The oxy-muriatic acid has been used to purify the air in fever hospitals and prisons, as it burns and de- stroys putrid effluvia of every kind. Tfre infedicn of the small pox is likewise destroyed by this gas, and matter that has been submitted to its influence will no longer generate that disorder. Caroline. Indeed, I think the remedy must he nearly as bad as the disease; the oxy-muriatic acid has such a dreadful suffocating smell. Airs. B. It is certainly extremely otTensive ; but, by keeping the mouth shut, and wetting the nostrils with liquid ammonia, in order to neutralize the va- pour as it reaches the nose, its prejudicial effeds mav be in some degree prevented. At any rate, however* 268 this mode of disinfedion can hardly be used in place* that are inhabited. And as the vapour of nitric acid, which is scarcely less efficacious for this purpose, is not at all prejudicial, it is usually preferred on such occasions. Amongst the compound salts formed by muriatic acid, the muriat of soda, or common salt, is the most interesting. The uses and properties of this ;salt are too well known to require much comment. Besides the pleasant flavour it imparts to the food, it is very wholesome, when not used to excess, as it greatly assists the process of digestion. Sea-water is the great source from which muriat of soda is extraded by evaporation. But it is also found in large solid masses in the bowels of the earth, in England, and in many other parts of the world. Emily. I thought that salts, when solid, were al- ways in a state of- crystals; but the common table salt is in the form of a coarse white powder. ' Airs. B. Crystallization depends, as you may re- colled, on the slow and regular reunion of particles dissolved in a fluid; common sea salt is only in a state of imperfed crystallization, because the process by which it is prepared is not favourable to the for- mation of regular crystals. But, if you melt it, and afterwards evaporate the water slowly, you will ob- tain a regular crystallization. Muriat of ammonia is another combination of this acid, which we have already mentioned as the prin- cipal source from which ammonia is derived- I can at once shew you the formation of this salt by the immediate combination of muriatic acid with ammonia.—These two glass jars contain, the one mu- riatic acid gas, the other ammoniacal gas, both pf which are perfedly invisible—now, if 1 mix them together, you see they immediately form an opaque white cloud, like smoke. If a thermometer were placed in the jar in which these gasses are mixed, you would perceive that some heat is at the same * 'time produced. *J9 Emily. The effeds of chymical combinations are indeed, wonderful—how extraordinary it is that two invisible bodies should become visible by their union. Mrs. B. This strikes you with wonder because it is a phenomenon which nature seldom exhibits to our view; but the most common of her operations are as wonderful, and it is their frequency only that prevents our regarding them with equal admiration. What would be more surprising for instance, than combustion, were it not rendered so familiar by cus- tom ? Emily. That is true.—But pray, Mrs. B. is this white cloud the salt that produces ammonia ? How* different it is from the solid muriat of ammonia which you once shewed us ! Mrs. B. It is the same substance which first ap- pears in the state of vapour, but will soon be con- densed, by cooling against the sides of the jar, in the form of very minute crystals. We may now proceed to the oxy-muriats. In this class of salts the oxy-muriat of potash is the most wor- thy of our attention, for its striking properties. The .acid, in this state of combination, contains a still greater proportion of oxygen than when alone. Caroline. But how can the oxy-muriatic acid ac- quire an increase of oxygen by combining with pot- ash ? Airs. B. It does- not really acquire an additional- quantity of oxygen, but it loses some of the muriatic acid, which produces the same effed, as the acid that remains is proportionably super-oxygenated. If this salt be mixed, and merely rubbed together with sulphur, phosphorus, charcoal, or indeed any other combustible, it explodes strongly. Caroline. Like gunpowder, I suppose, it is sudden- ly convened into elastic fluids ? Mrs. B. Yes ; but with this remarkable differ- ence, that no increase of temperature, any further than is produced by the gentle fridion, Is required in this instance. Can you tell me what gasses a^e z2 270 generated by the detonation of this salt with char- coal ? Emily. Let me consider......The oxy muri- atic acid parts with its excess of oxygen to the char- coal, by which means it is converted into muriatic acid gas ; whilst the charcoal, being burnt by the oxygen, is changed to carbonic acid gas—What be- comes of the potash I cannot tell. Airs. B. That is a fixed produd which remains in the vessel. Caroline. But since the potash does not enter into the new combinations, I do not understand of what use it is in this operation. Would not the oxy-mu- riatic acid and the charcoal produce the same effed without it ? , Mrs. B. No ; because there would not be that ,;.' very great concentration of oxygen which the com- .'■ binatien with the potash produces, as I have just ex- •;.! plained. P I mean to shew you this experiment, but T would advise you not to repeat it alone; for if care be not taken to mix only very small quantities at a time, the detonation will be extremely violent, and may be attended with dangerous effeds. You see I mix an . , exceedingly small quantity of the salt with a little powdered charcoal, in this Wedgwood mortar, and rub them together with the pestle— Caroline. Heavens ! How can such a loud explo- .. sion be produced by so small a quantity of matter ?' ^ Mrs. B- Yo« must consider that an extremely * small quantity of solicL substance may produce a very