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Conversations on Chemistry, V. 1-2
by Jane Marcet
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[Transcriber's Note:

DO NOT TRY THIS AT HOME.

This e-text comes in three different forms: unicode (UTF-8), Latin-1 and ascii-7. Use the one that works best on your text reader.

—If "oe" displays as a single character, and apostrophes and quotation marks are "curly" or angled, you have the utf-8 version (best). If any part of this paragraph displays as garbage, try changing your text reader's "character set" or "file encoding". If that doesn't work, proceed to: —In the Latin-1 version, "oe" is two letters, but the word "aeriform" is usually written with dieresis (dots) over the "e", and "ae" is a single letter. Apostrophes and quotation marks will be straight ("typewriter" form). Again, if you see any garbage in this paragraph and can't get it to display properly, use: —The ASCII-7 or rock-bottom version. All necessary text will still be there; it just won't be as pretty.

The full caption of each Plate is given after its first mention in the text—generally a few pages before the Plate's physical appearance, as specified in the caption.

Many terms used in this book are different from today's standard terminology. Note in particular:

oxy-muriatic acid = the element chlorine phosphat of lime = calcium diphosphate or the element calcium glucium = the element beryllium

muriatic acid = hydrochloric acid muriat of lime = calcium chloride oxymuriate of potash = potassium chlorate carbonic acid = carbon dioxide

Further details and more examples are at the end of the e-text.

Each Volume had its own table of contents. They have been merged for this e-text, but the Vol. II title page was retained. Some Conversations were renumbered between the 4th and 5th edition, resulting in the apparent disappearance of Conversations XI and XII.

Typographical errors are listed at the end of the text.]

* * * * * * * * * * * * * *

CONVERSATIONS ON CHEMISTRY;

In Which The Elements Of That Science Are Familiarly Explained And Illustrated By Experiments.

IN TWO VOLUMES.

The Fifth Edition, revised, corrected, and considerably enlarged.

VOL. I. ON SIMPLE BODIES.

London: Printed For Longman, Hurst, Rees, Orme, and Brown, Paternoster-Row. 1817.



Printed by A. Strahan, Printers-Street, London.



ADVERTISEMENT.

The Author, in this fifth edition, has endeavoured to give an account of the principal discoveries which have been made within the last four years in Chemical Science, and of the various important applications, such as the gas-lights, and the miner's-lamp, to which they have given rise. But in regard to doctrines or principles, the work has undergone no material alteration.

London, July, 1817.



PREFACE.

In venturing to offer to the public, and more particularly to the female sex, an Introduction to Chemistry, the author, herself a woman, conceives that some explanation may be required; and she feels it the more necessary to apologise for the present undertaking, as her knowledge of the subject is but recent, and as she can have no real claims to the title of chemist.

On attending for the first time experimental lectures, the author found it almost impossible to derive any clear or satisfactory information from the rapid demonstrations which are usually, and perhaps necessarily, crowded into popular courses of this kind. But frequent opportunities having afterwards occurred of conversing with a friend on the subject of chemistry, and of repeating a variety of experiments, she became better acquainted with the principles of that science, and began to feel highly interested in its pursuit. It was then that she perceived, in attending the excellent lectures delivered at the Royal Institution, by the present Professor of Chemistry, the great advantage which her previous knowledge of the subject, slight as it was, gave her over others who had not enjoyed the same means of private instruction. Every fact or experiment attracted her attention, and served to explain some theory to which she was not a total stranger; and she had the gratification to find that the numerous and elegant illustrations, for which that school is so much distinguished, seldom failed to produce on her mind the effect for which they were intended.

Hence it was natural to infer, that familiar conversation was, in studies of this kind, a most useful auxiliary source of information; and more especially to the female sex, whose education is seldom calculated to prepare their minds for abstract ideas, or scientific language.

As, however, there are but few women who have access to this mode of instruction; and as the author was not acquainted with any book that could prove a substitute for it, she thought that it might be useful for beginners, as well as satisfactory to herself, to trace the steps by which she had acquired her little stock of chemical knowledge, and to record, in the form of dialogue, those ideas which she had first derived from conversation.

But to do this with sufficient method, and to fix upon a mode of arrangement, was an object of some difficulty. After much hesitation, and a degree of embarrassment, which, probably, the most competent chemical writers have often felt in common with the most superficial, a mode of division was adopted, which, though the most natural, does not always admit of being strictly pursued—it is that of treating first of the simplest bodies, and then gradually rising to the most intricate compounds.

It is not the author's intention to enter into a minute vindication of this plan. But whatever may be its advantages or inconveniences, the method adopted in this work is such, that a young pupil, who should occasionally recur to it, with a view to procure information on particular subjects, might often find it obscure or unintelligible; for its various parts are so connected with each other as to form an uninterrupted chain of facts and reasonings, which will appear sufficiently clear and consistent to those only who may have patience to go through the whole work, or have previously devoted some attention to the subject.

It will, no doubt, be observed, that in the course of these Conversations, remarks are often introduced, which appear much too acute for the young pupils, by whom they are supposed to be made. Of this fault the author is fully aware. But, in order to avoid it, it would have been necessary either to omit a variety of useful illustrations, or to submit to such minute explanations and frequent repetitions, as would have rendered the work tedious, and therefore less suited to its intended purpose.

In writing these pages, the author was more than once checked in her progress by the apprehension that such an attempt might be considered by some, either as unsuited to the ordinary pursuits of her sex, or ill-justified by her own recent and imperfect knowledge of the subject. But, on the one hand, she felt encouraged by the establishment of those public institutions, open to both sexes, for the dissemination of philosophical knowledge, which clearly prove that the general opinion no longer excludes women from an acquaintance with the elements of science; and, on the other, she flattered herself that whilst the impressions made upon her mind, by the wonders of Nature, studied in this new point of view, were still fresh and strong, she might perhaps succeed the better in communicating to others the sentiments she herself experienced.

The reader will soon perceive, in perusing this work, that he is often supposed to have previously acquired some slight knowledge of natural philosophy, a circumstance, indeed, which appears very desirable. The author's original intention was to commence this work by a small tract, explaining, on a plan analogous to this, the most essential rudiments of that science. This idea she has since abandoned; but the manuscript was ready, and might, perhaps, have been printed at some future period, had not an elementary work of a similar description, under the tide of "Scientific Dialogues," been pointed out to her, which, on a rapid perusal, she thought very ingenious, and well calculated to answer its intended object.



Contents Of The First Volume.

ON SIMPLE BODIES.

CONVERSATION I. Page

ON THE GENERAL PRINCIPLES OF CHEMISTRY. 1

Connexion between Chemistry and Natural Philosophy. —Improved State of modern Chemistry. —Its use in the Arts. —The general Objects of Chemistry. —Definition of Elementary Bodies. —Definition of Decomposition. —Integrant and Constituent Particles. —Distinction between Simple and Compound Bodies. —Classification of Simple Bodies. —Of Chemical Affinity, or Attraction of Composition. —Examples of Composition and Decomposition.

CONVERSATION II.

ON LIGHT AND HEAT. 26

Light and Heat capable of being separated. —Dr. Herschel's Experiments. —Phosphorescence. —Of Caloric. —Its two Modifications. —Free Caloric. —Of the three different States of Bodies, solid, fluid, and aeriform. —Dilatation of solid Bodies. —Pyrometer. —Dilatation of Fluids. —Thermometer. —Dilatation of Elastic Fluids. —Air Thermometer. —Equal Diffusion of Caloric. —Cold a Negative Quality. —Professor Prevost's Theory of the Radiation of Heat. —Professor Pictet's Experiments on the Reflexion of Heat. —Mr. Leslie's Experiments on the Radiation of Heat.

CONVERSATION III.

CONTINUATION OF THE SUBJECT. 70

Of the different Power of Bodies to conduct Heat. —Attempt to account for this Power. —Count Rumford's Theory of the non-conducting Power of Fluids. —Phenomena of Boiling. —Of Solution in general. —Solvent Power of Water. —Difference between Solution and Mixture. —Solvent Power of Caloric. —Of Clouds, Rain, Dr. Wells' theory of Dew, Evaporation, &c. —Influence of Atmospherical Pressure on Evaporation. —Ignition.

CONVERSATION IV.

ON COMBINED CALORIC, COMPREHENDING SPECIFIC HEAT AND LATENT HEAT. 122

Of Specific Heat. —Of the different Capacities of Bodies for Heat. —Specific Heat not perceptible by the Senses. —How to be ascertained. —Of Latent Heat. —Distinction between Latent and Specific Heat. —Phenomena attending the Melting of Ice and the Formation of Vapour. —Phenomena attending the Formation of Ice, and the Condensation of Elastic Fluids. —Instances of Condensation, and consequent Disengagement of Heat, produced by Mixtures, by the Slaking of Lime. —General Remarks on Latent Heat. —Explanation of the Phenomena of Ether boiling, and Water freezing, at the same Temperature. —Of the Production of Cold by Evaporation. —Calorimeter. —Meteorological Remarks.

CONVERSATION V.

ON THE CHEMICAL AGENCIES OF ELECTRICITY. 160

Of Positive and Negative Electricity. —Galvani's Discoveries. —Voltaic Battery. —Electrical Machine. —Theory of Voltaic Excitement.

CONVERSATION VI.

ON OXYGEN AND NITROGEN. 181

The Atmosphere composed of Oxygen and Nitrogen in the State of Gas. —Definition of Gas. —Distinction between Gas and Vapour. —Oxygen essential to Combustion and Respiration. —Decomposition of the Atmosphere by Combustion. —Nitrogen Gas obtained by this Process. —Of Oxygenation in general. —Of the Oxydation of Metals. —Oxygen Gas obtained from Oxyd of Manganese. —Description of a Water-Bath for collecting and preserving Gases. —Combustion of Iron Wire in Oxygen Gas. —Fixed and volatile Products of Combustion. —Patent Lamps. —Decomposition of the Atmosphere by Respiration. —Recomposition of the Atmosphere.

CONVERSATION VII.

ON HYDROGEN. 214

Of Hydrogen. —Of the Formation of Water by the Combustion of Hydrogen. —Of the Decomposition of Water. —Detonation of Hydrogen Gas. —Description of Lavoisier's Apparatus for the formation of Water. —Hydrogen Gas essential to the Production of Flame. —Musical Tones produced by the Combustion of Hydrogen Gas within a Glass Tube. —Combustion of Candles explained. —Gas lights. —Detonation of Hydrogen Gas in Soap Bubbles. —Air Balloons. —Meteorological Phenomena ascribed to Hydrogen Gas. —Miner's Lamp.

[Transcriber's Note: The final two pages of the Table of Contents for Volume I were missing; everything after "Decomposition of Water" was supplied from earlier and later editions, compared against the body text. The section marked "Diamond" (Conv. IX) was called "Diamond is Carbon(e) in a state of perfect purity" in the 4th edn., "Diamond" alone in later editions.]

CONVERSATION VIII.

ON SULPHUR AND PHOSPHORUS. 256

Natural History of Sulphur. —Sublimation. —Alembic. —Combustion of Sulphur in Atmospheric Air. —Of Acidification in general. —Nomenclature of the Acids. —Combustion of Sulphur in Oxygen Gas. —Sulphuric Acid. —Sulphurous Acid. —Decomposition of Sulphur. —Sulphurated Hydrogen Gas. —Harrogate, or Hydro-sulphurated Waters. —Phosphorus. —History of its Discovery. —Its Combustion in Oxygen Gas. —Phosphoric Acid. —Phosphorus Acid. —Eudiometer. —Combination of Phosphorus with Sulphur. —Phosphorated Hydrogen Gas. —Nomenclature of Binary Compounds. —Phosphoret of Lime burning under Water.

CONVERSATION IX.

ON CARBON. 282

Method of obtaining pure Charcoal. —Method of making common Charcoal. —Pure Carbon not to be obtained by Art. —Diamond. —Properties of Carbon. —Combustion of Carbon. —Production of Carbonic Acid Gas. —Carbon susceptible of only one Degree of Acidification. —Gaseous Oxyd of Carbon. —Of Seltzer Water and other Mineral Waters. —Effervescence. —Decomposition of Water by Carbon. —Of Fixed and Essential Oils. —Of the Combustion of Lamps and Candles. —Vegetable Acids. —Of the Power of Carbon to revive Metals.

CONVERSATION X.

ON METALS. 314

Natural History of Metals. —Of Roasting, Smelting, &c. —Oxydation of metals by the Atmosphere. —Change of Colours produced by different degrees of Oxydation. —Combustion of Metals. —Perfect Metals burnt by Electricity only. —Some Metals revived by Carbon and other Combustibles. —Perfect Metals revived by Heat alone. —Of the Oxydation of certain Metals by the Decomposition of Water. Power of Acids to promote this Effect. —Oxydation of Metals by Acids. —Metallic Neutral Salts. —Previous oxydation of the Metal requisite. —Crystallisation. —Solution distinguished from Dissolution. —Five metals susceptible of acidification. —Meteoric Stones. —Alloys, Soldering, Plating, &c. —Of Arsenic, and of the caustic Effects of Oxygen. —Of Verdigris, Sympathetic Ink, &c. —Of the new Metals discovered by Sir H. Davy.

Contents Of The Second Volume.

ON COMPOUND BODIES.

CONVERSATION XIII. Page

ON THE ATTRACTION OF COMPOSITION. 1

Of the laws which regulate the Phenomena of the Attraction of Composition. —1. It takes place only between Bodies of a different Nature. —2. Between the most minute Particles only. —3. Between 2, 3, 4, or more Bodies. —Of Compound or Neutral Salts. —4. Produces a Change of Temperature. —5. The Properties which characterise Bodies in their separate State, destroyed by Combination. —6. The Force of Attraction estimated by that which is required by the Separation of the Constituents. —7. Bodies have amongst themselves different Degrees of Attraction. —Of simple elective and double elective Attractions. —Of quiescent and divellent Forces. —Law of definite Proportions. —Decomposition of Salts by Voltaic Electricity.

CONVERSATION XIV.

ON ALKALIES. 19

Of the Composition and general Properties of the Alkalies. —Of Potash. —Manner of preparing it. —Pearlash. —Soap. —Carbonat of Potash. —Chemical Nomenclature. —Solution of Potash. —Of Glass. —Of Nitrat of Potash or Saltpetre. —Effect of Alkalies on Vegetable Colours. —Of Soda. —Of Ammonia or Volatile Alkali. —Muriat of Ammonia. —Ammoniacal Gas. —Composition of Ammonia. —Hartshorn and Sal Volatile. —Combustion of Ammoniacal Gas.

CONVERSATION XV.

ON EARTHS. 44

Composition of the Earths. —Of their Incombustibility. —Form the Basis of all Minerals. —Their Alkaline Properties. —Silex; its Properties and Uses in the Arts. —Alumine; its Uses in Pottery, &c. —Alkaline Earths. —Barytes. —Lime; its extensive chemical Properties and Uses in the Arts. —Magnesia. —Strontian.

CONVERSATION XVI.

ON ACIDS. 69

Nomenclature of the Acids. —Of the Classification of Acids. —1st Class —Acids of simple and known Radicals, or Mineral Acids. —2d Class —Acids of double Radicals, or Vegetable Acids. —3d Class —Acids of triple Radicals or Animal Acids. —Of the Decomposition of Acids of the 1st Class by Combustible bodies.

CONVERSATION XVII.

OF THE SULPHURIC AND PHOSPHORIC ACIDS: OR, THE COMBINATIONS OF OXYGEN WITH SULPHUR AND WITH PHOSPHORUS; AND OF THE SULPHATS AND PHOSPHATS. 80

Of the Sulphuric Acid. —Combustion of Animal or Vegetable Bodies by this Acid. —Method of preparing it. — The Sulphurous Acid obtained in the Form of Gas. —May be obtained from Sulphuric Acid. —May be reduced to Sulphur. —Is absorbable by Water. —Destroys Vegetable Colours. —Oxyd of Sulphur. —Of Salts in general. —Sulphats. —Sulphat of Potash, or Sal Polychrest. —Cold produced by the melting of Salts. —Sulphat of Soda, or Glauber's Salt. —Heat evolved during the Formation of Salts. —Crystallisation of Salts. —Water of Crystallisation. —Efflorescence and Deliquescence of Salts. —Sulphat of Lime, Gypsum or Plaister of Paris. —Sulphat of Magnesia. —Sulphat of Alumine, or Alum. —Sulphat of Iron. —Of Ink. —Of the Phosphoric and Phosphorous Acids. —Phosphorus obtained from Bones. —Phosphat of Lime.

CONVERSATION XVIII.

OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATION OF OXYGEN WITH NITROGEN AND WITH CARBON; AND OF THE NITRATS AND CARBONATS. 100

Nitrogen susceptible of various Degrees of Acidification. —Of the Nitric Acid. —Its Nature and Composition discovered by Mr. Cavendish. —Obtained from Nitrat of Potash. —Aqua Fortis. —Nitric Acid may be converted into Nitrous Acid. —Nitric Oxyd Gas. —Its Conversion into Nitrous Acid Gas. —Used as an Eudiometrical Test. —Gaseous Oxyd of Nitrogen, or exhilarating Gas, obtained from Nitrat of Ammonia. —Its singular Effects on being respired. —Nitrats. —Of Nitrat of Potash, Nitre or Saltpetre. —Of Gunpowder. —Causes of Detonation. —Decomposition of Nitre. —Deflagration. —Nitrat of Ammonia. —Nitrat of Silver. —Of the Carbonic Acid. —Formed by the Combustion of Carbon. —Constitutes a component Part of the Atmosphere. —Exhaled in some Caverns. —Grotto del Cane. —Great Weight of this Gas. —Produced from calcareous Stones by Sulphuric Acid. —Deleterious Effects of this Gas when respired. —Sources which keep up a Supply of this Gas in the Atmosphere. —Its Effects on Vegetation. —Of the Carbonats of Lime; Marble, Chalk, Shells, Spars, and calcareous Stones.

CONVERSATION XIX.

ON THE BORACIC, FLUORIC, MURIATIC, AND OXYGENATED MURIATIC ACIDS; AND ON MURIATS. 131

On the Boracic Acid. —Its Decomposition by Sir H. Davy. —Its Basis Boracium. —Its Recomposition. —Its Uses in the Arts. —Borax or Borat of Soda. —Of the Fluoric Acid. —Obtained from Fluor; corrodes Siliceous Earth; its supposed Composition. —Fluorine; its supposed Basis. —Of the Muriatic Acid. —Obtained from Muriats. —Its gaseous Form. —Is absorbable by Water. —Its Decomposition. —Is susceptible of a stronger Degree of Oxygenation. —Oxygenated Muriatic Acid. —Its gaseous Form and other Properties. —Combustion of Bodies in this Gas. —It dissolves Gold. —Composition of Aqua Regia. —Oxygenated Muriatic Acid destroys all Colours. —Sir H. Davy's Theory of the Nature of Muriatic and Oxymuriatic Acid. —Chlorine. —Used for Bleaching and for Fumigations. —Its offensive Smell, &c. —Muriats. —Muriat of Soda, or common Salt. —Muriat of Ammonia. —Oxygenated Muriat of Potash. —Detonates with Sulphur, Phosphorus, &c. —Experiment of burning Phosphorus under Water by means of this Salt and of Sulphuric Acid.

CONVERSATION XX.

ON THE NATURE AND COMPOSITION OF VEGETABLES. 162

Of organised Bodies. —Of the Functions of Vegetables. —Of the Elements of Vegetables. —Of the Materials of Vegetables. —Analysis of Vegetables. —Of Sap. —Mucilage, or Gum. —Sugar. —Manna, and Honey. —Gluten. —Vegetable Oils. —Fixed Oils, Linseed, Nut, and Olive Oils. —Volatile Oils, forming Essences and Perfumes. —Camphor. —Resins and Varnishes. —Pitch, Tar, Copal, Mastic, &c. —Gum Resins. —Myrrh, Assafoetida, &c. —Caoutchouc, or Gum Elastic. —Extractive colouring Matter; its Use in the Arts of Dyeing and Painting. —Tannin; its Use in the Art of preparing Leather. —Woody Fibre. —Vegetable Acids. —The Alkalies and Salts contained in Vegetables.

CONVERSATION XXI.

ON THE DECOMPOSITION OF VEGETABLES. 202

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. —Analysis of Wine by Distillation. —Of Brandy, Rum, Arrack, Gin, &c. —Tartrit of Potash, or Cream of Tartar. —Liqueurs. —Chemical Properties of Alcohol. —Its Combustion. —Of Ether. —Of the Acetous Fermentation, the Product of which is Vinegar. —Fermentation of Bread. —Of the Putrid Fermentation, which reduces Vegetables to their Elements. —Spontaneous Succession of these Fermentations. —Of Vegetables said to be petrified. —Of Bitumens: Naphtha, Asphaltum, Jet, Coal, Succin, or Yellow Amber. —Of Fossil Wood, Peat, and Turf.

CONVERSATION XXII.

HISTORY OF VEGETATION. 243

Connexion between the Vegetable and Animal Kingdoms. —Of Manures. —Of Agriculture. —Inexhaustible Sources of Materials for the Purposes of Agriculture. —Of sowing Seed. —Germination of the Seed. —Function of the Leaves of Plants. —Effects of Light and Air on Vegetation. —Effects of Water on Vegetation. —Effects of Vegetation on the Atmosphere. —Formation of Vegetable Materials by the Organs of Plants. —Vegetable Heat. —Of the Organs of Plants. —Of the Bark, consisting of Epidermis, Parenchyma, and Cortical Layers. —Of Alburnum, or Wood. —Leaves, Flowers, and Seeds. —Effects of the Season on Vegetation. —Vegetation of Evergreens in Winter.

CONVERSATION XXIII.

ON THE COMPOSITION OF ANIMALS. 276

Elements of Animals. —Of the principal Materials of Animals, viz. —Gelatine, Albumen, Fibrine, Mucus. —Of Animal Acids. —Of Animal Colours, Prussian Blue, Carmine, and Ivory Black.

CONVERSATION XXIV.

ON THE ANIMAL ECONOMY. 297

Of the principal Animal Organs. —Of Bones, Teeth, Horns, Ligaments, and Cartilage. —Of the Muscles, constituting the Organs of Motion. —Of the Vascular System, for the Conveyance of Fluids. —Of the Glands, for the Secretion of Fluids. —Of the Nerves, constituting the Organs of Sensation. —Of the Cellular Substance which connects the several Organs. —Of the Skin.

CONVERSATION XXV.

ON ANIMALISATION, NUTRITION, AND RESPIRATION. 314

Digestion. —Solvent Power of the Gastric Juice. —Formation of a Chyle. —Its Assimilation, or Conversion into Blood. —Of Respiration. —Mechanical Process of Respiration. —Chemical Process of Respiration. —Of the Circulation of the Blood. —Of the Functions of the Arteries, the Veins, and the Heart. —Of the Lungs. —Effects of Respiration on the Blood.

CONVERSATION XXVI.

ON ANIMAL HEAT; AND OF VARIOUS ANIMAL PRODUCTS. 336

Of the Analogy of Combustion and Respiration. —Animal Heat evolved in the Lungs. —Animal Heat evolved in the Circulation. —Heat produced by Fever. —Perspiration. —Heat produced by Exercise. —Equal Temperature of Animals at all Seasons. —Power of the Animal Body to resist the Effects of Heat. —Cold produced by Perspiration. —Respiration of Fish and of Birds. —Effects of Respiration on Muscular Strength. —Of several Animal Products, viz. Milk, Butter, and Cheese; Spermaceti; Ambergris; Wax; Lac; Silk; Musk; Civet; Castor. —Of the putrid Fermentation. —Conclusion.



CONVERSATIONS

ON

CHEMISTRY.



CONVERSATION I.

ON THE GENERAL PRINCIPLES OF CHEMISTRY.

MRS. B.

As you have 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 imperfect idea of bodies from the study of the general laws by which they are governed, if we remain totally ignorant of their intimate nature.

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 the sciences which exhibit nature on a grand scale, to those that are confined to the minutiae of petty details. Can the studies which we have lately pursued, the general properties of matter, or the revolutions of the heavenly bodies, be compared to the mixing up of a few insignificant drugs? I grant, however, there may be entertaining experiments in chemistry, and should not dislike to try some of them: the distilling, for instance, of lavender, or rose water . . . . . .

MRS. B.

I rather imagine, my dear Caroline, that your want of taste for chemistry proceeds from the very limited idea you entertain of its object. You confine the chemist's laboratory to the narrow precincts of the apothecary's and perfumer's shops, whilst it is subservient to an immense 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 employed in chemical operations. You are surprised, Caroline, but I assure you that the most wonderful and the most interesting phenomena of nature are almost all of them produced by chemical powers. What Bergman, in the introduction to his history of chemistry, has said of this science, will give you a more just and enlarged idea of it. The knowledge of nature may be divided, he observes, into three periods. The first was that in which the attention of men was occupied in learning the external forms and characters of objects, and this is called Natural History. In the second, they considered the effects of bodies acting on each other by their mechanical power, as their weight and motion, and this constitutes the science of Natural Philosophy. The third period is that in which the properties and mutual action of the elementary parts of bodies was investigated. This last is the science of CHEMISTRY, and I have no doubt you will soon agree with me in thinking it the most interesting.

You may easily conceive, therefore, that without entering 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 instruction.

CAROLINE.

If this is the case, I have certainly been much mistaken in the notion I had formed of chemistry. I own that I thought it was chiefly confined to the knowledge and preparation of medicines.

MRS. B.

That is only a branch of chemistry which is called Pharmacy; and, though the study of it is certainly of great importance to the world at large, it belongs exclusively to professional men, and is therefore the last that I should advise you to pursue.

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 Alchemists, 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 beautiful science, to which art is entirely subservient. It is true, however, that we are indebted to the alchemists for many very useful discoveries, which sprung from their fruitless attempts to make gold, and which, undoubtedly, have proved of infinitely greater advantage to mankind than all their chimerical pursuits.

The modern chemists, instead of directing their ambition to the vain attempt of producing any of the original substances in nature, rather aim at analysing and imitating her operations, and have sometimes succeeded in forming combinations, or effecting decompositions, no instances of which occur in the chemistry of Nature. They have little reason to regret their inability to make gold, whilst, by their 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?

MRS. B.

There are many ways by which labour may be rendered more easy, independently of mechanics; but even the machine, the most wonderful in its effects, the Steam-engine, cannot be understood without the assistance of chemistry. In agriculture, a chemical knowledge of the nature of soils, and of vegetation, is highly useful; and, in those arts which relate to the comforts and conveniences 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 proved so beneficial to society?

MRS. B.

That would be an injudicious anticipation; for you would not comprehend the nature of such discoveries and useful applications, as well as you will do hereafter. Without a due regard to method, we cannot expect to make any progress in chemistry. I wish to direct your observations chiefly to the chemical operations 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 regularly. 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 confined science, which comprehends every thing material within our sphere.

CAROLINE.

On the contrary, it must be inexhaustible; and I am a loss to conceive how any proficiency can be made in a science whose objects are so numerous.

MRS. B.

If every individual substance were formed 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 composed 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 change chemistry has undergone since it has become a regular science. Within these thirty years especially, 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 has never been decomposed, that is to say, separated into other substances; and fire, air, earth, and water, are all of them susceptible 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 division. 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 ingredients of which it is made, the flour, the yeast, the salt, and the water, it would be very different from cutting or crumbling the loaf into pieces.

EMILY.

I understand you now very well. To decompose a body is to separate from each other the various elementary substances of which it consists.

CAROLINE.

But flour, water, and other materials of bread, according to our 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 body; 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 are called 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 consists of only one kind of substance, though it may be divided 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 substance?

MRS. B.

No, my dear; they are every one of them susceptible of being separated into various simple bodies. Instead of four, chemists now reckon upwards of forty elementary substances. The existence of most of these is established by the clearest experiments; but, in regard to a few of them, particularly the most subtle agents of nature, heat, light, and electricity, there is yet much uncertainty, and I can only give you the opinion which seems most probably deduced from the latest discoveries. After I have given you a list of the elementary bodies, classed according to their properties, we shall proceed to examine each of them separately, and then consider them in their combinations with each other.

Excepting the more general agents of nature, heat, light, and electricity, it would seem that the simple form of bodies is that of a metal.

CAROLINE.

You astonish me! I thought the metals were only one class of minerals, and that there were besides, earths, stones, rocks, acids, alkalies, vapours, fluids, and the whole of the animal and vegetable kingdoms.

MRS. B.

You have made a tolerably good enumeration, though I fear not arranged in the most scientific order. All these bodies, however, it is now strongly believed, may be ultimately resolved into metallic substances. Your surprise at this circumstance is not singular, as the decomposition of some of them, which has been but lately accomplished, has excited the wonder of the whole philosophical world.

But to return to the list of simple bodies—these being usually found in combination with oxygen, I shall class them according to their properties when so combined. This will, I think, facilitate their future investigation.

EMILY.

Pray what is oxygen?

MRS. B.

A simple body; at least one that is supposed to be so, as it has never been decomposed. It is always found united with the negative electricity. It will be one of the first of the elementary bodies whose properties I shall explain to you, and, as you will soon perceive, it is one of the most important in nature; but it would be irrelevant to enter upon this subject at present. We must now confine our attention to the enumeration and classification of the simple bodies in general. They may be arranged as follows:

CLASS I.

Comprehending the imponderable agents, viz.

HEAT or CALORIC, LIGHT, ELECTRICITY.

CLASS II.

Comprehending agents capable of uniting with inflammable bodies, and in most instances of effecting their combustion.

OXYGEN, CHLORINE, IODINE.*

[Footnote *: It has been questioned by some eminent chemists, whether these two last agents should not be classed among the inflammable bodies, as they are capable of combining with oxygen, as well as with inflammable bodies. But they seem to be more distinctly characterised by their property of supporting combustion than by any other quality.]

CLASS III.

Comprehending bodies capable of uniting with oxygen, and, forming with it various compounds. This class may be divided as follows:

DIVISION 1.

HYDROGEN, forming water.

DIVISION 2.

Bodies forming acids.

NITROGEN, forming nitric acid. SULPHUR, forming sulphuric acid. PHOSPHORUS, forming phosphoric acid. CARBON, forming carbonic acid. BORACIUM, forming boracic acid. FLUORIUM, forming fluoric acid. MURIATIUM, forming muriatic acid.

DIVISION 3.

Metallic bodies forming alkalies.

POTASSIUM, forming potash. SODIUM, forming soda. AMMONIUM, forming ammonia.

DIVISION 4.

Metallic bodies forming earths.

CALCIUM, or metal forming lime. MAGNIUM, forming magnesia. BARIUM, forming barytes. STRONTIUM, forming strontites. SILICIUM, forming silex. ALUMIUM, forming alumine. YTTRIUM, forming yttria. GLUCIUM, forming glucina. ZIRCONIUM, forming zirconi.*

[Footnote *: Of all these earths, three or four only have as yet been distinctly decomposed.]

DIVISION 5.

Metals, either naturally metallic, or yielding their oxygen to carbon or to heat alone.

Subdivision 1.

Malleable Metals.

GOLD, PLATINA, PALLADIUM, SILVER* MERCURY** TIN, COPPER, IRON, LEAD, NICKEL, ZINC.

[Footnote *: These first four metals have commonly been distinguished by the appellation of perfect or noble metals, on account of their possessing the characteristic properties of ductility, malleability, inalterability, and great specific gravity, in an eminent degree.]

[Footnote **: Mercury, in its liquid state, cannot, of course, be called a malleable metal. But when frozen, it possesses a considerable degree of malleability.]

Subdiv. 2.

Brittle Metals.

ARSENIC, BISMUTH, ANTIMONY, MANGANESE, TELLURIUM, COBALT, TUNGSTEN, MOLYBDENUM, TITANIUM, CHROME, URANIUM, COLUMBIUM or TANTALIUM, IRIDIUM, OSMIUM, RHODIUM.*

[Footnote *: These last four or five metallic bodies are placed under this class for the sake of arrangement, though some of their properties have not been yet fully investigated.]

CAROLINE.

Oh, what a formidable list! You will have much to do to explain it, Mrs. B.; for I assure you it is perfectly unintelligible to me, and I think rather perplexes than assists me.

MRS. B.

Do not let that alarm you, my dear; I hope that hereafter this classification will appear quite clear, and, so far from perplexing you, will assist you in arranging your ideas. It would be in vain to attempt forming a division that would appear perfectly clear to a beginner: for you may easily conceive that a chemical division being necessarily founded on properties with which you are almost wholly unacquainted, it is impossible that you should at once be able to understand its meaning or appreciate its utility.

But, before we proceed further, 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 Composition, consists in the peculiar tendency which bodies 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 aggregation, which you often mentioned to us, in former conversations?

MRS. B.

The attraction of cohesion exists only between particles of the same nature, whether simple or compound; thus it unites the particles of a piece of metal which is a simple substance, and likewise the particles of a loaf of bread which is a compound. The attraction of composition, on the contrary, unites and maintains, in a state of combination, 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 body: the attraction of composition that which combines the constituent particles. Is it not so?

MRS. B.

Precisely: and observe that the attraction of cohesion unites particles of a similar nature, 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 compound bodies, quite different from any of their constituents. 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 combine 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 already 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 resistance which the strong cohesion of the particles of copper opposes to their combination with it, but also to overcome 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.

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 into 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 different this compound is from either of its ingredients. 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 liquid; but when the union is completed, and the water with which the acid is diluted is evaporated, the compound 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.

[Footnote *: These crystals are more easily obtained from a mixture of sulphuric with a little nitric acid.]

CAROLINE.

How very beautiful they are, in colour, form, and transparency!

EMILY.

Nothing can be more striking than this example of chemical attraction.

MRS. B.

The term attraction has been lately introduced into chemistry as a substitute for the word affinity, to which some chemists have objected, because it originated in the vague notion that chemical combinations depended upon a certain resemblance, or relationship, between particles that are disposed to unite; and this idea is not only imperfect, but erroneous, as it is generally particles of the most dissimilar nature, that have the greatest tendency to combine.

CAROLINE.

Besides, there seems to be no advantage in using a variety of terms to express the same 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 meaning, 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 supposed), yet, as it really exists, it ought to be expressed.

MRS. B.

Well, let it be agreed that you may use the terms affinity, chemical attraction and attraction of composition, indifferently, provided you recollect that they have all the same meaning.

EMILY.

I do not conceive how bodies can be decomposed by chemical attraction. That this power should be the means of composing them, is very obvious; but that it should, at the same time, produce exactly the contrary effect, appears to me very singular.

MRS. B.

To decompose a body is, you know, to separate its constituent parts, which, as we have just observed, cannot be done by mechanical means.

EMILY.

No: because mechanical means separate only the integrant particles; they act merely against the attraction of cohesion, and only divide a compound into smaller parts.

MRS. B.

The decomposition of a body is performed by chemical powers. If you present to a body composed 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 the body is composed, A and B. If we present to it another ingredient 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 effected 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 be thrown down in 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 bluish liquid, like that contained in the glass, it will be covered with a thin coat of copper.

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, deposits or precipitates the copper on the surface of the blade.

EMILY.

But, cannot three or more substances combine together, without any of them being precipitated?

MRS. B.

That is sometimes the case; but, in general, the stronger affinity destroys the weaker; and it seldom happens that the attraction of several substances for each other is so equally balanced as to produce such complicated compounds.

CAROLINE.

But, pray, Mrs. B., what is the cause of the chemical attraction of bodies for each other? It appears to me more extraordinary or unnatural, if I may use the expression, than the attraction of cohesion, which unites particles of a similar nature.

MRS. B.

Chemical attraction may, like that of cohesion or gravitation, be one of the powers inherent in matter which, in our present state of knowledge, admits of no other satisfactory explanation than an immediate reference to a divine cause. Sir H. Davy, however, whose important discoveries have opened such improved views in chemistry, has suggested an hypothesis which may throw great light upon that science. He supposes that there are two kinds of electricity, with one or other of which all bodies are united. These we distinguish by the names of positive and negative electricity; those bodies are disposed to combine, which possess opposite electricities, as they are brought together by the attraction which these electricities have for each other. But, whether this hypothesis be altogether founded on truth or not, it is impossible to question the great influence of electricity in chemical combinations.

EMILY.

So, that we must suppose that the two electricities always attract each other, and thus compel the bodies in which they exist to combine?

CAROLINE.

And may not this be also the cause of the attraction of cohesion?

MRS. B.

No, for in particles of the same nature the same electricities must prevail, and it is only the different or opposite electric fluids that attract each other.

CAROLINE.

These electricities seem to me to be a kind of chemical spirit, which animates the particles of bodies, and draws them together.

EMILY.

If it is known, then, with which of the electricities bodies are united, it can be inferred which will, and which will not, combine together?

MRS. B.

Certainly. —I should not omit to mention, that some doubts have been entertained whether electricity be really a material agent, or whether it might not be a power inherent in bodies, similar to, or, perhaps identical with, attraction.

EMILY.

But what then would be the electric spark which is visible, and must therefore be really material?

MRS. B.

What we call the electric spark, may, Sir H. Davy says, be merely the heat and light, or fire produced by the chemical combinations with which these phenomena are always connected. We will not, however, enter more fully on this important subject at present, but reserve the principal facts which relate to it to a future conversation.

Before we part, however, I must recommend you to fix in your memory the names of the simple bodies, against our next interview.



CONVERSATION II.

ON LIGHT AND HEAT OR CALORIC.

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?

MRS. B.

Respecting the nature of light we have little more than conjectures. It is considered by most philosophers as a real substance, immediately emanating from the sun, and from all luminous bodies, from which it is projected in right lines with prodigious velocity. Light, however, being imponderable, it cannot be confined and examined by itself; and therefore it is to the effects it produces on other bodies, rather than to its immediate nature, that we must direct our attention.

The connection between light and heat is very obvious; indeed, it is such, that it is extremely difficult to examine the one independently of the other.

EMILY.

But, is it possible to separate light from heat; I thought they were only different degrees of the same thing, fire?

MRS. B.

I told you that fire was not now considered as a simple element. Whether light and heat be altogether different agents, or not, I cannot pretend to decide; but, in many cases, light may be separated from heat. The first discovery of this was made by a celebrated Swedish chemist, Scheele. Another very striking illustration of the separation of heat and light was long after pointed out by Dr. Herschell. This philosopher discovered that these two agents were emitted in the rays of the sun, and that heat was less refrangible than light; for, in separating the different coloured rays of light by a prism (as we did some time ago), he found that the greatest heat was beyond the spectrum, at a little distance from the red rays, which, you may recollect, are the least refrangible.

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 experiment 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 refracted 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.

EMILY.

Though I no longer doubt that light and heat can be separated, Dr. Herschell'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.

MRS. B.

No doubt there must be some difference in the various coloured rays. Even their chemical powers are different. The blue rays, for instance, have the greatest effect in separating oxygen from bodies, as was found by Scheele; and there exist also, as Dr. Wollaston has shown, rays more refrangible than the blue, which produce the same chemical effect, and, what is very remarkable, are invisible.

EMILY.

Do you think it possible that heat may be merely a modification of light?

MRS. B.

That is a supposition which, in the present state of natural philosophy, can neither be positively affirmed nor denied. Let us, therefore, instead of discussing theoretical points, be contented with examining what is known respecting the chemical effects of light.

Light is capable of entering into a kind of transitory union with certain substances, and this is what has been called phosphorescence. Bodies that are possessed of this property, after being exposed to the sun's rays, appear luminous in the dark. The shells of fish, the bones of land animals, marble, limestone, and a variety of combinations of earths, are more or less powerfully phosphorescent.

CAROLINE.

I remember being much surprised last summer with the phosphorescent appearance of some pieces of rotten wood, which had just been dug out of the ground; they shone so bright that I at first supposed them to be glow-worms.

EMILY.

And is not the light of a glow-worm of a phosphorescent nature?

MRS. B.

It is a very remarkable instance of phosphorescence in living animals; this property, however, is not exclusively possessed by the glow-worm. The insect called the lanthorn-fly, which is peculiar to warm climates, emits light as it flies, producing in the dark a remarkably sparkling appearance. But it is more common to see animal matter in a dead state possessed of a phosphorescent quality; sea fish is often eminently so.

EMILY.

I have heard that the sea has sometimes had the appearance of being illuminated, and that the light is supposed to proceed from the spawn of fishes floating on its surface.

MRS. B.

This light is probably owing to that or some other animal matter. Sea water has been observed to become luminous from the substance of a fresh herring having been immersed in it; and certain insects, of the Medusa kind, are known to produce similar effects.

But the strongest phosphorescence is produced by chemical compositions prepared for the purpose, the most common of which consists of oyster shells and sulphur, and is known by the name of Canton's Phosphorus.

EMILY.

I am rather surprised, Mrs. B., that you should have said so much of the light emitted by phosphorescent bodies without taking any notice of that which is produced by burning bodies.

MRS. B.

The light emitted by the latter is so intimately connected with the chemical history of combustion, that I must defer all explanation of it till we come to the examination of that process, which is one of the most interesting in chemical science.

Light is an agent capable of producing various chemical changes. It is essential to the welfare both of the animal and vegetable kingdoms; for men and plants grow pale and sickly if deprived of its salutary influence. It is likewise remarkable for its property of destroying colour, which renders it of great consequence in the process of bleaching.

EMILY.

Is it not singular that light, which in studying optics we were taught to consider as the source and origin of colours, should have also the power of destroying them?

CAROLINE.

It is a fact, however, that we every day experience; you know how it fades the colours of linens and silks.

EMILY.

Certainly. And I recollect that endive is made to grow white instead of green, by being covered up so as to exclude the light. But by what means does light produce these effects?

MRS. B.

This I cannot attempt to explain to you until you have obtained a further knowledge of chemistry. As the chemical properties of light can be accounted for only in their reference to compound bodies, it would be useless to detain you any longer on this subject; we may therefore pass on to the examination of heat, or caloric, with which we are somewhat better acquainted.

HEAT and LIGHT may be always distinguished by the different sensations they produce, Light affects the sense of sight; Caloric that of feeling; the one produces Vision, the other the sensation of Heat.

Caloric is found to exist in a variety of forms or modifications, and I think it will be best to consider it under the two following heads, viz.

1. FREE OR RADIANT CALORIC.

2. COMBINED CALORIC.

The first, FREE or RADIANT CALORIC, is also called HEAT OF TEMPERATURE; it comprehends all heat which is perceptible to the senses, and affects the thermometer.

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.

MRS. 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; this word, therefore, in the accurate language of science, 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 sun, without any reference to the sensation which they are capable of exciting.

It was in order to avoid the confusion which arose from thus confounding the cause and effect, that modern chemists adopted the new word caloric, to denote the principle which produces heat; yet they do not always, in compliance with their own language, limit the word heat to the expression of the sensation, since they still frequently employ it in reference to the other modifications of caloric which are quite independent of sensation.

CAROLINE.

But you have not yet explained to us what these other modifications of caloric are.

MRS. B.

Because you are not acquainted with the properties of free caloric, and you know that 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 subtle, that it enters and pervades all bodies whatever, forces itself between their particles, and not only separates them, but frequently drives them asunder 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 it has on bodies, therefore, is directly contrary to that of the attraction of cohesion; the one draws the particles together, the other drives them asunder.

MRS. B.

Precisely. There is a continual struggle between the attraction of aggregation, and the expansive 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 accordingly we find that most bodies are capable of passing from one of these forms to the other, merely in consequence of their receiving different quantities of caloric.

CAROLINE.

That is very curious; but I think I understand the reason of it. If a great quantity of caloric is added to a solid body, it introduces itself between the particles in such a manner as to overcome, in a considerable degree, the attraction of cohesion; and the body, from a solid, is then converted into a fluid.

MRS. B.

This is the case whenever a body is fused or melted; but if you add caloric to a liquid, can you tell me what is the consequence?

CAROLINE.

The caloric forces itself in greater abundance 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, and with all bodies that assume an aeriform state.

EMILY.

I do not well understand the word aeriform?

MRS. B.

Any elastic fluid whatever, whether it be merely vapour or permanent air, is called aeriform.

But each of these various states, solid, liquid, and aeriform, admit of many different degrees of density, or consistence, still arising (chiefly at least) from the different quantities of caloric the bodies contain. Solids are of various degrees of density, from that of gold, to that 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 yet acquainted) are susceptible 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?

MRS. B.

Undoubtedly; and this I can immediately show you by a very 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 increased 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 see how much it 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.

MRS. 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 expands, it increases 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 purpose equally well, if the bar, in dilating, pressed against the index, 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 far more conspicuous.

By submitting different bodies to the test of the pyrometer, it is found that they are far from dilating in the same proportion. Different metals expand in different degrees, and other kinds of solid bodies vary still more in this respect. But this different susceptibility of dilatation is still more remarkable 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, in order that the effect may be more conspicuous. The spirit of wine, you see, dilates by the warmth of my hand as I hold the bulb.

EMILY.

It certainly does, for I see it is rising into the tube. But water, it seems, is not so easily affected by heat; for scarcely any change is produced on it by the warmth of the hand.

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 ascend highest.

CAROLINE.

How rapidly it expands! Now it has nearly reached the top of the tube, though the water has hardly begun 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 subtractions, of caloric, than those of any other fluid.

CAROLINE.

Yet I have often seen coloured spirit of wine used in thermometers.

MRS. B.

The expansions 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 degree of cold has ever been known to freeze spirit of wine.

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 indicated 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 exposed 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 Fahrenheit, and of Reaumur; the first is generally preferred by the English, the latter by the French.

EMILY.

The variety of scale must be very inconvenient, and I should think liable to occasion confusion, when French and English experiments are compared.

MRS. B.

The inconvenience is but very trifling, because the different gradations of the scales do not 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 deg. corresponds with the freezing point, and 212 deg. with the point of boiling water: and that Reaumur's is divided only into 80 degrees, in which 0 deg. denotes the freezing point, and 80 deg. that of boiling water, it is easy to compare the two scales together, and reduce the one into the other. But, for greater convenience, thermometers are sometimes constructed with both these scales, one on either side of the tube; so that the correspondence of the different degrees of the two scales is thus instantly seen. Here is one of these scales, (PLATE II. Fig. 1.) by which you can at once perceive that each degree of Reaumur's corresponds to 2-1/4 of Fahrenheit's division. But I believe the French have, of late, given the preference to what they call the centigrade scale, in which the space between the freezing and the boiling point is divided into 100 degrees.



CAROLINE.

That seems to me the most reasonable division, and I cannot guess why the freezing point is called 32 deg., or what advantage is derived from it.

MRS. B.

There really is no advantage in it; and it originated in a mistaken opinion of the instrument-maker, Fahrenheit, who first constructed these thermometers. He mixed snow and salt together, and produced by that means a degree of cold which he concluded was the greatest possible, and therefore made his scale begin from that point. Between that and boiling water he made 212 degrees, and the freezing point was found to be at 32 deg.

EMILY.

Are spirit of wine, and mercury, the only liquids used in the construction of thermometers?

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 experiments in which a very quick and delicate test of the changes of temperature is required, air is the fluid sometimes employed. The bulb of air thermometers is filled with common air only, and its expansion and contraction are indicated by a small drop of any coloured liquor, which is suspended within the tube, and moves up and down, according as the air within the bulb and tube expands or contracts. But in general, air thermometers, however sensible to changes of temperature, are by no means accurate in their indications.

I can, however, show you an air thermometer of a very peculiar construction, which is remarkably well adapted for some chemical experiments, as it is equally delicate and accurate in its indications.

CAROLINE.

It looks like a double thermometer reversed, the tube being bent, and having a large bulb at each of its extremities. (PLATE II. Fig. 2.)

EMILY.

Why do you call it an air thermometer; the tube contains a coloured liquid?

MRS. B.

But observe that the bulbs are filled with air, the liquid being confined to a portion of the tube, and answering only the purpose of showing, by its motion in the tube, the comparative dilatation or contraction of the air within the bulbs, which afford an indication of their relative temperature. Thus if you heat the bulb A, by the warmth of your hand, the fluid will rise towards the bulb B, and the contrary will happen if you reverse the experiment.

But if, on the contrary, both tubes are of the same temperature, as is the case now, the coloured liquid, suffering an equal pressure on each side, no change of level takes place.

CAROLINE.

This instrument appears, indeed, uncommonly delicate. The fluid is set in motion by the mere approach of my hand.

MRS. B.

You must observe, however, that this thermometer cannot indicate the temperature of any particular body, or of the medium in which it is immersed; it serves only to point out the difference of temperature between the two bulbs, when placed under different circumstances. For this reason it has been called differential thermometer. You will see by-and-bye to what particular purposes this instrument applies.

EMILY.

But do common thermometers indicate the exact quantity of caloric contained either in the atmosphere, or in any body with which they are in contact?

MRS. B.

No: first, because there are other modifications of caloric which do not affect the thermometer; and, secondly, because the temperature of a body, as indicated by the thermometer, is only relative. When, for instance, the thermometer remains stationary at the freezing point, we know that the atmosphere (or medium in which it is placed, whatever it may be) is as cold as freezing water; 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 cold; and consequently we cannot determine that of the body in which the thermometer is placed.

CAROLINE.

I do not quite understand this explanation.

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 the well is unfathomable, it must be impossible to know the absolute 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 contains.

CAROLINE.

Now I comprehend it very well; nothing appears to me to explain a thing so clearly as a comparison.

EMILY.

But will thermometers bear any degree of heat?

MRS. B.

No; for if the temperature were 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 thermometer can be applied to temperatures higher than the boiling point of the liquid used in its construction, for the steam, on the liquid beginning to boil, would burst the tube. In furnaces, or whenever any very high temperature is to be measured, a pyrometer, invented by Wedgwood, 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 indicates 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.

This is not an exception to the rule. You must recollect that the bulk of the clay is not compared, whilst hot, with that which it has when cold; but it is from the change which the clay has undergone by having been heated that the indications of this instrument are derived. This change consists in a beginning fusion which tends to unite the particles of clay more closely, thus rendering it less pervious or spongy.

Clay is to be considered as a spongy body, having many interstices or pores, from its having contained water when soft. These interstices are by heat lessened, and would by extreme heat be entirely obliterated.

CAROLINE.

And how do you ascertain the degrees of contraction of Wedgwood's pyrometer?

MRS. B.

The dimensions of a piece of clay are measured by a scale graduated on the side of a tapered groove, formed in a brass ruler; the more the clay is contracted by the heat, the further it will descend into the narrow part of the tube.

Before we quit the subject of expansion, I must observe to you that, as liquids expand more readily than solids, so elastic fluids, whether air or vapour, are the most expansible of all bodies.

It may appear extraordinary that all elastic fluids whatever, undergo the same degree of expansion from equal augmentations of temperature.

EMILY.

I suppose, then, that all elastic fluids are of the same density?

MRS. B.

Very far from it; they vary in density, more than either liquids or solids. The uniformity of their expansibility, which at first may appear singular, is, however, readily accounted for. For if the different susceptibilities of expansion of bodies arise from their various degrees of attraction of cohesion, no such difference can be expected in elastic fluids, since in these the attraction of cohesion does not exist, their particles being on the contrary possessed of an elastic or repulsive power; they will therefore all be equally expanded by equal degrees of caloric.

EMILY.

True; as there is no power opposed to the expansive force of caloric in elastic bodies, its effect must be the same in all of them.

MRS. B.

Let us now proceed to examine the other properties of free caloric.

Free caloric always tends to diffuse itself equally, that is to say, when two bodies are of different temperatures, the warmer gradually parts with its heat to the colder, till they are both brought to the same temperature. Thus, when a thermometer is applied to a hot body, it receives caloric; when to a cold one, it communicates part of its own caloric, and this communication continues until the thermometer and the body arrive at the same temperature.

EMILY.

Cold, then, is nothing but a negative quality, simply implying the absence of heat.

MRS. B.

Not the total absence, but a diminution 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 marble table I feel it positively cold, and cannot conceive 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 which is exposed to it, but the table melts that part with which it is in contact.

CAROLINE.

But why does caloric tend to an equilibrium? It cannot be on the same principle as other fluids, since it has no weight?

MRS. B.

Very true, Caroline, that is an excellent objection. You might also, with some propriety, object to the term equilibrium being applied to a body that is without weight; but I know of no expression that would explain my meaning so well. You must consider it, however, in a figurative rather than a literal sense; its strict meaning is an equal diffusion. We cannot, indeed, well say by what power it diffuses itself equally, though it is not surprising that it should go from the parts which have the most to those which have the least. This subject is best explained by a theory suggested by Professor Prevost of Geneva, which is now, I believe, generally adopted.

According to this theory, caloric is composed of particles perfectly separate from each other, every one of which moves with a rapid velocity in a certain direction. These directions vary as much as imagination can conceive, the result of which is, that there are rays or lines of these particles moving with immense velocity in every possible direction. Caloric is thus universally diffused, so that when any portion of space happens to be in the neighbourhood of another, which contains more caloric, the colder portion receives a quantity of calorific rays from the latter, sufficient to restore an equilibrium of temperature. This radiation does not only take place in free space, but extends also to bodies of every kind. Thus you may suppose all bodies whatever constantly radiating caloric: those that are of the same temperature give out and absorb equal quantities, so that no variation of temperature is produced in them; but when one body contains more free caloric than another, the exchange 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 reciprocal radiation 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 unnatural that a hot body should receive any caloric from a cold one, even though it should return a greater quantity.

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, which, you know, must necessarily happen.

CAROLINE.

Well, Mrs. B—, I believe that I must give up the point. But I wish I could see these rays of caloric; 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 suffering a diminution, you are really making an acquisition 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, because 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 diminish the loss.

MRS. B.

Very well, indeed, Emily. Professor Pictet, of Geneva, has made some very interesting experiments, which prove not only that caloric radiates from all bodies whatever, but that these rays may be reflected, according to the laws of optics, in the same manner as light. I shall repeat these experiments before you, having procured mirrors fit for the purpose; and it will afford us an opportunity of using the differential thermometer, which is particularly well adapted for these experiments. —I place an iron bullet, (PLATE III. Fig. 1.) about two inches in diameter, and heated to a degree not sufficient to render it luminous, in the focus of this large metallic concave mirror. The rays of heat which fall on this mirror are reflected, agreeably to the property of concave mirrors, in a parallel direction, so as to fall on a similar mirror, which, you see, is placed opposite to the first, at the distance of about ten feet; thence the rays converge to the focus of the second mirror, in which I place one of the bulbs of this thermometer. Now, observe in what manner it is affected by the caloric which is reflected on it from the heated bullet. —The air is dilated in the bulb which we placed in the focus of the mirror, and the liquor rises considerably in the opposite leg.



EMILY.

But would not the same effect take place, if the rays of caloric from the heated bullet fell directly on the thermometer, without the assistance of the mirrors?

MRS. B.

The effect would in that case be so trifling, at the distance at which the bullet and the thermometer are from each other, that it would be almost imperceptible. The mirrors, you know, greatly increase the effect, by collecting a large quantity of rays into a focus; place your hand in the focus of the mirror, and you will find it much hotter there than when you remove it nearer to the bullet.

EMILY.

That is very true; it appears extremely singular to feel the heat diminish in approaching the body from which it proceeds.

CAROLINE.

And the mirror which produces so much heat, by converging the rays, is itself quite cold.

MRS. B.

The same number of rays that are dispersed over the surface of the mirror are collected by it into the focus; but, if you consider how large a surface the mirror presents to the rays, and, consequently, how much they are diffused in comparison to what they are at the focus, which is little more than a point, I think you can no longer wonder that the focus should be so much hotter than the mirror.

The principal use of the mirrors in this experiment is, to prove that the calorific emanation is reflected in the same manner as light.

CAROLINE.

And the result, I think, is very conclusive.

MRS. B.

The experiment may be repeated 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 must be interposed between the mirrors; for light, you know, passes with great facility through glass, whilst the transmission of caloric is almost wholly impeded by it. We shall find, however, in this experiment, that some few of the calorific rays pass through the glass together with the light, as the thermometer rises a little; but, as soon as the glass is removed, and a free passage left to the caloric, it will rise considerably higher.

EMILY.

This experiment, as well as that of Dr. Herschell's, proves that light and heat may be separated; for in the latter experiment the separation was not perfect, any more than in that of Mr. Pictet.

CAROLINE.

I should like to repeat this experiment, 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?

MRS. B.

That we shall see; I have procured some ice for the purpose.

EMILY.

The thermometer falls 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 be positive, since it is capable of reflection.

MRS. B.

So it at first appeared to Mr. Pictet; but upon a little consideration he found that it afforded only an additional proof of the reflection of heat: this I shall endeavour to explain to you.

According to Mr. Prevost's theory, we suppose that all bodies whatever radiate caloric; the thermometer used in these experiments therefore emits calorific rays in the same 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 receives, the consequence is, that its temperature is raised, whilst that of the surrounding bodies is proportionally lowered.

EMILY.

If, for instance, I was to bring a large piece of ice into this room, the ice would in time be melted, by absorbing caloric from the general radiation which is going on throughout the room; and as it would contribute very little caloric in return for what is absorbed, the room would necessarily be cooled by it.

MRS. B.

Just so; and as in consequence of the mirrors, a more considerable exchange of rays takes place between the ice and the thermometer, than between these and any of the surrounding bodies, the temperature of the thermometer must be more lowered than that of any other adjacent object.

CAROLINE.

I confess I do not perfectly understand your explanation.

MRS. B.

This experiment is exactly similar to that made with the heated bullet: for, if we consider the thermometer as the hot body (which it certainly is in comparison to the ice), you may then easily 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 thermometer.

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 exchange 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 caloric to the thermometer, must not the latter fall still lower when the ice is removed?

MRS. B.

No; for the space that the ice occupied, admits rays from all the surrounding bodies to pass through it; and those being of the same temperature as the thermometer, will not affect it, because as much heat now returns to the thermometer as radiates from it.

CAROLINE.

I must confess that you have explained this in so satisfactory a manner, that I cannot help being convinced now that cold has no real claim to the rank of a positive being.

MRS. B.

Before I conclude the subject of radiation I must observe to you that different bodies, (or rather surfaces,) possess the power of radiating caloric in very different degrees.

Some very curious experiments have been made by Mr. Leslie on this subject, and it was for this purpose that he invented the differential thermometer; with its assistance he ascertained that black surfaces radiate most, glass next, and polished surfaces the least of all.

EMILY.

Supposing these surfaces, of course, to be all of the same temperature.

MRS. B.

Undoubtedly. I will now show you the very simple and ingenious apparatus, by means of which he made these experiments. This cubical tin vessel or canister, has each of its sides externally covered with different materials; the one is simply blackened; the next is covered with white paper; the third with a pane of glass, and in the fourth the polished tin surface remains uncovered. We shall fill this vessel with hot water, so that there can be no doubt but that all its sides will be of the same temperature. Now let us place it in the focus of one of the mirrors, making each of its sides front it in succession. We shall begin with the black surface.

CAROLINE.

It makes the thermometer which is in the focus of the other mirror rise considerably. Let us turn the paper surface towards the mirror. The thermometer falls a little, therefore of course this side cannot emit or radiate so much caloric as the blackened side.

EMILY.

This is very surprising; for the sides are exactly of the same size, and must be of the same temperature. But let us try the glass surface.

MRS. B.

The thermometer continues falling, and with the plain surface it falls still lower; these two surfaces therefore radiate less and less.

CAROLINE.

I think I have found out the reason of this.

MRS. B.

I should be very happy to hear it, for it has not yet (to my knowledge) been accounted for.

CAROLINE.

The water within the vessel gradually cools, and the thermometer in consequence gradually falls.

MRS. B.

It is true that the water cools, but certainly in much less proportion than the thermometer descends, as you will perceive if you now change the tin surface for the black one.

CAROLINE.

I was mistaken certainly, for the thermometer rises again now that the black surface fronts the mirror.

MRS. B.

And yet the water in the vessel is still cooling, Caroline.

EMILY.

I am surprised that the tin surface should radiate the least caloric, for a metallic vessel filled with hot water, a silver teapot, for instance, feels much hotter to the hand than one of black earthen ware.

MRS. B.

That is owing to the different power which various bodies possess for conducting caloric, a property which we shall presently examine. Thus, although a metallic vessel feels warmer to the hand, a vessel of this kind is known to preserve the heat of the liquid within, better than one of any other materials; it is for this reason that silver teapots make better tea than those of earthen ware.

EMILY.

According to these experiments, light-coloured dresses, in cold weather, should keep us warmer than black clothes, since the latter radiate so much more than the former.

MRS. B.

And that is actually the case.

EMILY.

This property, of different surfaces to radiate in different degrees, appears to me to be at variance with the equilibrium of caloric; since it would imply that those bodies which radiate most, must ultimately become coldest.

Suppose that we were to vary this experiment, by using two metallic vessels full of boiling water, the one blackened, the other not; would not the black one cool the first?

CAROLINE.

True; but when they were both brought down to the temperature of the room, the interchange of caloric between the canisters and the other bodies of the room being then equal, their temperatures would remain the same.

EMILY.

I do not see why that should be the case; for if different surfaces of the same temperature radiate in different degrees when heated, why should they not continue to do so when cooled down to the temperature of the room?

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