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.

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 vi 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; vii 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 viii 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 ix 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 x 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.

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 minutiæ 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 3 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 4 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 5 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 6 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, 7 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 8 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, 9 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 10 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 11 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 12 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:

Comprehending the imponderable agents, viz.

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

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

Bodies forming acids.

Metallic bodies forming alkalies.

Metallic bodies forming earths.

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

Subdivision 1.

Malleable Metals.

Subdiv. 2.

Brittle Metals.

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 18 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 19 specimen, as I have prepared some for that purpose.

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 22 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 23 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 24 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 25 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.

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 27 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 29 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 32 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.

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? 34 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 35 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 aëriform 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 aëriform state.

EMILY.

I do not well understand the word aëriform?

MRS. B.

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

But each of these various states, solid, liquid, and aëriform, 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.

Plate I.

Vol. I. p. 38.

Fig. 1   A.A Bar of Metal.   1.2.3 Lamps burning.   B.B Wheel work.   C Index.
Fig. 2   A.A Glass tubes with bulbs.   B.B Glasses of water in which they are immersed.

Larger view

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. 42 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° corresponds with the freezing point, and 212° with the point of boiling water: and that Reaumur’s is divided only into 80 degrees, in which 0° denotes the freezing point, and 80° that of 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 43 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 2¼ 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.

Plate II.

Vol. I. p. 42.

Larger view

CAROLINE.

That seems to me the most reasonable division, and I cannot guess why the freezing point is called 32°, 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°.

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 46 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 48 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 49 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 50 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? 52 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 53 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 55 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.

Plate III.

Vol. I. p. 54

A.A. & B.B Concave mirrors fixed on stands.   C Heated Bullet placed in the focus of the mirror A.   D Thermometer, with its bulb placed in the focus of the mirror B.
1.2.3.4 Rays of Caloric radiating from the bullet & falling on the mirror A.   5.6.7.8 The same rays reflected from the mirror A to the mirror B.   9.10.11.12 The same rays reflected by the mirror B to the Thermometer.

Larger view

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 56 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 59 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 62 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?

MRS. B.

You have started a difficulty, Emily, which certainly requires explanation. It is found by experiment that the power of absorption corresponds with and is proportional to that of radiation; so that under equal temperatures, bodies compensate for the greater loss they sustain in consequence of their greater radiation by their greater absorption; so that if you were to make your experiment in an atmosphere heated like the canisters, to the temperature of boiling water, though it is true that the canisters would radiate in different degrees, no change of temperature would be produced in them, because they would each absorb caloric in proportion to their respective radiation.

EMILY.

But would not the canisters of boiling water also absorb caloric in different degrees in a room of the common temperature?

MRS. B.

Undoubtedly they would. But the various bodies in the room would not, at a lower temperature, furnish either of the canisters with a sufficiency of caloric to compensate for the loss they undergo; for, suppose the black canister to absorb 400 rays of caloric, whilst the metallic one absorbed only 200; yet if the former radiate 800, whilst the latter radiates only 400, the black canister will be the first cooled down to the temperature of the room. But from the moment the equilibrium of temperature has taken place, the black canister, both receiving and giving out 400 rays, and the metallic one 200, no change of temperature will take place.

EMILY.

I now understand it extremely well. But what becomes of the surplus of calorific rays, which good radiators emit and bad radiators refuse to receive; they must wander about in search of a resting-place?

MRS. B.

They really do so; for they are rejected and sent 67 back, or, in other words, reflected by the bodies which are bad radiators of caloric; and they are thus transmitted to other bodies which happen to lie in their way, by which they are either absorbed or again reflected, according as the property of reflection, or that of absorption, predominates in these bodies.

CAROLINE.

I do not well understand the difference between radiating and reflecting caloric, for the caloric that is reflected from a body proceeds from it in straight lines, and may surely be said to radiate from it?

MRS. B.

It is true that there at first appears to be a great analogy between radiation and reflection, as they equally convey the idea of the transmission of caloric.

But if you consider a little, you will perceive that when a body radiates caloric, the heat which it emits not only proceeds from, but has its origin in the body itself. Whilst when a body reflects caloric, it parts with none of its own caloric, but only reflects that which it receives from other bodies.

EMILY.

Of this difference we have very striking examples before us, in the tin vessel of water, and the concave mirrors; the first radiates its own heat, 68 the latter reflect the heat which they receive from other bodies.

CAROLINE.

Now, that I understand the difference, it no longer surprises me that bodies which radiate, or part with their own caloric freely, should not have the power of transmitting with equal facility that which they receive from other bodies.

EMILY.

Yet no body can be said to possess caloric of its own, if all caloric is originally derived from the sun.

MRS. B.

When I speak of a body radiating its own caloric, I mean that which it has absorbed and incorporated either immediately from the sun’s rays, or through the medium of any other substance.

CAROLINE.

It seems natural enough that the power of absorption should be in opposition to that of reflection, for the more caloric a body receives, the less it will reject.

EMILY.

And equally so that the power of radiation should correspond with that of absorption. It is, in fact, cause and effect; for a body cannot radiate 69 heat without having previously absorbed it; just as a spring that is well fed flows abundantly.

MRS. B.

Fluids are in general very bad radiators of caloric; and air neither radiates nor absorbs caloric in any sensible degree.

We have not yet concluded our observations on free caloric. But I shall defer, till our next meeting, what I have further to say on this subject. I believe it will afford us ample conversation for another interview.

MRS. B.

In our last conversation, we began to examine the tendency of caloric to restore an equilibrium of temperature. This property, when once well understood, affords the explanation of a great variety of facts which appeared formerly unaccountable. You must observe, in the first place, that the effect of this tendency is gradually to bring all bodies that are in contact to the same temperature. Thus, the fire which burns in the grate, communicates its heat from one object to another, till every part of the room has an equal proportion of it.

EMILY.

And yet this book is not so cold as the table on which it lies, though both are at an equal distance from the fire, and actually in contact with each other, so that, according to your theory, they should be exactly of the same temperature.

CAROLINE.

And the hearth, which is much nearer the fire than the carpet, is certainly the colder of the two.

MRS. B.

If you ascertain the temperature of these several bodies by a thermometer (which is a much more accurate test than your feeling), you will find that it is exactly the same.

CAROLINE.

But if they are of the same temperature, why should the one feel colder than the other?

MRS. B.

The hearth and the table feel colder than the carpet or the book, because the latter are not such good conductors of heat as the former. Caloric finds a more easy passage through marble and wood, than through leather and worsted; the two former will therefore absorb heat more rapidly from your hand, and consequently give it a stronger sensation of cold than the two latter, although they are all of them really of the same temperature.

CAROLINE.

So, then, the sensation I feel on touching a cold body, is in proportion to the rapidity with which my hand yields its heat to that body?

MRS. B.

Precisely; and, if you lay your hand successively on every object in the room, you will discover which are good, and which are bad conductors of heat, by the different degrees of cold you feel. But, in order to ascertain this point, it is necessary that the several substances should be of the same temperature, which will not be the case with those that are very near the fire, or those that are exposed to a current of cold air from a window or door.

EMILY.

But what is the reason that some bodies are better conductors of heat than others?

MRS. B.

This is a point not well ascertained. It has been conjectured that a certain union or adherence takes place between the caloric and the particles of the body through which it passes. If this adherence be strong, the body detains the heat, and parts with it slowly and reluctantly; if slight, it propagates it freely and rapidly. The conducting power of a body is therefore, inversely, as its tendency to unite with caloric.

EMILY.

That is to say, that the best conductors are those that have the least affinity for caloric.

MRS. B.

Yes; but the term affinity is objectionable in this case, because, as that word is used to express a chemical attraction (which can be destroyed only by decomposition), it cannot be applicable to the slight and transient union that takes place between free caloric and the bodies through which it passes; an union which is so weak, that it constantly yields to the tendency which caloric has to an equilibrium. Now you clearly understand, that the passage of caloric, through bodies that are good conductors, is much more rapid than through those that are bad conductors, and that the former both give and receive it more quickly, and therefore, in a given time, more abundantly, than bad conductors, which makes them feel either hotter or colder, though they may be, in fact, both of the same temperature.

CAROLINE.

Yes, I understand it now; the table, and the book lying upon it, being really of the same temperature, would each receive, in the same space of time, the same quantity of heat from my hand, were their conducting powers equal; but as the table is the best conductor of the two, it will absorb the heat from my hand more rapidly, and consequently produce a stronger sensation of cold than the book.

MRS. B.

Very well, my dear; and observe, likewise, that if you were to heat the table and the book an equal number of degrees above the temperature of your body, the table, which before felt the colder, would now feel the hotter of the two; for, as in the first case it took the heat most rapidly from your hand, so it will now impart heat most rapidly to it. Thus the marble table, which seems to us colder than the mahogany one, will prove the hotter of the two to the ice; for, if it takes heat more rapidly from our hands, which are warmer, it will give out heat more rapidly to the ice, which is colder. Do you understand the reason of these apparently opposite effects?

EMILY.

Perfectly. A body which is a good conductor of caloric, affords it a free passage; so that it penetrates through that body more rapidly than through one which is a bad conductor; and consequently, if it is colder than your hand, you lose more caloric, and if it is hotter, you gain more than with a bad conductor of the same temperature.

MRS. B.

But you must observe that this is the case only when the conductors are either hotter or colder than your hand; for, if you heat different conductors 75 to the temperature of your body, they will all feel equally warm, since the exchange of caloric between bodies of the same temperature is equal. Now, can you tell me why flannel clothing, which is a very bad conductor of heat, prevents our feeling cold?

CAROLINE.

It prevents the cold from penetrating . . . . . . . .

MRS. B.

But you forget that cold is only a negative quality.

CAROLINE.

True; it only prevents the heat of our bodies from escaping so rapidly as it would otherwise do.

MRS. B.

Now you have explained it right; the flannel rather keeps in the heat, than keeps out the cold. Were the atmosphere of a higher temperature than our bodies, it would be equally efficacious in keeping their temperature at the same degree, as it would prevent the free access of the external heat, by the difficulty with which it conducts it.

EMILY.

This, I think, is very clear. Heat, whether external or internal, cannot easily penetrate flannel; 76 therefore in cold weather it keeps us warm; and if the weather was hotter than our bodies, it would keep us cool.

MRS. B.

The most dense bodies are, generally speaking, the best conductors of heat; probably because the denser the body the greater are the number of points or particles that come in contact with caloric. At the common temperature of the atmosphere a piece of metal will feel much colder than a piece of wood, and the latter than a piece of woollen cloth; this again will feel colder than flannel; and down, which is one of the lightest, is at the same time one of the warmest bodies.

CAROLINE.

This is, I suppose, the reason that the plumage of birds preserves them so effectually from the influence of cold in winter?

MRS. B.

Yes; but though feathers in general are an excellent preservative against cold, down is a kind of plumage peculiar to aquatic birds, and covers their chest, which is the part most exposed to the water; for though the surface of the water is not of a lower temperature than the atmosphere, yet, as it is a better conductor of heat, it feels much 77 colder, consequently the chest of the bird requires a warmer covering than any other part of its body. Besides, the breasts of aquatic birds are exposed to cold not only from the temperature of the water, but also from the velocity with which the breast of the bird strikes against it; and likewise from the rapid evaporation occasioned in that part by the air against which it strikes, after it has been moistened by dipping from time to time into the water.

If you hold a finger of one hand motionless in a glass of water, and at the same time move a finger of the other hand swiftly through water of the same temperature, a different sensation will be soon perceived in the different fingers.

Most animal substances, especially those which Providence has assigned as a covering for animals, such as fur, wool, hair, skin, &c. are bad conductors of heat, and are, on that account, such excellent preservatives against the inclemency of winter, that our warmest apparel is made of these materials.

EMILY.

Wood is, I dare say, not so good a conductor as metal, and it is for that reason, no doubt, that silver teapots have always wooden handles.

MRS. B.

Yes; and it is the facility with which metals 78 conduct caloric that made you suppose that a silver pot radiated more caloric than an earthen one. The silver pot is in fact hotter to the hand when in contact with it; but it is because its conducting power more than counterbalances its deficiency in regard to radiation.

We have observed that the most dense bodies are in general the best conductors; and metals, you know, are of that class. Porous bodies, such as the earths and wood, are worse conductors, chiefly, I believe, on account of their pores being filled with air; for air is a remarkably bad conductor.

CAROLINE.

It is a very fortunate circumstance that air should be a bad conductor, as it tends to preserve the heat of the body when exposed to cold weather.

MRS. B.

It is one of the many benevolent dispensations of Providence, in order to soften the inclemency of the seasons, and to render almost all climates habitable to man.

In fluids of different densities, the power of conducting heat varies no less remarkably; if you dip your hand into this vessel full of mercury, you will scarcely conceive that its temperature is not lower than that of the atmosphere.

CAROLINE.

Indeed I know not how to believe it, it feels so extremely cold.—But we may easily ascertain its true temperature by the thermometer.—It is really not colder than the air;—the apparent difference then is produced merely by the difference of the conducting power in mercury and in air.

MRS. B.

Yes; hence you may judge how little the sense of feeling is to be relied on as a test of the temperature of bodies, and how necessary a thermometer is for that purpose.

It has indeed been doubted whether fluids have the power of conducting caloric in the same manner as solid bodies. Count Rumford, a very few years since, attempted to prove, by a variety of experiments, that fluids, when at rest, were not at all endowed with this property.

CAROLINE.

How is that possible, since they are capable of imparting cold or heat to us; for if they did not conduct heat, they would neither take it from, nor give it to us?

MRS. B.

Count Rumford did not mean to say that fluids would not communicate their heat to solid bodies; 80 but only that heat does not pervade fluids, that is to say, is not transmitted from one particle of a fluid to another, in the same manner as in solid bodies.

EMILY.

But when you heat a vessel of water over the fire, if the particles of water do not communicate heat to each other, how does the water become hot throughout?

MRS. B.

By constant agitation. Water, as you have seen, expands by heat in the same manner as solid bodies; the heated particles of water, therefore, at the bottom of the vessel, become specifically lighter than the rest of the liquid, and consequently ascend to the surface, where, parting with some of their heat to the colder atmosphere, they are condensed, and give way to a fresh succession of heated particles ascending from the bottom, which having thrown off their heat at the surface, are in their turn displaced. Thus every particle is successively heated at the bottom, and cooled at the surface of the liquid; but as the fire communicates heat more rapidly than the atmosphere cools the succession of surfaces, the whole of the liquid in time becomes heated.

CAROLINE.

This accounts most ingeniously for the propagation 81 of heat upwards. But suppose you were to heat the upper surface of a liquid, the particles being specifically lighter than those below, could not descend: how therefore would the heat be communicated downwards?

MRS. B.

If there were no agitation to force the heated surface downwards, Count Rumford assures us that the heat would not descend. In proof of this he succeeded in making the upper surface of a vessel of water boil and evaporate, while a cake of ice remained frozen at the bottom.

CAROLINE.

That is very extraordinary indeed!

MRS. B.

It appears so, because we are not accustomed to heat liquids by their upper surface; but you will understand this theory better if I show you the internal motion that takes place in liquids when they experience a change of temperature. The motion of the liquid itself is indeed invisible from the extreme minuteness of its particles; but if you mix with it any coloured dust, or powder, of nearly the same specific gravity as the liquid, you may judge of the internal motion of the latter by that of the coloured dust it contains.—Do you see the 82 small pieces of amber moving about in the liquid contained in this phial?

CAROLINE.

Yes, perfectly.

MRS. B.

We shall now immerse the phial in a glass of hot water, and the motion of the liquid will be shown, by that which it communicates to the amber.

EMILY.

I see two currents, the one rising along the sides of the phial, the other descending in the centre: but I do not understand the reason of this.

MRS. B.

The hot water communicates its caloric, through the medium of the phial, to the particles of the fluid nearest to the glass; these dilate and ascend laterally to the surface, where, in parting with their heat, they are condensed, and in descending, form the central current.

CAROLINE.

This is indeed a very clear and satisfactory experiment; but how much slower the currents now move than they did at first?

MRS. B.

It is because the circulation of particles has 83 nearly produced an equilibrium of temperature between the liquid in the glass and that in the phial.

CAROLINE.

But these communicate laterally, and I thought that heat in liquids could be propagated only upwards.

MRS. B.

You do not take notice that the heat is imparted from one liquid to the other, through the medium of the phial itself, the external surface of which receives the heat from the water in the glass, whilst its internal surface transmits it to the liquid it contains. Now take the phial out of the hot water, and observe the effect of its cooling.

EMILY.

The currents are reversed; the external current now descends, and the internal one rises.—I guess the reason of this change:—the phial being in contact with cold air instead of hot water, the external particles are cooled instead of being heated; they therefore descend and force up the central particles, which, being warmer, are consequently lighter.

MRS. B.

It is just so. Count Rumford hence infers that no alteration of temperature can take place in a fluid, without an internal motion of its particles, 84 and as this motion is produced only by the comparative levity of the heated particles, heat cannot be propagated downwards.

But though I believe that Count Rumford’s theory as to heat being incapable of pervading fluids is not strictly correct, yet there is, no doubt, much truth in his observation, that the communication is materially promoted by a motion of the parts; and this accounts for the cold that is found to prevail at the bottom of the lakes in Switzerland, which are fed by rivers issuing from the snowy Alps. The water of these rivers being colder, and therefore more dense than that of the lakes, subsides to the bottom, where it cannot be affected by the warmer temperature of the surface; the motion of the waves may communicate this temperature to some little depth, but it can descend no further than the agitation extends.

EMILY.

But when the atmosphere is colder than the lake, the colder surface of the water will descend, for the very reason that the warmer will not.

MRS. B.

Certainly: and it is on this account that neither a lake, nor any body of water whatever, can be frozen until every particle of the water has risen to the surface to give off its caloric to the colder 85 atmosphere; therefore the deeper a body of water is, the longer will be the time it requires to be frozen.

EMILY.

But if the temperature of the whole body of water be brought down to the freezing point, why is only the surface frozen?

MRS. B.

The temperature of the whole body is lowered, but not to the freezing point. The diminution of heat, as you know, produces a contraction in the bulk of fluids, as well as of solids. This effect, however, does not take place in water below the temperature of 40 degrees, which is 8 degrees above the freezing point. At that temperature, therefore, the internal motion, occasioned by the increased specific gravity of the condensed particles, ceases; for when the water at the surface no longer condenses, it will no longer descend, and leave a fresh surface exposed to the atmosphere: this surface alone, therefore, will be further exposed to its severity, and will soon be brought down to the freezing point, when it becomes ice, which being a bad conductor of heat, preserves the water beneath a long time from being affected by the external cold.

CAROLINE.

And the sea does not freeze, I suppose, because 86 its depth is so great, that a frost never lasts long enough to bring down the temperature of such a great body of water to 40 degrees?

MRS. B.

That is one reason why the sea, as a large mass of water, does not freeze. But, independently of this, salt water does not freeze till it is cooled much below 32 degrees, and with respect to the law of condensation, salt water is an exception, as it condenses even many degrees below the freezing point. When the caloric of fresh water, therefore, is imprisoned by the ice on its surface, the ocean still continues throwing off heat into the atmosphere, which is a most signal dispensation of Providence to moderate the intensity of the cold in winter.

CAROLINE.

This theory of the non-conducting power of liquids, does not, I suppose, hold good with respect to air, otherwise the atmosphere would not be heated by the rays of the sun passing through it?

MRS. B.

Nor is it heated in that way. The pure atmosphere is a perfectly transparent medium, which neither radiates, absorbs, nor conducts caloric, but transmits the rays of the sun to us without in any way 87 diminishing their intensity. The air is therefore not more heated, by the sun’s rays passing through it, than diamond, glass, water, or any other transparent medium.

CAROLINE.

That is very extraordinary! Are glass windows not heated then by the sun shining on them?

MRS. B.

No; not if the glass be perfectly transparent. A most convincing proof that glass transmits the rays of the sun without being heated by them is afforded by the burning lens, which by converging the rays to a focus will set combustible bodies on fire, without its own temperature being raised.

EMILY.

Yet, Mrs. B., if I hold a piece of glass near the fire it is almost immediately warmed by it; the glass therefore must retain some of the caloric radiated by the fire? Is it that the solar rays alone pass freely through glass without paying tribute? It seems unaccountable that the radiation of a common fire should have power to do what the sun’s rays cannot accomplish.

MRS. B.

It is not because the rays from the fire have more power, but rather because they have less, that 88 they heat glass and other transparent bodies. It is true, however, that as you approach the source of heat the rays being nearer each other, the heat is more condensed, and can produce effects of which the solar rays, from the great distance of their source, are incapable. Thus we should find it impossible to roast a joint of meat by the sun’s rays, though it is so easily done by culinary heat. Yet caloric emanated from burning bodies, which is commonly called culinary heat, has neither the intensity nor the velocity of solar rays. All caloric, we have said, is supposed to proceed originally from the sun; but after having been incorporated with terrestrial bodies, and again given out by them, though its nature is not essentially altered, it retains neither the intensity nor the velocity with which it first emanated from that luminary; it has therefore not the power of passing through transparent mediums, such as glass and water, without being partially retained by those bodies.

EMILY.

I recollect that in the experiment on the reflection of heat, the glass skreen which you interposed between the burning taper and the mirror, arrested the rays of caloric, and suffered only those of light to pass through it.

CAROLINE.

Glass windows, then, though they cannot be 89 heated by the sun shining on them, may be heated internally by a fire in the room? But, Mrs. B., since the atmosphere is not warmed by the solar rays passing through it, how does it obtain heat; for all the fires that are burning on the surface of the earth would contribute very little towards warming it?

EMILY.

The radiation of heat is not confined to burning bodies: for all bodies, you know, have that property; therefore, not only every thing upon the surface of the earth, but the earth itself, must radiate heat; and this terrestrial caloric, not having, I suppose, sufficient power to traverse the atmosphere, communicates heat to it.

MRS. B.

Your inference is extremely well drawn, Emily; but the foundation on which it rests is not sound; for the fact is, that terrestrial or culinary heat, though it cannot pass through the denser transparent mediums, such as glass or water, without loss, traverses the atmosphere completely: so that all the heat which the earth radiates, unless it meet with clouds or any foreign body to intercept its passage, passes into the distant regions of the universe.

CAROLINE.

What a pity that so much heat should be wasted!

MRS. B.

Before you are tempted to object to any law of nature, reflect whether it may not prove to be one of the numberless dispensations of Providence for our good. If all the heat which the earth has received from the sun, since the creation had been accumulated in it, its temperature by this time would, no doubt, have been more elevated than any human being could have borne.

CAROLINE.

I spoke indeed very inconsiderately. But, Mrs. B., though the earth, at such a high temperature, might have scorched our feet, we should always have had a cool refreshing air to breathe, since the radiation of the earth does not heat the atmosphere.

EMILY.

The cool air would have afforded but very insufficient refreshment, whilst our bodies were exposed to the burning radiation of the earth.

MRS. B.

Nor should we have breathed a cool air; for though it is true that heat is not communicated to the atmosphere by radiation, yet the air is warmed by contact with heated bodies, in the same manner as solids or liquids. The stratum of air which is immediately in contact with the earth is heated by 91 it; it becomes specifically lighter and rises, making way for another stratum of air which is in its turn heated and carried upwards; and thus each successive stratum of air is warmed by coming in contact with the earth. You may perceive this effect in a sultry day, if you attentively observe the strata of air near the surface of the earth; they appear in constant agitation, for though it is true the air is itself invisible, yet the sun shining on the vapours floating in it, render them visible, like the amber dust in the water. The temperature of the surface of the earth is therefore the source from whence the atmosphere derives its heat, though it is communicated neither by radiation, nor transmitted from one particle of it to another by the conducting power; but every particle of air must come in contact with the earth in order to receive heat from it.

EMILY.

Wind then by agitating the air should contribute to cool the earth and warm the atmosphere, by bringing a more rapid succession of fresh strata of air in contact with the earth, and yet in general wind feels cooler than still air?

MRS. B.

Because the agitation of the air carries off heat from the surface of our bodies more rapidly than 92 still air, by occasioning a greater number of points of contact in a given time.

EMILY.

Since it is from the earth and not the sun that the atmosphere receives its heat, I no longer wonder that elevated regions should be colder than plains and valleys; it was always a subject of astonishment to me, that in ascending a mountain and approaching the sun, the air became colder instead of being more heated.

MRS. B.

At the distance of about a hundred million of miles, which we are from the sun, the approach of a few thousand feet makes no sensible difference, whilst it produces a very considerable effect with regard to the warming the atmosphere at the surface of the earth.

CAROLINE.

Yet as the warm air rises from the earth and the cold air descends to it, I should have supposed that heat would have accumulated in the upper regions of the atmosphere, and that we should have felt the air warmer as we ascended?

MRS. B.

The atmosphere, you know, diminishes in density, and consequently in weight, as it is more distant 93 from the earth; the warm air, therefore, rises only till it meets with a stratum of air of its own density; and it will not ascend into the upper regions of the atmosphere until all the parts beneath have been previously heated. The length of summer even in warm climates does not heat the air sufficiently to melt the snow which has accumulated during the winter on very high mountains, although they are almost constantly exposed to the heat of the sun’s rays, being too much elevated to be often enveloped in clouds.

EMILY.

These explanations are very satisfactory; but allow me to ask you one more question respecting the increased levity of heated liquids. You said that when water was heated over the fire, the particles at the bottom of the vessel ascended as soon as heated, in consequence of their specific levity: why does not the same effect continue when the water boils, and is converted into steam? and why does the steam rise from the surface, instead of the bottom of the liquid?

MRS. B.

The steam or vapour does ascend from the bottom, though it seems to arise from the surface of the liquid. We shall boil some water in this Florence flask, (Plate IV. Fig. 1.) in order that 94 you may be well acquainted with the process of ebullition;—you will then see, through the glass, that the vapour rises in bubbles from the bottom. We shall make it boil by means of a lamp, which is more convenient for this purpose than the chimney fire.

Plate IV.

Vol. I. p. 84.

Fig. 2. Boiling water in a flask over a Patent lamp.

Larger view (complete Plate)

EMILY.

I see some small bubbles ascend, and a great many appear all over the inside of the flask; does the water begin to boil already?

MRS. B.

No; what you now see are bubbles of air, which were either dissolved in the water, or attached to the inner surface of the flask, and which, being rarefied by the heat, ascend in the water.

EMILY.

But the heat which rarefies the air inclosed in the water must rarefy the water at the same time; therefore, if it could remain stationary in the water when both were cold, I do not understand why it should not when both are equally heated?

MRS. B.

Air being much less dense than water, is more easily rarefied; the former, therefore, expands to a great extent, whilst the latter continues to occupy 95 nearly the same space; for water dilates comparatively but very little without changing its state and becoming vapour. Now that the water in the flask begins to boil, observe what large bubbles rise from the bottom of it.

EMILY.

I see them perfectly; but I wonder that they have sufficient power to force themselves through the water.

CAROLINE.

They must rise, you know, from their specific levity.

MRS. B.

You are right, Caroline; but vapour has not in all liquids (when brought to the degree of vaporization) the power of overcoming the pressure of the less heated surface. Metals, for instance, mercury excepted, evaporate only from the surface; therefore no vapour will ascend from them till the degree of heat which is necessary to form it has reached the surface; that is to say, till the whole of the liquid is brought to a state of ebullition.

EMILY.

I have observed that steam, immediately issuing from the spout of a teakettle, is less visible than at a further distance from it; yet it must be more 96 dense when it first evaporates, than when it begins to diffuse itself in the air.

MRS. B.

When the steam is first formed, it is so perfectly dissolved by caloric, as to be invisible. In order however to understand this, it will be necessary for me to enter into some explanation respecting the nature of SOLUTION. Solution takes place whenever a body is melted in a fluid. In this operation the body is reduced to such a minute state of division by the fluid, as to become invisible in it, and to partake of its fluidity; but in common solutions this happens without any decomposition, the body being only divided into its integrant particles by the fluid in which it is melted.

CAROLINE.

It is then a mode of destroying the attraction of aggregation.

MRS. B.

Undoubtedly.—The two principal solvent fluids are water, and caloric. You may have observed that if you melt salt in water, it totally disappears, and the water remains clear, and transparent as before; yet though the union of these two bodies appears so perfect, it is not produced by any chemical combination; both the salt and the water remain unchanged; and if you were to separate 97 them by evaporating the latter, you would find the salt in the same state as before.

EMILY.

I suppose that water is a solvent for solid bodies, and caloric for liquids?

MRS. B.

Liquids of course can only be converted into vapour by caloric. But the solvent power of this agent is not at all confined to that class of bodies; a great variety of solid substances are dissolved by heat: thus metals, which are insoluble in water, can be dissolved by intense heat, being first fused or converted into a liquid, and then rarefied into an invisible vapour. Many other bodies, such as salt, gums, &c. yield to either of these solvents.

CAROLINE.

And that, no doubt, is the reason why hot water will melt them so much better than cold water?

MRS. B.

It is so. Caloric may, indeed, be considered as having, in every instance, some share in the solution of a body by water, since water, however low its temperature may be, always contains more or less caloric.

EMILY.

Then, perhaps, water owes its solvent power merely to the caloric contained in it?

MRS. B.

That, probably, would be carrying the speculation too far; I should rather think that water and caloric unite their efforts to dissolve a body, and that the difficulty or facility of effecting this, depend both on the degree of attraction of aggregation to be overcome, and on the arrangement of the particles which are more or less disposed to be divided and penetrated by the solvent.

EMILY.

But have not all liquids the same solvent power as water?

MRS. B.

The solvent power of other liquids varies according to their nature, and that of the substances submitted to their action. Most of these solvents, indeed, differ essentially from water, as they do not merely separate the integrant particles of the bodies which they dissolve, but attack their constituent principles by the power of chemical attraction, thus producing a true decomposition. These more complicated operations we must consider in another place, and confine our attention 99 at present to the solutions by water and caloric.

CAROLINE.

But there are a variety of substances which, when dissolved in water, make it thick and muddy, and destroy its transparency.

MRS. B.

In this case it is not a solution, but simply a mixture. I shall show you the difference between a solution and a mixture, by putting some common salt into one glass of water, and some powder of chalk into another; both these substances are white, but their effect on the water will be very different.

CAROLINE.

Very different indeed! The salt entirely disappears and leaves the water transparent, whilst the chalk changes it into an opaque liquid like milk.

EMILY.

And would lumps of chalk and salt produce similar effects on water?

MRS. B.

Yes, but not so rapidly; salt is, indeed, soon melted though in a lump; but chalk, which does not mix so readily with water, would require a 100 much greater length of time; I therefore preferred showing you the experiment with both substances reduced to powder, which does not in any respect alter their nature, but facilitates the operation merely by presenting a greater quantity of surface to the water.

I must not forget to mention a very curious circumstance respecting solutions, which is, that a fluid is not nearly so much increased in bulk by holding a body in solution, as it would by mere mixture with the body.

CAROLINE.

That seems impossible; for two bodies cannot exist together in the same space.

MRS. B.

Two bodies may, by condensation, occupy less space when in union than when separate, and this I can show you by an easy experiment.

This phial, which contains some salt, I shall fill with water, pouring it in quickly, so as not to dissolve much of the salt; and when it is quite full I cork it.—If I now shake the phial till the salt is dissolved, you will observe that it is no longer full.

CAROLINE.

I shall try to add a little more salt.—But now, you see, Mrs. B., the water runs over.

MRS. B.

Yes; but observe that the last quantity of salt you put in remains solid at the bottom, and displaces the water; for it has already melted all the salt it is capable of holding in solution. This is called the point of saturation; and the water in this case is said to be saturated with salt.

EMILY.

I think I now understand the solution of a solid body by water perfectly: but I have not so clear an idea of the solution of a liquid by caloric.

MRS. B.

It is probably of a similar nature; but as caloric is an invisible fluid, its action as a solvent is not so obvious as that of water. Caloric, we may conceive, dissolves water, and converts it into vapour by the same process as water dissolves salt; that is to say, the particles of water are so minutely divided by the caloric as to become invisible. Thus, you are now enabled to understand why the vapour of boiling water, when it first issues from the spout of a kettle, is invisible; it is so, because it is then completely dissolved by caloric. But the air with which it comes in contact, being much colder than the vapour, the latter yields to it a quantity of its caloric. The particles of vapour being thus in a great measure deprived 102 of their solvent, gradually collect, and become visible in the form of steam, which is water in a state of imperfect solution; and if you were further to deprive it of its caloric, it would return to its original liquid state.

CAROLINE.

That I understand very well. If you hold a cold plate over a tea-urn, the steam issuing from it will be immediately converted into drops of water by parting with its caloric to the plate; but in what state is the steam, when it becomes invisible by being diffused in the air?

MRS. B.

It is not merely diffused, but is again dissolved by the air.

EMILY.

The air, then, has a solvent power, like water and caloric?

MRS. B.

This was formerly believed to be the case. But it appears from more recent enquiries that the solvent power of the atmosphere depends solely upon the caloric contained in it. Sometimes the watery vapour diffused in the atmosphere is but imperfectly dissolved, as is the case in the formation of clouds and fogs; but if it gets into a region sufficiently warm, it becomes perfectly invisible.

EMILY.

Can any water dissolve in the atmosphere without its being previously converted into vapour by boiling?

MRS. B.

Unquestionably; and this constitutes the difference between vaporization and evaporation. Water, when heated to the boiling point, can no longer exist in the form of water, and must necessarily be converted into vapour or steam, whatever may be the state and temperature of the surrounding medium; this is called vaporization. But the atmosphere, by means of the caloric it contains, can take up a certain portion of water at any temperature, and hold it in a state of solution. This is simply evaporation. Thus the atmosphere is continually carrying off moisture from the surface of the earth, until it is saturated with it.

CAROLINE.

That is the case, no doubt, when we feel the atmosphere damp.

MRS. B.

On the contrary, when the moisture is well dissolved it occasions no humidity: it is only when in a state of imperfect solution and floating in the atmosphere, in the form of watery vapour, that it produces dampness. This happens more frequently 104 in winter than in summer; for the lower the temperature of the atmosphere, the less water it can dissolve; and in reality it never contains so much moisture as in a dry hot summer’s day.

CAROLINE.

You astonish me! But why, then, is the air so dry in frosty weather, when its temperature is at the lowest?

EMILY.

This, I conjecture, proceeds not so much from the moisture being dissolved, as from its being frozen; is not that the case?

MRS. B.

It is; and the freezing of the watery vapour which the atmospheric heat could not dissolve, produces what is called a hoar frost; for the particles descend in freezing, and attach themselves to whatever they meet with on the surface of the earth.

The tendency of free caloric to an equilibrium, together with its solvent power, are likewise connected with the phenomena of rain, of dew, &c. When moist air of a certain temperature happens to pass through a colder region of the atmosphere, it parts with a portion of its heat to the surrounding air; the quantity of caloric, therefore, which served to keep the water in a state of 105 vapour, being diminished, the watery particles approach each other, and form themselves into drops of water, which being heavier than the atmosphere, descend to the earth. There are also other circumstances, and particularly the variation in the weight of the atmosphere, which may contribute to the formation of rain. This, however, is an intricate subject, into which we cannot more fully enter at present.

EMILY.

In what manner do you account for the formation of dew?

MRS. B.

Dew is a deposition of watery particles or minute drops from the atmosphere, precipitated by the coolness of the evening.

CAROLINE.

This precipitation is owing, I suppose, to the cooling of the atmosphere, which prevents its retaining so great a quantity of watery vapour in solution as during the heat of the day.

MRS. B.

Such was, from time immemorial, the generally received opinion respecting the cause of dew; but it has been very recently proved by a course of ingenious experiments of Dr. Wells, that the deposition 106 of dew is produced by the cooling of the surface of the earth, which he has shown to take place previously to the cooling of the atmosphere; for on examining the temperature of a plot of grass just before the dew-fall, he found that it was considerably colder than the air a few feet above it, from which the dew was shortly after precipitated.

EMILY.

But why should the earth cool in the evening sooner than the atmosphere?

MRS. B.

Because it parts with its heat more readily than the air; the earth is an excellent radiator of caloric, whilst the atmosphere does not possess that property, at least in any sensible degree. Towards evening, therefore, when the solar heat declines, and when after sunset it entirely ceases, the earth rapidly cools by radiating heat towards the skies; whilst the air has no means of parting with its heat but by coming into contact with the cooled surface of the earth, to which it communicates its caloric. Its solvent power being thus reduced, it is unable to retain so large a portion of watery vapour, and deposits those pearly drops which we call dew.

EMILY.

If this be the cause of dew, we need not be apprehensive 107 of receiving any injury from it; for it can be deposited only on surfaces that are colder than the atmosphere, which is never the case with our bodies.

MRS. B.

Very true; yet I would not advise you for this reason to be too confident of escaping all the ill effects which may arise from exposure to the dew; for it may be deposited on your clothes, and chill you afterwards by its evaporation from them. Besides, whenever the dew is copious, there is a chill in the atmosphere which it is not always safe to encounter.

CAROLINE.

Wind, then, must promote the deposition of dew, by bringing a more rapid succession of particles of air in contact with the earth, just as it promotes the cooling of the earth and warming of the atmosphere during the heat of the day?

MRS. B.

Yes; provided the wind be unattended with clouds, for these accumulations of moisture not only prevent the free radiation of the earth towards the upper regions, but themselves radiate towards the earth; under these circumstances much less dew is formed than on fine clear nights, when the radiation of the earth passes without obstacle through the atmosphere to the distant regions of space, whence it 108 receives no caloric in exchange. The dew continues to be deposited during the night, and is generally most abundant towards morning, when the contrast between the temperature of the earth and that of the air is greatest. After sunrise the equilibrium of temperature between these two bodies is gradually restored by the solar rays passing freely through the atmosphere to the earth; and later in the morning the temperature of the earth gains the ascendency, and gives out caloric to the air by contact, in the same manner as it receives it from the air during the night.—Can you tell me, now, why a bottle of wine taken fresh from the cellar (in summer particularly), will soon be covered with dew; and even the glasses into which the wine is poured will be moistened with a similar vapour?

EMILY.

The bottle being colder than the surrounding air, must absorb caloric from it; the moisture therefore which that air contained becomes visible, and forms the dew which is deposited on the bottle.

MRS. B.

Very well, Emily. Now, Caroline, can you inform me why, in a warm room, or close carriage, the contrary effect takes place; that is to say, that the inside of the windows is covered with vapour?

CAROLINE.

I have heard that it proceeds from the breath of those within the room or the carriage; and I suppose it is occasioned by the windows which, being colder than the breath, deprive it of part of its caloric, and by this means convert it into watery vapour.

MRS. B.

You have both explained it extremely well. Bodies attract dew in proportion as they are good radiators of caloric, as it is this quality which reduces their temperature below that of the atmosphere; hence we find that little or no dew is deposited on rocks, sand, water; while grass and living vegetables, to which it is so highly beneficial, attract it in abundance—another remarkable instance of the wise and bountiful dispensations of Providence.

EMILY.

And we may again observe it in the abundance of dew in summer, and in hot climates, when its cooling effects are so much required; but I do not understand what natural cause increases the dew in hot weather?

MRS. B.

The more caloric the earth receives during the day, the more it will radiate afterwards, and consequently the more rapidly its temperature will be reduced in the evening, in comparison to that of the 110 atmosphere. In the West-Indies especially, where the intense heat of the day is strongly contrasted with the coolness of the evening, the dew is prodigiously abundant. During a drought, the dew is less plentiful, as the earth is not sufficiently supplied with moisture to be able to saturate the atmosphere.

CAROLINE.

I have often observed, Mrs. B., that when I walk out in frosty weather, with a veil over my face, my breath freezes upon it. Pray what is the reason of that?

MRS. B.

It is because the cold air immediately seizes on the caloric of your breath, and, by robbing it of its solvent, reduces it to a denser fluid, which is the watery vapour that settles on your veil, and there it continues parting with its caloric till it is brought down to the temperature of the atmosphere, and assumes the form of ice.

You may, perhaps, have observed that the breath of animals, or rather the moisture contained in it, is visible in damp weather, or during a frost. In the former case, the atmosphere being over-saturated with moisture, can dissolve no more. In the latter, the cold condenses it into visible vapour; and for the same reason, the steam arising from water that is warmer than the atmosphere, 111 becomes visible. Have you never taken notice of the vapour rising from your hands after having dipped them into warm water?

CAROLINE.

Frequently, especially in frosty weather.

MRS. B.

We have already observed that pressure is an obstacle to evaporation: there are liquids that contain so great a quantity of caloric, and whose particles consequently adhere so slightly together, that they may be rapidly converted into vapour without any elevation of temperature, merely by taking off the weight of the atmosphere. In such liquids, you perceive, it is the pressure of the atmosphere alone that connects their particles, and keeps them in a liquid state.

CAROLINE.

I do not well understand why the particles of such fluids should be disunited and converted into vapour, without any elevation of temperature, in spite of the attraction of cohesion.

MRS. B.

It is because the degree of heat at which we usually observe these fluids is sufficient to overcome their attraction of cohesion. Ether is of this description; 112 it will boil and be converted into vapour, at the common temperature of the air, if the pressure of the atmosphere be taken off.

EMILY.

I thought that ether would evaporate without either the pressure of the atmosphere being taken away, or heat applied; and that it was for that reason so necessary to keep it carefully corked up?

MRS. B.

It is true it will evaporate, but without ebullition; what I am now speaking of is the vaporization of ether, or its conversion into vapour by boiling. I am going to show you how suddenly the ether in this phial will be converted into vapour, by means of the air-pump.—Observe with what rapidity the bubbles ascend, as I take off the pressure of the atmosphere.

CAROLINE.

It positively boils: how singular to see a liquid boil without heat!

MRS. B.

Now I shall place the phial of ether in this glass, which it nearly fits, so as to leave only a small space, which I fill with water; and in this state I put it again under the receiver. 113 (Plate IV. Fig. 1.)* You will observe, as I exhaust the air from it, that whilst the ether boils, the water freezes.

Plate IV.

Vol. I. p. 84.

Fig. 1. Ether evaporated & water frozen in the air pump.   A Phial of Ether.   B Glass vessel containing water.   C.C Thermometers, one in the Ether, the other in the water.

Larger view (complete Plate)

CAROLINE.

It is indeed wonderful to see water freeze in contact with a boiling fluid!

EMILY.

I am at a loss to conceive how the ether can pass to the state of vapour without an addition of caloric. Does it not contain more caloric in a state of vapour, than in a state of liquidity?

MRS. B.

It certainly does; for though it is the pressure of the atmosphere which condenses it into a liquid, it is by forcing out the caloric that belongs to it when in an aëriform state.

EMILY.

You have, therefore, two difficulties to explain, Mrs. B.—First, from whence the ether obtains the caloric necessary to convert it into vapour when it is relieved from the pressure of the atmosphere; and, secondly, what is the reason that the water, in which the bottle of ether stands, is frozen?

CAROLINE.

Now, I think, I can answer both these questions. The ether obtains the addition of caloric required, from the water in the glass; and the loss of caloric, which the latter sustains, is the occasion of its freezing.

MRS. B.

You are perfectly right; and if you look at the thermometer which I have placed in the water, whilst I am working the pump, you will see that every time bubbles of vapour are produced, the mercury descends; which proves that the heat of the water diminishes in proportion as the ether boils.

EMILY.

This I understand now very well; but if the water freezes in consequence of yielding its caloric to the ether, the equilibrium of heat must, in this case, be totally destroyed. Yet you have told us, that the exchange of caloric between two bodies of 115 equal temperature, was always equal; how, then, is it that the water, which was originally of the same temperature as the ether, gives out caloric to it, till the water is frozen, and the ether made to boil?

MRS. B.

I suspected that you would make these objections; and, in order to remove them, I enclosed two thermometers in the air-pump; one which stands in the glass of water, the other in the phial of ether; and you may see that the equilibrium of temperature is not destroyed; for as the thermometer descends in the water, that in the ether sinks in the same manner; so that both thermometers indicate the same temperature, though one of them is in a boiling, the other in a freezing liquid.

EMILY.

The ether, then, becomes colder as it boils? This is so contrary to common experience, that I confess it astonishes me exceedingly.

CAROLINE.

It is, indeed, a most extraordinary circumstance. But pray, how do you account for it?

MRS. B.

I cannot satisfy your curiosity at present; for before we can attempt to explain this apparent 116 paradox, it is necessary to become acquainted with the subject of LATENT HEAT: and that, I think, we must defer till our next interview.

CAROLINE.

I believe, Mrs. B., that you are glad to put off the explanation; for it must be a very difficult point to account for.

MRS. B.

I hope, however, that I shall do it to your complete satisfaction.

EMILY.

But before we part, give me leave to ask you one question. Would not water, as well as ether, boil with less heat, if deprived of the pressure of the atmosphere?

MRS. B.

Undoubtedly. You must always recollect that there are two forces to overcome, in order to make a liquid boil or evaporate; the attraction of aggregation, and the weight of the atmosphere. On the summit of a high mountain (as Mr. De Saussure ascertained on Mount Blanc) much less heat is required to make water boil, than in the plain, where the weight of the atmosphere is 117 greater.* Indeed if the weight of the atmosphere be entirely removed by means of a good air-pump, and if water be placed in the exhausted receiver, it will evaporate so fast, however cold it maybe, as to give it the appearance of boiling from the surface. But without the assistance of the air-pump, I can show you a very pretty experiment, which proves the effect of the pressure of the atmosphere in this respect.

Observe, that this Florence flask is about half full of water, and the upper half of invisible vapour, the water being in the act of boiling.—I take it from the lamp, and cork it carefully—the water, you see, immediately ceases boiling.—I shall now dip the flask into a bason of cold water.†

CAROLINE.

But look, Mrs. B., the hot water begins to boil again, although the cold water must rob it more and more of its caloric! What can be the reason of that?

MRS. B.

Let us examine its temperature. You see the thermometer immersed in it remains stationary at 180 degrees, which is about 30 degrees below the boiling point. When I took the flask from the lamp, I observed to you that the upper part of it was filled with vapour; this being compelled to yield its caloric to the cold water, was again condensed into water—What, then, filled the upper part of the flask?

EMILY.

Nothing; for it was too well corked for the air to gain admittance, and therefore the upper part of the flask must be a vacuum.

MRS. B.

The water below, therefore, no longer sustains the pressure of the atmosphere, and will consequently boil at a much lower temperature. Thus, you see, though it had lost many degrees of heat, it began boiling again the instant the vacuum was formed above it. The boiling has now ceased, the temperature of the water being still farther reduced; if it had been ether, instead of water, it would have continued boiling much longer, for ether boils, under the usual atmospheric pressure, at a temperature as low as 100 degrees; and in a vacuum it boils at almost any temperature; but 119 water being a more dense fluid, requires a more considerable quantity of caloric to make it evaporate quickly, even when the pressure of the atmosphere is removed.

EMILY.

What proportion of vapour can the atmosphere contain in a state of solution?

MRS. B.

I do not know whether it has been exactly ascertained by experiment; but at any rate this proportion must vary, both according to the temperature and the weight of the atmosphere; for the lower the temperature, and the greater the pressure, the smaller must be the proportion of vapour that the atmosphere can contain.

To conclude the subject of free caloric, I should mention Ignition, by which is meant that emission of light which is produced in bodies at a very high temperature, and which is the effect of accumulated caloric.

EMILY.

You mean, I suppose, that light which is produced by a burning body?

MRS. B.

No: ignition is quite independent of combustion. Clay, chalk, and indeed all incombustible 120 substances, may be made red hot. When a body burns, the light emitted is the effect of a chemical change which takes place, whilst ignition is the effect of caloric alone, and no other change than that of temperature is produced in the ignited body.

All solid bodies, and most liquids, are susceptible of ignition, or, in other words, of being heated so as to become luminous; and it is remarkable that this takes place pretty nearly at the same temperature in all bodies, that is, at about 800 degrees of Fahrenheit’s scale.

EMILY.

But how can liquids attain so high a temperature, without being converted into vapour?

MRS. B.

By means of confinement and pressure. Water confined in a strong iron vessel (called Papin’s digester) can have its temperature raised to upwards of 400 degrees. Sir James Hall has made some very curious experiments on the effects of heat assisted by pressure; by means of strong gun-barrels, he succeeded in melting a variety of substances which were considered as infusible: and it is not unlikely that, by similar methods, water itself might be heated to redness.

EMILY.

I am surprised at that: for I thought that the force of steam was such as to destroy almost all mechanical resistance.

MRS. B.

The expansive force of steam is prodigious; but in order to subject water to such high temperatures, it is prevented by confinement from being converted into steam, and the expansion of heated water is comparatively trifling.—But we have dwelt so long on the subject of free caloric, that we must reserve the other modifications of that agent to our next meeting, when we shall endeavour to proceed more rapidly.

MRS. B.

We are now to examine the other modifications of caloric.

CAROLINE.

I am very curious to know of what nature they can be; for I have no notion of any kind of heat that is not perceptible to the senses.

MRS. B.

In order to enable you to understand them, it will be necessary to enter into some previous explanations.

It has been discovered by modern chemists, that bodies of a different nature, heated to the same temperature, do not contain the same quantity of caloric.

CAROLINE.

How could that be ascertained? Have you not told us that it is impossible to discover the absolute quantity of caloric which bodies contain?

MRS. B.

True; but at the same time I said that we were enabled to form a judgment of the proportions which bodies bore to each other in this respect. Thus it is found that, in order to raise the temperature of different bodies the same number of degrees, different quantities of caloric are required for each of them. If, for instance, you place a pound of lead, a pound of chalk, and a pound of milk, in a hot oven, they will be gradually heated to the temperature of the oven; but the lead will attain it first, the chalk next, and the milk last.

CAROLINE.

That is a natural consequence of their different bulks; the lead being the smallest body, will be heated soonest, and the milk, which is the largest, will require the longest time.

MRS. B.

That explanation will not do, for if the lead be the least in bulk, it offers also the least surface to the caloric, the quantity of heat therefore which can enter into it in the same space of time is proportionally smaller.

EMILY.

Why, then, do not the three bodies attain the temperature of the oven at the same time?

MRS. B.

It is supposed to be on account of the different capacity of these bodies for caloric.

CAROLINE.

What do you mean by the capacity of a body for caloric?

MRS. B.

I mean a certain disposition of bodies to require more or less caloric for raising their temperature to any degree of heat. Perhaps the fact may be thus explained:

Let us put as many marbles into this glass as it will contain, and pour some sand over them—observe how the sand penetrates and lodges between them. We shall now fill another glass with pebbles of various forms—you see that they arrange themselves in a more compact manner than the marbles, which, being globular, can touch each other by a single point only. The pebbles, therefore, will not admit so much sand between them; and consequently one of these glasses will necessarily contain more sand than the other, though both of them be equally full.

CAROLINE.

This I understand perfectly. The marbles and the pebbles represent two bodies of different kinds, and the sand the caloric contained in them; 125 it appears very plain, from this comparison, that one body may admit of more caloric between its particles than another.

MRS. B.

You can no longer be surprised, therefore, that bodies of a different capacity for caloric should require different proportions of that fluid to raise their temperatures equally.

EMILY.

But I do not conceive why the body that contains the most caloric should not be of the highest temperature; that is to say, feel hot in proportion to the quantity of caloric it contains?

MRS. B.

The caloric that is employed in filling the capacity of a body, is not free caloric; but is imprisoned as it were in the body, and is therefore imperceptible: for we can feel only the caloric which the body parts with, and not that which it retains.

CAROLINE.

It appears to me very extraordinary that heat should be confined in a body in such a manner as to be imperceptible.

MRS. B.

If you lay your hand on a hot body, you feel only the caloric which leaves it, and enters your hand; for it is impossible that you should be sensible of that which remains in the body. The thermometer, in the same manner, is affected only by the free caloric which a body transmits to it, and not at all by that which it does not part with.

CAROLINE.

I begin to understand it: but I confess that the idea of insensible heat is so new and strange to me, that it requires some time to render it familiar.

MRS. B.

Call it insensible caloric, and the difficulty will appear much less formidable. It is indeed a sort of contradiction to call it heat, when it is so situated as to be incapable of producing that sensation. Yet this modification of caloric is commonly called SPECIFIC HEAT.

CAROLINE.

But it certainly would have been more correct to have called it specific caloric.

EMILY.

I do not understand how the term specific applies to this modification of caloric?

MRS. B.

It expresses the relative quantity of caloric which different species of bodies of the same weight and temperature are capable of containing. This modification is also frequently called heat of capacity, a term perhaps preferable, as it explains better its own meaning.

You now understand, I suppose, why the milk and chalk required a longer portion of time than the lead to raise their temperature to that of the oven?

EMILY.

Yes: the milk and chalk having a greater capacity for caloric than the lead, a greater proportion of that fluid became insensible in those bodies: and the more slowly, therefore, their temperature was raised.

CAROLINE.

But might not this difference proceed from the different conducting powers of heat in these three bodies, since that which is the best conductor must necessarily attain the temperature of the oven first?

MRS. B.

Very well observed, Caroline. This objection would be insurmountable, if we could not, by reversing the experiment, prove that the milk, the chalk, and the lead, actually absorbed different 128 quantities of caloric, and we know that if the different time they took in heating, proceeded merely from their different conducting powers, they would each have acquired an equal quantity of caloric.

CAROLINE.

Certainly. But how can you reverse this experiment?

MRS. B.

It may be done by cooling the several bodies to the same degree in an apparatus adapted to receive and measure the caloric which they give out. Thus, if you plunge them into three equal quantities of water, each at the same temperature, you will be able to judge of the relative quantity of caloric which the three bodies contained, by that, which, in cooling, they communicated to their respective portions of water: for the same quantity of caloric which they each absorbed to raise their temperature, will abandon them in lowering it; and on examining the three vessels of water, you will find the one in which you immersed the lead to be the least heated; that which held the chalk will be the next; and that which contained the milk will be heated the most of all. The celebrated Lavoisier has invented a machine to estimate, upon this principle, the specific heat of bodies in a more perfect manner; but I cannot 129 explain it to you, till you are acquainted with the next modification of caloric.

EMILY.

The more dense a body is, I suppose, the less is its capacity for caloric?

MRS. B.

This is not always the case with bodies of different nature; iron, for instance, contains more specific heat than tin, though it is more dense. This seems to show that specific heat does hot merely depend upon the interstices between the particles; but, probably, also upon some peculiar constitution of the bodies which we do not comprehend.

EMILY.

But, Mrs. B., it would appear to me more proper to compare bodies by measure, rather than by weight, in order to estimate their specific heat. Why, for instance, should we not compare pints of milk, of chalk, and of lead, rather than pounds of those substances; for equal weights may be composed of very different quantities?

MRS. B.

You are mistaken, my dear; equal weight must contain equal quantities of matter; and when we wish to know what is the relative quantity of caloric, 130 which substances of various kinds are capable of containing under the same temperature, we must compare equal weights, and not equal bulks of those substances. Bodies of the same weight may undoubtedly be of very different dimensions; but that does not change their real quantity of matter. A pound of feathers does not contain one atom more than a pound of lead.

CAROLINE.

I have another difficulty to propose. It appears to me, that if the temperature of the three bodies in the oven did not rise equally, they would never reach the same degree; the lead would always keep its advantage over the chalk and milk, and would perhaps be boiling before the others had attained the temperature of the oven. I think you might as well say that, in the course of time, you and I should be of the same age?

MRS. B.

Your comparison is not correct, Caroline. As soon as the lead reached the temperature of the oven, it would remain stationary; for it would then give out as much heat as it would receive. You should recollect that the exchange of radiating heat, between two bodies of equal temperature, is equal: it would be impossible, therefore, for the lead to accumulate heat after having attained 131 the temperature of the oven; and that of the chalk and milk therefore would ultimately arrive at the same standard. Now I fear that this will not hold good with respect to our ages, and that, as long as I live, I shall never cease to keep my advantage over you.

EMILY.

I think that I have found a comparison for specific heat, which is very applicable. Suppose that two men of equal weight and bulk, but who required different quantities of food to satisfy their appetites, sit down to dinner, both equally hungry; the one would consume a much greater quantity of provisions than the other, in order to be equally satisfied.

MRS. B.

Yes, that is very fair; for the quantity of food necessary to satisfy their respective appetites, varies in the same manner as the quantity of caloric requisite to raise equally the temperature of different bodies.

EMILY.

The thermometer, then, affords no indication of the specific heat of bodies?

MRS. B.

None at all: no more than satiety is a test of the quantity of food eaten. The thermometer, as 132 I have repeatedly said, can be affected only by free caloric, which alone raises the temperature of bodies.

But there is another mode of proving the existence of specific heat, which affords a very satisfactory illustration of that modification. This, however, I did not enlarge upon before, as I thought it might appear to you rather complicated.—If you mix two fluids of different temperatures, let us say the one at 50 degrees, and the other at 100 degrees, of what temperature do you suppose the mixture will be?

CAROLINE.

It will be no doubt the medium between the two, that is to say, 75 degrees.

MRS. B.

That will be the case if the two bodies happen to have the same capacity for caloric; but if not, a different result will be obtained. Thus, for instance, if you mix together a pound of mercury, heated at 50 degrees, and a pound of water heated at 100 degrees, the temperature of the mixture, instead of being 75 degrees, will be 80 degrees; so that the water will have lost only 12 degrees, whilst the mercury will have gained 38 degrees; from which you will conclude that the capacity of mercury for heat is less than that of water.

CAROLINE.

I wonder that mercury should have so little specific heat. Did we not see it was a much better conductor of heat than water?

MRS. B.

And it is precisely on that account that its specific heat is less. For since the conductive power of bodies depends, as we have observed before, on their readiness to receive heat and part with it, it is natural to expect that those bodies which are the worst conductors should absorb the most caloric before they are disposed to part with it to other bodies. But let us now proceed to LATENT HEAT.

CAROLINE.

And pray what kind of heat is that?

MRS. B.

It is another modification of combined caloric, which is so analogous to specific heat, that most chemists make no distinction between them; but Mr. Pictet, in his Essay on Fire, has so clearly discriminated them, that I am induced to adopt his view of the subject. We therefore call latent heat that portion of insensible caloric which is employed in changing the state of bodies; that is to say, in converting solids into liquids, or liquids; into vapour. When a body changes its state from 134 solid to liquid, or from liquid to vapour, its expansion occasions a sudden and considerable increase of capacity for heat, in consequence of which it immediately absorbs a quantity of caloric, which becomes fixed in the body which it has transformed; and, as it is perfectly concealed from our senses, it has obtained the name of latent heat.

CAROLINE.

I think it would be much more correct to call this modification latent caloric instead of latent heat, since it does not excite the sensation of heat.

MRS. B.

This modification of heat was discovered and named by Dr. Black long before the French chemists introduced the term caloric, and we must not presume to alter it, as it is still used by much better chemists than ourselves. And, besides, you are not to suppose that the nature of heat is altered by being variously modified: for if latent heat and specific heat do not excite the same sensations as free caloric, it is owing to their being in a state of confinement, which prevents them from acting upon our organs; and consequently, as soon as they are extricated from the body in which they are imprisoned, they return to their state of free caloric.

EMILY.

But I do not yet clearly see in what respect latent heat differs from specific heat; for they are both of them imprisoned and concealed in bodies.

MRS. B.

Specific heat is that which is employed in filling the capacity of a body for caloric, in the state in which this body actually exists; while latent heat is that which is employed only in effecting a change of state, that is, in converting bodies from a solid to a liquid, or from a liquid to an aëriform state. But I think that, in a general point of view, both these modifications might be comprehended under the name of heat of capacity, as in both cases the caloric is equally engaged in filling the capacities of bodies.

I shall now show you an experiment, which I hope will give you a clear idea of what is understood by latent heat.

The snow which you see in this phial has been cooled by certain chemical means (which I cannot well explain to you at present), to 5 or 6 degrees below the freezing point, as you will find indicated by the thermometer which is placed in it. We shall expose it to the heat of a lamp, and you will see the thermometer gradually rise, till it reaches the freezing point——

EMILY.

But there it stops, Mrs. B., and yet the lamp burns just as well as before. Why is not its heat communicated to the thermometer?

CAROLINE.

And the snow begins to melt, therefore it must be rising above the freezing point?

MRS. B.

The heat no longer affects the thermometer, because it is wholly employed in converting the ice into water. As the ice melts, the caloric becomes latent in the new-formed liquid, and therefore cannot raise its temperature; and the thermometer will consequently remain stationary, till the whole of the ice be melted.

CAROLINE.

Now it is all melted, and the thermometer begins to rise again.

MRS. B.

Because the conversion of the ice into water being completed, the caloric no longer becomes latent; and therefore the heat which the water now receives raises its temperature, as you find the thermometer indicates.

EMILY.

But I do not think that the thermometer rises so quickly in the water as it did in the ice, previous to its beginning to melt, though the lamp burns equally well?

MRS. B.

That is owing to the different specific heat of ice and water. The capacity of water for caloric being greater than that of ice, more heat is required to raise its temperature, and therefore the thermometer rises slower in the water than in the ice.

EMILY.

True; you said that a solid body always increased its capacity for heat by becoming fluid; and this is an instance of it.

MRS. B.

Yes, and the latent heat is that which is absorbed in consequence of the greater capacity which the water has for heat, in comparison to ice.

I must now tell you a curious calculation founded on that consideration. I have before observed to you that though the thermometer shows us the comparative warmth of bodies, and enables us to determine the same point at different times and places, it gives us no idea of the absolute quantity of heat in any body. We cannot tell how low it ought to fall by the privation of all heat, but an 138 attempt has been made to infer it in the following manner. It has been found by experiment, that the capacity of water for heat, when compared with that of ice, is as 10 to 9, so that, at the same temperature, ice contains one tenth of caloric less than water. By experiment also it is observed, that in order to melt ice, there must be added to it as much heat, as would, if it did not melt it, raise its temperature 140 degrees. This quantity of heat is therefore absorbed when the ice, by being converted into water, is made to contain one-ninth more caloric than it did before. Therefore 140 degrees is a ninth part of the heat contained in ice at 30 degrees; and the point of zero, or the absolute privation of heat, must consequently be 1260 degrees below 32 degrees.

This mode of investigating so curious a question is ingenious, but its correctness is not yet established by similar calculations for other bodies. The points of absolute cold, indicated by this method in various bodies, are very remote from each other; it is however possible, that this may arise from some imperfection in the experiments.

CAROLINE.

It is indeed very ingenious—but we must now attend to our present experiment. The water begins to boil, and the thermometer is again stationary.

MRS. B.

Well, Caroline, it is your turn to explain the phenomenon.

CAROLINE.

It is wonderfully curious! The caloric is now busy in changing the water into steam, in which it hides itself, and becomes insensible. This is another example of latent heat, producing a change of form. At first it converted a solid body into a liquid, and now it turns the liquid into vapour!

MRS. B.

You see, my dear, how easily you have become acquainted with these modifications of insensible heat, which at first appeared so unintelligible. If, now, we were to reverse these changes, and condense the vapour into water, and the water into ice, the latent heat would re-appear entirely, in the form of free caloric.

EMILY.

Pray do let us see the effect of latent heat returning to its free state.

MRS. B.

For the purpose of showing this, we need simply conduct the vapour through this tube into this vessel of cold water, where it will part with its latent heat and return to its liquid form.

EMILY.

How rapidly the steam heats the water!

MRS. B.

That is because it does not merely impart its free caloric to the water, but likewise its latent heat. This method of heating liquids, has been turned to advantage, in several economical establishments. The steam-kitchens, which are getting into such general use, are upon the same principle. The steam is conveyed through a pipe in a similar manner, into the several vessels which contain the provisions to be dressed, where it communicates to them its latent caloric, and returns to the state of water. Count Rumford makes great use of this principle in many of his fire-places: his grand maxim is to avoid all unnecessary waste of caloric, for which purpose he confines the heat in such a manner, that not a particle of it shall unnecessarily escape; and while he economises the free caloric, he takes care also to turn the latent heat to advantage. It is thus that he is enabled to produce a degree of heat superior to that which is obtained in common fire-places, though he employs less fuel.

EMILY.

When the advantages of such contrivances are so clear and plain, I cannot understand why they are not universally used.

MRS. B.

A long time is always required before innovations, however useful, can be reconciled with the prejudices of the vulgar.

EMILY.

What a pity it is that there should be a prejudice against new inventions; how much more rapidly the world would improve, if such useful discoveries were immediately and universally adopted!

MRS. B.

I believe, my dear, that there are as many novelties attempted to be introduced, the adoption of which would be prejudicial to society, as there are of those which would be beneficial to it. The well-informed, though by no means exempt from error, have an unquestionable advantage over the illiterate, in judging what is likely or not to prove serviceable; and therefore we find the former more ready to adopt such discoveries as promise to be really advantageous, than the latter, who having no other test of the value of a novelty but time and experience, at first oppose its introduction. The well-informed, however, are frequently disappointed in their most sanguine expectations, and the prejudices of the vulgar, though they often retard the progress of knowledge, yet sometimes, it must be 142 admitted, prevent the propagation of error.—But we are deviating from our subject.

We have converted steam into water, and are now to change water into ice, in order to render the latent heat sensible, as it escapes from the water on its becoming solid. For this purpose we must produce a degree of cold that will make water freeze.

CAROLINE.

That must be very difficult to accomplish in this warm room.

MRS. B.

Not so much as you think. There are certain chemical mixtures which produce a rapid change from the solid to the fluid state, or the reverse, in the substances combined, in consequence of which change latent heat is either extricated or absorbed.

EMILY.

I do not quite understand you.

MRS. B.

This snow and salt, which you see me mix together, are melting rapidly; heat, therefore, must be absorbed by the mixture, and cold produced.

CAROLINE.

It feels even colder than ice, and yet the snow is melted. This is very extraordinary.

MRS. B.

The cause of the intense cold of the mixture is to be attributed to the change from a solid to a fluid state. The union of the snow and salt produces a new arrangement of their particles, in consequence of which they become liquid; and the quantity of caloric, required to effect this change, is seized upon by the mixture wherever it can be obtained. This eagerness of the mixture for caloric, during its liquefaction, is such, that it converts part of its own free caloric into latent heat, and it is thus that its temperature is lowered.

EMILY.

Whatever you put in this mixture, therefore, would freeze?

MRS. B.

Yes; at least any fluid that is susceptible of freezing at that temperature. I have prepared this mixture of salt and snow for the purpose of freezing the water from which you are desirous of seeing the latent heat escape. I have put a thermometer in the glass of water that is to be frozen, in order that you may see how it cools.

CAROLINE.

The thermometer descends, but the heat which the water is now losing, is its free, not its latent heat.

MRS. B.

Certainly; it does not part with its latent heat till it changes its state and is converted into ice.

EMILY.

But here is a very extraordinary circumstance! The thermometer is fallen below the freezing point, and yet the water is not frozen.

MRS. B.

That is always the case previous to the freezing of water when it is in a state of rest. Now it begins to congeal, and you may observe that the thermometer again rises to the freezing point.

CAROLINE.

It appears to me very strange that the thermometer should rise the very moment that the water freezes; for it seems to imply that the water was colder before it froze than when in the act of freezing.

MRS. B.

It is so; and after our long dissertation on this circumstance, I did not think it would appear so surprising to you. Reflect a little, and I think you will discover the reason of it.

CAROLINE.

It must be, no doubt, the extrications of latent heat, at the instant the water freezes, that raises the temperature.

MRS. B.

Certainly; and if you now examine the thermometer, you will find that its rise was but temporary, and lasted only during the disengagement of the latent heat—now that all the water is frozen it falls again, and will continue to fall till the ice and mixture are of an equal temperature.

EMILY.

And can you show us any experiments in which liquids, by being mixed, become solid, and disengage latent heat?

MRS. B.

I could show you several; but you are not yet sufficiently advanced to understand them well. I shall, however, try one, which will afford you a striking instance of the fact. The fluid which you see in this phial consists of a quantity of a certain salt called muriat of lime, dissolved in water. Now, if I pour into it a few drops of this other fluid, called sulphuric acid, the whole, or very nearly the whole, will be instantaneously converted into a solid mass.

EMILY.

How white it turns! I feel the latent heat escaping, for the bottle is warm, and the fluid is changed to a solid white substance like chalk!

CAROLINE.

This is, indeed, the most curious experiment we have seen yet. But pray what is that white vapour that ascends from the mixture?

MRS. B.

You are not yet enough of a chemist to understand that.—But take care, Caroline, do not approach too near it, for it has a very pungent smell.

I shall show you another instance similar to that of the water, which you observed to become warmer as it froze. I have in this phial a solution of a salt called sulphat of soda or Glauber’s salt, made very strong, and corked up when it was hot, and kept without agitation till it became cold, as you may feel the phial is. Now when I take out the cork and let the air fall upon it, (for being closed when boiling, there was a vacuum in the upper part) observe that the salt will suddenly crystallize. . . .

CAROLINE.

Surprising! how beautifully the needles of salt have shot through the whole phial!

MRS. B.

Yes, it is very striking—but pray do not forget the object of the experiment. Feel how warm the phial has become by the conversion of part of the liquid into a solid.

EMILY.

Quite warm I declare! this is a most curious experiment of the disengagement of latent heat.

MRS. B.

The slakeing of lime is another remarkable instance of the extrication of latent heat. Have you never observed how quick-lime smokes when water is poured upon it, and how much heat it produces?

CAROLINE.

Yes; but I do not understand what change of state takes place in the lime that occasions its giving out latent heat; for the quick-lime, which is solid, is (if I recollect right) reduced to powder, by this operation, and is, therefore, rather expanded than condensed.

MRS. B.

It is from the water, not the lime, that the latent heat is set free. The water incorporates with, and becomes solid in the lime; in consequence of which, the heat, which kept it in a liquid state, is disengaged, and escapes in a sensible form.

CAROLINE.

I always thought that the heat originated in the lime. It seems very strange that water, and cold water too, should contain so much heat.

EMILY.

After this extrication of caloric, the water must exist in a state of ice in the lime, since it parts with the heat which kept it liquid.

MRS. B.

It cannot properly be called ice, since ice implies a degree of cold, at least equal to the freezing point. Yet as water, in combining with lime, gives out more heat than in freezing, it must be in a state of still greater solidity in the lime, than it is in the form of ice; and you may have observed that it does not moisten or liquefy the lime in the smallest degree.

EMILY.

But, Mrs. B., the smoke that rises is white; if it was only pure caloric which escaped, we might feel, but could not see it.

MRS. B.

This white vapour is formed by some of the particles of lime, in a state of fine dust, which are carried off by the caloric.

EMILY.

In all changes of state, then, a body either absorbs or disengages latent heat?

MRS. B.

You cannot exactly say absorbs latent heat, as the heat becomes latent only on being confined in the body; but you may say, generally, that bodies, in passing from a solid to a liquid form, or from the liquid state to that of vapour, absorb heat; and that when the reverse takes place, heat is disengaged.*

EMILY.

We can now, I think, account for the ether boiling, and the water freezing in vacuo, at the same temperature.†

MRS. B.

Let me hear how you explain it.

EMILY.

The latent heat, which the water gave out in freezing, was immediately absorbed by the ether, during its conversion into vapour; and therefore, from a latent state in one liquid, it passed into a latent state in the other.

MRS. B.

But this only partly accounts for the result of the experiment; it remains to be explained why the 150 temperature of the ether, while in a state of ebullition, is brought down to the freezing temperature of the water.—It is because the ether, during its evaporation, reduces its own temperature, in the same proportion as that of the water, by converting its free caloric into latent heat: so that, though one liquid boils, and the other freezes, their temperatures remain in a state of equilibrium.

EMILY.

But why does not water, as well as ether, reduce its own temperature by evaporating?

MRS. B.

The fact is that it does, though much less rapidly than ether. Thus, for instance, you may often have observed, in the heat of summer, how much any particular spot may be cooled by watering, though the water used for that purpose be as warm as the air itself. Indeed so much cold may be produced by the mere evaporation of water, that the inhabitants of India, by availing themselves of the most favourable circumstances for this process which their warm climate can afford, namely, the cool of the night, and situations most exposed to the night breeze, succeed in causing water to freeze, though the temperature of the air be as high as 60 degrees. The water is put into shallow earthen trays, so as to expose an extensive 151 surface to the process of evaporation, and in the morning, the water is found covered with a thin cake of ice, which is collected in sufficient quantity to be used for purposes of luxury.

CAROLINE.

How delicious it must be to drink liquids so cold in those tropical climates! But, Mrs. B., could we not try that experiment?

MRS. B.

If we were in the country, I have no doubt but that we should be able to freeze water, by the same means, and under similar circumstances. But we can do it immediately, upon a small scale, in this very room, in which the thermometer stands at 70 degrees. For this purpose we need only place some water in a little cup under the receiver of the air-pump (Plate V. fig. 1.), and exhaust the air from it. What will be the consequence, Caroline?

Plate V.

Vol. I. page 138.

Fig. 1.   The air-pump & receiver for M^r. Leslie’s experiment.   C a saucer with sulphuric Acid.   B a glass or earthen cup containing Water.   D a stand for the cup with its legs made of Glass.   A a Thermometer.

Larger view (complete Plate)

CAROLINE.

Of course the water will evaporate more quickly, since there will no longer be any atmospheric pressure on its surface: but will this be sufficient to make the water freeze?

MRS. B.

Probably not, because the vapour will not be carried off fast enough; but this will be accomplished without difficulty if we introduce into the receiver (fig. 1.), in a saucer, or other large shallow vessel, some strong sulphuric acid, a substance which has a great attraction for water, whether in the form of vapour, or in the liquid state. This attraction is such that the acid will instantly absorb the moisture as it rises from the water, so as to make room for the formation of fresh vapour; this will of course hasten the process, and the cold produced from the rapid evaporation of the water, will, in a few minutes, be sufficient to freeze its surface.* We shall now exhaust the air from the receiver.

EMILY.

Thousands of small bubbles already rise through the water from the internal surface of the cup; what is the reason of this?

MRS. B.

These are bubbles of air which were partly attached to the vessel, and partly diffused in the water itself; and they expand and rise in consequence of the atmospheric pressure being removed.

CAROLINE.

See, Mrs. B.; the thermometer in the cup is sinking fast; it has already descended to 40 degrees!

EMILY.

The water seems now and then violently agitated on the surface, as if it was boiling; and yet the thermometer is descending fast!

MRS. B.

You may call it boiling, if you please, for this appearance is, as well as boiling, owing to the rapid formation of vapour; but here, as you have just observed, it takes place from the surface, for it is only when heat is applied to the bottom of the vessel that the vapour is formed there.—Now crystals of ice are actually shooting all over the surface of the water.

CAROLINE.

How beautiful it is! The surface is now entirely frozen—but the thermometer remains at 32 degrees.

MRS. B.

And so it will, conformably with our doctrine of latent heat, until the whole of the water is frozen; but it will then again begin to descend lower and lower, in consequence of the evaporation which goes on from the surface of the ice.

EMILY.

This is a most interesting experiment; but it would be still more striking if no sulphuric acid were required.

MRS. B.

I will show you a freezing instrument, contrived by Dr. Wollaston, upon the same principle as Mr. Leslie’s experiment, by which water may be frozen by its own evaporation alone, without the assistance of sulphuric acid.

This tube, which, as you see (Plate V. fig. 2.), is terminated at each extremity by a bulb, one of which is half full of water, is internally perfectly exhausted of air; the consequence of this is, that the water in the bulb is always much disposed to evaporate. This evaporation, however, does not proceed sufficiently fast to freeze the water; but if the empty ball be cooled by some artificial means, so as to condense quickly the vapour which rises from the water, the process may be thus so much promoted as to cause the water to freeze in the other ball. Dr. Wollaston has called this instrument Cryophorus.

Plate V.

Vol. I. page 138.

Fig. 2. D^r. Wollaston’s Cryophorus.
Fig. 5. D^r. Marcet’s mode of using the Cryophorus.
Fig. 3. & 4. the different parts of Fig. 5. seen separate.

Larger view (complete Plate)

CAROLINE.

So that cold seems to perform here the same part which the sulphuric acid acted in Mr. Leslie’s experiment?

MRS. B.

Exactly so; but let us try the experiment.

EMILY.

How will you cool the instrument? You have neither ice nor snow.

MRS. B.

True: but we have other means of effecting this.* You recollect what an intense cold can be produced by the evaporation of ether in an exhausted receiver. We shall inclose the bulb in this little bag of fine flannel (fig. 3.), then soke it in ether, and introduce it into the receiver of the air-pump. (Fig. 5.) For this purpose we shall find it more convenient to use a cryophorus of this shape (fig. 4.), as its elongated bulb passes easily through a brass plate which closes the top of the receiver. If we now exhaust the receiver quickly, you will see, in less than a minute, the water freeze in the other bulb, out of the receiver.

EMILY.

The bulb already looks quite dim, and small drops of water are condensing on its surface.

CAROLINE.

And now crystals of ice shoot all over the water. This is, indeed, a very curious experiment!

MRS. B.

You will see, some other day, that, by a similar method, even quicksilver may be frozen.—But we cannot at present indulge in any further digression.

Having advanced so far on the subject of heat, I may now give you an account of the calorimeter, an instrument invented by Lavoisier, upon the principles just explained, for the purpose of estimating the specific heat of bodies. It consists of a vessel, the inner surface of which is lined with ice, so as to form a sort of hollow globe of ice, in the midst of which the body, whose specific heat is to be ascertained, is placed. The ice absorbs caloric from this body, till it has brought it down to the freezing point; this caloric converts into water a certain portion of the ice which runs out through an aperture at the bottom of the machine; and the quantity of ice changed to water is a test of the quantity of caloric which the body has given out in descending from a certain temperature to the freezing point.

CAROLINE.

In this apparatus, I suppose, the milk, chalk, 157 and lead, would melt different quantities of ice, in proportion to their different capacities for caloric?

MRS. B.

Certainly: and thence we are able to ascertain, with precision, their respective capacities for heat. But the calorimeter affords us no more idea of the absolute quantity of heat contained in a body, than the thermometer; for though by means of it we extricate both the free and combined caloric, yet we extricate them only to a certain degree, which is the freezing point; and we know not how much they contain of either below that point.

EMILY.

According to the theory of latent heat, it appears to me that the weather should be warm when it freezes, and cold in a thaw: for latent heat is liberated from every substance that it freezes, and such a large supply of heat must warm the atmosphere; whilst, during a thaw, that very quantity of free heat must be taken from the atmosphere, and return to a latent state in the bodies which it thaws.

MRS. B.

Your observation is very natural; but consider that in a frost the atmosphere is so much colder than the earth, that all the caloric which it takes 158 from the freezing bodies is insufficient to raise its temperature above the freezing point; otherwise the frost must cease. But if the quantity of latent heat extricated does not destroy the frost, it serves to moderate the suddenness of the change of temperature of the atmosphere, at the commencement both of frost, and of a thaw. In the first instance, its extrication diminishes the severity of the cold; and, in the latter, its absorption moderates the warmth occasioned by a thaw: it even sometimes produces a discernible chill, at the breaking up of a frost.

CAROLINE.

But what are the general causes that produce those sudden changes in the weather, especially from hot to cold, which we often experience?

MRS. B.

This question would lead us into meteorological discussions, to which I am by no means competent. One circumstance, however, we can easily understand. When the air has passed over cold countries, it will probably arrive here at a temperature much below our own, and then it must absorb heat from every object it meets with, which will produce a general fall of temperature.

CAROLINE.

But pray, now that we know so much of the 159 effects of heat, will you inform us whether it is really a distinct body, or, as I have heard, a peculiar kind of motion produced in bodies?

MRS. B.

As I before told you, there is yet much uncertainty as to the nature of these subtle agents. But I am inclined to consider heat not as mere motion, but as a separate substance. Late experiments too appear to make it a compound body, consisting of the two electricities, and in our next conversation I shall inform you of the principal facts on which that opinion is founded.

MRS. B.

Before we proceed further it will be necessary to give you some account of certain properties of electricity, which have of late years been discovered to have an essential connection with the phenomena of chemistry.

CAROLINE.

It is ELECTRICITY, if I recollect right, which comes next in our list of simple substances?

MRS. B.

I have placed electricity in that list, rather from the necessity of classing it somewhere, than from any conviction that it has a right to that situation, for we are as yet so ignorant of its intimate nature, that we are unable to determine, not only whether it is simple or compound, but whether it is in fact a material agent; or, as Sir H. Davy has hinted, whether it may not be merely a property inherent 161 in matter. As, however, it is necessary to adopt some hypothesis for the explanation of the discoveries which this agent has enabled us to make, I have chosen the opinion, at present most prevalent, which supposes the existence of two kinds of electricity, distinguished by the names of positive and negative electricity.

CAROLINE.

Well, I must confess, I do not feel nearly so interested in a science in which so much uncertainty prevails, as in those which rest upon established principles; I never was fond of electricity, because, however beautiful and curious the phenomena it exhibits may be, the theories, by which they were explained, appeared to me so various, so obscure and inadequate, that I always remained dissatisfied. I was in hopes that the new discoveries in electricity had thrown so great a light on the subject, that every thing respecting it would now have been clearly explained.

MRS. B.

That is a point which we are yet far from having attained. But, in spite of the imperfection of our theories, you will be amply repaid by the importance and novelty of the subject. The number of new facts which have already been ascertained, and the immense prospect of discovery 162 which has lately been opened to us, will, I hope, ultimately lead to a perfect elucidation of this branch of natural science; but at present you must be contented with studying the effects, and in some degree explaining the phenomena, without aspiring to a precise knowledge of the remote cause of electricity.

You have already obtained some notions of electricity: in our present conversation, therefore, I shall confine myself to that part of the science which is of late discovery, and is more particularly connected with chemistry.

It was a trifling and accidental circumstance which first gave rise to this new branch of physical science. Galvani, a professor of natural philosophy at Bologna, being engaged (about twenty years ago) in some experiments on muscular irritability, observed, that when a piece of metal was laid on the nerve of a frog, recently dead, whilst the limb supplied by that nerve rested upon some other metal, the limb suddenly moved, on a communication being made between the two pieces of metal.

EMILY.

How is this communication made?

MRS. B.

Either by bringing the two metals into contact, or by connecting them by means of a metallic conductor. 163 But without subjecting a frog to any cruel experiments, I can easily make you sensible of this kind of electric action. Here is a piece of zinc, (one of the metals I mentioned in the list of elementary bodies)—put it under your tongue, and this piece of silver upon your tongue, and let both the metals project a little beyond the tip of the tongue—very well—now make the projecting parts of the metals touch each other, and you will instantly perceive a peculiar sensation.

EMILY.

Indeed I did, a singular taste, and I think a degree of heat: but I can hardly describe it.

MRS. B.

The action of these two pieces of metal on the tongue is, I believe, precisely similar to that made on the nerve of a frog. I shall not detain you by a detailed account of the theory by which Galvani attempted to account for this fact, as his explanation was soon overturned by subsequent experiments, which proved that Galvanism (the name this new power had obtained) was nothing more than electricity. Galvani supposed that the virtue of this new agent resided in the nerves of the frog, but Volta, who prosecuted this subject with much greater success, shewed that the phenomena did not depend on the organs of the frog, but upon 164 the electrical agency of the metals, which is excited by the moisture of the animal, the organs of the frog being only a delicate test of the presence of electric influence.

CAROLINE.

I suppose, then, the saliva of the mouth answers the same purpose as the moisture of the frog, in exciting the electricity of the pieces of silver and zinc with which Emily tried the experiment on her tongue.

MRS. B.

Precisely. It does not appear, however, necessary that the fluid used for this purpose should be of an animal nature. Water, and acids very much diluted by water, are found to be the most effectual in promoting the developement of electricity in metals; and, accordingly, the original apparatus which Volta first constructed for this purpose, consisted of a pile or succession of plates of zinc and copper, each pair of which was connected by pieces of cloth or paper impregnated with water; and this instrument, from its original inconvenient structure and limited strength, has gradually arrived at its present state of power and improvement, such as is exhibited in the Voltaic battery. In this apparatus, a specimen of which you see before you (Plate VI. fig. 1.), the plates of zinc and copper are soldered together in pairs, each pair being placed at regular 165 distances in wooden troughs and the interstices being filled with fluid.

Plate VI.

p. 151.

Fig. 1. 2. & 4.   Voltaic Batteries

Larger view (complete Plate)

CAROLINE.

Though you will not allow us to enquire into the precise cause of electricity, may we not ask in what manner the fluid acts on the metals so as to produce it?

MRS. B.

The action of the fluid on the metals, whether water or acid be used, is entirely of a chemical nature. But whether electricity is excited by this chemical action, or whether it is produced by the contact of the two metals, is a point upon which philosophers do not yet perfectly agree.

EMILY.

But can the mere contact of two metals, without any intervening fluid, produce electricity?

MRS. B.

Yes, if they are afterwards separated. It is an established fact, that when two metals are put in contact, and afterwards separated, that which has the strongest attraction for oxygen exhibits signs of positive, the other of negative electricity.

CAROLINE.

It seems then but reasonable to infer that the 166 power of the Voltaic battery should arise from the contact of the plates of zinc and copper.

MRS. B.

It is upon this principle that Volta and Sir H. Davy explain the phenomena of the pile; but notwithstanding these two great authorities, many philosophers entertain doubts on the truth of this theory. The principal difficulty which occurs in explaining the phenomena of the Voltaic battery on this principle, is, that two such plates show no signs of different states of electricity whilst in contact, but only on being separated after contact. Now in the Voltaic battery, those plates that are in contact always continue so, being soldered together: and they cannot therefore receive a succession of charges. Besides, if we consider the mere disturbance of the balance of electricity by the contact of the plates, as the sole cause of the production of Voltaic electricity, it remains to be explained how this disturbed balance becomes an inexhaustible source of electrical energy, capable of pouring forth a constant and copious supply of electrical fluid, though without any means of replenishing itself from other sources. This subject, it must be owned, is involved in too much obscurity to enable us to speak very decidedly in favour of any theory. But, in order to avoid perplexing you with different explanations, 167 I shall confine myself to one which appears to me to be least encumbered with difficulties, and most likely to accord with truth.*

This theory supposes the electricity to be excited by the chemical action of the acid on the zinc; but you are yet such novices in chemistry, that I think it will be necessary to give you some previous explanation of the nature of this action.

All metals have a strong attraction for oxygen, and this element is found in great abundance both in water and in acids. The action of the diluted acid on the zinc consists therefore in its oxygen combining with it, and dissolving its surface.

CAROLINE.

In the same manner I suppose as we saw an acid dissolve copper?

MRS. B.

Yes; but in the Voltaic battery the diluted acid is not strong enough to produce so complete 168 an effect; it acts only on the surface of the zinc, to which it yields its oxygen, forming upon it a film or crust, which is a compound of the oxygen and the metal.

EMILY.

Since there is so strong a chemical attraction between oxygen and metals, I suppose they are naturally in different states of electricity?

MRS. B.

Yes; it appears that all metals are united with the positive, and that oxygen is the grand source of the negative electricity.

CAROLINE.

Does not then the acid act on the plates of copper, as well as on those of zinc?

MRS. B.

No; for though copper has an affinity for oxygen, it is less strong than that of zinc; and therefore the energy of the acid is only exerted upon the zinc.

It will be best, I believe, in order to render the action of the Voltaic battery more intelligible, to confine our attention at first to the effect produced on two plates only. (Plate VI. fig. 2.)

If a plate of zinc be placed opposite to one of copper, or any other metal less attractive of oxygen, 169 and the space between them (suppose of half an inch in thickness), be filled with an acid or any fluid capable of oxydating the zinc, the oxydated surface will have its capacity for electricity diminished, so that a quantity of electricity will be evolved from that surface. This electricity will be received by the contiguous fluid, by which it will be transmitted to the opposite metallic surface, the copper, which is not oxydated, and is therefore disposed to receive it; so that the copper plate will thus become positive, whilst the zinc plate will be in the negative state.

This evolution of electrical fluid however will be very limited; for as these two plates admit of but very little accumulation of electricity, and are supposed to have no communication with other bodies, the action of the acid, and further developement of electricity, will be immediately stopped.

EMILY.

This action, I suppose, can no more continue to go on, than that of a common electrical machine, which is not allowed to communicate with other bodies?

MRS. B.

Precisely; the common electrical machine, when excited by the friction of the rubber, gives out both the positive and negative electricities.—(Plate VI. Fig. 3.) The positive, by the rotation 170 of the glass cylinder, is conveyed into the conductor, whilst the negative goes into the rubber. But unless there is a communication made between the rubber and the ground, but a very inconsiderable quantity of electricity can be excited; for the rubber, like the plates of the battery, has too small a capacity to admit of an accumulation of electricity. Unless therefore the electricity can pass out of the rubber, it will not continue to go into it, and consequently no additional accumulation will take place. Now as one kind of electricity cannot be given out without the other, the developement of the positive electricity is stopped as well as that of the negative, and the conductor therefore cannot receive a succession of charges.

Plate VI.

p. 151.

Fig. 3.   A the Cylinder.   B the Conductor.   R the Rubber.   C the Chain.

Larger view (complete Plate)

CAROLINE.

But does not the conductor, as well as the rubber, require a communication with the earth, in order to get rid of its electricity?

MRS. B.

No; for it is susceptible of receiving and containing a considerable quantity of electricity, as it is much larger than the rubber, and therefore has a greater capacity; and this continued accumulation of electricity in the conductor is what is called a charge.

EMILY.

But when an electrical machine is furnished with two conductors to receive the two electricities, I suppose no communication with the earth is required?

MRS. B.