Chapter V: Preface (4)

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 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, 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 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 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 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 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 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.*

[Footnote *: This rule, if not universal, admits of very few
exceptions.]

EMILY.

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

[Footnote †: See page 102.]

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 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 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?

Fig. 1.
The air-pump & receiver for Mr. 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.

Fig. 2. Dr. Wollaston’s Cryophorus.

Fig. 5. Dr. Marcet’s mode of using the Cryophorus.

Fig. 3. & 4. the different parts of Fig. 5. seen separate.]

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.

[Footnote *: This experiment was first devised by Mr. Leslie, and
has since been modified in a variety of forms.]

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_.

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.

[Footnote *: This mode of making the experiment was proposed, and
the particulars detailed, by Dr. Marcet, in the 34th vol. of
Nicholson’s Journal, page 119.]

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, 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 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 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.

CONVERSATION V.

ON THE CHEMICAL AGENCIES OF ELECTRICITY.

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 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 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. 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 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 distances in wooden troughs and the interstices being filled with fluid.

Fig. 1. Voltaic Battery.
Fig. 2.
Fig. 4.
Fig. 1. 2. & 4. Voltaic Batteries

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

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 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, 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.

[Footnote *: This mode of explaining the phenomena of the Voltaic
pile is called the _chemical theory_ of electricity, because it
ascribes the cause of these phenomena to certain chemical changes
which take place during their appearance. In the preceding edition
of this work, the same theory was presented in a more elaborate,
but less easy form than it is in this. The mode of viewing the
subject which is here sketched was long since suggested by Dr.
Bostock, of whose theory, however, this is by no means to be
considered as a complete statement.]

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 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, 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 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.

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.

Certainly not, until the two are fully charged; for the two conductors will receive equal quantities of electricity.

CAROLINE.

I thought the use of the chain had been to convey the electricity _from_ the ground into the machine?

MRS. B.

That was the idea of Dr. Franklin, who supposed that there was but one kind of electricity, and who, by the terms positive and negative (which he first introduced), meant only different quantities of the same kind of electricity. The chain was in that case supposed to convey electricity _from_ the ground through the rubber into the conductor. But as we have adopted the hypothesis of two electricities, we must consider the chain as a vehicle to conduct the negative electricity into the earth.

EMILY.

And are both kinds of electricity produced whenever electricity is excited?

MRS. B.

Yes, invariably. If you rub a tube of glass with a woollen cloth, the glass becomes positive, and the cloth negative. If, on the contrary, you excite a stick of sealing-wax by the same means, it is the rubber which becomes positive, and the wax negative.

But with regard to the Voltaic battery, in order that the acid may act freely on the zinc, and the two electricities be given out without interruption, some method must be devised, by which the plates may part with their electricities as fast as they receive them. --Can you think of any means by which this might be effected?

EMILY.

Would not two chains or wires, suspended from either plate to the ground, conduct the electricities into the earth, and thus answer the purpose?

MRS. B.

It would answer the purpose of carrying off the electricity, I admit; but recollect, that though it is necessary to find a vent for the electricity, yet we must not lose it, since it is the power which we are endeavouring to obtain. Instead, therefore, of conducting it into the ground, let us make the wires, from either plate, meet: the two electricities will thus be brought together, and will combine and neutralize each other; and as long as this communication continues, the two plates having a vent for their respective electricities, the action of the acid will go on freely and uninterruptedly.

EMILY.

That is very clear, so far as two plates only are concerned; but I cannot say I understand how the energy of the succession of plates, or rather pairs of plates, of which the Galvanic trough is composed, is propagated and accumulated throughout a battery?

MRS. B.

In order to shew you how the intensity of the electricity is increased by increasing the number of plates, we will examine the action of four plates; if you understand these, you will readily comprehend that of any number whatever. In this figure (PLATE VI. Fig. 4.), you will observe that the two central plates are united; they are soldered together, (as we observed in describing the Voltaic trough,) so as to form but one plate which offers two different surfaces, the one of copper, the other of zinc.

Now you recollect that, in explaining the action of two plates, we supposed that a quantity of electricity was evolved from the surface of the first zinc plate, in consequence of the action of the acid, and was conveyed by the interposed fluid to the copper plate, No. 2, which thus became positive. This copper plate communicates its electricity to the contiguous zinc plate, No. 3, in which, consequently, some accumulation of electricity takes place. When, therefore, the fluid in the next cell acts upon the zinc plate, electricity is extricated from it in larger quantity, and in a more concentrated form, than before. This concentrated electricity is again conveyed by the fluid to the next pair of plates, No. 4 and 5, when it is farther increased by the action of the fluid in the third cell, and so on, to any number of plates of which the battery may consist; so that the electrical energy will continue to accumulate in proportion to the number of double plates, the first zinc plate of the series being the most negative, and the last copper plate the most positive.

CAROLINE.