CAROLINE.

But when I lay my hand on this marble table I feel it _positively_ cold, and cannot conceive that there is any caloric in it.

MRS. B.

The cold you experience consists in the loss of caloric that your hand sustains in an attempt to bring its temperature to an equilibrium with the marble. If you lay a piece of ice upon it, you will find that the contrary effect will take place; the ice will be melted by the heat which it abstracts from the marble.

CAROLINE.

Is it not in this case the air of the room, which being warmer than the marble, melts the ice?

MRS. B.

The air certainly acts on the surface which is exposed to it, but the table melts that part with which it is in contact.

CAROLINE.

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

MRS. B.

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

According to this theory, caloric is composed of particles perfectly separate from each other, every one of which moves with a rapid velocity in a certain direction. These directions vary as much as imagination can conceive, the result of which is, that there are rays or lines of these particles moving with immense velocity in every possible direction.

Caloric is thus universally diffused, so that when any portion of s.p.a.ce happens to be in the neighbourhood of another, which contains more caloric, the colder portion receives a quant.i.ty of calorific rays from the latter, sufficient to restore an equilibrium of temperature. This radiation does not only take place in free s.p.a.ce, 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 quant.i.ties, 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 quant.i.ty.

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 quant.i.ty of heat we receive in exchange from the colder body, because it serves only to diminish the loss.

MRS. B.

Very well, indeed, Emily. Professor Pictet, of Geneva, has made some very interesting experiments, which prove not only that caloric radiates from all bodies whatever, but that these rays may be reflected, according to the laws of optics, in the same manner as light. I shall repeat these experiments before you, having procured mirrors fit for the purpose; and it will afford us an opportunity of using the differential thermometer, which is particularly well adapted for these experiments.

--I place an iron bullet, (PLATE III. Fig. 1.) about two inches in diameter, and heated to a degree not sufficient to render it luminous, in the focus of this large metallic concave mirror. The rays of heat which fall on this mirror are reflected, agreeably to the property of concave mirrors, in a parallel direction, so as to fall on a similar mirror, which, you see, is placed opposite to the first, at the distance of about ten feet; thence the rays converge to the focus of the second mirror, in which I place one of the bulbs of this thermometer. Now, observe in what manner it is affected by the caloric which is reflected on it from the heated bullet. --The air is dilated in the bulb which we placed in the focus of the mirror, and the liquor rises considerably in the opposite leg.

[Ill.u.s.tration: Plate III. Vol. I. p. 54 Mr. Pictet's Apparatus for the Reflection of Heat.

Fig. 1.

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.]

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 a.s.sistance of the mirrors?

MRS. B.

The effect would in that case be so trifling, at the distance at which the bullet and the thermometer are from each other, that it would be almost imperceptible. The mirrors, you know, greatly increase the effect, by collecting a large quant.i.ty 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 princ.i.p.al 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 gla.s.s must be interposed between the mirrors; for light, you know, pa.s.ses with great facility through gla.s.s, 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 pa.s.s through the gla.s.s together with the light, as the thermometer rises a little; but, as soon as the gla.s.s is removed, and a free pa.s.sage left to the caloric, it will rise considerably higher.

EMILY.

This experiment, as well as that of Dr. Hersch.e.l.l'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.