I should like to repeat this experiment, with the difference of subst.i.tuting 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 subst.i.tuting a piece of ice in the place of the heated bullet.

CAROLINE.

Well, Mrs. B., and what was the result?

MRS. B.

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

EMILY.

The thermometer falls considerably!

CAROLINE.

And does not that prove that cold is not merely a _negative_ quality, implying simply an inferior degree of heat? The cold must be _positive_, since it is capable of reflection.

MRS. B.

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

According to Mr. Prevost's theory, we suppose that all bodies whatever radiate caloric; the thermometer used in these experiments therefore emits calorific rays in the same manner as any other substance. When its temperature is in equilibrium with that of the surrounding bodies, it receives as much caloric as it parts with, and no change of temperature is produced. But when we introduce a body of a lower temperature, such as a piece of ice, which parts with less caloric than it receives, the consequence is, that its temperature is raised, whilst that of the surrounding bodies is proportionally lowered.

EMILY.

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

MRS. B.

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

CAROLINE.

I confess I do not perfectly understand your explanation.

MRS. B.

This experiment is exactly similar to that made with the heated bullet: for, if we consider the thermometer as the hot body (which it certainly is in comparison to the ice), you may then easily understand that it is by the loss of the calorific rays which the thermometer sends to the ice, and not by any cold rays received from it, that the fall of the mercury is occasioned: for the ice, far from emitting rays of cold, sends forth rays of caloric, which diminish the loss sustained by the thermometer.

Let us say, for instance, that the radiation of the thermometer towards the ice is equal to 20, and that of the ice towards the thermometer to 10: the exchange in favour of the ice is as 20 is to 10, or the thermometer absolutely loses 10, whilst the ice gains 10.

CAROLINE.

But if the ice actually sends rays of caloric to the thermometer, must not the latter fall still lower when the ice is removed?

MRS. B.

No; for the s.p.a.ce that the ice occupied, admits rays from all the surrounding bodies to pa.s.s 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 a.s.sistance he ascertained that black surfaces radiate most, gla.s.s next, and polished surfaces the least of all.

EMILY.

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

MRS. B.

Undoubtedly. I will now show you the very simple and ingenious apparatus, by means of which he made these experiments. This cubical tin vessel or canister, has each of its sides externally covered with different materials; the one is simply blackened; the next is covered with white paper; the third with a pane of gla.s.s, 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 gla.s.s 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.