Drawn by the Author

cording to the laws of optics, in the same manner as light. I wish I could repeat these experiments before you, but the difficulty of procuring mirrors fit for the purpose puts it out of my power; you must therefore be satisfied with an account of them, illustrated by this diagram: (Plate III. Fig. 4.)-He placed an iron bullet, about two inches in diameter, and heated to a degree not sufficient to render it luminous, in the focus of a large metallic mirror. The rays of heat which fell on this mirror were reflected, agreeably to the property of concave mirrors, in a parrallel direction, so as to fall on a similar mirror, which was placed opposite the first, at the distance of about twelve feet; thence they converged to the focus of the second mirror, in which the bulb of a thermometer was placed, the consequence of which was, that the thermometer immediately rose sevral degrees.

Emily. But would not the same effect have taken place, if the rays of caloric from the heated bullet had fallen directly on the thermometer, without the assistance of the mirrors?

Mrs. B. The effect would in that case have been so trifling, at the distance at which the bullet and the thermometer were from each other, as would probably have rendered it imperceptible. The mirrors, you know, greatly increase the effect, by collecting a large, quantity of rays into a focus; but their principal use was to prove that the calorific emanation was reflected in the same manner as light.

Caroline. And the result I think was very conclusive.

Mrs. B. The experiment was afterwards repeated with a wax taper instead of the bullet, with a view of

PLATE III.

A A. and B B. Concave mirrors fixed on stands. C. heated bullet placed in the focus of the mirror A, D. The thermometer with its bulb placed in the focus of the mirror B. 1 2 3 4. Rays of caloric radiating from the bullet and falling on the mirror A. 567 8. The same rays reflected from the mirror A to mirror B. 9 10 11 12. The same rays reflected by the mirror B to the thermometer.

separating the light from the caloric. For this purpose a transparent plate of glass was interposed between the mirrors; for light you know passes with great fecility through glass, whilst the transmission of caloric is considerably impeded by it. It was found however, in this experiment, that some of the calorific rays passed through the glass together with the light, as the thermometer rose a few degrees; but as soon as the glass was removed, and a free passage left to the caloric, it rose immediately double the number of degrees.

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 Mr. Pictet's experiments, 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. The thermometer fell considerably.

Caroline. And does that not 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; but upon a little consideration it was found that it afforded only an additional proof of the reflection of heat: this I shall endeavour to explain to you.

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 proportionably lowered; and as, from the effect 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 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 air which will fill the space that the ice occupied, being of the same temperature as the thermometer, will emit and receive an equal quantity of caloric, so that no alteration of temperature will be produced.

Caroline. I must confess that you have explained this in so satisfactory a manner that I cannot help being convinced that cold has no real claim to the rank of a positive being. So now we may proceed to the other modifications of caloric.

D

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

Conversation III.

Continuation of the Subject.

Mrs. B.

You

In our last conversation, we began to examine the constant tendency of free 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. 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,