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 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.