We have already alluded to the experiments of Count Rumford, which were published almost immediately after those of Professor Leslie, and which so generally agree with them, that although they are both numerous and in genious, it will not be necessary to give any minute de scription of them. Like Professor Leslie, Count Rumford found, that polished metal radiates heat in a much less de gree than a metallic surface, at the same temperature, but covered with paint or varnish ; that a covering of linen, flan nel, paper, or in short of any animal or vegetable substance, tends to promote the emission of heat ; and he farther found, that the radiation and absorption of heat bore a di rect ratio to each other, and likewise an inverse ratio to the power of reflection. In one particular, indeed, the Count's experiments had a different result from Professor Leslie's ; this latter experimentalist compared the radiating effect of various metals, and conceived that they differed from each other in this respect, while Rumford could • not perceive any difference between them ; upon the whole, however, we are disposed to regard Professor Leslie's results as the most correct. Count Rumford, according to his usual custom, deduces some important practical conclusions from his ex periments, which are the more valuable, because some of them are precisely contrary to the previous ideas that were entertained upon the subject. In putting heated bodies into vessels or tubes, our object is sometimes to retain the heat as long as possible : but in others, on the contrary, we arc desirous that the heat should be quickly dispersed through the contiguous air. If we wish to confine the heat, we must employ metal, and have its surface highly polished, a fact fully proved by the above experiments, but directly contrary to the conclusion that might seem to fol low from the superior conducting power of a metallic body. On the other hand, if the object be to cool the vessels with their contents, or to transfer their heat to the surrounding medium, we must cover them with paint or varnish, or with some kind of soft coating, not of a metallic nature. As an example of the two cases, we shall mention that of tubes conveying steam, which may, in the one instance, be for the purpose of transporting heat from one vessel to an other, arid where, of course, it is an object that none should be lost in the passage ; here we must use bright metallic tubes, which will radiate as little as possible. But if, on the contrary, we introduce steam tubes for the purpose of warming an apartment, here we wish to promote the radia tion as much as possible, and we should therefore use the tubes unpolished, and varnished, painted, or even rusted. In the same way bright metallic vessels should be employ ed, when we wish to preserve the heat in fluids, where they are used for the purposes of cooking or manufactures, while the opposite plan is to be followed, when the object is to promote their cooling. (Phil. Trans. 1804, p. 177, et seq.) On the cooling of bodies we shall have occasion to speak more at large, in a subsequent part of the article.
We must now advert to a train of phenomena, connected with the radiating power of bodies, although perhaps de pending upon a different principle, and leading to some new ideas respecting the nature of heat, the radiation of cold. By the radiation of cold we mean simply to express the fact, that when a cold body is placed in the focus of a concave mirror, a thermometer will fall that is suspended in the focus of an opposite mirror. This singular circum stance was first noticed by the members of the Florentine Academy, and was very distinctly described by them, al though they were so much surprised at the effect, as almost to doubt the accuracy of their own experiment. (Saggi di nat. Esfter. p. 176.) They do not seem to have made any atttempt to explain it ; no farther notice was taken of it at the time, and it appears to have been almost forgotten, when Pictet, in the course of his experiments, to which we have referred above, after having ascertained the radia tion of heat and its reflection by his apparatus, placed a vessel filled with ice in one of the foci, and observed the thermometer in the other focus instantly to sink several de grees ; when the ice was removed, the thermometer rose again to its former elevation. By adding nitric acid to the
snow, and thus producing a more intense cold, he•found that the effect on the thermometer was augmented. Essay on Fire. § 69. See our article COLD.
The apparent radiation of cold has been since confirmed by Professor Leslie, and what may, at first view, appear still more remarkable, it seems to be acted upon by bodies in the same manner with radiant heat. It is promoted and retarded by the same kind of surfaces which promote and retard the radiation of caloric, and also in the same propor tion. The canister, which had been employed in the for mer experiments, was now filled with ice or snow, and its different sides in turn exposed to the mirror, the differential thermometer being, as before, suspended in the opposite focus. The cold produced by the varnished side was the greatest, while that from the uncovered side was the least, the glass and the paper .being intermediate between the two, exactly in the same manner as when the vessel had been filled with boiling water. The results also were simi lar when the thermometer was covered with different sub stances, so as to affect its power of absorbing heat, and also when the surface of the mirror was changed, so as to change its power of producing reflection. For example, when the thermometer was coated with a leaf of metal, it fell less, and when coated with varnish, more than in its ordinary state. And with respect to .the mirror, the reflection of cold was most considerable from the bright metallic sur face, less when a mirror of glass was employed, and still less when it was varnished. Here the power of the mirror in radiating cold, was exactly in the inverse ratio of its re flecting power, just as is the case with the radiation of heat. Lastly, the interposition of screens of different kinds, and with different surfaces, between the ice and the mirror, had effects which were precisely analogous to those men tioned above ; so that under all circumstances, Professor Leslie found the strictest coincidence between the two kinds of radiation : (Inquiry, p. 23.) It may be proper to men tion, that experiments of a similar nature, and with similar results, were performed on the radiation of cold by Count Rumford. They are such as sufficiently establish the facts, and might be esteemed valuable, were they not, for the most part, superseded by those of Professor Leslie. Phil. Trans. 1804, p. 170.
The general conclusions that may be deduced from the experiments of Pictet, Rumford, and Leslie, are, that bo dies possess a power which generates cold, or reduces ac tual temperature ; and that this power is emitted in right lines, and may be reflected, condensed, or intercepted, in its passage from one body to another, like the rays of radi ant heat. It also follows the same laws with respect to the action of surfaces upon it, and bears the same relation to their reflecting property. Certainly the most obvious in ference would be, that cold, like heat, depends upon the presence of a real material agent, capable of being trans ferred from one body to another, and subject to similar laws of radiation and reflection ; but this supposition is so strongly opposed by many other considerations, which seem to prove, in the most decisive manner, that cold is merely a negative property, and signifies nothing more than the abstraction of heat, that we ate compelled to look out for the explanation of the phenomena upon other principles : these we shall afterwards endeavour to elucidate.