An interesting train of experiments, on the manner in which heat radiates, or escapes from the surfaces of bo dies, was performed by Rumford, in which he shewed, with his usual address and dexterity, the different effects which •are produced by a difference in the nature of the radiating surface : •(Phil. Trans. 1804, p. 89 et seq) By a singular coincidence, nearly about the same time that this essay was published in the Phil. Trans. a still more complete view of the subject was taken by Professor Leslie. They both supported their peculiar opinions by a number of con trived and well executed experiments, which led to many curious and unexpected results, and which have, in some measure, altered our previous notions respecting the na ture of heat, or at least respecting some of its most re markable properties. It had been before known, that the nature of the surface of a body materially affects its power of admitting caloric to enter into it ; and this power was .now extended to the emission or radiation of heat.
There is so much similarity between the experiments of Rumford and Leslie, that it will not be necessary to refer to both of them, considering the narrow limits to which we are confined in this article ; and as those of the latter are generally the most decisive, and, for the most part, per formed with the most accurate instruments, we shall prin cipally employ them in our examination of the subject.
Professor Leslie's researches illustrate, in a striking man ner, the effect of the peculiar nature of the surfaces of bo dies, both .upon the emission and reception of radiant heat, bodies of equal temperatures discharging and absorbing it in very different degrees. When a body sends off rays of heat, we may conceive that it parts with a portion of caloric that was previously united to it ; and that when it receives the rays of heat, a quantity of caloric becomes combined with it, which was before in a free state. These two operations, although the reverse of each other, seem to exist in the same proportion, and in all respects to bear an exact ratio to each other. Professor Leslie employed in his experiments a species of air thermometer of a new construction, which, besides possessing the advantage of being an lost! ument of gi eat delicacy, and being sensible to very minute variations of heat, has also the useful pro perty of indicating, at all times, any variation that occurs in the temperature of the portion of air in which it is immersed, and of adapting, as it were, its own scale to this new temperature, so that the apparent effect is the same, whatever be the actual temperature at which the ex periment is performed. It was from this property that he gave to his instrument the name of differential thermome ter, as not indicating the actual degree of heat, but only the degree in which it differs from that of the atmosphere. (It is shewn in Plate CXLI. Fig. I.) Rumford employed an instrument very similar in its nature, which he called a thermoscolie.
In ascertaining the quantity of heat emitted from the surfaces of bodies, Professor Leslie generally examined the rays after they had been reflected by concave mirrors.
Those which he used were composed of polished block tin ; and by means of a mould, upon which they were form ed, they were made to constitute portions of the parabolic curve. The substance from which the heat was emitted was boiling water, contained in a cubical canister of block tin. This was provided with a thermometer to ascertain its temperature ; and the apparatus being placed in the focus of an elliptical tin reflector, the effect was noticed upon the differential thermometer, situated in the opposite focus. The canister had four sides of equal dimensions ; and these being prepared in different ways, either polished, or left rough, varnished, or covered with paper, or some other substance, afforded an opportunity of accurately ex amining and comparing the effect of different kinds of sur faces on the radiation of heat : (Inquiry, p. 17.) The appa ratus is shewn in Plate CCLXXXIX. Fig. 11. Professor Leslie begins by ascertaining what was the effect of the canister of boiling water, when simply placed in the focus of the reflector, in what length of time the maximum of heat was produced, and how long the process of cooling occupied. He likewise observed the effect produced on the thermometer, by employing water of different tempe ratures, and also by the degree in which the temperature of the water exceeds that of the temperature of the air of the chamber in which the apparatus is situated. The ves sel that contained the water was a cube of six inches; and when it was at the boiling heat, and was placed at the dis tance of about three feet from the mirror, the rise of the differential thermometer was equivalent to what would have been of Fahrenheit's scale. He also found that the greater was the excess of the heat of the water above the temperature of the room, exactly in the same propor tion was its action on the thermometer. Hitherto the ves sel had been employed with its uncovered side turned to wards the reflector ; and he next proceeded to contrast with this the effect of the other three sides of the canister, one of which hada plate of glass cemented to it, anothei had writing paper pasted on it, and the fourth was covered with a varnish of lamp black. The effect produced by these different surfaces was very singular, and was, to the expe rimentalist himself, very unexpected. The uncovered side, which had the usual polish of a metallic surface, produced an effect upon the thermometer equal to 12°, the side to which the vlass had been applied to 90°, that covered with paper to 98°, while the varnished side was equal to 100°. From these experiments we arrive at the itnpoi tam con clusion, that heat radiates from a polished metallic surface with not quite gth part of the energy that it does from a surface that is covered with some substance, which takes away the effect of the polish. Inquiry, p. 18.