The properties of caloric are of three kinds : Those that are strictly mechanical, or such as may be conceived analogous to the laws of gravitation or impulse ; chemical, or those that tend directly to effect a chemical change in bodies ; and a third class, which may be regarded as speci fic, and which do not bear an exact resemblance to either of the two former.
Among the mechanical properties cf caloric are its ra diation, rellexion, and refraction, which bear a very near re semblance to the same affections of light. Some obscure intimation of the radiation of heat may be met with in the authors of the latter part of the 17th and beginning of the 18th centuries, but the subject was not much attended to until the time of Scheele. This distinguished philosopher, in his investigation of the nature of fire, performed some new and decisive experiments, which completely establish ed the existence of this property, how it dif fered from the power which hot bodies piAsess of commu tating warmth by contact. TIc found, that when glass is interposed between the face and a quantity of burning fuel, although the light passes through without interruption, the heat, at least for a certain space of time, is entirely stop ped. Heat, he observed, radiates through air, without com municating any warmth to it, and its passage does not ap pear to be interrupted by any currents in the atmosphere. He found, that a transparent mirror, which concentrates the rays of light, does not produce any increase of tempera ture in the focus until it has absorbed a sufficient portion of heat, but it then becomes a radiating body, and emits heat in certain directions. The conclusion which may be deduced from Scheele's experiments is, that caloric is sent off in rays from all hot bodies, and moves through the air with great velocity, but that in this transmission it does not necessarily communicate warmth to it, and is not di verted from the straight course by any currents or motion of the air itself: (On Fire, p. 70, et seq.) His experiments also tended considerably to elucidate another point respect ing heat, which had been the subject of much controversy, whether it was not identical with light, or only differing from it in consequence of some slight modification of its properties. Heat and light are so frequently observed in connection with each other, emanating from the same sour ces, and produced, as it would appear, by the same agents, that the opinion of their identity was generally adopted, at the commencement of the last century. Some distinguish ed experimentalists had indeed embraced the contrary doc trine, and some facts were brought forward in its support. Instances perpetually occurred, of high degrees of heat being produced, which were not accompanied by light ; and, on the other hand, it was found, that there were some luminous bodies which were not hot, and especially, that the moon's rays might be concentrated by a lens, or mirror, so as to produce a very brilliant light, yet that no sensation of heat was excited. These, however, and some other analo gous facts, were not deemed sufficiently conclusive to es tablish the point. Some authors regarded heat and light
as the same substance in different states ; while those, who did not admit their materiality, conceived that they depend ed upon the same affection of matter, but differing in its degree or intensity. Scheele's experiments seemed to prove, that heat and light, when closely united together, as in the rays of the sun, or of a burning body, may be com pletely separated, so that the specific effects of each may be obtained distinct from the other. We have already refer red to his observation of the manner in which a sheet of glass permits the light to pass through it, while it inter cepts the transmission of the heat ; and he also found, that if rays composed both of heat and light, as they were sent off in combustion, be received upon a glass mirror, the heat is absorbed, while the light alone is reflected.
The distinct nature of light and heat is rendered still more probable, by some late experiments of Dr Herschel's, in which he separated them from the state of combination in which they exist in the sun's rays. He was led to ob serve that the differently coloured rays possess different powers of producing heat ; the least refrangible rays, those which excite the sensation of red, possess the great est heating power ; and the power diminishes, until we arrive at the violet, or most refrangible rays, which excite the least heat, the intermediate colours possessing an in termediate power in this respect. But he not only disco vered that the rays of heat and of light were thus very dif ferently affected by the prism ; he farther found, that the effect upon the thermometer was most considerable in a point beyond the red-making rays, entirely out of the li mits of the spectrum, and, of course, in a spot on which no part of the luminous ray was received. Beyond this point, where the temperature was at its maximum, the heat gradually diminished, until it was no longer percepti ble : (Phil. Trans, 1806, p. 286. et seq.) From these ex periments, the results of which were verified by Sir H. Englcfield, who employed an apparatus of rather a differ ent kind, we learn, not only that heat is emitted in rays from the sun, but that solar heat may be obtained separate from light ; and although, like light, it is possessed of the pow er of being refracted, yet it possesses this power in a dif ferent degree. In Sir H. Englefield's experiments, where the blue rays raised the thermometer to 56°, the green to 58', the yellow to 62°, and the red to 72°, the space beyond the prismatic spectrum elevated it to 79°, and it sunk to when returned into the red rays : (Journ. of Royal Inst. i. p. 203.) Similar results were obtained by M. Be rard ; but he observed, that the maximum of heat was at the very extremity of the red rays, when the bulb of the thermometer was completely covered with them ; and that beyond the red extremity, where Herschel found the heat to be a maximum, it was only 1-5th above that of the am bient air. See AprErtnix to this article.