We have very few remarks to make respecting the two other mechanical properties of heat, its reflexibility and its refrangibility. The reflection of heat has been abundantly proved by the numerous facts that we have stated, in which the concave mirrors were employed, as well as in all those where the nature of the surface absorbing the rays of heat affected this property ; for it has been shewn, that in as much as the surfaces were unfavourable for absorption, they were, in the same proportion, favourable for the reflection of heat. The refraction of heat, as distinct from light, was clearly exhibited in Herschel's experiments ; by its pass age through the prism, it was diverted from its straight course like light, only in a greater degree, as the calorific rays arc found to be dispersed over a larger space than the spectrum formed by the visible rays. The rays proceed ing from a candle, or from burning fuel, were also found to be capable of refraction, like those of solar heat. We may conclude that the solar heat consists of rays of different de grees of refrangibility, although no other difference has been detected in their nature, as is the case with the dif ferently coloured rays of light. This conclusion follows from the fact, that the space occupied by the rays after they have passed through the prism is greater than before they entered it. See APPENDIX to this article.
Before we conclude our account of what we have styled the mechanical properties of heat, we shall offer a very few remarks upon the velocity with which it moves. That the velocity must be very great, is a point of which there can be no reasonable doubt ; yet it will be found very difficult to assign the actual velocity. It is perhaps the most pro bable conjecture, that heat, when radiating from a body, moves at the same rate with light ; yet we know of no de cisive arguments from which this can be inferred, as more than a conjecture. Pictet made sonic experiments upon this subject ; hut they only prove that no perceptible inter val elapses in the passage of heat through about 70 feet, a space much too small to prove any thing important.
The second class of properties, which we announced as belonging to heat, are its chemical properties, or those that tend directly to produce a chemical change in bodies. We shall, however, postpone the consideration of these to a fu ture part of the article, because we shall then be better prepared to determine, what properties ought to be regard ed as chemical, after we have made ourselves aquainted with the effects of heat, as well as with the other properties that have not yet been considered, which we have styled specific. These we shall now proceed to examine.
The specific properties of heat may be classed under two heads : its tendency to diffuse itself equally among bodies, or its mode of communication from one body to another ; and the peculiar manner in which it passes through bodies. Whatever be our opinion respecting the real nature of heat, it is almost impossible to enter into any investigation con cerning it, without using language that would seem to im ply, that it proceeds from the operation of a material cause. If, therefore, we fall into these forms of expression, it must he ascribed, not to our decidedly adopting this hypothesis of the nature of caloric, but to the extreme difficulty of avoiding them, although they must be admitted to be incor rect. With this preliminary caution, we may be allowed
to say, that heat differs from other bodies, in its tendency to diffuse or distribute itself uniformly through all kinds of matter. When any substance possesses a different portion of free caloric from the substances in its vicinity, either in immediate contact, or connected with it by the intervention of a third substance, the superabundant portion of heat will have a tendency to pass from the first to the second, and the one will give and the other receive heat, until they ar rive at a common temperature. This is one of the most fa miliar occurrences, and one of which we perpetually avail ourselves in the arts of life. If we wish to impart heat to a body, we bring it near a substance that is hotter than itself, when it immediately begins to receive heat, and continues to acquire it, as long as it remains in its new situation, or until it shall have experienced some change, which renders it incapable of the farther reception of heat. When, on the contrary, we wish to cool a body, we remove it into the neighbourhood of one which is cooler than itself, when an operation the reverse of the former will ensue ; the cold body will abstract heat from the warmer, until the'common temperature be gained.
The cause of this tendency in heat to fly o, from bodies, 1 or to pass from one to the other, and thus diffuse itself among them, is attributed to its possessing an inherent re pulsive power. The particles of all kinds of ponderable matter are necessarily attracted to each other, and conse quently, under all circumstances, they have a tendency to be drawn and held together, unless some counteracting cause prevents their union. This is equally exemplified in the attraction which prevails between large masses of mat ter, by which the planets are kept in their orbits, called the attraction of gravitation, and the attraction which exists be tween the individual particles of matter, and influences ma ny of the minute operations of nature, under the denomina tion of chemical attraction. The repulsive power, which appears to be an inherent quality of heat, may be regarded in general as the cause of its diffusion among bodies ; but the manner in which it is distributed, or the particular law which it follows in passing off from one body, and attaching itself to others, seems to depend upon a different principle, or at least to be modified in a way that cannot be referred to repulsion. It has been conceived, that the phenomena might be explained upon the idea, that there is a combina tion of the two powers of repulsion and attraction, the heat escaping from a body, in consequence of the repulsive power that exists between its particles, while, at the same time, it is attracted by the particles of the body into which it enters. And perhaps this kind of double operation will serve to explain most of the facts, or at least will enable us to announce them in language which implies no contradic tion, and gives an idea of their relation to each other. The equal distribution of heat, as it has been called by some writers, or the equilibrium of caloric, as it has been styled by others, constituting one of the specific properties which we enumerated above, has been the subject of much obser vation and experiment, and has also given rise to much hy pothetical discussion.