All the phenomena concur to skew, that the rays of caloric move with a very considerable velocity, though the rate has not been ascertained in a satisfactory manner. Some experiments of Mr. Leslie would lead us to conclude, that they move with the same velocity as sound. The following experiment of M. Pictet indi cates a very considerable velocity. He placed two concave mirrors at the dis tance of 69 feet from each other ; the one of tin, the other of plaster gilt, and 18 inches in diameter. Into the focus of this last mirror he put an air thermometer, and a hot bullet of iron into that of the other. A few inches from the face of the tin mirror there was placed a thick screen, which was removed as soon as the bullet reached the focus. The thermometer rose the instant the screen was removed without any perceptible interval, conse quently the time which caloric takes in moving 69 feet is too minute to be mea sured. The velocity of caloric, if it is equal to that of light, would prove that its particles must be equally minute. Therefore; neither the addition of caloric, nor its abstraction, can sensibly affect the weight of bodies.
Caloric agrees with light in another pro perty no less peculiar : its particles are never found cohering together in masses.; and whenever they are forcibly accumu lated, they fly off in all directions, and se parate from each other with inconceiv able rapidity. This property necessarily supposes the existence of a mutual repul sion between the particles of caloric. Thus it appears that caloric and light re semble each other in a great number of properties. Both are emitted from the sun in rays, with the velocity of 200,000 miles in a second ; both of them are re fracted by transparent bodies, and re flected by polished surfaces ; both of them consist of particles which mutually repel each other, and which produced no sensible effect 'upon the weight of other bodies. They differ, however, in this particular : light produces in us the sen sation of vision ; caloric, on the contrary, the sensation of heat. Upon the whole, we are authorized, by th e above statement of facts, to conclude, that.the solar light is composed of three distinct substances, in some measure separable by the prism, on account of the difference of their re frangibility. The colorific rays are the least refrangible, the deoxidizing rays are most refrangible, and the calorific rays possess a mean degree of refrangi bility. Hence the rays in the middle of the spectrum have the greatest illuminat ing power ; those beyond the red end the greatest heating power; and those be yond the violet end the greatest deoxi dizing power : and the heating power on the one hand, and the deoxidizing pow er on the other, gradually increase, as we approach that end of the spectrum where the maximum of each is concen trated. These different bodies resemble each other in so many particulars, that the same reasoning respecting refrangi bility, reflexibility, esc. may be applied
to all ; but they effects upon those bodies on which they act. Little progress has yet been made in the investigation of these effects ; but we may look forward to this subject as likely to correct many vague and unmeaning opinions, which are at present in vogue among chemists.
From this account of the nature of ca loric we learn, that it is capable, like light, of radiating in all directions from the surfaces of bodies ; and that when thus radiated, it moves with a very con siderable velocity. Like light, too, it is liable to be absorbed when it impinges against the surfaces of bodies. When it has thus entered, it is capable of making its way through all bodies ; but its mo tion in this case is comparatively slow. Heat then moves at two very different rates. 1. It escapes from the surfaces of bodies. 2. It is conducted, or passes through bodies.
When bodies artificially heated are ex posed to the open air, they immediately begin to emit heat, and continue to do so till they become nearly of the tempera. ture of the surrounding atmosphere. That different substances, when placed in this situation, cool down with very dif ferent degrees of rapidity, could not have escaped the most careless observer; but the influence of the surface of the hot body in accelerating or retarding the cooling process, was not suspected till lately. For this curious and important part of the doctrine of heat, we are in debted to the sagacity of Mr. Leslie, who has already brought it to a great degree of perfection. To whose work we refer the philosophical reader for much useful and highly interesting matter.
Although caloric is incapable of moving in rays through solid bodies, yet it is well known that all bodies whatever are pervious to it. Through solids, then, it must pass in a different manner. In ge neral, its passage through them is re markably slow. Thus, if we put the end of a bar of iron, 20 inches long, into a common fire, while a thermometer is at tached to the other extremity, four mi nutes elapse before the thermometer be gins to ascend, and 15 minutes by the time it has risen 15°. In this case, the caloric takes four minutes to pass through a bar of iron 20 inches in length. When caloric passes in this slow manner, it is said to be conducted through bodies. It is in this manner alone that it passes through non-elastic bodies ; and though it often moves by radiation through elas tic media, yet we shall find that it is ca pable of being conducted through them likewise. As the velocity of caloric,when it is conducted through bodies, is greatly retarded, it is clear that it does not move through them without restraint. It must be detained for some time by the parti cles of the conducting body, and conse quently must be attracted by them.— Hence it follows, that there is an affinity or attraction between caloric and every conductor. It is in consequence of this affinity that it is conducted through the body.