RADIATION OF HEAT is a motion of its particles in rectilinear directions, diverging every way from a heated body, either luminous or not ; and it is imagined to arise from the existence of a strongly repul sive power by which the particles are made to recede from each other with great velocity.
The intensity of heat thus emitted from a point of radiation is obviously the fame at equal distances from the point; and at unequal distances, it is inversely proportional to the squares of the distances. The radiating particles falling upon the surfaces of any bodies in the vicinity of that from which they emanate, are, according to the nature of those bodies, absorbed in them, or transmitted through them, or again they may be reflected from them ; and, in the two last cases, the radiant heat, as it is called, appears to suffer modifications analogous to those which, in like circumstances, take place in light. According to M. Prevost (` Easai sur Calorique Itayonnante, 1809), the radiation of heat is a process which is perpetually going on among all the bodies in nature ; those which are of equal temperature mutually inter changing equal quantities of caloric ; but, with respect to two bodies which are unequally heated, that which has the greatest quantity sends forth emanations in greater abundance:than the other ; the differ ence however diminishing as both bodies approach to an equality of temperature.
The particles of beat (calorific particles) appear to move with perfect freedom through a vacuum, and to be impeded, but in an insensible degree, in their progress through air or any of the gases; they are also found capable of being transmitted, though in small quantities through transparent media of the denser kinds, as glass, rock-crystal, &e.; and in passing through air they produce no sensible effect on the temperature of the latter. If a body be heated to any degree of temperature, and be placed in an absolute vacuum, it is evident that, in consequence of the repulsive power above mentioned, the heat must at length be entirely dissipated by the radiation merely ; and if the body be placed in any fluid, it may be readily concluded that the abstraction of the heat must be influenced by the conducting power of the fluid.
The first direct experiments which appear to have been made on the radiation and reflection of heat are those of 31ariotte, and an account of them is given in the 3I6moires de l*Acad.; 1682. He caused the heat of a fire to fall on the surface of a concave mirror, and observed that it was concentrated in the focus of the latter ; and on placing a plate of glass between the fire and the mirror, he perceived that the rays of heat were intercepted. The subject does not appear to have
been much attended to till about the middle of the 18th century, when Lambert, in his Photometria' (1760), states that, on placing a large glass lens before a fire, the heat was scarcely sensible at the focus, while the reflected heat of burning charcoal set fire to combustibles at a considerable distance ; and Scheele, in his treatise on air and fire, which appears to have beeu written in 1775, describes radiant heat as differing from ordinary heat by disseminating itself in right lines whose directions are not changed by the agitations of the air, and by being reflected from polished metallic mirrors, while it is absorbed in those of glass, and in the others when their surfaces are blackened.
The experiments of Scheele were varied by MM. Saussure and Pictet, who by an appropriate apparatus endeavoured to ascertain the laws of the radiation of caloric. They employed for this purpose two concave mirrors of polished tin, in the focus of one of which was placed the bulb of a R6aumur's thermometer, and in that of the other a ball of iron heated below the degree necessary to render it luminous ; and, by a comparison of the height of the mercury in the thermometer with that in one which was placed out of the focus, but at an equal distance from the iron, the difference was found to be equal to about 8 degrees. M. Pictet employed also, in place of the heated ball, a glass flask containing boiling water, in order to avoid the risk of any light being combined with the heat; and the effect produced on the thermometer sufficiently proved that the calorific rays exist in dependently of those of light. (Pictet, Essai sur he Feu, 1790.) In pursuance of the experiments of Scheele with a blackened mirror, 3T. Pictet covered with lampblack the bulb of the thermometer in the focus of one of his mirrors, and found that the concentrated heat from the flask, which when the bulb was bright raised the mercury degrees, now raised it 41 degrees. The same experimenter placed a flask of snow in the focus of one metallic reflector, and a thermometer in that of another ; and the observed fall of the mercury was then con sidered as an indication that cold is susceptible of radiation and reflec tion like heat. The last experiment belongs however to an earlier date, as may be seen by reference to a letter from Oldenburg, dated 1665, in Boyle's Works (fo. 1744).