RADIATION

(Lat. radius, a ray.) In Physics and irology, the emi ion of rays of light or heat from a luminous or heated body. The theory of the radiation and conduction of heat, which long remained one of the most obscure parts of physical inqolry, has been reduced, by the succe ive labours of PreL< slie, Fourier, Biot Laplace, Poi on, Melloni, Forbes, and others, to a purely mathematical form, and thereby I in the same rank with physical optics, with which, ny princlpli Tii" ■ni.-rii laws of tin ' which have 'sed by experiment, are the following: 1. Like all Olhel emanations, Its intensity, in a vacuum, varies in the ratio of the inverse square of the distance from the rad i;.int. In air and the gases, the decrease is a little ice of a partial absorption. 2. The amount of tadil lion, or the rate at which B body parts with its heat, is projx rtional to the exce of the temperature of the body above that of the medium In which it is placed. This principle was a umed by Newton; and it follows from it that If the 'imes of cooling lie tiken In an arithmetical progre ion, the beat will decrease in a geometrical progre The principle, how ever, is found by experiment to hold good only within a certain range' of temperature, not exceeding 5ip of Fahrenheit At the higher tenfperatores Dulong and Petit found the rate of cooling to be more rapid than in tin,' ted, 3. All bodies placed in an enclosed space a ume in time the temperature of the enclosure. 4. Heat is i oiii every point of the surface of a hot body in all directions, and the intensity of the heating ray is as the sine of the angle which it makes with the surface. This result, which is by no means obvious, was discovered by Leslie; end il< physical cause is, that radiation takes place not from the surface alone, but from particles situated within a certain minute but sensible depth, which is different for different surfaces. 5. The intensity of radiation varies with the nainre of the radiating body, and the state of its surface with regard to polish, colour, source of heat, . It is for rough and dark surfaces, and least for bright surfaces of polished metal. The velocity with which radiated heat is propagated through space is entirely unknown. It is certain, however, mat it is very great, and probably not inferior to that of light. The doctrine of the radiation of heat was first stated in a Mil satisfactory manner by Prevost of Geneva, about R90. Its leading principle being that all bodies are iierpetuinnging their heat with one another, it is sometimes called the theory of exchanges. As this emi ion of heat cannot be attributed to the body which receives it, but for the same radiating body and the same state of surface depend! only on the temperature of the body, we are led to ■appose that radiation takes place with greater or le intensity at all temperatures; that it is reciprocal between distant hodies; and that it subsists when the temperatures are equal though in this case no alteration of temperature ■ e. By a close attention to the phenomena, we are also led to infer that the faculty of the emi ion (and also of of heat belongs to all the molecules of s body; and, consequently, that radiation takes place not only at the siirl aces, but also iu the interior of solids and liquids, in the same manner as it takes place in air, or differing only in con- [s. 1031]