RADIATION

(Lat. radius, a ray.) In Physics and Meteorology, 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 inquiry, has been reduced by the succe ive labours of Prevost, Leslie, Fourier, Biot, Laplace, Poi (in, Melloni, Forbes, and others to a purely mathematical form, and thereby placed in the, same rank with physical optics, with which, indeed, it has many principles in common. The general laws of the radiation of heat, which have been established by experiment, are the following: — 1. Like all other emanations, its intensity, in a vacuum, varies in the ratio of the inverse square of the distance from the radiating point. In air and the gases, the decrease is a little faster in consequence of a partial absorption. 2. The amount of radiation, or the rate at which a body parts with its heat, is proportional 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 times of cooling be taken in an arithmetical progre ion, the heat will decrease in a geometrical progre ion. The principle, however, is found by experiment to hold good only within a certain range of temperature, not exceeding 50° of Fahrenheit. At the higher temperatures Dulong and Petit found the rate of cooling to be more rapid than in the ratio stated. 3. All bodies placed in an enclosed space a ume in time the temperature of the enclosure. 4. Heat is emitted from 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; and its 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 nature of the radiating body, and the state of its surface with regard to polish, colour, source of heat, 1012 RADIATION. . It is greatest 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, that it is very great, and probably not inferior to that of light. The doctrine of the radiation of heat was first stated in a precise and satisfactory manner by Prevost of Geneva, about 1790. Its leading principle being that all bodies are perpetually exchanging their heat with one another, it is sometimes called the theory qf 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 depends only on the temperature of the body, we are led to suppose that radiation takes place with greater or le intensity at all temperatures; that it is reciprocal between distant bodies; and that it subsists when the temperatures are equal, though in this case no alteration of temperature takes place. By a close attention to the phenomena, we are also led to infer that the faculty of the emi ion (and also of the absorption) of heat belongs to all the molecules of a body; and consequently that radiation takes place not only at the surfaces, but also in the interior of solids and liquids, in the same manner as it takes place in air, or differing only in conseqtience of its more rapid absorption. Fourier and Laplace were thus led to view the constituent molecules of all bodies as so many foci of radiating heat. This heat is radiated by every molecule in every direction, and is propagated through the pores or void spaces of ponderable matter, until it is entirely absorbed by the molecules which it encounters. In solids and liquids the absorption takes place at very small distances; in air and the gases at very great distances. The mathematical theory of heat is founded on this hypothesis of molecular radiation. Solar and Terrestrial Radiation.— The measure of heat received from the sun, and of that which is constantly escaping from the earth into the regions of space, are among the most important elements of meteorology. If the earth were not surrounded with the atmosphere, the quantity of solar light received at any time on a given portion of its surface would be proportional to the inclination of the ray to the surface, or to the cosine of the sun's zenith distance; but owing to the modifying effects of the atmosphere, it would appear from the results of experiments that so far from this law holding good, the force of the sun's direct radiation rather increases with the latitude. Profe or Daniell {Meteorological E ays, p. 228.) suggests that as the cooling power of the air has been proved to be in proportion to its elasticity, it is reasonable to suppose that the difficulty with which heat pa es through the atmospcre is in the same ratio. The proportion of solar heat absorbed in traversingt he atmosphere vertically has been estimated by I^eslie and Pouillet at 2.5 parts in 100; by Kamptz at 32; and by Prof. Forbes at 29The force of solar radiation is measured by the exce of the temperature which a body a umes wiien exposed to the direct action of the sun's rays above that which it would have in the shade. This exce may be roughly measured by two common thermometers, one placed in the shade, and the other exposed to the sun and having its bulb covered to prevent reflexion. The most accurate measurement is, however, given by the actinomcter, an instrument invented by Sir J. Herschel, in 1824, for the dynamical measure of solar radiation. A description of this valuable instrument, and full directions for the method of using it, are given in the Report of the President and Council of the Royal Society on the Objects of Scientific Inquiry in Physics and Meteorology, 1840. P'rom certain experiments made by Pouillet at Paris, he inferred that the whole amount of solar heat annually radiated to the earth is equal to that which would suffice to melt a stratum of ice about 14 metres or 46 feet thick encrusting the whole earth ( EUmens de Physique et de Miteorologie); but in a subsequent memoir {Sur la Chaleur Solaire) he estimated the same quantity at 31 metres, or 102 feet. The measure of terrestrial radiation is of equal importance with that of solar radiation, but no perfect instrument has yet been contrived for its determination. The best means consist in the daily register of the minimum temperature shown by a register thermometer, the bulb of which is placed in the focus of a parabolic metallic mirror pointed towards the clear aspect of the sky, and defended from currents. It may be supposed that the same atmospheric causes which obstruct the pa age of radiant heat from the sun oppose also its transmi ion from the earth into space. From the theory of terrestrial radiation, a curious deduction was made by Fourier, with regard to the temperature of the region of space through which the earth moves in its orbital revolution. On computing the temperature which it is nece ary to suppose the planetary spaces to po es a in order that the thermometrical state of the earth's surface may be such as is actually observed. [s. 1025]