Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/232

214 RADIATION 8. Thus radiation is one phenomenon, and (as we shall find) the spectrum of a black body (a conception roughly realized in the carbon poles of an electric lamp) is continu- ous from the longest possible wave-length to the shortest which it is hot enough to emit. These various groups of rays, however, are perceived by us in very different ways, whether by direct impressions of sense or by the different modes in which they effect physical changes or transforma- tions. The only way as yet known to us of treating them all alike is to convert their energy into the heat-form and measure it as such. This we can do in a satisfactory manner by the thermo-electric pile and galvanometer. 9. Of the history of the gradual development of the theory of radiation we can give only the main features. The apparent concentration of cold by a concave mirror, which had been long before observed by Porta, was redis- covered by Pictet, and led to the extremely important enunciation of the Law of Exchanges by Prevost in 1791. As we have already seen, Prevost's idea of the nature of radiation was a corpuscular one, no doubt greatly influenced in this direction by the speculations of Lesage (see ATOM). But the value of his theory as a concise statement of facts and a mode of co-ordinating them is not thereby materially lessened. We give his own statements in the following close paraphrase, in which the italics are re- tained, from sect. ix. of his Du Calorique Rayonnant (Geneva, 1809). "1. Free caloric is a radiant fluid. And because caloric becomes free at the surfaces of bodies every point of the surface of a body is a centre, towards and from which filaments (filets) of caloric move in all directions. " 2. Heat equilibrium between two neighbouring free spaces consists in equality of exchange. " 3. When equilibrium is interfered with it is re-established by inequalities of exchange. And, in a medium of constant temper- ature, a hotter or a colder body reaches this temperature according to the law that difference of temperature diminishes in geometrical 2)rogression in successive equal intervals of time. " 4. If into a locality at uniform temperature a reflecting or refract- ing surface is introduced, it has no effect in the way of changing the temperature at any point in that locality. "5. If into a locality otherwise at uniform temperature there is introduced a warmer or a colder body, and next a reflecting or refract- ing surface, the points on which the rays emanating from tlie body are thrown by these surfaces will be affected, in the sense of being wanned if the body is warmer, and cooled if it is colder. " 6. A reflecting body, heated or cooled in its interior, will acquire the surrounding temperature more slowly than would a non-reflector. "7. A reflecting body, heated or cooled in its interior, will less affect (in the way of heating or cooling it) another body placed at a little distance than would a non-reflecting body under the same circumstances. " All these consequences have been verified by experiment, except that which regards the refraction of cold. This experiment remains to be made, and I confidently predict the result, at least if the refraction of cold can be accurately observed. This result is indi- cated in the fourth and fifth consequences [above], and they might thus be sibjected to a new test. It is scarcely necessary to point out here the precautions requisite to guard against illusory results of all kinds in this matter." 10. There the matter rested, so far as theory is con- cerned, for more than half a century. Leslie and, after him, many others added fact by fact, up to the time of De la Provostaye and Desains, whose experiments pointed to a real improvement of the theory in the form of specializa- tion. But, though such experiments indicated, on the whole, a proportionality between the radiating and absorb- ing powers of bodies and a diminution of both in the case of highly reflecting surfaces, the anomalies frequently met with (depending on the then unrecognized colour-differences of various radiations) prevented any grand generalization. The first real step of the general theory, in advance of what Prevost had achieved, and it was one of immense import, was made by Balfour Stewart in 1858. Before we take it up, however, we may briefly consider Provost's state- ments, putting aside his erroneous views as to the nature of heat ; and we must also introduce some results of the splendid investigations of Sadi Carnot (1824), which cast an entirely new light on the whole subject of heat. 11. Prevost's leading idea was that all bodies, whether cold or hot, are constantly radiating heat. This of itself was a very great step. It is distinctly enunciated in the term " exchange " which he employs. And from the way in which he introduces it it is obvious that he means (though he does not expressly say so) that the radiation from a body depends on its own nature and temperature alone, and is independent altogether of the nature and temperature of any adjacent body. This also was a step in advance, and of the utmost value. It will be seen later that Prevost was altogether wrong in his assumption of the geometrical rate of adjustment of differences of temperature, a statement originally made by Newton, but true only approximately, and even so for very small temperature differences alone. Newton in the Queries to the third book of his Optics distinctly recognizes the pro- pagation of heat from a hot body to a cold one by the vibrations of an intervening medium. But he says no- thing as to bodies of the same temperature. 12. To Carnot we owe the proposition that the thermal motivity of a system cannot be increased by internal actions. A system in which all the parts are at the same tempera- ture has no thermal motivity, for bodies at different temperatures are required in order to work a heat-engine, so as to convert part of their heat into work. Hence, if the contents of an enclosure which is impervious to heat are at any instant at one and the same temperature, no changes of temperature can take place among them. This is certainly true so far as our modes of measurement are concerned, because the particles of matter (those of a gas, for instance) are excessively small in comparison with the dimensions of any of our forms of apparatus for mea- suring temperatures. Something akin to this statement has often been assumed as a direct result of experiment : a number of bodies (of any kinds) within the same imper- vious enclosure^ which contains no source of heat, mill ulti- mately acquire the same temperature. This form is more general than that above, inasmuch as it involves con- siderations of dissipation of energy. Either of them, were it strictly true, would suffice for our present purpose. But neither statement can be considered as rigorously true. We may employ them, however, in our reasoning as true in the statistical sense ; but we must not be surprised if we should find that the assumption of their rigorous truth may in some special cases lead us to theoretical results which are inconsistent with experimental facts, i.e., if we should find that deviations from an average, which are on far too minute a scale to be directly detected by any of our most delicate instruments, may be seized upon and converted into observable phenomena by some of the almost incomparably more delicate systems which we call individual particles of matter. 13. The next great advance was made by Balfour Stewart. 1 The grand novelty which he introduced, and from which all his varied results follow almost intuitively, is the idea of the absolute uniformity (qualitative as well as quantitative) of the radiation at all points, and in all direc- tions, within an enclosure impervious to heat, when thermal equilibrium has once been arrived at. (So strongly does he insist on this point that he even states that, whatever be the nature of the bodies in the enclosure, the radiation there will, when equilibrium is established, be that of a black body at the same temperature. He does not expressly say that the proposition will still be true even if the bodies can radiate, and therefore absorb, one definite wave-length only ; but this is a legitimate deduction from his state- 1 Trans. R. S. ., 1858 ; see also Phil. Mag., 1863, i. p. 354.