SOME CONTEMPORARY ADVANCES IN PHYSlCS-VIl 285 



tlu'ir vibrators, the electromagnetic theory would receive a most 

 valuable supplement. And. much as a competent theory of the 

 binding-forces was to be desired, a continuing failure to produce one 

 would not impugn the electromagnetic theory, which in itself was a 

 coherent system, self-sustaining and self-sufficient. 



This was the state of affairs in the late nineties. The wave-con- 

 ception of light had existed for more than two centuries, and it was 

 se\'enty-five years since any noticeable opposition had been raised 

 against it. The electromagnetic theory of light had existed for about 

 thirty years, and now that the electron had been discovered to serve as 

 a source for the waves which in their propagation through space 

 had already been so abundantly explained, there was no effective oppo- 

 sition to it. Not all the facts of emission and absorption had been 

 accounted for, but there was no reason to believe that any particular 

 one of them was unaccountable. Authoritative people thought 

 that the epoch of great discoveries in physics was ended. It was 

 only beginning. 



In the year 1900, Max Planck published the result of a long series 

 of researches on the character of the radiation inside a completely- 

 enclosed or nearly-enclosed cavity, surrounded by walls maintained 

 at an even temperature. Every point within such a cavity is tra- 

 versed by rays of a wide range of wave lengths, moving in all direc- 

 tions. By the "character" of the radiation, I mean the absolute 

 intensities of the rays of all the various frequencies, traversing such a 

 point. The character of the radiation, in this sense, is perfectly 

 determinate; experiment shows that it depends only on the temper- 

 ature of the walls of the cavity, not on its material. According to 

 the electromagnetic theory of radiation, as completed by the adoption 

 of the electron, the walls of the cavity are densely crowded with bound 

 electrons; nor are these electrons all bound in the same manner, so 

 that they would all have the same natural frequency of oscillation— 

 they are bound in all sorts of different ways with all magnitudes of 

 restoring-forces, so that every natural frequency of oscillation over a 

 wide range is abundantly represented among them. Now the con- 

 clusion of Planck's long study was this: 



// the bound electrons in the walls of the cavity (i.e., in any solid body) 

 did really radiate while and as they oscillate, in the fashion prescribed 

 by the electromagnetic theory, then the character of the radiation in the 

 cavity would be totally different from that which is observed.* 



' The belief that the character of radiation within a cavity could not be explained 

 without doing some violence to the "classical mechanics" had already been gaining 

 ground for some years, by reason of extremely recondite speculations of a statistical 

 nature. It is very difficult to gauge the exact force and bearing of such considerations. 



