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IV. The Theory of Radiation. By S. B. McLaken, M.A., 

 Assistant Lecturer in Mathematics in the University of 

 Birmingham *. 



Page 



§ 1. Introduction 43 



§ 2. Statistical Theory 44 



§ 3. Interaction of Matter and Radiation 46 



§ 4. Emission and Absorption 50 



§ 5. Complete Radiation 52 



§ 6. Theory of a Simple Wave-Train 54 



§ 7. Induced Magnetization 55 



§ 8. Conclusions 56 



§ 1. Introduction. 



rT*HE unrest o£ our time has invaded even the world of 

 JL Physics, where scarcely one of the principles long 

 accepted as fundamental passes unchallenged by all. The 

 spirit of revolution is seen at its boldest in the theory of 

 radiation. It is not only that Einstein's idea of the quantum 

 is destructive of the continuous medium and all that was 

 built upon it in the nineteenth century. His form of atomism 

 excludes what has been fundamental in Physical science, the 

 ideal of mechanical explanation. Mechanism tries to resolve 

 into motion every form of change ; matter is to be simply 

 what moves and the laws of its motion are the whole of 

 Physics. But the behaviour of Einstein's quanta is strictly 

 analogous to a chemical process. It is governed by laws of 

 change which are not laws of motion. We have always 

 been used to think of the past and the present states of the 

 material universe as joined with no gap between. Einstein, 

 on the contrary, requires us to allow a sudden break from 

 one to the other. The changes of which he speaks are not 

 motions, they need no longer be even continuous. 



This paper is an attempt to save the classical view of 

 radiation as a continuous wave motion. If that can be 

 done, it seems to me a small thing to sacrifice the ordinary 

 mechanical notions of matter. Indeed, I am convinced that 

 some such idea as that of Einstein's quantum is necessary to 

 explain its most elementary properties. It need be no 

 obscure inference from the laws of radiation. The various 

 forms of matter are in no continuous series; the gaps between 

 may be crossed perhaps in a radioactive transformation, but 

 they are not occupied. Matter is stamped in a few definite 

 moulds, and always retains their impress. It is hard to 

 imagine how continuous law 7 s of motion can lead to such 

 results. And the theory of relativity increases the difficulty, 



* Communicated bv the Author. 



