PHYSICS IN 1913 



WITH SPECIAL REFERENCE TO THE DIFFRACTION 

 OF X-RAYS BY CRYSTALS 



By E. N. da C. ANDRADE, B.Sc, Ph.D. 

 {John Harling Fellow of the University of Manchester) 



Perhaps the most important, certainly the most striking, advance 

 in physics during the period of the last year or so is the demon- 

 stration of the regular diffraction of X-rays, the experimental 

 confirmation, that is, of a theory often tentatively put forward, 

 that X-rays are a disturbance of the same nature as light, 

 differing only, as far as we can judge, in their wave-length from 

 visible light. While evidence for this was being accumulated, 

 however, the nature of light radiation itself has become more 

 and more mysterious, and the recent investigations as to the 

 laws and nature of radiant energy have rather served to demon- 

 strate the defects of present theories than to provide us with 

 any very convincing and comprehensive ideas on the subject. 

 Planck's theory of quanta, which assumes that radiant energy is 

 emitted not continuously but in discrete units whose magnitude 

 depends only on the frequency of the radiation in question, still 

 provides the reigning hypothesis, although the conception of 

 the discontinuous absorption of energy in such units has been 

 abandoned on account of the insuperable difficulties it presents. 

 Many atom models have been put forward, most of which 

 have a very limited application, and would seem designed with 

 a view of imitating one set of phenomena only. Apart from 

 these ephemeral fancies is Rutherford's nucleus atom, consisting 

 of a small positive nucleus, surrounded by rings of electrons at 

 distances from it very large compared to the dimensions of the 

 nucleus itself. This has shown itself very successful so far, and 

 a mathematical treatment by Bohr, based on an application of 

 the quantum theory to the radiation from an atom of this type 

 has attracted considerable attention, and will be described in 

 more detail in the following account. Two sensational announce- 



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