Connexion between j3 and 7 Ray Spectra. 307 



to consist of a great number of lines, about 50 of which were 

 measured. It was pointed out that there appeared to be 

 certain simple numerical relations between a number of the 

 groups of /3 rays from radium C. In the meantime, the 

 problem had been attacked from another direction. According 

 io the theory, the 7 rays emitted from a radioactive substance 

 should consist of types of characteristic X radiations which 

 should be exponentially absorbed by a light substance like 

 aluminium. This question has been examined in detail by 

 Mr. H. Hichardson and myself *, and the results obtained 

 have fully confirmed this point of view. The 7 rays from each 

 radioactive substance can be analysed into a number of 

 distinct groups. Some of these groups undoubtedly corre- 

 spond to the characteristic radiations to be expected from 

 elements of their atomic weight; but attention was drawn 

 to the evidence of the existence of other types of character- 

 istic radiation not previously observed by workers with 

 X rays. It was found that the different radioactive sub- 

 stances showed great variety in the types of 7 rays emitted, 

 but they could be classified by their power of penetration as 

 belonging to certain general types of characteristic radiations. 

 Mr. H. Richardson has continued these investigations and 

 has recently obtained evidence of the excitation of character- 

 istic radiations in a large number of elements when the 

 ,/3 rays of active matter fall upon them. 



The discovery of Laue of the diffraction of X rays and 

 the subsequent work of W. H. and W. L. Bragg and of 

 Moseleyand Darwin and others, have placed into our hands a 

 powerful and simple method for determining the wave-lengths 

 of the X rays. If the 7 rays from radioactive matter con- 

 sisted of groups of characteristic rays, it v^as to be anticipated 

 that the rays would show a line spectrum when reflected 

 from a crystal surface. This point of view has been com- 

 pletely confirmed by subsequent researches of Dr. Andrade 

 and myself. In the first paper t we gave an account of the 

 examination of the spectrum of the solt 7 rays from radium B, 

 and adduced evidence that the strong lines of the spectrum 

 of this substance were identical with the characteristic " L " 

 spectrum of lead. In a subsequent research J we have 

 determined the wave-lengths of the penetrating 7 rays from 

 radium B and radium C and verified the results by the 

 adoption of a new experimental method. 



* Rutherford & Richardson, Phil. Mag. xxv. p. 722 ; xxvi. p. 324 ; 

 xxvi. p. 937 (1913). Richardson, Phil. Mag. xxvii. p. 252 (1914). 

 t Rutherford and Andrade, Phil. Mag. xxvii. p. 854 (1914). 

 J Rutherford and Andrade, Phil. Mag. August 1914. 



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