Thin Films of Elements exposed to Rontgen Hays. 121 



corpuscles varied as the fourth power of the velocity with 

 which they emerged from the metal. 



In the later work of Barkla and Shearer *, reference was 

 made to the fact that the electrons emitted from metal plates 

 had different associations according to whether the X-radiation 

 to which the plate was exposed caused it to emit the K, L, 

 M, etc. series, or only the L, M, etc. series of tertiary 

 X-radiations. This would depend primarily upon the 

 relationship between the wave-length of the exciting radiation 

 and that of the K, L, M, etc. emission lines in the radiation 

 of the screen f acted upon. In spite of the different 

 associations, it was found that the maximum velocity of 

 emergence did not depend upon the type of X-radiation set 

 up in the screen, the controlling factor being the wave- 

 length of the incident X-rays ; nevertheless, the authors 

 significantly remarked that "although the exact shape of 

 the curve depended upon the substance of the screen, the 

 pressure at which the carve became straight was identical 

 for all screens exposed to a particular radiation " J. 



The work of Moseley and others on the X-ray spectra of 

 the elements has shown the subject to be of greater com- 

 plexity. There must be a distinct /3-ray emission corre- 

 sponding to each of the X-ray spectral lines of a given 

 element. Interest lies in the study, not only of the maximum 

 speed of emission, but also of any real sub-speeds that may 

 exist (such sub-speeds not arising from any diminution in 

 energy through the electrons having emerged from deep 

 layers of material). 



There is supporting evidence for the existence of a /3-ray 

 "' spectrum " from the non-radioactive elements. Hahn § 

 produced some interesting photographs of the /3-ray "spec- 

 trum " from a thin film of radioactive matter. Reference has 

 already been made to the statement of Barkla and Shearer 

 that their absorption-curves varied slightly in shape along 

 their curved portions. 0. W. Richardson || pointed out that 



* Phil. Mag-, ser. 6, vol. xxx. p. 745 (1915). 



t The elements emitting the electrons will be referred to throughout 

 as " the screens." 



X Ibid. p. 748. The curves referred to are those connecting pressure 

 and ionization in a flat ionization chamber backed with the various 

 screens. The ionization is due to the effect of the /3-rays from the 

 screens upon the gas in the chamber. The pressure at which the curve 

 becomes straight measures the range and therefore the fourth power of 

 the velocitv of the particles. 



§ Reproduced in Rutherford's 'Radioactive Substances and their 

 Radiations' (1913), figs. 70 A and B. 



!| Proc. Roy. Soc. A. vol. xciv. p. 272 (19 IS). 



