Thin Films of Elements exposed to Rontgen Hays. 123 



According to this scheme it must be noted that the so-called 

 tC K." electron is the slowest one, whilst the most rapidly 

 ejected electron is the one that comes from the periphery 

 of the atom, i. e. the so-called photo-electron *. 



In a previous paper f I pointed out that the absorption 

 coefficients of these particles in various gases derived by 

 Beatty and Sadler must be regarded as mean values. If the 

 above scheme is correct, not only has the mean to be taken 

 over the various speeds, but the matter is complicated further 

 by our not knowing the* relative numbers possessing the 

 respective speeds. As one might say, we do not know 

 the relative intensities of the respective /3-ray spectral lines. 

 I also produced experimental evidence in support of the 

 above views. 



Although it has often been stated that- there is a definite 

 " K" group of electrons associated with "K" X-radiation, 

 and an "L" group associated with "L" X-radiation, yet I 

 have been unable to find any experimental determination as 

 to which group will have the greater velocity if the parent 

 atoms are emitting simultaneously both " K " and "L" 

 X-radiations. The nearest approach to any definite statement 

 was made by Sadler, who pointed out that under certain 

 conditions intense electronic emission from a screen was not 

 necessarily associated with the emission of any (tertiary) 

 X-radiations that could be detected. " For instance, when 

 the secondary exciting beam from silver itself falls upon 

 silver as tertiary radiator, no homogeneous Rontgen radiation" 

 is produced, and yet a considerable emission of corpuscular 

 radiation occurs" J. 



Finally, some extremely important deductions have been 

 made from the work quoted above, e. g. it is deduced that 

 the maximum velocity of the /3-rays is never greater than 

 that of the parent cathode ray within the discharge-tube. 



* It must be pointed out that this statement is not contrary to the 

 experimental results of Barkla and Shearer, who excited first K radiation 

 in a given screen by the incidence of a still harder radiation and then 

 L radiation in a screen of higher atomic weight, by the same radiation 

 (too soft to produce the K radiation in this latter screen). They then 

 studied the maximum Telocity of emergence of the /3-rays from the 

 two screens and found them to be equal. These experiments support 

 this conclusion, but an attempt has been made to push the subject 

 farther. Concomitantly with K emission, a given screen must be 

 emitting L, M, etc. X-radiations, and what one calls the '• K" electron 

 is probably -that emerging from the atom with the smallest velocity of 

 the groups of electrons associated with the emission of the various 

 X-rav spectral lines. 



t Trans. Roy. Sue. S. Africa, vol. viii. pt. 1, p. 82 (1919). 



X Ibid. p. 35-1. 



