374 



Prof. C. G. Barkla on Typical 



are more correctly indicated if we assume that the absorption 

 of various beams o£ X-rays in air is proportional to the 



Fhr. 1. 



*3 



1 



*■ 





























1 





co 2 









\ © » 













L 





C.,E-JJr 

















200 



50 



15 30 45 60 



ionization produced in air. With this assumption we get 

 the relative values for the ionizations produced by the various 

 primary beams of equal intensity as given in columns 5 and 6 

 (Table I.). 



The curves exhibiting the relation between the ionization 

 in a thin film of gas or vapour and the absorbability of the 

 X-radiation are given in fig. 2. The accuracy of course 

 depends upon the truth of the assumption made, but of the 

 marked features there can be no doubt. These curves 

 should be compared with those showing the variation of 

 absorption in various elements with variation in penetrating- 

 power of the X-rays used *. 



The marked deviation from simple proportionality of 

 ionization in C 2 H 5 Br to ionization in air or to absorbability 

 of the primary radiation is thus due to the presence of Br, 

 which has a characteristic radiation within the range of 

 penetrating power used in these experiments. 



Similar deviations occur in the case of CH 3 I at the 

 particular penetrating power characteristic of the secondary 

 radiation from I, in the case of SnCl 2 at that characteristic 

 of Sn, and in Se 2 Cl 2 at that characteristic of Se. Carbon, 



* Barkla and Sadler, Phil. Mag. May 1909, pp. 739-760 ; Barkla, 

 Proc. Camb. Phil. Soc. May 1909, pp. 257-268. 



