﻿Absorption of the j3 Rays from Uranium. 



391 





E/I X 100. 



Substance. 



Eadium. 



Uranium. 



Lead 



Gold 



49 



47 

 42-5 

 36 

 24 



15 

 14o 

 145 

 12 

 4 

 very small. 



Platinum 



Tin 



Copper 



Aluminium 





It will be at once seen that the values for uranium are con- 

 siderably smaller than those for radium rays, and the ratio 

 also seems to decrease more rapidly with the atomic weight 

 for the rays from the former substance. 



These differences may possibly be due in part to the 

 presence in the radium radiation of comparatively slowly 

 moving /3 rays. Becquerel has shown that the amount of 

 secondary radiation produced decreases rapidly as the velocity 

 of the primary rays is increased. It seems possible therefore 

 that a large proportion of the secondary radiation due to the 

 radium rays may be set up by the more slowly moving portions 

 of the rays, while the more penetrating portion corresponding 

 to the uranium rays in velocity may produce comparatively- 

 little. 



Whatever the cause^ both the absorption-curves and the 

 direct experiments seem to show that the secondarv radiation 

 set up by the uranium rays is distinctly less in amount than 

 that produced by radium radiation. 



No attempt has been made to calculate what McClelland 

 has called the true coefficient of absorption for the ravs. 



McClelland* has shown that, it* secondary radiation is set 

 up, the true coefficient of absorption /j, of the ravs is con- 

 nected with the coefficient \ as previously defined, by the 

 equation 



\ = jjl x /1 — k, 



where k is the proportion of absorbed energy which is restored 

 to the stream of rays as secondary radiation. He further 

 shows that this quantity k is connected with the total return 



* McClelland, he. cit. 



