the Absorption of the Gamma Rays. 141 



The mean value of p, for each element is also given in the 

 Table. The mass scattering a/p and the mass absorption p,/p 

 were calculated. 



In the calculations given in this paper it has been 

 implicitly assumed that the scattered radiations are exactly 

 of the same type as the primary radiation, and have not 

 suffered any transformation. It should, however, be em- 

 phasized that Florance found distinct evidence that there 

 was a real change of type in the radiation which accom- 

 panied scattering. For example, he found the radia- 

 tion scattered backward from the plate was more easily 

 absorbed than primary radiation, and therefore was of 

 longer wave-length. It is difficult to take any account of 

 this transformation in deducing the scattering coefficient. 

 If, however, we disregard the change of type, the radiation 

 scattered backward should be taken into account. It has 



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i 



1 















Ph 























s — ' 

































1- '" e 

















^sffi 







































~ 









-<£_ 





. 





' r- 

















L 













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K \ 



























































1-5 2 



"* pt.in gr. cm: 



8 



\® 



already been mentioned that the amount of the radiation 

 scattered backwards was about 15 per cent, of the radiation 

 scattered in the forward direction. Making this correction 

 for the observed values, we conclude that the values of a 

 and afp are as follows : — 



Aluminium 



Iron 



Lead 



Table VIII. 



(T. 



-121 



-334 



-397 



a/p. 



•045*- 



•042 



•034 



* Florance (Phil. Mag. xxvii. p. 225, 1914) estimated the value of alp for 

 aluminium to be 0'05 approximately. 



