the Absorption of the Gamma Rays. 131 



absorption o£ X rays of high frequencies, using a Coolidge 

 tube as a source of radiation. Under the experimental 

 conditions, where the scattered rays did not enter the 

 testing vessel, they found that the apparent mass absorption 

 coefficient \jp was given by an equation of the following 

 form : — 



X _ fju <x 



P " P P' 

 They assumed that /ifa, or the " mass transformation co- 

 efficient," was proportional to the cube of the wave-length 

 of the X ray, and that ajp was a universal constant, inde- 

 pendent of the wave-length of the ray and of the material ; 

 so that 



- =al* + b, 

 P 



where I is the wave-length. From Barkla's data, b was 

 taken as 012. They showed that the values of X in 

 aluminium and copper, which they measured, satisfied 

 approximately the following equations : 



G)^ 14-9^0-12, 



(*\ = 150 Z 3 + 0-12, 



and 



1 P 



respectively, where I was in A.U. They found that the 

 absorption of X rays in lead was complicated by the 

 appearance of a marked absorption band, which appeared 

 when the wave-length was 0*149 A.U. For the lowest 

 wave-length examined, viz. 0*122, the value of X\p was 

 3*00 for lead. 



The wave-lengths of the penetrating gamma rays from 

 radium have been experimentally determined by Sir E. 

 Rutherford and Andrade *. The two shortest wave-lengths 

 observed in their experiments were *071 and '099 ^.U. 

 Since the value of \/p found in this present paper is 

 about *076, i. e. about ^- 6 of the value found by Hull and 

 Rice for \ = 0*122, it seems probable that the penetrating 

 gamma rays from radium C contain waves of much shorter 

 length than those observed experimentally |. Hull and 



* Sir E. Rutherford and Andrade, Phil. Mag. xxviii. p. 263 (1914). 

 t See Sir E. Kutherford's British Association Address, ■ En°ineerine- ' 

 Oct. 6, 1916. . = 5 ' 



K2 



