the Absorption of the Gamma Rays. 145 



Another experiment, similar to that described in § 3, was 

 performed with mercury. The mercury was filled in a 

 glass trough, the thickness of the mercury plate being 

 2*06 cm. The source was the same as that used in the 

 first experiment, but it was screened by a lead plate 1 cm. 

 thick. The ionization current in the electroscope was as 

 follows : — 



The source alone . . , 100 per cent., 



The mercury plate in position R x (fig. 2) 17*71 „ 

 The mercury plate in position R 2 (fig. 2) 31*24 „ 



which gave : 



/jl-\-ct = '86 cm. -1 , /ul = *56 cm. -1 , S = 13*5 p. c; 



<Td= *30 cm."^ o- K = *37 cm. -1 ; 



lip = *041 cm. 2 gr.- 1 , - = *027 cm. 2 gr. -1 . 



r 



The results confirm the above-mentioned conclusion. 



In the second experiment only one thickness of mercury 

 was tried, and therefore the final values of cr and fi for 

 small thickness could not be obtained. 



(3) Comparison of the scattering coefficients for the 

 ordinary X rays and for the gamma rays of very high 

 frequencies : — In the case of light elements, whose atomic 

 weight is not greater than 32, the absolute values of the 

 mass-scattering of X rays were found to be between 0*1 and 

 0*4 cm. 2 gr. -1 , and to be about 1*2 cm. 2 gr. -1 for aluminium 

 by Crowther. For the heavier elements the values were 

 greater, according to Barkla's and Crcwther's results. The 

 writer's results in Table VIII. show that the mass-scattering 

 for the gamma rays of very high frequencies is far less than 

 that for the ordinary X-rays. The difference is of great 

 interest, but further investigation is required before any 

 explanation can be offered. 



4. The scattering coefficient of the gamma rays of very 

 high frequencies is comparable in value with the true ab- 

 sorption coefficient of an element. This difference between 

 gamma rays and ordinary X rays indicates the importance 

 of taking into account the scattering of gamma rays in all 

 measurements of their absorption by matter. 



Phil. Mag. S. 6. Vol. 33. No. 193. Jan. 1917. L 



