﻿Absolution of Homogeneous ft Rays. 733 



W. Wilson * has discussed the question of absorption of 

 swift /3 rays by aluminium in two papers. The results given 

 in his first communication agree approximately with those 

 given in the present paper. He examined this question again 

 in a second investigation, in which he believed that he obtained 

 more homogeneous ft rays. In this paper he finds that the 

 absorption curve for fast ravs in aluminium rises to a 

 maximum for a small thickness of matter and then decreases 

 in an approximately linear relation. His method of measuring 

 the ionization due to the /3 rays was to subtract the reading 

 of the electroscope with a thick screen from the reading with 

 the screen in which the absorption of the j3 rays was under 

 examination. Thus he assumes that the 7 radiation produces 

 the same quantity of secondary /3 radiation in a thick plate 

 as in a thin plate. In general this is not the case. The 

 conditions of his experiment are not sufficiently clear to 

 form an idea of the magnitude of this correction ; for it will 

 depend on what material he placed in front of his ionization 

 vessel (behind the aluminium screen), hi my experiments, 

 with celluloid in this position, the secondary /3-ray effect 

 was greater with a thick aluminium plate than with a thin 

 one, as is shown in curve BB (fig. 2). If this were the case 

 in Wilson's experiment, the rise of his absorption curve to 

 a maximum receives a simple explanation. This is rendered 

 the more probable since he mentions that the 7-ray effect 

 was relatively strong, thus giving a relatively great import- 

 ance to the secondary /3 rays from the absorbing screens. I 

 have tried to obtain his results by narrowing the slits to 

 produce greater purity of rays, but have been unsuccessful. 

 In Wilson's experiment the air was not exhausted from the 

 apparatus. To test the effect of this, readings were taken 

 with air in the box B (fig. 1), but the only obvious effect 

 was a diminution in intensity of the j3 radiation. 



In conclusion I wish to thank Prof. Sir Ernest Ruther- 

 ford for suggesting this research and for many valuable 

 suggestions during its progress. I also wish to thank 

 Dr. N. Bohr for his great interest and help in connexion with 

 the theory of absorption of ft particles. 



The Physics Laboratory, 



Victoria University of Manchester, 

 March 4, 1915. 



* W. Wilson, Proc. 'Roy. Sue. A. lxxxii. \\ 612 (1909) ; Ixxxvii. p. 810 

 (1912). 



