oc Particles with Light Atoms. 



575 



ol particles, and this is borne out by experiment. In the 

 case of air, the active disk coated with radium G of 7-ray 

 activity about 30 mg. Ra, was mounted with its plane 

 vertical at a distance of about 7 cm. from a zinc sulphide 

 screen in the open air. Both the source and screen were 

 placed between the poles of a large electromagnet to deflect 

 the ft rays. The vertical convection currents due to the 

 heated electromagnet prevented any contamination of the 

 screen by active matter escaping from the source. 



The eud oi' the range of the a particles was sharply denned, 

 but numerous bright scintillations were observed for distances 

 nearly 2 cm. beyond the range of the a particles. There was 

 a steady decrease both in number and brightness up to 9 cm. 

 of air, and beyond that distance the small number of scintil- 

 lations observed, due to H atoms from the source and from 

 the water vapour in the air, fell off slowly. 



The range of these atoms was best determined by placing 

 the screen just outside the range of the a particles (7*1 cm. 

 at 15° C) and then adding thin screens of aluminium foil 

 close to the zinc sulphide screen. The variation of number 

 with absorption in terms of cms. of air is shown in fig. 1. 



Fig. 1. 

 10 





































































A, 



Hatoms 



t 





70 



7-4 



9-0 



9-4 



7-8 8-2 8-6 



Absorption in cms. of air. 



It will be seen that the scintillations fall off at first slowly 

 with increase of absorption and more rapidly near the end 

 of their range, which was equal to 9'0 cm, of air at normal 



9-8 



