Recoil of Hydrogen Nuclei from Swift a Particles. 893 



aluminium faces the source of a rays. In the first case the 

 ionization is due to 7 rays and also to recoil atoms produced 

 along the whole path of the a rays ; in the second the con- 

 ditions are the same, except that the a particles in the first 

 0*69 cm. of their range are passing through aluminium, and 

 are therefore not able to excite recoil atoms. The difference 

 in the ionization thus represents the effect of the recoil atoms 

 from the swiftest ex. particles only. 



2. Apparatus, (Fig. 1.) 



The separate parts of the apparatus are screwed to the 

 under side of a wooden frame which rests on top of the electro- 

 magnet used to remove the /3-rays. The source, a small metal 



Fia-. 1. 



^*\ A 1 u minium 

 Radioactive J)$\ Screen 

 Source 



Compensating 

 / tonisation 

 Cnamder 



Dummy 

 f/ectroscope 



plate whose face is covered with the active deposit o( radium 

 or with thorium C, fits in a brass holder, its active surface 

 facing the ionization chambers. The paraffin screen slips 



