of Gamma Rays from Radium B and Radium C. 725 



The source of y rays was in all cases more than 9 cm. dis- 

 tant from the vessel CD. The absorbing metal screens were 

 placed between the pole-pieces of the magnet close to the 

 source. The {3 rays escaping from the absorbing material 



Fie:. 1. 



Sect /on of pole p/ece 



were removed by the magnetic field before entering the 

 ionization vessel. The y rays which entered the ionization 

 vessel passed nearly normally through the absorbing screens, 

 so that no correction for obliquity was necessary in deter- 

 minino- the absorption coefficient of the rays. 



Preliminary experiments showed that if the ionization 

 vessel were filled with air, the effect of the penetrating 

 y rays from radium C was large compared with that due to 

 the softer types of radiation that were present. When air 

 was used, the reduction of the ionization by using absorbing- 

 screens of aluminium of different thicknesses is shown in 

 fig. 2 A (p. 726), where the logarithms of the ionization are 

 plotted as ordiuates and the thickness of aluminium as 

 abscissae. It will be observed that in this case there is a 

 rapid drop of the ionization corresponding to about 10 per cent. 

 oH the whole effect. 



In order to bring out prominently the effect of the softer 

 types of 7 radiation present, the vapour of methyl iodide 

 was used instead of air. As it was impossible to exhaust 

 the ionization vessel on account of the thin mica covering of 



Phil Mag. S. 6. Vol. 25. No. 149. May 1913. 3 D 



