[pound] absorption OF THE DIFFERENT TYPES OF BETA RAYS 67 



in a similar way, i.e., with one direction of the field the axis of the 

 pencil of secondary rays corresponding to RA would be turned anti- 

 clockwise, while with the field reversed this pencil would undergo a 

 clockwise deflection. From the diagram shewn in Fig. 17 it can be 

 seen that when the pencil of maximum intensity RA is given a clock- 

 wise rotation the air path traversed by it will be lessened, and con- 

 sequently the ionisation produced by it reduced. On the other hand, 

 with the anti-clockwise rotation the length of path traversed by this 

 pencil will be increased, and hence one should not expect the magni- 

 tude of the decrease in ionisation following the application of the field 

 producing this deflection to be as great as when the field applied caused 

 the rays to be deflected in the opposite sense. It is evident, too, that 

 the tertiary rays excited on the walls of the chamber by the aluminium 

 secondary rays would be greater in the case of the anti-clockwise 

 rotation of the secondary rays than in experiments when the rotation 

 of these rays was in the opposite direction. One naturally inquires 

 why this effect did not appear in the experiments when tin and lead 

 were used as coverings for the openings into the chamber, and also 

 when a thickness of 0.0784 mms. of tinfoil was placed below the 

 aluminium cover. The probable explanation is that the transmitted 

 secondary rays from tin and lead are not so effective ionising agents 

 or so good exciters of tertiary rays as the secondary rays from alu- 

 minium. The eiïect even in the case of aluminium is small although 

 quite noticeable, and it is probable therefore, with the weaker second- 

 ary rays from the tin and lead that the effect would be very much less, 

 and consequently masked by the other influences present. 



The experiments which have just been described are also inter- 

 esting for the light which they throw on the nature of the transmitted 

 secondary radiation excited in the metals aluminium, tin, and lead by 

 Y rays. According to the argument which has been presented, it 

 follows from Bragg's conclusions, since the secondary rays from 

 aluminium are better ionisers than those from tin and lead, that the 

 particles constituting these secondary rays must be endowed with 

 smaller velocities than those constituting the secondary radiation from 

 the other two metals. The transmitted y excited secondary rays 

 from aluminium should therefore, from this point of view, be more 

 easily absorbed than those emitted by tin and lead. 



This conclusion regarding the character of the transmitted 

 secondary radiation excited in aluminium by y rays is in accord with 

 the conclusions of McClelland,^ Starke^ and others, who have found 



1 McClelland, Trans. Roy. Dublin Soc. 8, p. 169, 1905. 

 ■^ Starke, Le Radium, Feb., 1908. 



