the Corpuscular Hypothesis of the y and X Rai/s, 



399 



action of the 7 rays. The fact is, however, that the division 

 of the ionization into these two terms is not quite right, even 

 supposing the ionization due to the 7 rays to include the 

 ionization due to the ft rays generated by the 7 rays in 

 the gas. 



Let us consider so far as we can what should be the amount 

 of ionization in a gas through which 7 rays are passing, 

 assuming the entity hypothesis and its consequences. There 

 are two cases at least in which the solution is fairly easy and 

 satisfactory. The easier one is the case of an ionization 

 vessel lined completely with any material, provided only that 

 it is so thick that ft rays cannot cross it. The other is the 

 case of a large but shallow ionization vessel, the top and 

 bottom of which consist of two parallel plates, one of which 

 is made of a substance having about the same atomic weight 

 as the air which the vessel contains. Let us take the latter 

 case first. 



It simplifies considerations of this kind to remember that 

 the spacing of atoms plays a subordinate part in them. 

 Suppose, for example, that a stream of ft rays passes up 

 normally to a plate through an opening in it at A, and that 



Fig. 2. 



wiiiiummv/jimiA 



wzzzzmmnzmm 



ft Wj» 



B, C, and D are imaginary surfaces in the air parallel to the 

 plate. The ft rays cause a certain ionization in the air 

 between the planes B and (). It would make no difference 

 in this amount if the air between C and D were compressed 

 into a thin layer lying along C or indeed anywhere above it, 

 so long as the air between B and C remained in a uniform 

 layer between and parallel to B and C. It would be the 

 same even if air were brought down from above C and laid 

 in a layer along C in such quantities that no ft rays could 

 get through it ; or if a plate composed of atoms of nearly the 



