398 Prof. W. H. Bragg on the Consequences of 



and compared the ionization current in the two cases. He 

 supposed that the ionization could be assigned to two sources, 

 one the direct action of the 7 rays on the gas, the other the 

 secondary rays caused by the 7 rays to issue from the metal 

 sides of the chamber. The former would be the same for 

 the two chambers, let it be called Ip: the second would not, 

 let it be I S l for the chamber which is all lead, and I SA for the 

 one which is lined with aluminium. He then assumed that 

 Isa= '286 xIsl, since Eve had shown that when 7 rays fell 

 on lead and aluminium plates the returned ft ray s were in the 

 proportion of 100 to 28' 6. 

 Thus : 



Ip + Isl= 90*05 (total ionization in the lead chamber). 

 Ip + IsA = 49"5 (the aluminium lining having been inserted). 

 I SA =-286xI SL . 



Hence he found that I P = 33*05, I SL =57 00, I S a=16*3; 

 and concluded that Ip, that is to say the result of the direct 

 action of the 7 rays upon the gas, was very considerable. 



The source of error in this calculation is the assumption 

 that I SA = '286 xIsl- It was not known at that time that 

 this relation only holds in respect to the ft radiations from 

 the front face on which 7 rays fall : the ft radiations which 

 issue from the face of a plate from which 7 rays are emerging 

 may even be greater for aluminium than for lead : and 

 McLennan's results depended on both incidence and emer- 

 gence rays. It was not right to use Eve's figures, which 

 referred to a special case of incidence rays ; and there is no 

 contradiction of the deduction we have drawn from the entity 

 hypothesis, viz. that Ip is zero. 



Again, W.Wilson records * measurements of the ionization 

 in an electroscope made partly of aluminium and partly of 

 brass when the pressure of the air was varied from one to 

 forty atmospheres : the 7 rays came from RaC. He supposes 

 that " the total ionization due to the secondary ft rays at 

 different pressures will be given by B(l — e~ kap ) where B is a 

 constant, p the pressure and X the coefficient of absorption," 

 and further that " the ionization due to the 7 rays will be 

 given by a term of the form Ap, where A is a constant." He 

 then finds that B must be 6'6 times A, and that the ionization 

 due to the secondary rays is therefore several times the 

 ionization due to the direct action of the 7 rays on the gas. 

 This is of course nearer than McLennan's result to what we 

 now expect, but it still ascribes some effect to the direct 



* Phil. Mag. Jan. 1909. 



