﻿650 Direct or Indirect Nature of Ionization by X-rays. 

 k 1 w l + k z w 2 , and that the ionization is given by 



(k 1 w 1 -\-k 2 w 2 ) 



\ 1 tv l + \ 2 W$ 



This is not the same as JciW^/jl + k%iv 2 , and in the case o£ 

 Ag rajs acting on mixtures of air and ethyl bromide the 

 difference should be observable, on the assumption made by 

 Dr. Barkla that X 1 = A 2 . For instance, when k ± is much 

 greater than Jc 2 > as * n this case : and when iv± = iv 2 the ratio 

 of the two expressions is 0*85 nearly instead o£ unity : 

 assuming /i=l'4. 



I admit at once that, if work and assumptions are all 

 correct, the difference should have been apparent. But two 

 things may be pointed out. In the first place, Dr. Barkla's 

 results would show that there is no indirect action, whereas 

 it is well known that there is much o£ it, The fact is evident 

 from the form of such curves as Beatty obtained (Proc. Roy, 

 Soc. lxxxv* p. 230). If the ionization in the gas were all due 

 to direct action it would be simply proportional to the 

 pressure of the gas_, and Beatty shows that this is far from 

 being the case* Mr* Porter and I have made many similar 

 experiments ; in several of them Ag rays acted on ethyl 

 bromide in a chamber lined with carbon or aluminium. 

 Until the pressure of the gas is such that the cathode rays 

 made in the gas can complete most of their paths within the 

 ionization chamber, the increase of ionization with pressure 

 is far less than it afterwards becomes. If Dr* Barkla's 

 experiments do not reveal the existence of the large amount 

 of indirect action which is known to exist, they cannot be 

 held to prove anything definite as to the relative amounts of 

 direct and indirect action. 



The second point is that the whole of Dr. Barkla's argu* 

 ment turns on the supposition that \ x =\ 2 in all cases. This 

 is Lenard's law ; but it is not known for certain that it can 

 be applied so generally. I think we must wait for further 

 knowledge of the absorption — so-called — of cathode rays 

 before we are influenced by arguments which assume the 

 law of equal densities. I gather from the concluding- 

 sentences of Dr. Barkla's paper that he himself is impressed 

 by this consideration-. 



Yours faithfully, 



W. H. Bragg. 



Leeds, March 1L 



