﻿648 Prof. W. H. Bragg on the Direct or Indirect 



indirect. Dr. Barkla says that we ought to have allowed 

 more for the scattering of the X-rays by the screens of lower 

 atomic weight, that if we had done so we should have found 

 a value for the ratio nearly twice as great as that which is 

 stated above, and therefore that we had not proved onr 

 point. 



It is qnite true that the screens of low atomic weight 

 scatter the X-rays (in this case, Sn X-rays) to a considerable 

 extent : but the silver screen also scatters the X-rays. 

 "Whether the process is exactly of the same nature is not of 

 importance : experiment shows that the ratio of the absorp- 

 tion coefficients of silver and of (say) carbon is very little 

 altered if the screens are placed close to the ionization 

 chamber so that scattered rays enter it, or farther away so 

 that they do not. If silver and carbon plates are used in 

 turn as that wall of the ionization chamber on which the 

 X-rays fall, the returned radiation is not very different in 

 either quality or quantity. I think we were sufficiently 

 correct in adopting the value 36*3, and in drawing our main 

 conclusion. 



Dr. Barkla would leave the question of the directness or 

 indirectness of the ionization an open one, arguing that our 

 experiments furnished no direct evidence in favour of the 

 view to which they led us. I venture to think that this 

 attitude is not strictly logical* When the passage of X-rays 

 through a gas was first found to be accompanied by the 

 ionization of the gas, it was assumed that the former process 

 was the cause of the latter. This was the right thing to do ; 

 but when it appeared further that the X-rays produced /3 or 

 cathode rays which were already known to possess ionizing 

 powers, all evidence for the direct action disappeared. It 

 became improper to assert the direct action until it had been 

 shown that the newly discovered indirect action was in- 

 sufficient ; and such evidence, it seems to me, is still 



wanting. 



Meanwhile, it appears from energy considerations which I 

 have tried to state at various times, and which Dr. Barkla 

 does not attack, that the X-rays cannot spend much energy 

 in their flight, and that therefore thev cannot have much 

 ionizing action : unless, indeed, ionization can be accom- 

 plished without the expenditure of energy. 



The experiments of Mr. Porter and myself were an attempt 

 to show r that an inference arrived at on other grounds was 

 consistent with facts so far as they could be ascertained. 

 I do not suppose that they supply a test of more than 

 a ten per cent, accuracy : in an earlier paper of mine (Phil. 



