37(5 Prof. C. G. Barkla on Typical 



absorption in A is approximately proportional to the absorption 

 in B, the ionization in A proportional to the ionization in B, 

 and the intensity of secondary X-radiation already excited 

 in A proportional to the intensity of secondary X-radiation 

 already excited in B. But as the penetrating power of the 

 primary X-radiation becomes just greater than that of the 

 radiation characteristic of an element in A say, this new type 

 of X-radiation begins to be emitted by A, the absorption of 

 the primary radiation in A begins to increase, the ionization 

 in A begins to increase, the intensity of corpuscular radiation 

 from A begins to increase. All these increases occur 

 together, and they are, in general, very considerable in 

 magnitude. There is every indication of all these quan- 

 tities ultimately settling down to proportionality again with 

 the corresponding quantities in B, though in this higher 

 ratio. 



The question naturally arises as to the possibility of the 

 great increase in ionization being produced not by the direct 

 action of the primary rays, but of the secondary rays — (X or 

 corpuscular) — which are connected with the increase in 

 ionization. It may easily be shown that the secondary 

 X-radiation did not produce more than a very slight increase. 

 The effect of the corpuscular radiation will, however, be 

 considered as it leads to an interesting result. 



During the transmission of X-rays through a gas, each 

 thin layer of gas, unless within about 1 millimetre of the 

 boundary in these experiments, is subject to the corpuscular 

 radiation from two thick plates of its own substance — one on 

 each side. 



Let X and V be the coefficients of absorption of the primary 

 X-radiation and of the secondary corpuscular radiation 

 respectively in the gas or vapour, as defined by the equations 



l=I e- Xz and I' = I ^- Vx . 



Let k' be the coefficient of transformation of X-radiation 

 into corpuscular radiation, as defined by the equation 



dE' = k'Idx. 



d¥j being the total energy of the primary radiation of unit 

 cross-sectional area transformed into corpuscular radiation 

 per second in a layer of: depth dx. 



If /i is the fraction of this directed towards the face of 

 incidence of the primary beam, the total intensity of this 

 corpuscular radiation emerging from a thick layer through 



