﻿994 



Prof. C. G. Barkla and Mr. V. Collier on th 



weight than 160, neither the K nor the L radiation is excited, 

 and again the absorption is less than what it would have 

 been if these radiations had been excited. 



Similar curves may be obtained by using any homogeneous 

 X-radiation, but if the radiation is of more penetrating type, 

 all the maxima and minima are displaced to the right, and if 

 more absorbable to the left. 



Absorption in Elements of Low Atomic Weight. 



The absorption by light elements is interesting, because 

 when penetrating rays are transmitted through them by far 

 the greater proportion of the radiation is merely scattered, 

 and if surrounded by light elements rescattered. It has 

 been shown by one of us that the energy of Rontgen radia- 

 tion lost by scattering during transmission through a thick- 

 ness dx of a light element is given by the expression 



dFi= —sEidx, 

 where s is a constant for rays of all penetrating powers, and 

 is proportional simply to the density of the light scattering 



substance. Thus, if — is the mass-absorption coefficient, we 



r § 



may by analogy call the mass-scattering coefficient. 



This is constant for all matter made up of light atoms. The 

 lightest elements absorb much less than heavier ones, mass 



for mass, so that — diminishes with the atomic weight of the 

 absorbing element. It also diminishes as the penetrating- 

 power of the radiation increases. As - remains constant, 



sip s P 



~~ or =r (which is the fraction of the absorption coefficient 



Xjp A, v L 



due to scattering) becomes great when penetrating rays are 

 passed through light elements. 



Now, as the penetrating power of a radiation varies, the ab- 

 sorptions in any two substances remain approximately propor- 

 tional, provided the range of penetrating power experimented 

 upon does not include or approach on the more penetrating 

 side the penetrating power of a fluorescent radiation charac- 

 teristic of either A or B. If we m ly be allowed, for the sake 

 of simplicity, to express this in the terms applied to light, it 

 may be stated thus : — The absorptions of X-rays in two 

 substances A and B bear an approximately constant ratio, 

 one to the other over a range of wave-lengths not including 

 a spectral line of either and not near to a spectral line of 

 either on the side of shorter wave-lengths. As the scattering 

 is independent of the wave-length, we should expect this law 

 to cease to hold when the loss bv scattering is an appreciable 





