230 Prof. Barkla and Miss J. G. Dunlop on the 



It is not necessary to give the detailed results obtained in 

 this investigation as some points need further examination. 

 We have, however, established the approximate constancy 

 of the scattering coefficient in aluminium for radiation 

 within the range of wave-length used in these experiments. 



Not only these experiments but a number of independent 

 investigations indicate a slight increase of scattering from 

 aluminium with an increase in the wave-length of the 

 radiation. Within the range of wave-length of these experi- 

 ments it is, however, certainly small ; the error will be of 

 no moment if we neglect it. 



Conclusions. 



The conclusions to be drawn from the results of Table II, 

 may be summarized as follows : — 



The scattering of X-rays of very short wave-length by 

 equal masses of various substances varies only little with the 

 atomic weight of the scattering element. There is a slight 

 increase of the mass scattering coefficient with an increase 

 in the atomic weight of the scatterer. (Elements of low 

 atomic weight have previously been shown to scatter to 

 approximately the same extent mass for mass not only 

 X-radiation of short wave-length, but X-radiation of any 

 wave-length between widely separated limits.) 



The increase of the scattering with the atomic weight of 

 the scattering substance becomes more marked with 

 X-radiation of greater wave-length, until for long waves it 

 becomes very considerable; that is, the heavier elements 

 scatter much more mass for mass than light elements, when 

 the radiation is of long wave-length ("soft"). 



Thus the intensity of X-radiation of given wave-length 

 scattered by equal masses of different elements increases 

 with the atomic weight of the element, and the intensity of 

 X-radiation scattered from a given material increases with 

 the wave-length of the radiation of given intensity. 



As previously stated, these variations are inappreciable 

 among elements oE low atomic weight for radiations of the 

 wave-length which may be regarded as ordinary Rontgen 

 radiation. 



In attempting to interpret these results, it should be 

 pointed out that the scattering is proportional to the number 

 of scattering electrons only when these act independently of 

 one another. The condition is apparently satisfied in light 

 atoms when traversed by Rontgen radiation of any wave- 

 length within wide limits. Each electron is then influenced 



