242 Mr. D. C. H. Florance on 



of the secondary radiation from the zinc measured in a 

 screen of lead 0*084 cm. thick is given in Table II. It is 

 seen that the secondary y radiation is gradually changing in 

 type and that the radiation becomes less penetrating the 

 more its path is removed from the original direction of the 

 primary beam. This secondary y radiation is not only 

 completely heterogeneous taken as a whole, but it is also 

 heterogeneous when a limited region is examined. This 

 heterogeneity is most marked when lead is used as the 

 absorbing screen. For radiators such as aluminium and 

 zinc characteristic radiations were not expected ; and in 

 these experiments there was no indication of such a 

 radiation. 



The general view at the present time regarding the 

 scattered X rays is that they are of the same penetrating- 

 power as the primary X rays. Webster * has put forward 

 an explanation for the distribution of scattered X rays, and 

 he points out that a diminution in the penetrating power of 

 the radiation is to be expected. If the secondary y radiation 

 we have been examining is a result o£ scattering, then the 

 scattering of y rays must involve some modification in the 

 primary y rays. The early view that scattering consisted in 

 the sifting out of the various components of the original 

 primary beam is no longer tenable. This is obvious from 

 the fact that after the primary beam has passed through 

 1 cm. of lead and then through a lead radiator 1 cm. thick, 

 there is still present a soft secondary radiation which must 

 have been completely absorbed had it been an untransformed 

 constituent of the primary beam. In other words, the y rays 

 emerging from any thickness of material appear to possess 

 the same power of producing secondary rays. A similar 

 conclusion has been arrived at by Gray j". 



We seem, therefore, to be reduced to two conclusions, that 

 either these secondary y rays are the primary rays scattered 

 and in the course of scattering the rays have lost energy and 

 have become altered in type, or a complete transformation 

 has taken place and these radiations are true secondary 

 radiations. This latter hypothesis is not supported by 

 experimental evidence. The secondary radiation is not in 

 any way characteristic of the material from which it emerges. 

 The secondary y rays do not appear therefore to be intimately 

 connected with the atoms of the radiator apart from the fact 

 that the atoms act as scattering centres. Moreover, although 



* Webster, Phil. Mag-. February 1913. 

 f Gray, Phil. Mag. October 1913. 



