Symmetry shown by Secondary JL-Rays. S6% 



incidence (1) curves for Pt, Cu, and Fe ; the other the 

 corresponding curves for Sn, Al, and celluloid. 



The experiments described in this paper show that a very 

 marked want of symmetry occurs in the case of secondary 

 X-rays, the emergence rays being generally greater than the 

 incidence. This is another instance of the close parallelism 

 between X- and y rays. On a material theory of X- and 

 y rays the effect is easily explained, and is to be classed with 

 the scattering to which /?, and also, as lately shown clearly 

 by Greiger, a rays are subject. But if the X- and y rays 

 consist of energy bundles of very small volume, as suggested 

 by J. J. Thomson, then these bundles must be capable of 

 deflexions in going through atoms — that is to say, swung 

 out of their paths by the electrical forces to be found within 

 the atoms, just as neutral pairs would be in virtue of their 

 electrical fields. It seems hard to understand the distinction 

 between such bundles and entities generally classed as 

 material. 



In the course of this investigation we have made a number 

 of experiments on the quantities and qualities of the secondary 

 radiations. This subject has been fully treated by Barkla, 

 some of whose recent papers have not yet reached us, and 

 any discussion we gave might be merely a duplication of 

 part of his inquiry. There is, however, one point to which 

 we should like to refer. 



Very hard y rays follow a density law of absorption, 

 treating all atoms alike, except in respect to weight. Soft 

 y rays are not independent of atomic groupings of matter, 

 and are far more strongly absorbed by heavy atoms than by 

 light, after allowance has been made for weight. The same 

 is generally true of X-rays ; but in the case of very soft 

 X-rays there is a tendency to revert to the density law again. 

 For instance, X-rays that have passed through the glass of 

 the bulb are soft to copper, silver, tin, and so on, but hard 

 to aluminium, carbon, and low atomic weight generally. 

 No doubt those rays which are soft to such light atoms have 

 already been absorbed by the glass. But secondary X-rays 

 from most substances are softer than anything emerging 

 from the bulb and contained in the primary ray. The 

 difference is not very great when the absorption is measured 

 with the aid of screens made of substances of the higher 

 atomic weights, because to these the primary rays are soft 

 already. But if the screens are made of aluminium, still 

 more of filter-paper, the difference now seems to be very 

 great, for the secondary rays are soft even to low atomic 

 weights. For example, in one experiment, a sheet of copper 



