from a Plate exposed to Rays from Radium. 187 



conclude that the process is not a single action, that the 

 primary ray is not simply deflected, nor simply absorbed 

 (consider, for instance, the production of j3 rays by y rays, 

 and the reverse) : but that the mechanical process is some- 

 thing more like a convective transference o£ energy, in a 

 manner suggested by electrolytic convection. As a working 

 hypothesis one might say that the atoms which are bombarded 

 absorb energy until internal instability is reached, and then 

 a sort of explosion takes place with the expulsion of electrons 

 and 7 rays ; these in turn provide energy to be absorbed by 

 the next layer ; and so on through the plate. Since the 

 limiting layer required for maximum '•reflected'*' rays at 

 the angle here employed is small compared with the maximum 

 layer capable o£ giving "transmitted" rays, we would have 

 to assume that the propagation of the energy obeyed a law 

 somewhat similar to that known for the propagation of wave- 

 energy, a maximum in the direction of and a minimum in 

 the opposite direction to the stream of entering energy. The 

 primary beam of rays would produce, as it were, an E.M.F. 

 in the direction of the beam. 



The relatively large intensity of these " transmitted " rays 

 would seem to have a bearing on the method of investigating 

 spontaneous ionization. The general results of this paper 

 are in agreement with those of Co eke and others on the 

 influence of the secondary rays due to the radiations coming 

 from outside the ionization-vessel ; bat they do not seem to 

 fall in with some of the observations of Campbell * on the 

 influence of plates of ordinary materials placed before the 

 thin aluminium side of his testing-vessel. 



The method of measuring the absorption of a beam of 

 radiation caused by a plate of given material, by noting the 

 change of ionization in a vessel when different thicknesses of 

 that material are put in the path of the beam, gives not the 

 absorbing power of the material, at least in the sense one is 

 apt to associate with the term, but the difference between 

 what it absorbs and what it radiates ; and as the latter 

 quantity can be quite large, the method is open to objections. 



Physical Laboratory, Dalhousie University. 

 May 9th, 1907. 



Phil. Mag. [6] vol. ix. April 1905. 



