i64 



and that in addition there is in some cases a lack of sym- 

 metry in the quality of the emergence and incidence y radia- 

 tion. 



Kleeman, from a study of the incidence radiation, has 

 advanced a theory of selective absorption, not only for the 

 secondary but also for the primary y rays. In the light of 

 the more comprehensive information which can be obtained 

 from a study of both the emergence and the incidence radia- 

 tion, an attempt will be made in the present paper to show 

 that most of the secondary ray effects can be explained, if 

 we suppose that from the radium there are originally emit- 

 ted two sets of homogeneous y rays, which each subsequently 

 suffer modification by the process of scattering, becoming 

 softened and in some cases broken up, giving rise to )8 rays. 



The present paper is intended to give a preliminary out- 

 line of the experimental work and general theory. 



The measurements are in many cases small and difficult 

 to make with any very great accuracy, but the effects to be 

 described seem well marked. It will be necessary to extend 

 with greater care many of the details of the work before it 

 can be considered as at all complete. 



§1- 



The arrangement of apparatus employed in the first ex- 

 periments is shown in fig. i. The radium is placed near the 



To Ear!h 



To Elec frometcr^^ 



To Cells ^ 

 Insulation 



Y 



/y\ "C /// 



^ 



fIG. t. 



The 7 radiation proceeds from the Ra through the conical 

 hole. Radiators may be placed in its path as at e. The quality 

 of the secondary radiation may be tested by the domes h,h or the 

 cylinders i,i. When measuring the effects of the secondary inci- 

 dence radiation, the ionization chamber is inverted and radiators 

 placed as at p,'p. 



