192 



myself for the /S rays which are produced from the y rays 

 of Ra. It was found that the value of A. for the hard and 

 soft )3 rays was approximately as one to four. 



Summary. 



1. The y rays of Ra and probably of Th appear to con- 

 sist of two distinct homogeneous bundles, the value of Xj A 

 for the soft set being approximately four times that for the 

 hard. 



2. For each set of rays the value of A/ A is constant and 

 practically independent of the nature of the absorbing mate- 

 rial with which A is measured, provided that in the case of 

 the soft rays secondary effects be excluded. 



3. Secondary y radiation appears on both sides of a plate 

 which is penetrated by a stream of y rays. There exists a 

 marked lack of symmetry between the amount of secondary 

 radiation which proceeds from the two sides. 



4. A lack of symmetry exists in the case of some sub- 

 stances between the quality of the radiation on the two sides. 



5. The last results seem very difficult to reconcile with 

 a pulse theory. On the ''material" theory propounded by 

 Professor Bragg no such difficulty arises. 



6. The secondary y radiation appears to be derived from 

 the primary by a process of scattering. This process gene- 

 rally involving a reduction in the subsequent penetrating 

 power of the ray affected. 



7. There appears to be reason to believe that the dis- 

 tribution of the scattered radiation depends to some extent 

 upon the hardness of the radiation which is scattered ; also 

 upon the nature of the material in which the scattering is 

 produced. The softer radiation appears to be turned back 

 to a somewhat greater extent than the hard. Materials of 

 high atomic weight seem to be able to produce more com- 

 plete scattering than those of lower atomic weight. 



8. The absorption of y radiation which has already passed 

 through a thickness of one substance by screens of a differ- 

 ent substance may not in all cases give a true measure of 

 the absorption of the original radiation which has been 

 effected by the first screens. 



In conclusion, it is a pleasant duty for me to acknow- 

 ledge my indebtedness and sincere thanks to Professor Bragg 

 for his keen interest and advice during the progress of these 

 experiments. 



University, Adelaide, 

 Sept., 1908. 



