304 



The area of the curve is given by A/Aj^ + B/A.2, and the 

 relative values of the areas are placed in the third column, 

 that of C being first subtracted and that of Pb being set 

 equal to 200. In the last column are the corresponding 

 figures when the amount of radiation is measured by A + B, 

 i.e., the ionisation found when one layer of tinfoil is placed 

 over the chamber. It is clear that there is a considerable 

 diiference. 



When tinfoil is placed over the radium its^elf so as to 

 cut out a large proportion of the slower )8 rays, the secondary 

 radiation is also hardened correspondingly. 



The figures here given are not sufiiciently accurate to be 

 taken as absolute. The measurements were made as care- 

 fully as possible, but more num.erous and more varied experi- 

 ments are needed before it will be possible to map out the 

 phenomena with exactness. But the results show clearly 

 that — 



(1) The secondary radiation from different substances, 

 due to the /3 and y rays of radium, varies in quality, those 

 of lower atomic weight returning a greater proportion of less 

 penetrating rays. 



(2) When the less penetrating portion of a primary beam 

 of 13 rays from radium is sorted out, the secondary rays also 

 become more penetrating. 



(3) In view of these considerations the ionisation cur- 

 rent due to secondary radiation, measured in a chamber of 

 the ordinary form, does not give a simple or accurate deter- 

 mination of the nature of that radiation. 



