Prof. Owens on Thorium Radiation. 



Fig. 4. — Screening effect of thin Aluminium foil. 

 Thorium Oxide. Thin layer. 



371 



O 50 



i 5 (5 



Layers of foil. 



An inspection of the curve (fig. 3) at once shows that 

 the radiation is at first absorbed rapidly, but as successive 

 layers are added they produce a smaller and smaller pro- 

 portional diminution of the current. 



Evidently the radiation consists of a readily absorbable 

 kind, forming the greater part of the whole, and a more 

 penetrating kind small in amount as compared with the first. 

 If the radiation is homogeneous a simple absorption law 

 should apply, that is, the current should decrease in geo- 

 metrical progression as the thickness of the absorbing layer 

 increases in arithmetical progression, or if one of the two 

 kinds is homogeneous the current should approximately obey 

 the law when it is mainly due to this kind. As the first part 

 of the curve is nearly logarithmic, we may infer that the first 

 or more absorbable part of the radiation is nearly if not quite 

 of one kind. 



Again, if a thin layer of the active material is used, the 

 conduction-current will be smaller and the total amount of 



2 D 2 



