140 HUXLEY MEMORIAL LECTURES 



radius as simply representing the range of the ray 

 in biotite. The furthest-reaching ray will define 

 the radius of the halo. In the case of the uranium 

 family this will be radium C, and in the case of 

 thorium it will be thorium C 2 . Now here we pos- 

 sess a means of at once confirming or rejecting 

 the view that the halo is a radioactive phenomenon 

 and occasioned by alpha radiation; for we can 

 calculate what the range of these rays will be in 

 biotite, availing ourselves of Bragg's additive 

 law, already referred to. When we make this 

 calculation we find that radium C just penetrates 

 0*033 mm. and thorium C 2 0*040 mm. The proof 

 is complete that we are dealing with the effects 

 of alpha rays. Observe now that not only is the 

 coincidence of measurement and calculation a 

 proof of the view that alpha radiation has occa- 

 sioned the halo, but it is a very complete verifica- 

 tion of the important fact stated by Bragg, that 

 the stopping power depends solely on the atomic 

 weight of the atoms traversed by the ray. 



We have seen that our examination of the 

 rocks reveals only the two sorts of halo : the 

 radium halo and the thorium halo. This is not 

 without teaching. For why not find an actinium 

 halo ? Now Rutherford long ago suggested that 

 this element and its derivatives were probably an 

 offspring of the uranium family; a side branch, 

 as it were, in the formation of which relatively 

 few transforming atoms took part. On Ruther- 

 ford's theory then, actinium should always accom- 



