24 



SCIENCE PROGRESS 



having gone. Now we are in a position to say which a stream 

 belongs to each substance. The block which is left behind must 

 be due to radium ; so the particles from radium have an extreme 

 range of 3'5 cm. and are less energetic than those of any of its 

 derivatives. The particles of longest range are those of Ra C ; 

 the two intermediate ranges belong to the emanation and to 

 RaA. Between these last two this experimental method can- 

 not easily decide, though it might be made to do so. But 

 H. W. Schmidt has shown by an independent method that the 

 shorter range belongs to the emanation. Each of the successors 



lorvitsoriorv. 



Fig. 6. 



Fig. 7. 



of radium up to Ra C therefore expels its a particle more 

 violently than those that have gone before. 



Now for a last and very interesting test of Rutherford's 

 theory. The radium plate which has been cleared of its 

 products will grow them again ; therefore if we continue our 

 experiments and examine the curve from time to time we 

 ought to see it recovering gradually its old form. The time 

 of growth is governed by the life of the emanation. Since 

 it has an average existence of between three and four days, 

 it will rise to half its final value in about that time : formed 

 at a regular rate, it accumulates at a diminishing rate, since 



