510 Prof. W. H. Bragg on the Influence of the 



half a millimetre, which is well outside the limits o£ experi- 

 mental error, provided there is no mistake in the cause to which 

 it is ascribed. At this point I ought to say that the method, 

 in which the whole ionization curve of radium is used, 

 generally shows more of the effect under discussion than the 

 methods referred to earlier in this paper. When the loss of 

 range of the a particles of BaC is compared with the loss 

 of range of those of Ba, more difference between the two 

 losses is generally found than if the former loss is compared 

 with that of a. particle of BaC whose velocity has been cut 

 down so as to equal the velocity of the particles of Ba itself. 

 I have not yet been able to find a reason for this : the work 

 is of course on the extreme boundary of what can be done 

 under present experimental conditions. There is certainly 

 no justification at present for supposing that there is here 

 any proof that the a particles of BaC and of Ba are dis- 

 similar in nature or condition : but when greater accuracy 

 is attainable it will be proper to make a careful examination 

 of the question. 



This point being reserved, it will be clear that on the 

 whole the stopping-power of aluminium relative to air is at 

 least nearly independent of the velocity of the a. particle. 



The effects differ considerably in the case of gold. In 

 fig. 2 are shown two curves representing the results of an 



Fig. 2. — Showing that the stopping-power of a certain gold sheet is 

 altered by varying its distance from the radium. 



experiment which differed from that recorded in fig. 1 in 

 one particular only, viz. that Au was used in place of Al. 



