318 PROCEEDING? OP SECTION A. 



4._0N THE INFLUENCE OF THE SPEED OF THE a- 

 PARTICLE UPON THE STOPPING-POWER OF THE 

 MEDIUM THROUGH WHICH IT PASSES. 



By W. H. BRAGG, M.A., Elder Professor of Mathematics and Physics in the 

 University of Adelaide. 



(Abstract.) 



When the stopping-powers of substances were first measured by 

 Bragg and Kleeman {Phil. Mag., Sept., 1905) it was observed that the 

 swifter a-particles from Ra.C. were more hindered in passing through 

 a metal sheet than the slower particles of Ra. itself. Kucera and 

 Masek have recently {Phys. Zeit., 7, Nos. 18 and 19) investigated this 

 effect, and have concluded that the stopping-powers of metals are 

 approximately proportional to the speed of the a-particle. 



It is now pointed out that the results of Kucera and Masek are in- 

 consistent v,'ith the results obtained by Rutherford, Levin, and others. 

 For Rutherford has shown {Phil. Mag., Aug., 1906) that the particle 

 spends its energy at a uniform rate on its way through aluminium ; 

 and this must be true of air also, since the loss of range caused by a 

 sheet of aluminium is proportional to its thickness. And further it 

 might be inferred from experiments described by Kucera and Masek 

 that this is true also of Au and Pt, since the loss of range caused by 

 two sheets — one of Al and the other of Au or Pt — is independent of 

 the order in which the particle passes through them. 



The author has made further experiments on this point, from which 

 he draws conclusions differing in some respects from those of Kucera 

 and Masek. The stopping-power of Al is nearly independent of the 

 speed of the particle : on the other hand, the stopping-power of Sn 

 increases with the speed ; and the stopping-powers of Au and Pt do 

 so to a more marked degree than that of tin. Consequently the 

 stopping-power of two sheets of different metals must depend on the 

 order in which the particle passes through them ; and this appears to 

 be the case from the author's experiments, though Kucera and Masek 

 did not find it to be so. 



It is further shown that this result will probably account for the 

 experimental results of Mme. Curie, who found different values for the 

 ionisation current above two metal sheets depending on their order. 



5.— THE BRITISH GRAVITATION SYSTEM OF UNITS. 

 By If. R. JAMIESON, M.A., B.Sc. 



6.— THE DEFINITION OF THE CO-EFFICIENT OF 



EXPANSION. 



By W. R. JAMIESON, M.A., B.Sc. 



