508 Prof. W. H. Bragg on the Influence of tlie 



indirectly, by finding the thickness of the air stratum which 

 the particle could still penetrate. 



2. It has been shown by McOlung, by Levin, and by 

 Kncera and Masek (§ 1 of the paper quoted), that each layer 

 of Al foil which. is laid upon a radioactive surface diminishes 

 the range of the a. particle by an equal amount. That is to 

 say, if one layer cuts off *5 cm., two cut off 1 cm., three 

 1*5 cm., and so on. Taken in conjunction with Rutherford's 

 experiment this implies that in air, as in aluminium, the loss 

 of energy in crossing a given layer is independent of the 

 speed of the particle, for if the loss of range in air is pro- 

 portional to the thickness of the foil, then the relation 

 between speed and stopping-power must be the same in 

 metal as in air. 



3. Kucera and Masek describe experiments from which it 

 would appear that the range lost by an a particle in going 

 through two sheets of dissimilar metals such as Al and Au 

 or Al and Pt (§ 6 of the paper quoted), is independent of the- 

 order in which it passes through them. Now if it crosses 

 the Al first, its velocity whilst passing through this metal is 

 greater than when passing through the gold, and vice versa. 

 Since speed is a matter of indifference when it crosses the 

 Al, it must be so also when crossing the Au or Pt. There- 

 fore for these metals also the stopping-power is independent 

 of the speed of the particle. 



We have thus reached a conclusion which is directly 

 opposed to that of Kucera and Masek ; and it is clear that 

 there must be an error somewmere. 



Some experiments which I have made recently seem to 

 suggest a probable explanation of the inconsistencies. In 

 agreement with the work of Kucera and Masek, and with the 

 original experiments of Kleeman and myself, they show T that 

 the stopping-power of a metal sheet is not independent of the 

 speed. But they show also that this effect is not equally 

 great in different metals ; it is small in Al, but larger in Au 

 and Pt. It follows that the reversal of two layers of different 

 metals must alter the stopping- power of the combination; 

 and this I find to be well shown under suitable circumstances. 



Let us first consider some experiments with aluminium. 

 The behaviour of this metal is of special interest since it was 

 used by Rutherford in the research already mentioned (Phil. 

 Mag. Aug. 1906). My experiments show, on the whole, 

 that the velocity of the a particle has only a small influence 

 on the loss of range in going through a given sheet. It is 

 fairly clear that there is some influence ; but the experimental 

 errors are often large enough to hide it. 



& — — to 



