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XL. The Influence of the Velocity of the a particle upon the 

 Stojjping Power of the Substance through which it p>asses. 

 By W. H. Bragg, M.A., Elder Professor of Mathematics 

 and Physics in the University of Adelaide** 



A PAPER " On the a particles of Radium and their loss 

 o£ range in passing through various atoms and mole- 

 cules " was published by Mr. Kleeman and myself in the 

 Philosophical Magazine for September 1905. On p. 337 of 

 this paper it was pointed out that the ionization curve ob- 

 tained when a metal sheet was placed over the radium did 

 not show exactly the same drop from the normal throughout 

 its entire length : the slower a particles were a little less 

 affected than the swifter. That is to say, the a particle 

 ejected by RaO lost rather more of its range in going through 

 a plate placed over the radium than the particle from Ra 

 itself; the former particle being the swifter of the two. 

 We remarked that this point deserved further enquiry. 



The question has been further discussed in a paper by 

 Kucera and Masek, " Ueber die Strahlung des Radio- 

 tellurs II.," pp. 630-640, of No. 19 of the Physikalischen 

 Zeitschrift for 1906. In § 7 of their paper they state that 

 the stopping-power of Al is lowered by 10 per cent., and 

 that of Pt by 12*5 per cent, if the rays, instead of entering 

 the metal directly after leaving the radiostellurium, first pass 

 through 1*9 cm. of air. Since the a. particle would lose 

 about 11 per cent, of its velocity in crossing such a layer, 

 the authors suggest that probably the stopping-power is pro- 

 portional to the velocity of the a. particle (see also paragraph 

 7 of their summing up) . 



This hypothesis will of course explain the effects observed 

 by Kleeman and myself. But in the form in which it is 

 stated by Kucera and Masek it seems to me to be inconsistent 

 with certain other experiments made by Rutherford, McClung, 

 and Levin, and indeed by the authors themselves. 



I think this is clearly seen if the various results are set 

 out as follows : — 



1. Rutherford has shown (Phil. Mag. Aug. 1906) that the 

 <x particle loses energy at a uniform rate on its way through 

 aluminium. Rutherford measured the velocity of the particle 

 directly; so that his experiments may be said to be absolute 

 as compared with those made by Kleeman and myself. Our 

 measurements were relative, since we determined the energy 



* Communicated by the Author. Read before the Australasian 

 Association for the Advancement of Science, Jan. 9, 1907. 



