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LX XXVII. On the Emission of Electrons by Metals under 

 the Influence of Alpha Rays. By H. A. Bumstead *. 



IT was observed by Bragg and Kleeman f that when a thin 

 sheet o£ a metal, such as gold, was interposed in the 

 path of a pencil of a -rays, the diminution in range produced 

 was less when the metal sheet was far from the source of 

 rays than when it was near ; in other words, the rays seemed 

 to be " stopped " less by the metal when they were going 

 slowly than when they were moving with greater speed. 

 T. S. Taylor J made a careful study of the phenomenon and 

 found that this differential effect increased regularly with the 

 atomic weight of the metal. Substances whose atomic 

 weight was much greater than that of air showed the effect 

 very markedly ; substances, such as paper, whose average 

 atomic weight was nearly equal to that of air did not show 

 the effect at all ; and when a layer of hydrogen was used 

 instead of a solid obstacle, the effect was reversed. By 

 plotting the ionization curves of the a-rays in air and in 

 hydrogen, Taylor showed that there was exactly the same 

 relation between the ionization in the two gases at different 

 parts of the range as between their relative stopping powers. 

 Where the ionization in hydrogen was greater than in air, 

 the hydrogen was more effective in retarding the rays, and 

 vice versa ; and the numerical ratios for the two effects were 

 equal. These results led naturally to the hypothesis that the 

 energy lost by the a-particles in their progress through a gas 

 was mainly, if not wholly, consumed in the production of 

 ions, and that where the ionization was great (as at the 

 " knee " of the Bragg curve) the a-particles lost energy 

 more rapidly than where the ionization was less. As the 

 knee is more conspicuous in hydrogen than in air the relative 

 stopping effects of these two gases are easily accounted for 

 on this hypothesis. 



Shortly afterward, Geiger § published the results of a 

 repetition of Rutherford's measurements of the velocities of 

 the a-particle at different points in its range. The measure- 

 ments were made under more favourable conditions than 

 were possible at the time of Rutherford's original experi- 

 ments, and Geiger was able to show that the loss of energy 

 by the particles in passing through a layer of air was at least 



* Communicated bv the Author. 

 t Phil. Mag. x. p. 318 (1905). 



t Amer. Journ. Sci. xxviii. p. 357 (1909); Phil. Mag. xviii. p. 604 

 (1909). 



§ Proc. Roy. ^oc. Ixxxiii. p. 505 (1910). 



