516 Velocity of the at, particle and its Stopping Power. 



importance, relatively. It is clear, therefore, that the 

 observed difference of 7 per cent, in the ionizations can easily 

 be accounted for. 



This is of course but one instance. Yet so far as it goes 

 it leads to the conclusion that the effects observed by Madame 

 Curie, and by Kucera and Masek, can be explained by as- 

 cribing them to the slight alteration in the stopping-power 

 of two metals when inverted. We should expect, on this 

 hypothesis, the alteration to be relatively greater the thicker 

 the plates, the greater the distance from the radioactive 

 material, and probably also the greater the contrast between 

 the atomic weights of the metals. This agrees very well with 

 actual observation. 



Kucera and Masek ascribe the effects to a difference in the 

 amount of the scattering of the rays by the two metals. I 

 do not quite follow their argument as illustrated by the 

 diagram in § 3 of their third paper (Phys. Zeit. vii. 19. 

 pp. 650-654). They point out that if the scattering power 

 of two metals is unequal, then there must be a difference in 

 the paths of the outside rays of the pencil due to inversion 

 of the plates. But the scattering is small, and this differential 

 effect can only be of the second order of small quantities. 

 Also the outside rays are not fairly representative of all the 

 rays. And again, they compare the passage of a bundle of 

 a rays through air or metal to the passage of a ray of light 

 through a somewhat turbid medium. But the effect of two 

 such media upon a ray of light which passes through them 

 in succession is independent of the order in which it does so. 



If the stopping-power of a substance depends upon the 

 speed of the a particle, it is clear that the proportionality 

 between the stopping-power and the square root of the atomic 

 weight cannot be exact, except possibly at some particular 

 speed. But the stopping-power does not vary much with 

 the speed ; and the general truth of the law is not affected 

 by any of the results here described. 



It may be added that when the stopping-power of some 

 metals were measured by Kleeman and myself, we placed the 

 sheets about 1*5 cm. from the radium. They were usually 

 equivalent to about 3 cm. of air, so that they were fairly in 

 the centre of the range of the particle of EaC, and the 

 average velocity of the particle when crossing them was 

 nearly equal to the average velocity of the particle throughout 

 its whole course. 



