938 Dr. J. Chad wick and Mr. B. S. Bieler on th 



first sight, very similar to the curves of fig. 5. The curves 

 for a particles of different velocities are, however,' much 

 closer together than our experimental curves : in other 

 words, the elastic sphere gives a much smaller variation 

 in number of projected H particles with velocity of the 

 a-rays than we require. .By comparison of the two sets 

 of J>, 6 curves, it is easy to find the radius of an elastic 

 sphere which will give approximately both the observed 

 number and the distribution of the H particles for any one 

 velocity of the a. particle. For a particles of range 6'6 cm. 

 this radius is 8 x 10 ~ 13 cm. ; while for a, particles of range 

 4'3 cm. it is oxlO -13 cm. If we consider the elastic 

 sphere as a simple representation of a discontinuous field 

 of force, in which the repulsive forces increase very rapidly 

 at a short distance from the a particle, we should expect an 

 effective radius smaller at high velocities than at low. 

 Since the effective radius varies in the opposite way, we are 

 forced to reject the elastic sphere. If, as Darwin suggests, 

 the elastic sphere represents in a general way systems of any 

 shape, but orientated equally in all directions, we may con- 

 clude that the a particle is probably an orientated system so 

 arranged as to throw more of the H particles forwards. 



This view is strengthened by consideration of the collision 

 relation for the elastic plate, worked out by Darwin. This 

 model of a particle repels the H particle with a force varying 

 as the inverse square of the distance from the centre, and 

 the centre is surrounded by a circular plate from which the 

 H particle rebounds elastically. The p, 6 curves for such 

 an elastic plate (Darwin, p. 504) show a greater variation 

 with the velocity of the u particle than our experimental 

 curves : that is, they differ in the opposite sense to the curves 

 for the elastic sphere. 



In fig. 6, curve S is the p, (V /V) s curve for = 31°'3 for 

 an elastic sphere of radius 4 x 10~ 13 cm., and curve P that 

 for an elastic plate of radius 8 x 10 _13 cm. At the point o£ 

 intersection they turn into the inverse square law line B'. 

 The dimensions are so chosen as to give a deviation from the 

 inverse square law in the region given by the observations. 

 The corresponding experimental curve lies about midway 

 between these curves. 



As a first approximation, we may say that the a particle 

 behaves in these collisions as a body with properties inter- 

 mediate between the elastic sphere and the elastic plate, 

 and compare it with an elastic oblate spheroid of semi- 

 axes about 8 x 10 -13 cm. and 4 x 10~ 13 cm. respectively, 

 moving in the direction of its minor axis. On this view, 



