May ilth, 1919.J PROCEEDINGS. xix. 



with velocity is in good agreement with the inverse square 

 law. On the nucleus theory, the collision of a swift atom, 

 like an a particle, with a light atom of matter gives rise 

 to very intense forces due to the close approach of the two 

 nuclei in a direct impact. In a colhsion of a swift a par- 

 ticle of mass 4 with a hydrogen atom of mass i, the 

 hydrogen nucleus should be set in swift motion with a 

 maximum velocity i -6 times that of the a particle. Marsden 

 has shown that such swift H atoms can be detected by the 

 scintillations on a zinc sulphide screen a,t a distance in 

 hydrogen about four times greater than the Irange of the /a 

 particle. Since in such collisions, the "nuclei must approach 

 Avithin a distance of the order of io-i3 cms., the distribution 

 and number of the H atoms set in motion should throw 

 light on the dimensions of the nuclei and on the magnitude 

 of the forces involved. 



In order to examine these points, a metal disc coated 

 with radium C. served as a powerful source of homo- 

 geneous a radiation. The a rays after passing through a 

 column of hydrogen were stopped by a silver plate and t,be 

 scintillations observed on the zinc sulphide screen beyond 

 were due to sw:ift H atoms set in motion by the a 

 particles. If the nucleus of the a particle and H atom 

 could be regarded as point charges at such small distances, 

 the beam of H atoms should be very heterogeneous with 

 regard to velocity and H atoms of low velocity and short 

 range should preponderate. The distribution with velocity 

 of the H atoms was found to be very different; for with a 

 rays of range 7 cm. the H atoms appeared to be projected 

 mainly in the line of the a rays and to be of nearly equal 

 velocity. For a particles of lower velocity, the beam of 

 H atoms became more heterogeneous with regard to 

 velocity, but in all cases the actual number was greatly in 

 excess of that to be expected on the simple theory. The 

 results indicated that each a particle of range 7 cms. in 

 air shot at a perpendicular distance of 2.4x10-^3 cms. from 

 the H nucleus- set the latter in swift motion nearly in the 

 direction of the a particle. The general results were in 

 accord with the view that the a particle must have dimen- 

 sions of the order of the diameter of the electron (3.6X 

 IO-13 cms.), and thaJ: the forces between the nuclei aug- 

 mented rapidly for distances less than 3.4x10-^3 cms. Such 

 a conclusion is in accord with general ideas !of the nucleus 

 structure of hehum which must be complex probably con- 

 taining four H nuclei and two negative electrons. 



The velocity of the H atoms were measured and found 

 to be in good accord with the value expected from the 

 theory of impact. 



