a Particles with Light Atoms. 559 



spectrum. The experiments of Geiger and Marsden * on 

 the large angle scattering of heavy atoms like those of gold 

 showed that the nucleus of the gold atom could be regarded 

 as a point charge for distances of the order of 3 X 10" 12 cm., 

 and that the law of inverse squares held up to that distance 

 within the limits of experimental error. In the present 

 experiments on the collision of particles with hydrogen 

 atoms, the atomic nuclei approach still closer, viz. to a 

 distance of the order of 3xl0~ 13 cm. It is to be antici- 

 pated that for such small distances of the order of the 

 diameter of the electron, the structure of the helium nucleus 

 can no longer be regarded as a point, and this is borne out 

 by experiment. Such a conclusion in no way invalidates 

 the nucleus theory as ordinarily understood ; but a study 

 of the forces close to the nucleus is of great importance in 

 throwing light on its actual dimensions. 



It is clear from the results given in this paper that a close 

 collision between an a particle and a hydrogen nucleus is an 

 exceedingly rare occurrence. Only 1 in 100000 of the 

 a particles passing through 1 cm. of hydrogen at normal 

 pressure and temperature gives rise to a high-speed H atom, 

 while in the same distance each a particle on an average 

 passes through the sphere of action of about 10000 hydrogen 

 molecules. Thus for every 10° collisions with the molecules, 

 in only one case does the « particle pass close enough to the 

 nucleus to give rise to a swift H atom. No doubt a much 

 greater number of H atoms are set into comparatively swift 

 motion by less direct collisions, but these do not give rise 

 to H atoms which can be detected beyond the range of the 

 a particle. 



It is clear that for such close collisions, each hydrogen 

 atom in any complex molecule acts as an independent unit, 

 so that swift H atoms should be liberated by a particles from 

 every substance containing free or combined hydrogen. 

 This is fully borne out by experiment. 



In seeking for an explanation of these anomalous results, 

 there are tw^o salient facts to bear in mind, viz., that (1) the 

 H atoms produced by a particles of range greater than 

 6 cm. are projected mainly in the direction of the a particles 

 and over a narrow range of velocity, and (2) the number of 

 such swift H atoms is far in excess of the number on the 

 simple theory of point charges. 



\$ we consider the nuclei of the atoms in collision to act 



* Geiger and Marsden, Phil. Mag-, xxv. p. 604 (1913). 



