572 Professor Sir Ernest Rutherford [June 6, 



of 2*4 X 10"^^ cm. of the centre of the H nucleus in order to set it 

 in swift motion. This is a distance less than the diameter of the 

 electron, viz. 3'6 x 10"^^ cm. The general results obtained with 

 a-rays of range 7 cm. are similar to those to be expected if the 

 a-particle behaves like a charged disc, of radius of about the dia- 

 meter of an electron, travelling with its plane perpendicular to the 

 direction of motion. 



It is clear from the experiments with hydrogen that, for distances 

 of the order of the diameter of the electron, the a-particle no longer 

 behaves like a point charge, but that the a-particles must have 

 dimensions of the order of that of the electron. The closest distance 

 of approach in these collisions in hydrogen is about oue-tenth the 

 corresponding distances in the case of a collision of an a-particle 

 with an atom of gold. 



The results obtained with hydrogen in no way invalidate the 

 nucleus theory as used to explain the scattering of a-rays by heavy 

 atoms, but show% as we should expect, that the theory breaks down 

 when we approach very close to the nucleus structure. In our 

 ignorance of the constitution of the nucleus of the a-particle, we 

 can only speculate as to its structure and the distribution of forces 

 very close to it. If we take the a-particles of mass 4 to consist of 

 four positively charged H nuclei and two negative electrons, w^e 

 should expect it to have dimensions of the order of the diameter of 

 the electron, supposing, as seems probable, that the H nucleus is of 

 much smaller dimensions than the electron itself. When we con- 

 sider the enormous magnitude of the forces between the a-particle 

 and the H nucleus in a close collision — amountiug to 6 kg. of weight 

 — it is to be expected that the structure of the a-particle should be 

 much deformed, and that the law of force may undergo very marked 

 changes in direction and magnitude for small changes in the close- 

 ness of approach of the two colliding nuclei. Such considerations 

 offer a reasonable explanation of the anomalies shown in the number 

 and distribution with velocity of the H atoms exhibited for different 

 velocities of the a-particles. 



When we consider the enormous forces between the nuclei, it is 

 not so much a matter of surprise that the nuclei should be deformed 

 as that the structure of the a-particle or helium nucleus escapes 

 disruption into its constituent parts. Such an effect has been 

 carefully looked for, but so far no definite evidence of such a dis- 

 integration has l)een observed. If this be the case, the helium 

 nucleus must be a very stable structure to stand the strain of the 

 gigantic forces involved in a close collision. 



We have seen that the recoil atoms of all elements of atomic 

 mass less than 18 should travel beyond the range of the a-particle, 

 provided they carry a single charge. Preliminary experiments, in 

 which the a-particles passed through pure helium, showed that no 

 long-range recoil atoms w^ere present, indicating that after recoil the 



