November 21, 1919] 



SCIENCE 



471 



travelling with its plane perpendicular to the 

 direction of motion. 



It is clear from the experiments with hy- 

 drogen 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 hy- 

 drogen is about one tenth the corresponding 

 distances in the ease 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, we 

 should expect it to have dimensions of the 

 order of the diameter of the electron, sup- 

 posing, as seems probable, that the H nucleus 

 is of much smaller dimensions than the elec- 

 tron itself. When we consider the enormous 

 magnitude of the forces between the a-particle 

 and the H nucleus in a close collision — 

 amounting to 6 kg. of weight — it is to be ex- 

 pected 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 closeness of approach of the two collid- 

 ing nuclei. Such considerations offer a rea- 

 sonable explanation of the anomalies shown 

 in the number and distribution with velocity 

 of the H atoms exhibited for different veloc- 

 ities of the a-particles. 



When we consider the enormous forces be- 

 tween the nuclei, it is not so much a matter 

 of surprise that the nuclei should be deformed 

 as that the structure of the a-partiele 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 disintegration has been 



observed. If this be the case, the helium 

 nucleus must be a very stable structure to 

 stand the strain of the gigantic forces in- 

 volved 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, pro- 

 vided they carry a single charge. Preliminary 

 experiments, in which the a-particles passed 

 through pure helium, showed that no long- 

 range recoil atoms were present, indicating 

 that after recoil the helium atom carries a 

 double charge. In a similar way no certain 

 evidence has been obtained of long-range 

 recoil atoms from lithium, boron, or beryl- 

 lium. It is difficult in experiments with 

 solids or solid compounds to be sure of the 

 absence of hydrogen or water-vapor, which 

 results in the production of numerous swift 

 H atoms. These difficulties are not present 

 in the case of nitrogen and oxygen, and a 

 special examination has been made of recoil 

 atoms in these gases. Bright scintillations 

 were observed in both these gases about 2 cm. 

 beyond the range of the a-particle. These 

 scintillations are, presumably, due to swift N" 

 and O atoms carrying a single charge, for the 

 ranges observed are about those to be expected 

 for such atoms. The scintillations due to re- 

 coil atoms of N" and O are much brighter 

 than H scintillations, although the actual 

 energy of the flying atom is greater in the 

 later case. This difference in brightness is 

 probably connected with the much weaker ion- 

 ization per unit of path due to the swifter H 

 atom. 



The corresponding range of the recoil 

 atoms was about the same in oxygen, nitrogen 

 and carbon dioxide. Theoretically, it is to be 

 anticipated that the N" recoil atom should give 

 a somewhat greater range than the atom. 

 The recoil atoms observed in carbon dioxide 

 are apparently due to oxygen, for if the 

 carbon atoms carried a single charge they 

 should be detected beyond the range of O 

 atoms. 



The number of recoil atoms in nitrogen 

 and oxygen and their absorption indicate that 

 these atoms, like H atoms, are shot forward 

 mainly in the direction of the a-particles. It 



