236 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 5 



Not SO, however, with the nucleus itself. Despite the new facts 

 brought to light in the last fertile 5 years the nucleus still remains a 

 bundle of mysteries and even contradictions. Eather than simpli- 

 fying the situation the newer discoveries have shown the nucleus to 

 possess a more varied and complicated personality than previously 

 could have been permitted in the dreams of even the most imagi- 

 native 



Before attempting a consideration of the recent discoveries, let us 

 recall the picture which in the mind of the physicist represented the 

 1929 model of the nucleus. It was an agglomerate of electrons and 

 protons fastened together into a very small and very tightly bound 

 structure. The electron, now so familiar to everyone, is a unit of 

 negative electricity. No matter what its origin, it is always found to 

 possess the same definite charge of negative electricity and the same 

 mass. It is one of the building stones of all matter. It is a funda- 

 mental particle. The proton was considered the second and only other 

 fundamental particle. It possesses an electric charge equal in mag- 

 nitude to that of the electron, but opposite in sign and therefore posi- 

 tive; and has a mass nearly 2,000 times as great as that of the elec- 

 tron. Hydrogen, the simplest of all atoms, has for its nucleus a single 

 proton ; and therefore contains a single extranuclear electron to bal- 

 ance the charge of the proton and render the net charge of the atom 

 equal to zero. After the experiments in 1919 by the present Lord 

 Rutherford, in which he succeeded in observing protons driven out 

 of the nuclei of nitrogen by bombardment with alpha particles — 

 which are nuclei of helium atoms spontaneously emitted from radio- 

 active substances — and, after it was shown that protons could be ob- 

 tained in a similar manner from various other elements as well, it 

 was generally assumed that all nuclei were built up of protons and 

 electrons. On the basis of such a concept of the structure of nuclei, 

 it was possible, if one were not too critical, to explain many of the 

 known properties of nuclei. 



Each nucleus was assumed to contain a number of protons equal 

 to its atomic weight. But, to affix a correct value to the electric charge 

 of the nucleus, it was necessary to assume also that in all nuclei, with 

 the exception of hydrogen, were imprisoned a number of electrons 

 which canceled part of the positive charge of the protons. The latter 

 assumption, however, presented a grave difficulty. The same theories 

 which so successfully explained the behavior of the extranuclear elec- 

 trons would not permit any nucleus to contain so much even as one 

 electron. It was impossible to see how a particle of the mass of an 

 electron could be confined to a region of space as minute as a nucleus. 

 This difficulty cannot be resolved on the basis of present theories. 

 They were developed to describe the properties of an atom in which 



