ELECTKICAL STRUCTURE OF MATTER RUTHERFORD 179 



is slightly departed from. Such variations in mass may ultimately 

 prove of great importance in throwing light on the arrangement and 

 closeness of packing of the protons and electrons, and for this reason 

 it is to be ho]Ded that it may soon prove possible to compare atomic 

 masses of the elements with much greater precision even than at 

 present. 



While we may be confident that the proton and the electron are the 

 ultimate units which take part in the building up of all nuclei, and 

 can deduce with some certainty the number of protons and electrons 

 in the nuclei of all atoms, we have little, if any, information on the 

 distribution of these units in the atom or on the nature of the forces 

 that hold them in equilibrium. While it is known that the law of 

 the inverse square holds for the electrical forces some distance from 

 the nucleus, it seems certain that this law breaks down inside the 

 nucleus. A detailed study of the collisions betvv^een a particles and 

 hydrogen atoms, where the nuclei approach very close to each other, 

 shows that the forces between nuclei increase ultimately much more 

 rapidly than is to be expected from the law of the inverse square, 

 and it may be that new and unexpected forces may come into impor- 

 tance at the very small distances separating the protons and elec- 

 trons in the nucleus. Until we gain more information on the nature 

 and law of variation of the forces inside the nucleus, further prog- 

 ress on the detailed structure of the nucleus may be difficult. At 

 the same time, there are still a number of hopeful directions in which 

 an attack may be made on this most difficult of problems. A detailed 

 study of the y rays from radioactive bodies may be expected to 

 yield information as to the motion of the electrons inside the nucleus, 

 and it may be, as Ellis has suggested, that quantum laws are opera- 

 tive inside as well as outside the nucleus. From a study of the rela- 

 tive proportions of the elements in the earth's crust, Harkins has 

 shown that elements of even atomic number are much more abundant 

 than elements of odd number, suggesting a marked difference of 

 stability in these two classes of elements. It seems probable that 

 any process of stellar evolution must be intimately connected with 

 the building up of complex nuclei from simpler ones, and its study 

 may thus be expected to throw much light on the evolution of the 

 elements. 



The nucleus of a heavy atom is undoubtedly a very complicated 

 system, and in a sense a world of its own, little, if at all, influenced by 

 the ordinary physical and chemical agencies at our command. When 

 we consider the mass of a nucleus compared with its volume it seems 

 certain that its density is many billions of times that of our heaviest 

 element. Yet, if we could form a magnified picture of the nucleus, 

 we should expect that it would show a discontinuous structure, occu- 



