Supplement to ''Nature'' September 15, 1923 



417 



are still a number of hopeful directions in which an 

 attack may be-made on this most difficult of problems. 

 J A detailed study of the y-rays from radioactive bodies 

 lay be expected to yield information as to the motion 

 the electrons inside the nucleus, and it may be, as 

 ^llis has suggested, that quantum laws are operative 

 iside as well as outside the nucleus. From a study 

 )f the relative 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, 

 Kttle, 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, occupied but 

 not filled by the minute building units, the protons and 

 electrons, in ceaseless rapid motion controlled by their 

 mutual forces. 



Before leaving this subject it is desirable to say a 

 few words on the important question of the energy 

 relations involved in the formation and disintegration 

 of atomic nuclei, first opened up by the study of 

 radioactivity. For example, it is well known that the 

 total evolution of energy during the complete disinte- 

 gration of one gram of radium is many millions of 

 times greater than in the complete combustion of an 

 equal weight of coal. It is known that this energy is 

 initially mostly emitted in the kinetic form of swift a- 

 and /i-particles, and the energy of motion of these 

 bodies is ultimately converted into heat when they 

 are stopped by matter. Since it is believed that the 

 radioactive elements are analogous in structure to 

 the ordinary inactive elements, the idea naturally arose 

 that the atoms of all the elements contained a similar 

 concentration of energy, which would be available for 

 use if only some simple method could be discovered of 

 promoting and controlling their disintegration. This 

 possibility of obtaining new and cheap sources of 

 energy for practical purposes was naturally an alluring 

 prospect to the lay and scientific man alike. It is quite 

 true that, if we were able to hasten the radioactive 

 processes in uranium and thorium so that the whole 

 < ycle of their disintegration could be confined to a few 

 days instead of being spread over thousands of millions 

 of years, these elements would provide very convenient 

 sources of energy on a sufficient scale to be of consider- 

 able practical importance. Unfortunately, although 

 many experiments have been tried, there is no evidence 

 that the rate of disintegration of these elements can 

 be altered in the slightest degree by the most powerful 

 laboratory agencies. With increase in our knowledge 

 of atomic structure there has been a gradual change of 

 our point of view on this important question, and there 

 is by no means the same certainty to-day as a decade 



ago that the atoms of an element contain hidden stores 

 of energy. It may be worth while to spend a few 

 minutes in discussing the reason for this change in 

 outlook. This can best be illustrated by considering 

 an interesting analogy between the transformation of 

 a radioactive nucleus and the changes in the electron 

 arrangement of an ordinary atom. It is now well 

 known that it is possible by means of electron bombard- 

 ment or by appropriate radiation to excite an atom in 

 such a way that one of its superficial electrons is dis- 

 placed from its ordinary stable position to another 

 temporarily stable position further removed from the 

 nucleus. This electron in course of time falls back into 

 its old position, and its potential energy is converted 

 into radiation in the process. There is some reason for 

 believing that the electron has a definite average life in 

 the displaced position, and that the chance of its return 

 to its original position is governed by the laws of prob- 

 ability. In some respects an " excited ",^tom of this 

 kind is thus analogous to a radioactive atom, but of 

 course the energy released in the disintegration of a 

 nucleus is of an entirely different order of magnitude 

 from the energy released by return of the electron in 

 the excited atom. It may be that the elements, 

 uranium and thorium, represent the sole survivals in 

 the earth to-day of types of elements that were common 

 in the long-distant ages, when the atoms now composing 

 the earth were in course of formation. A fraction of 

 the atoms of uranium and thorium formed at that time 

 has survived over the long interval on account of their 

 very slow rate of transformation. It is thus possible to 

 regard these atoms as having not yet completed the 

 cycle of changes which the ordinary atoms have long 

 since passed through, and that the atoms are still in the 

 " excited " state where the nuclear units have not yet 

 arranged themselves in positions of ultimate equili- 

 brium, but still have a surplus of energy which can only 

 be released in the form of the characteristic radiation 

 from active matter. On such a view, the presence of 

 a store of energy ready for release is not a property of 

 all atoms, but only of a special class of atoms like the 

 radioactive atoms which have not yet reached the final 

 state for equilibrium. 



It may be urged that the artificial disintegration of 

 certain elements by bombardment with swift a-particles 

 gives definite evidence of a store of energy in some of the 

 ordinary elements, for it is known that a few of the 

 hydrogen nuclei, released from aluminium for example, 

 are expelled with such swiftness that the particle has a 

 greater individual energy than the a-particle which 

 causes their liberation. Unfortunately, it is very 

 difficult to give a definite answer on this point until we 

 know more of the details of this disintegration. 



On the other hand, another method of attack on 

 this question has become important during the last 

 few years, based on the comparison of the relative 

 masses of the elements. This new point of view can 

 best be illustrated by a comparison of the atomic 

 masses of hydrogen and helium. As we have seen, it 

 seems very proliable that helium is not an ultimate unit 

 in the structure of nuclei, but is a very close combina- 

 tion of four hydrogen nuclei and two electrons. The 

 mass of the helium nucleus, 4*00 in terms of = i6, is 

 considerably less than the mass, 4*03, of four hydrogen 

 nuclei. On modem views there is believed to be a very 



