176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



and to explain in considerable detail all the properties of an atom, 

 without any clear understanding of the underlying processes which 

 lead to these results. It is natural to hope that with advance of 

 knowledge we may be able to grasp the details of the process which 

 leads to the emission of radiation, and to understand why the orbits 

 of the electrons in the atom are defined by the quantum relations. 

 Some, however, are inclined to take the view that in the present 

 state of knowledge it may be quite impossible in the nature of 

 things to form that detailed picture in space and time of succes- 

 sive events that we have been accustomed to consider as so import- 

 ant a part of a complete theory. The atom is naturally the most 

 fundamental structure presented to us. Its properties must explain 

 the properties of all more complicated structures, including matter 

 in bulk, but we may not, therefore, be justified in expecting that 

 its processes can be explained in terms of concepts derived entirely 

 from a study of molar properties. The atomic processes involved 

 may be so fundamental that a complete understanding may be 

 denied us. It is early yet to be pessimistic on this question, for 

 we may hope that our difficulties may any day be resolved by further 

 discoveries. 



We must now turn our attention to that new and comparatively 

 unexplored territory, the nucleus of the atom. In a discussion on 

 the structure of the atom ten years ago, in answer to a question on 

 the structure of the nucleus, I was rash enough to say that it was 

 a problem that might well be left to the next generation, for at that 

 time there seemed to be few obvious methods of attack to throw light 

 on its constitution. While much more progress has been made 

 than appeared possible at that time, the problem of the structure 

 of the nucleus is inherently more difficult than the allied problem 

 already considered of the structure of the outer atom, where we 

 have a wealth of information obtained from the study of light 

 and X-ray spectra and from the chemical properties to test the 

 accuracy of our theories. 



In the case of the nucleus, we know its resultant charge, fixed by 

 Moseley's law, and its mass, which is very nearly equal to the mass 

 of the whole atom, since the mass of the planetary electrons is rela- 

 tively very small and may for most purposes be neglected. We 

 know that the nucleus is of size minute compared with that of the 

 whole atom, and can with some confidence set a maximum limit to 

 its size. The study of radioactive bodies has provided us with very 

 valuable information on the structure of the nucleus, for we know 

 that the a and ^ particles must be expelled from it, and there is 

 strong evidence that the very penetrating y rays represent modes of 

 vibration of the electrons contained in its structure. In the long 

 series of transformations which occur in the uranium atom, eight 



