180 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



pied 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 forma- 

 tion and disintegration of atomic nuclei, first opened up by the study 

 of radioactivity. For example, it is well known that the total evo- 

 lution of energy during the complete disintegration of 1 gram of 

 radium is many millions of times greater than in the complete com- 

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

 is initially mostly emitted in the kinetic form of swift a and ^ par- 

 ticles, and the energy of motion of these bodies is ultimately con- 

 verted into heat when they are stopped by matter. Since it is be- 

 lieved that the radioactive elements were 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 dis- 

 covered of promoting and controlling their disintegration. This 

 possibility of obtaining new and cheap sources of energy for prac- 

 tical 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 

 cycle 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 considerable 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 bombardment or 

 by appropriate radiation to excite an atom in such a way that one 

 of its superficial electrons is displaced from its ordinary stable posi- 

 tion 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 proba- 



