196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 9 



of new discoveries and new techniques 6 or 8 years ago. In 1930 

 two of Rutherford's students, Cockcroft and Walton, showed that 

 we do not have to depend on the helium particles shot off by radium 

 to produce these effects, but that we can accelerate charged atoms 

 of hydrogen and helium in electric fields until they gain sufficient 

 speed to disintegrate other atoms that they encounter. These par- 

 ticles available in large quantities and with controllable speeds are 

 the chief agents of modern research in this field. The fact that 

 Rutherford's work showed that the law of conservation of momen- 

 tum is applicable to this submicroscopic world enabled Chadwick, 

 one of his colleagues, to prove in 1932 the existence of a hitherto 

 unsuspected atomic particle with a mass closely the same as that of 

 hydrogen but incapable of acquiring an electric charge. Hence this 

 received the name "neutron." In the same year, at Pasadena, An- 

 derson demonstrated the existence of a new light electronic particle 

 having a characteristic positive charge, the opposite of that of the 

 ordinary electron which had been known for 30 years or so. On 

 the heels of these discoveries progress has been so rapid that I can 

 only indicate briefly our present picture of atomic nuclei and some 

 of the fascinating possibilities that are emerging as a result of these 

 researches. 



The fundamental and as yet indivisible particles that have 

 been discovered by research and of which all atoms are probably 

 composed in various proportions are the proton, which is the hydro- 

 gen nucleus, and the neutron. When recited baldly, our knowledge of 

 these particles does not seem extensive. We know their masses, 

 which are closely the same, and their electrical characteristics, the 

 proton having a characteristic positive charge and the neutron none 

 at all. We also know very roughly the volume of space they occupy 

 and that they have certain characteristic angular momenta, that is, 

 they behave somewhat like minute spinning tops. As the proton 

 has a charge and is rotating, we would expect it to behave like a 

 small magnet, which indeed it does, and somewhat to our surprise 

 the uncharged neutron also appears to have magnetic properties. 

 This is all we can say about these particles except that there are ap- 

 parently forces between them that bind them in the characteristic 

 complexes that we know as the nuclei of the atoms of all the differ- 

 ent elements. These forces apparently have no analogs in our large- 

 scale world, being of such a short range that they only become evident 

 on an atomic scale, but they are the object of present research and 

 speculation by atomic physicists in many laboratories. When we 

 know more about them, which we can only do by further experimental 

 work, we will be able to understand much more clearly the many 



