272 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



and 146 electrons, etc. We also know that the nucleus is very small 

 in comparison with the overall atomic dimensions, i.e., much smaller 

 than 10"^" centimeters in diameter — probably less than 10~" centi- 

 meters. 



We have good reason for thinking that some atomic nuclei are 

 magnets, with a magnetic moment equal to that of one electron, and 

 that this is true if there is an odd number of electrons in the nucleus. 

 But there are some phenomena that have not as yet been reconciled 

 with this idea of the magnetic properties of the nucleus. Further- 

 more, there is reason to believe that the proton configurations in the 

 nucleus may also contribute a magnetic moment far smaller than that 

 due to the electrons. 



We know that atomic nuclei are deformable under the action of 

 intense forces, such as can only be exerted by electrified particles, 

 like alpha particles from radium, which are shot toAvard the nuclei 

 with such tremendous velocities that they may come very close before 

 being deflected away by the rep)ulsive force between nucleus and 

 alpha particle. When their distances are greater than 10^^° centi- 

 meters, this force varies inversely as the square of the distance, as 

 nearly as we can tell, which shows us that tlie nuclei are practically 

 electrified points so far as distances greater than 10"" centimeters 

 are concerned. With closer approach, however, the force departs 

 more and more from the inverse square law, showing that the nuclei 

 have a structure or arrangement of electricity within their tiny do- 

 main, and that this structure may be deformed by strong electrical 

 forces. All this information is inferred from studies of the angular 

 distribution of scattering when alpha particles pass through thin 

 films of matter. 



We know that the nuclei are the seats of tremendous energies, as 

 evidenced directly by phenomena of radioactivity and indirectly 

 by certain aspects of the theory of relativity to which I will refer 

 later. From radioactivity, also, we find that groups of 4 protons 

 and 2 electrons (helium nuclei) appear to be particularly stable con- 

 figurations within the larger structure of the nuclei of heavy atoms. 

 We call these groups " alpha particles." 



Having said these things, we have told almost everything that is 

 known about atomic nuclei. Many other things we would tremen- 

 dously like to Imow. How are the protons and electrons arranged 

 in the nucleus? What is their state of motion? What forces hold 

 them together? How is their energy stored away? Under what 

 conditions can the nucleus be disrup)ted or this energy released, or 

 the configuration changed? To all of these questions we must con- 

 fess almost total ignorance. 



