ATOMIC NUCLEI HARNWELL 193 



through the work of Sir J. J. Thomson at the beginning of the 

 century that electrons are constituents of all atoms. What is an 

 electron? All we know about it is that there is a characteristic 

 minute invariable electric charge associated with it and that it has 

 an effective mass which is about a two-thousandth that of the hydro- 

 gen atom, the lightest one known. Furthermore it acts as if it 

 were a minute electrically charged spinning top for it possesses an- 

 gular momentum and a magnetic moment. In addition to these 

 facts, we know rouglily indeed the volume of space it occupies and 

 that is absolutely all we know about it of this nature. As the 

 electrons associated with an atom represent less than a two-thousandth 

 of the mass of the composite system, practically all of the atomic 

 mass is associated with whatever is left after the electrons are 

 removed. Also, as the electrons have a negative sign by the conven- 

 tion generally adopted and the atom as a whole is neutral, the massive 

 part of the atom must carry a positive charge. Atoms of hydrogen, 

 helium, lithium, and so on up to uranium are distinguished from one 

 another by their properties, which we find are almost entirely de- 

 termined by the number of electrons they contain. This number is 

 determined by finding how many of them we can remove in the 

 simple cases and less directly for the heavier more complex atoms. 



How are the electrons and the more massive constituent or con- 

 stituents held together to form an atom? As these two components 

 have opposite charges and we know opposite electrical charges attract 

 one another, we have here one possible type of force. Our investiga- 

 tions have shown that there are other types of which we know very 

 little. In the first place the identical nature of the electrons ap- 

 parently gives rise to forces between them. In addition the spinning 

 or vortical motion that appears to be associated with these elementary 

 particles gives rise to spin forces of interaction in somewhat the same 

 manner as two smoke rings can be bounced from one another. The 

 fact that I cannot describe these forces better shows that we have 

 much to learn about them. But that they necessitate the performance 

 of work to alter an atomic system is clearly established. The amount 

 of work that must be done to produce various changes is something 

 that is very accurately known through our study of the light that 

 can be emitted or absorbed by various types of atoms. For the color, 

 or wave length, of the light emitted by an atom determines directly 

 the energy change taking place. This type of spectroscopic investi- 

 gation tells us many important things about the electron configuration 

 of an atom from the energy point of view which is the most valuable 

 type of information we can obtain, but nothing of course as to what 

 the atom "looks" like. 



We have at our disposal certain bullets or projectiles that travel 

 so rapidly that the ordinary atoms composing matter are relatively at 

 rest with respect to them. One source of these is the natural radio- 



