354 BELL SYSTEM TECHNICAL JOURNAL 



lessly around it. So long have the chemists been on the search for new 

 elements, and so completely have they searched, that we may believe them 

 when they say that apart from the works of the "atom-smashers," no 

 nucleus exists having more than 238 particles altogether, 92 of which are 

 protons. Even the atom-smashers or (as I should rather call them) the 

 transmuters, for all the wonder and power of their art, have not forced the 

 total number of protons upward by more than two or the total number of 

 particles altogether upward by more than one. Moreover all of the two 

 dozen or so most massive nuclei known are subject to explosion — to explo- 

 sions quite terrific, some of them spontaneous, others touched off by what 

 seems a very minor cause. It may therefore be taken as nearly certain 

 that there is an upper limit to the size of nuclei, and probable that it is 

 electrostatic force that sets the limit. 



Now we come down to the short-range repulsion. Such a one there must 

 be, for again we can rehearse the ancient argument. A piece of iron does 

 not shrink into a point; therefore the iron atoms must either exert a force 

 of repulsion or else be more-or-less compressible pellets. A nucleus does 

 not shrink into a point, but offers an impenetrable front, measurable though 

 small, to an oncoming neutron; therefore the nuclear particles — but why 

 repeat the words? 



Shall we interpret neutrons and protons alike as systems of particles 

 still smaller, acting on one another by electromagnetic forces, to be treated 

 by quantum mechanics? Alas, if there is one surety in this field, it is that 

 we cannot play quite the same game twice. Quantum mechanics may not 

 be used up (some think that it is) but the electromagnetic forces certainly 

 are. In this direction we have as yet no leadership. 



Shall we then adopt the compressible globule or the point-particle with 

 a curious field of force surrounding it? Though the language of nuclear 

 theorists verges sometimes on the former, it is the latter practice which is 

 common — a fact which will hardly surprise the reader. In the specialized 

 literature, one finds many a speculation and (what is of more moment) 

 many an inference about the force-field which is drawn pretty directly from 

 reliable data. As a rule the inferences are expressed in language very 

 different from the phrases of this lecture: "interaction" is used instead 

 of "force-field," and there are queer and slightly comic technical terms 

 such as "potential- well." When you read of a "rectangular potential- 

 well," interpret that what I have been calling the "cohesive force" becomes 

 suddenly enormous at a certain specific radius; when of an "error-well" (!) 

 understand that the cohesive force increases rapidly according to a certain 

 law with decline of distance; when of a "Coulomb interaction" realize that 

 it is the inverse-square force-field of the electrostatic repulsion between 

 proton and proton. Of these interactions I will give only two facts: first. 



