346 BELL SYSTEM TECHNICAL JOURNAL 



of the force-field curiously devised which I have been describing? Formally, 

 there is not. But in respect of the path which the mind next tries to follow, 

 there is a difference, and a great one. 



The compressible atom being accepted, one asks, of what is it made? 

 and finds that one is thinking of a continuous substance, elastic and dense. 

 One who is trying to become a thoroughgoing atomist is hardly pleased to 

 discover a continuum at the base of the theory. The displeasure would not 

 be long-lasting, if by assigning a few simple qualities to the continuum one 

 could arrive at the right numerical values for things that can be measured — 

 if one could infer, for instance, that the continuum by its nature divides itself 

 into globules of just the same radii as the structure of crystals demands for 

 the atoms. We are to meet in nuclear physics with a calculation singularly 

 like this— but in general, the feat has not been done. It is not an adequate 

 retort to say that the thoroughgoing atomist is obliged to assign to his 

 atoms the sizes and the masses which they actually have, without giving 

 any deeper reason. He manages to avoid the question; it becomes im- 

 perious, when the continuum is brought upon the scene. The road to 

 success may lie by way of the continuum, but it is a road that has not been 

 successfully trodden. 



The force-field around the point-particle being accepted, one asks, why 

 this so curious force-field? An inverse-square field would seem so natural 

 as not even to ask for further explanation (but this is probably because the 

 human mind has had two and a half centuries for getting accustomed to it). 

 This combination of a short-range attraction with a repulsion still shorter in 

 range cries out from explanation. Could one but somehow reduce it all to 

 inverse-square forces, one would be more contented. This road seems 

 impassable, but already it has been trodden— built and trodden— to splendid 

 successes. Therefore I lay aside the compressible atom scooped out of a 

 continuum, mentioning that even now we have not heard the last of it. Two 

 stages of preparation are now required. 



First, I must take more care henceforward in using the words "atom" and 

 "particle". Hitherto I have used them interchangeably; from this moment 

 on, "atom" is to have one meaning and "particle" another. Of the two, 

 it will be "atom" which comes the closer to meaning what both words have 

 meant up to now. Atom will attract atom by the force of cohesion; atom 

 will repel atom by the nameless short-range force. The atoms in their turn 

 will be made up of more elementary particles, bearing such names as "nu- 

 cleus" and "electron". As to the forces between them, — that is the topic to 

 which we are coming. 



Second, I must introduce at long last the forces which the reader has so 

 long been missing from this discourse : the electromagnetic. 



Of these, it is the "electrostatic" force which stationary charges exert on 



