FORCES AND ATOMS 351 



ing again toward a continuum-theory. The ground is slippery, and I step 

 hastily from it into the last section of this lecture, into nuclear physics 

 proper. 



All of the theory of nuclei is firmly grounded on one basic statement, which 

 is this: the masses of all nuclei are nearly integer multiples of a common 

 unit, this being slightly less than the mass of the lightest among them. 



Here is a statement bitterly disappointing! the little word "nearly" and 

 the three little words "slightly less than" conjointly make a bright hope 

 stillborn. Were it not for those words, we should already have joyously 

 leaped to the conclusion that all nuclei are clusters of a single kind of 

 fundamental particle, different clusters differing only in how many of the 

 particles they comprise. The conclusion is so tempting that one is quite 

 unable to resist it, hoping against hope that the words of frustration can 

 somehow or other be cancelled. Soothing the reader with this veiled as- 

 surance, I adopt the conclusion. 



The conclusion itself must be tempered at once, for there is a second 

 basic statement coequal with the first: the charges of all nuclei are integer 

 multiples of a common unit of charge. No pernicious adverbs here! this 

 statement is an exact one, to the best of our knowledge and belief. The 

 common unit of charge, as nearly everyone knows, is equal to the electron- 

 charge and positive in sign. 



The conclusion would still be sound, if the charges of all nuclei were pro- 

 portionate to their masses (we should merely attribute an equal charge to 

 every particle). Definitely this is not so, being most strikingly denied by 

 the fact of "isobars": there are nucleus- types agreeing in mass, disagreeing 

 in charge. We seek the next simplest assumption, and find that it suffices: 

 Two t>^es of fundamental particles — equal in mass — the one of them 

 charged positively, the other neutral — each nucleus to be distinguished by 

 two integers, one being the number of the charged component particles 

 of the cluster, the other the number of the neutrals — "proton" and "neu- 

 tron" for the names of the two. 



This is the beginning of the programme for nuclear theory. Having 

 taken the first step by writing it down, we enter upon the second — and find 

 ourselves on the very road which our ancestors trod when atomic theory was 

 new, facing the same ascents, the same passes and the same morasses. 

 The long-range forces — the short-range forces — the cohesion — the repulsion 

 — the more-or-less-incompressible particle — the troubles of the concept 

 of the point-particle — the countervailing troubles of the continuum carven 

 into globules — the dream of reducing everything to long-range forces and 

 motion holding each other in balance — every one of these rejoins us on our 

 journey. The mighty difference is, that the road still ends in the darkness, 

 and the dream is still a dream. Therefore it is that the language of nuclear 



