FORCES AND ATOMS 355 



that the short-range attraction is confined within a very few times 10~^' 

 cm of the centre of the proton or neutron, whereas the cohesive attraction 

 of atom for atom spreads over a radius a hundred thousand times as great; 

 second, that the three short-range attractions of proton for proton, neutron 

 for neutron and neutron for proton are nearly the same. 



Shall we adopt the force-fields as given to us by experiment, with some 

 plausible assumptions added (for one cannot as yet do without them) 

 and operate on them by the procedures of quantum mechanics, hoping to 

 arrive at (say) values of binding-energies compatible with the data? This 

 is the present, or perhaps I should say the recent, programme of nuclear 

 theory. If one reads the theoretical papers of any one year out of the last 

 ten, one may readily get the impression that success is just around the 

 corner. But if one reads the papers of two or more years and takes note 

 of the rapid changes, the prospect does not look quite so rosy — nor when 

 one overhears the conversations of the theorists themselves. I will not 

 conduct the reader down the paths which are as yet so tortuous and hazy; 

 it will be better to fill in the picture with a few of the many remaining details. 



Mass was the first of properties (along with hardness) to be assigned to 

 the elementary particles; the second was charge; to these have lately been 

 added angular momentum and magnetic moment. It is difficult to say 

 when the idea of a spinning atom was first propounded (one recalls the 

 vortices in a continuous fluid which Kelvin introduced as one of the most 

 brilliant of all attempts to contrive a continuum and atoms as a part of it) 

 but easy to fix the time when the idea of the spinning electron became so 

 definite and sharp, as to be successfully used in explaining crucial data; 

 this was 1925. The electron, the proton and the neutron all have equal 

 angular momentum; its amount, common to these three which at present 

 claim most strongly the rank of elementary particle, is one of the universal 

 constants. When protons and neutrons are assembled in a nucleus, their 

 axes of spin all point in an identical direction, though not by any means 

 necessarily in the same sense in that direction. It is possible for a nucleus 

 to have zero angular momentum, through half of its particles setting them- 

 selves in the one sense and half in the other; the lightest nucleus for which 

 this happens is the alpha-particle, composed of two protons and two neu- 

 trons. The magnetic moments of the three elementary particles are very 

 far from equal, that of the electron being some seven hundred times as 

 great as that of the proton, which in turn is half again as great as that 

 of the neutron. One of the tragedies of theoretical physics occurred in 

 this connection. A principle of quantum mechanics had been proposed, 

 superbly capable of serving as a basis for most of the incomplete principles 

 which had already so well justified themselves in atomic physics, and 

 including among its parts the actual values of the angular. momentum and 



