FORCES AND ATOMS 349 



reasonably be claimed that they are so complex as to defy any theory. I 

 do not allege that our theory of massive particles, electromagnetic forces 

 and quantum mechanics has done even this. It has, however, done a great 

 deal, so much that it takes a rather skeptical physicist to deny it in the 

 realms to which it lays claim. 



In the light of this theory, let us consider the situation of the several 

 forces. 



Gravity remains apart and inaccessible, one of the ultimate forces, quite 

 probably a quality of space as Einstein has proposed. 



The electromagnetic forces remain ultimate, not explained in terms of 

 anything else, united among themselves by the theory of relativity, responsi- 

 ble for the incessant passage of energy to and fro between matter and light 

 which is one of the major features of the world. The ionization of atoms, 

 the generation and the absorption of light, show us these forces at work 

 within the atoms, holding together the electrified particles of which the 

 atoms are made, balanced by motion and by their own dual character of 

 attractions and repulsions. 



Cohesion, and the chemical forces which bind atoms into molecules and 

 grade insensibly into cohesion, and the nameless repulsive force which holds 

 the balance to them and led many to the concept of the more-or-less-com- 

 pressible atom: these are derivable from the electromagnetic forces between 

 the elementary particles whereof the atoms are made up. I repeat : derivable 

 from the electromagnetic forces, with the aid of quantum mechanics, — without 

 which aid they would not have been derived. In the literature one finds 

 incessant reference to "exchange forces"; these are not a novel category, but 

 a step in the derivation.^ Here are the fields of research where work is the 

 most active. The theory of chemical forces, which some call "quantum 

 chemistry", is well advanced; the theory of metals, not so well. Much 

 earlier and much more often than we like, do we impinge on the class of 

 phenomena, for which it can all too reasonably be claimed that they are so 

 complex as to defy the theorist probably for all time. Yet there are many 

 simple ones which have brilliantly been explained, and there is satisfaction 

 on the whole — until one raises the eyes and looks ahead: for the nuclear 

 phenomena are still before us. 



As a prelude to these we may view the electron itself. Hardly have we 

 begun to "look narrowly" upon it, before we see the spectre rising up of 

 that old antithesis between the point-atom and the atom carven out of a 

 continuum; nor is it long before the spectre grows more frightful than it was 

 in the earlier case. If the point-electron is adopted, all the old conceptual 



» There is also a strange quality of Nature bearing in quantum-mechanics the name 

 of "the exclusion-principle of Pauli," which to some extent resembles a repulsive force 

 acting between similar particles such as electron and electron or proton and proton, under 

 very special conditions. 



