250 Mr. W. Sutherland on Molecular 



The diagram represents these relations in perspective for a 

 central molecule and its six neighbours. Let R be its dis- 

 tance from its neighbours. Then it attracts the two axial 



Fiff. 1. 



neighbours with a force GeV/R 4 and its four lateral with a 

 force 3<?V/R 4 . The mean attraction is 4tf 2 s 2 /R 4 . Concerning 

 the forces between the central molecule and the more remote 

 ones we see that they are either repulsions or attractions 

 whose average effect can be calculated. I propose to treat 

 it as negligible in comparison with the attractions of the six 

 immediate neighbours. The reason for doing so is this. 

 The molecules of Nature are in motion, the directions of 

 their axes are changing. Our cubical arrangement of the 

 molecules and the assumed directions of the axes becomes a 

 closer representation of the facts of Nature, the smaller the 

 multiple of R to which it is extended from a central molecule. 

 Even for a molecule and its six nearest neighbours at any- 

 instant the cubical arrangement is not a true picture. The 

 real state of affairs is a succession of distorted cubical arrange- 

 ments with a strictly cubical arrangement for a mean. We 

 deprive our schematic representation of plasticity if we make 

 the one set of dividing planes apply to a large number of 

 molecules. The best way of stating the position is to say 

 that near a molecule the arrangement of other molecules is 

 approximately cubical at any instant, but that the accumu- 

 lated effects of small departure from the strict cubical arrange- 

 ment make the relations between any molecule and those 

 which are not its immediate neighbours not expressible by 

 means of a single cubical arrangement. For these reasons 

 then I propose to investigate molecular potential energy on 



