Forces between Atoms and Chemical Affinity. 759 



Sutherland found that for, the non-metallic elements the 

 electrostatic moment is proportional to the volume of the 

 atom, while for metals the moment is proportional to the 

 linear dimensions o£ the atom. 



The theory is tested by taking some physical quantity, 

 say the magnitude of the surface tension, which can be 

 expressed in terms of the C's — the electrostatic moment of 

 the atom — the values of the C's for say hydrogen, oxygen, and 

 carbon are found by equating the theoretical value of the 

 surface tension in terms of the Cs to the observed values 

 in three liquid compounds of these liquids. Having got 

 the C's we can calculate the surface tension for as many 

 liquid compounds of these elements as can be obtained, and 

 compare them with the observed values. The agreement, 

 though very far from perfect, is yet good enough to suggest 

 that in many cases the forces around the atom may be repre- 

 sented in this way. 



Methods for representing the forces between the atoms 

 have also been developed on somewhat similar lines to this 

 by Einstein * and also by Kleeman f . 



The forces exerted by a molecule AB can, however, only 

 be represented as the resultant of forces due to doublets of 

 constant moment in A and B when both A and B are 

 electrically neutral, i.e. when neither of them contains an 

 excess of one kind of electricity over that of the opposite. 

 For if one atom A has an excess of positive electricity, the 

 other B an excess of negative, the two atoms will give rise 

 to a doublet one of whose poles is on A while the other is 

 on B ; the distance between the poles of this doublet, and 

 therefore the force exerted by it, will depend on the distance 

 between the atoms in the molecule. The force will thus 

 depend not merely on the kind of atoms there are in the 

 molecule but also on the way they are arranged, and can 

 not be calculated in the same way as forces due to invariable 

 doublets contained within the atoms themselves. I shall 

 endeavour to show that chemical compounds may be divided 

 into two great classes — in one class the atoms are electrically 

 neutral, in the other they are charged, some positively 

 others negatively, — and that the properties of compounds are 

 strikingly different according as they belong to one class or 

 the other. We might expect that this would be the case, 

 for the poles of the doublet in the compounds of class II. 

 (when the atoms are charged) will be more widely separated 

 than those in compounds of class I., where the two poles of 



* Einstein, Ann. der Phys, iv. p. 513 (1901). 

 t Kleeman, Phil. Mag. [6] vol. xviii. p. 491. 

 3 E 2 



