626 Mr. W. Sutherland on the 



attractive. There is this fundamental distinction in the effects 

 of attractive and repulsive forces whose strength decreases 

 with increasing distance, that the attractive forces by their 

 own operation tend to increase themselves, while the repulsive 

 tend to decrease themselves. Consider, for example, a diatomic 

 gas in which each molecule contains a single electric doublet 

 formed by the opposite electrons holding its two atoms 

 together. If two neighbouring molecules are approaching 

 one another with their electric axes similarly directed in the 

 straight line of relative motion, then the attractive force acts 

 with strength increasing to the maximum possible value at 

 the instant of collision. If their electric axes were oppositely 

 directed in the line of motion the forces would be repulsive, 

 and could reverse the motion before their maximum possible 

 strength had been attained. In general there is the same 

 tendency for attractive forces to increase their strength and 

 for repulsive to diminish. This causes attraction to pre- 

 ponderate. The idea that molecular polarity might account 

 for the inverse fourth power law of molecular attraction 

 encouraged me when first investigating that law, though I 

 failed to see how the attractive forces due to polarity could 

 on the average be of more importance than the repulsive. It 

 therefore seemed better at that time to work inductively at 

 the accumulating mass of experimental material on molecular 

 force, than to follow out a deductive theory of molecular 

 attraction founded on molecular polarity, of which until re- 

 cently we have had little convincing evidence. But with recent 

 developments of the electron theory, which forces on our con- 

 sideration electric doublets as a prominent feature in molecular 

 structure, it becomes imperative to follow the deductive path. 

 For if with Helmholtz we regard everv chemical bond as con- 

 sisting of a # and a b electron (Phil. Mag. [6] iii. Feb. 1902), 

 we see that every junction of atom to atom involves the 

 existence of an electric doublet #b, analogous to a magnet and 

 exercising on every other such doublet a force of attraction 

 or repulsion varying inversely as the fourth power of the 

 distance between them. The electron theory supplies us 

 therefore with a true cause of molecular attraction on a 

 very simple basis, namely, the inverse square law of electric 

 force and the existence of the two sorts of electrons in equal 

 numbers. The results to be interpreted in the light of the 

 electron theory are contained in the following communi- 

 cations in the Phil. Mag. : series [5] "A Kinetic Theory of 

 Solids," vol. xxxii., " The Laws of Molecular Force," xxxv., 

 " The Viscosity of Gases and Molecular Force," xxxvi., " The 

 Attraction of Unlike Molecules," xxxviii., "Further Studies 

 on Molecular Force," xxxix., "The Fundamental Atomic 



