6c2 Mr. W. Sutherland on the 



I proposed that L might mark the linear dimensions of 

 molecular swarms, but according to the present line of 

 reasoning the idea of a molecular swarm gives place to that 

 of a range limited to a small multiple of the average distance 

 between two neighbours. Thus the range for an ordinary- 

 vapour would be different from that in its liquid. In steam 

 over boiling water the average distance between neighbour 

 molecules is about twelve times that in the water, but we 

 cannot assert that because steam still shows measurable 

 effects of molecular attraction therefore the range of mole- 

 cular attraction in water extends to at least twelve times the 

 distance between two neighbour molecules. The correct 

 deduction according to the principles under discussion would 

 be that in steam the range of molecular attraction must be 

 taken to be about twelve times as large as in water. In the 

 next section we shall introduce a great simplification by 

 treating the range of molecular attraction as the average 

 distance between two neighbour molecules, thereby express- 

 ing that attractions which eventuate in collisions are by far 

 the most important of the forces we are now considering. 



2. Comparison of Results with known Laws of Molecular 

 Attraction. 



It will now be shown that the laws of force so far dis- 

 covered satisfy the conditions required by the electron theory. 

 In the first place, although the force between two molecules 

 was on the analogy of gravitation written by me in the form 

 3Am 2 /?" 4 , it soon appeared from its application to experi- 

 mental data that 3Am 2 ought to be regarded as a single 

 parameter of molecular force 3a 2 which had no direct de- 

 pendence on the mass m of a molecule. This marks a strong 

 contrast to the law of gravitation, and satisfies the first con- 

 dition of the electron theory, namely, that molecular mass 

 does not enter into the expression for molecular attraction. 

 In the second place it has been proved (xxxviii.) that the 

 force between two unlike molecules 1 and 2 is Sa^/r*, a± 

 and a 2 being characteristic constants of 1 and 2. In the 

 third place an almost direct proof of the electronic origin of 

 molecular attraction is furnished by the law of the parameter 

 a 2 for binary molecules of the type RS W (Phil. Mag. [5] 

 xxxix. pp. 25 & 45), namely, 



a' 2 /n={pln + a) 2 , (A) 



p and a being parameters having definite values for each 

 element R and each element S, because, to quote from the 



