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LVII. On the Law of Molecular Attraction. 



To the Editors of the Philosophical Magazine. 



Cambridge, Feb. 21, 1912. 

 Gentlemen, — 



N a paper to the Philosophical Magazine of January 

 . Mr. Tvrer has investigated whether the function 



T z \ 



in the law of molecular attraction does really 



HA) 



contain the temperature T. The conclusions he arrives at 

 are based on the statement in his paper, page 109, that " if 

 the total change of energy of a liquid which expands at 

 constant temperature is not entirely made up of the changes 

 in the potential energies of attraction and of external pres- 

 sure, then we should expect to find that the specific heat of 

 a liquid at constant volume varies with the density." But 

 this statement is wrong. 



Let us suppose that the attraction between two molecules 

 depends on their temperature. The energy necessary to 

 separate the molecules of a mass of substance from one 

 another by an infinite distance against the molecular attrac- 

 tion may then be written ^(T, p), where p denotes the 

 density of the substance. The expression for the specific 

 heat at constant volume will then contain the term 



{' rv r • The specific heat is therefore not 



(11 J p= constant 



independent of p, and the above statement does not hold. 

 One of the conclusions at which Mr. Tyrer arrives, namely 

 that the attraction between two molecules is independent 

 of their temperature, thus underlies his reasoning as an 

 assumption. 



It is very probable, however, that the change in potential 

 energy of a substance with change of volume consists in part 

 of a change in the internal energies of the molecules. But 

 the latter quantity and the potential energy due to molecular 

 attraction always occur together, and it is difficult therefore 

 to disentangle them. However, I have obtained fairly con- 

 clusive evidence now that the attraction between two molecules 

 depends on their temperature. A simple explanation of this 

 property can be given. 



Yours faithfully, 



R. D. Kleeman. 



