84 Dr. R. D. Kleeman : Dete? , minations of the Law of 



It can be shown strictly mathematically that it is impos- 

 sible to determine completely the law of attraction between 

 atoms from latent heat or surface tension data ; in other words, 

 the law deduced must contain an arbitrary function of the 

 distance between the attracting molecules and their tempe- 

 rature. Thus, let yjr x (z, T) denote the internal heat of 

 evaporation of a liquid into a vacuum, where z is the distance 

 of separation of the molecules in the liquid and T is the 

 temperature, which is supposed to be deduced from the true 

 law of molecular attraction on the supposition that the in- 

 ternal latent heat is the work done against the attraction of 

 the molecules on their getting separated *. Since z may be 

 expressed in terms of the density of the liquid this expression 

 may be written ^ 2 (p, T). The above statement can most 



in 



conveniently be proved graphically. Let the curve Aj A 

 fig. 1 denote the graph of the equation L = i/r 2 (p, T) for a 



* We are assuming that the attraction between two molecules depends 

 on their temperature as well as on their distance of separation. 



