Atomic Laws of Thermochemistry. 5 



resulting from the attractions of molecules for one another 

 takes the form — § r, where k is nearly equal to the cri- 

 tical volume, and I is a constant for each substance but different 

 for different substances (a parameter) ; at volume k this 

 becomes — §Z/2&, and for volumes below k, that is for the 

 liquid state, the term retains the form— ^l/2v. The con- 

 stant I is thus an important measure of molecular attraction 

 amongst like molecules, and five chief methods along with 

 some subsidiary ones are given for calculating its value from 

 available data, a large number of values being tabulated 

 (Phil. Mag. 5th ser. vol. xxxv. March 1893) in the form M% 

 where M is the ordinary molecular mass (weight) of the 

 substance. The equation of one of these methods throws 

 light on the matter in hand ; it is that of the third method, 



M//^ = 66-5MX-101T 6 , .... (3) 



where X is the latent heat of vaporization of a gramme of 

 liquid at its ordinary boiling-point T b reckoned from absolute 

 zero, Vj being the volume of a gramme in cubic centimetres 

 at that temperature : with X in calories this equation gives I 

 in terms of 10 6 dynes as unit of force. In connexion with 

 this equation it is shown that for a large number of liquids 

 the approximate relation MA,= 19'4T 6 holds; that is, the 

 molecular latent heat is proportional to the absolute boiling- 

 point, a relation discovered empirically by Pictet in 1876. 

 Using this to eliminate Ts from our equation, we have ap- 

 proximately 



Mi/^ = 61'3MX. (4) 



This shows how, if we can obtain values of I, we can derive 

 the latent heat of vaporization of the substance as liquid. 

 The latent heat of fusion of solid to liquid is for most bodies 

 only a fraction of the heat of vaporization of the liquid ; so 

 that if in the last equation we replace v\, the volume of the 

 liquid at its boiling-point, by v the volume of the solid, we 

 shall have an approximate equation for the heat of vaporization 

 of the solid. The problem of finding the latent heat of vapo- 

 rization of solids is thus reduced to that of finding values of I. 

 The fifth method of finding I given in the Laws of Molecular 

 .Force is the only one of the five which is applicable to exist- 

 yig data for solids : the equation of that method is 



IssceafrfM}**; (5) 



where a. is the surface-tension of the liquid measured in 

 grammes weight per linear metre at two thirds of the absolute 



