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IV. Latent Heat and Surface Energy. — Part II. By 

 D. L. Hammick, Chemical Laboratory, The College, 

 Winchester *. 



IT has been shown in Part I. (Phil. Mag. xxxviii. p. 240 

 (1919)) that the work done in getting the molecules in 

 a gramme molecule of liquid into the surface layer is, at low 



temperatures, — -, where p is surface energy in ergs x cm. 2 ,. 



V is the molecular volume, and d is the molecular diameter 

 calculated from properties of the vapour. This work was 

 shown to be one-sixth of the internal latent heat of the 

 gramme molecule, which is the work that must be- done in 

 order to move all the molecules in the volume V apart from 

 one another against inter-molecular forces (internal pressure) 

 until the liquid has become a vapour. The equation 



~- . t = -7T can be used for the calculation of latent heat at 

 d J 6 



low temperatures only, because in deriving it the assumption 



is made of contiguity of molecules in the surface layer and 



absence of "vapour effects" (loc. cit. above). The equation 



given by Bakker, however, (Dissertation, Schiedam, 1888), 



is of general applicability. Bakker's equation has the 



general form 



I 2 Kdv=\i, 



. / V, 



where K is internal pressure per unit area across any section 

 in the interior of the liquid. K-dv thus represents the work 

 done against internal pressure when the system expands 



by dv, and 1 K dv, where i\ is the volume of 1 gramme of 



liquid and ,v 2 the volume of 1 gramme of vapour, gives the 

 work done in pulling all the molecules in 1 gramme of 

 liquid apart until the liquid has become vapour. In the 

 absence of an external atmosphere, this work is the internal 

 latent heat per gramme, X{. Integrating on the assumption 



that K is a function of v of the form -^, we get 

 * Communicated by the Author. 



