794 Dr. R. D. Kleeman on the Nature of the 



the surface. This will be at once evident when it is pointed 

 out that in bringing the molecule to the surface no work is 

 done against the attraction of the surface layer of the liquid 

 equal in thickness to the radius of the sphere of action of a 

 molecule. Now the investigation of the surface-tension gave 

 that <j)(z) must be of the form 



It follows, therefore, that the internal heat of evaporation 

 per gram of substance, which we will denote by L, is given 

 by an equation of the form 



the value of <j> 3 1 — a , ft J being the same for all liquids at cor- 

 responding states. The formula is of the same form as 

 equation (6) given at the beginning of the paper. It was 

 deduced from the equations 



=«tJ$ 



,J*, f=4 E=LT™^- 5-57x10-1 



(Phil. Mag. Oct. 1909, p. 491), and was found to agree well 

 with the facts. 



It is necessary next to consider the effect of the 

 surrounding vapour on the magnitude of the heat of evapo- 

 ration of a liquid. The work done in removing a molecule 

 from the interior of a slab of vapour of density p 2 to an infinite 

 distance is, according to the above investigation, equal to 



<wer*fc*> 



where x h is the distance of separation of the vapour molecules* 

 The equation for the internal heat of evaporation, therefore,, 

 strictly is 



i-=w{(£)"*&')-®""*©')}- 



But since p 2 = ap 1 the equation may be reduced to the previous 

 form 



L=B*g) 4 W, 



where B 2 is a constant which is the same for all liquids at 

 corresponding states. 



