610 Dr. R. D. Kleeman on the Equation of Continuity 



Let us next obtain the internal heat o£ evaporation on this 

 supposition, making use of the same notation. Suppose a 

 thin layer of liquid of unit area and thickness dx is removed 

 from the surface of the liquid and distributed in a space of 

 infinite extent, the layer being taken so thin that the work done 

 in distributing the matter in space is small in comparison with 

 the work done in removing the layer. If L„ denote the 

 internal latent heat per unit volume, 



n oo /"» oo 



L v dx = I dx . f 0) . ds = dx >fr(z) d; 

 Jo Jo 



L>= ffO)<L-». 





 /» oo 



L> = f 



Jo 



( lomparing this equation with the above expression for the 

 intrinsic pressure, wo see that 



P* = L> (5) 



This equation, obtained on the supposition that matter is 



evenly distributed in space, gives on comparing it with 



equation (4) that K3 = I. Whether this supposition is 



admissible in the above investigation can be tested by 



calculating P n by equation (5) and comparing it with that 



obtained bv equation (2). Thus, in the case of ether at a 



2Tc 

 temperature of -.-,- equation (5) gives 

 o 



^ 75*4 x 4-2 xl0 7 x -6007 Q __ . 2 



P n = Ycii = 21b7 atmos. per cm/, 



where 75*4 is the internal latent heat of evaporation f in 



* Stefan has shown (Wied. Ann. xxix. p. 665) that the internal heat 

 of evaporation of a molecule is equal to the work done in moving it from 

 the interior of the liquid to the surface and then to an infinite distance 

 from the liquid. This is true, however, only when matter is not evenly 

 distributed in space, i. e. consists of molecules, in which case a molecule 

 must be brought from the interior of the liquid to fill up the gap made by 

 removing one from the surface. When matter is evenly distributed in 

 space, however, we may suppose that during evaporation infinitely thin 

 layers of liquid are successively removed from the whole surface of the 

 liquid. The radii of the spheres of action of a set of different molecules 

 calculated by the writer, Phil. Mag. pp. 840-846, June 1910, on the 

 supposition that matter is evenly distributed in space and using Stefan's 

 result, therefore really denote their diameters. 



t The internal latent heat of evaporation and density data used in this 

 paper are taken from a paper by Mills, Jo urn. of Phys. Chem. vol. viii. 

 p. 405 (1904), who has calculated the internal latent heat at different 

 temperatures for a number of substances, using the density and pressure 

 data of Rarnsav and Youngr. 



