516 

 Hence 



Dr. G. Bakker on the 

 H 



Pi-P 

 We will now demonstrate that, if F (fig. 4) is the point on 

 the theoretical isotherm such that 



surface NHGMLN = surface LFGM, 

 we can put j/ = RF. 



Fio-. 4. 



'vzdxi's 



F is the point where yvdp = 0, and where also the thermo- 



dynamical potential has the same value as in the homogeneous 

 liquid and vapour phase. If /a 1 is this value, w r e put generally 



^vdp = fi-fx v 



If p denotes the pressure in a homogeneous phase having 

 the same density as in a point of the capillary layer, we 

 shall have 



pz=$ — ap 2 or dp = dd — 2apdp, 



where 6 designs the thermic pressure. If Y denotes further 

 the potential of the forces of cohesion, we have : 



d6=-pdY. 

 Eliminating dO between these differential equations : 



vdp + 2adp= — dY. 

 Integrating: ^vdp +2a(p—p 1 )=V 1 — Y, . . . (3) 



where the index belongs to the liquid phase. Now for this 

 phase we have 



Y 1 — —2api (Gauss and van der Waals) 



and equation (3) becomes : 



Y + 2ap = f i l - f i (4) 



