Theory of Surface Forces. 517 



For the potential V of the forces of cohesion I have found * 



Hence 







(5) 

 (6) 



The member on the right being for a point on the theoretical 

 isotherm nothing but the integral — \ x vdp, is given by the 

 superficies (for the point F for instance): 



-KEGM + LFCM. 



The member on the right becomes also null in the three 



points 



H, F, and K, 



d 2 V 

 and therefore we have for these points equally: -jjj = 0. 



The curve, which presents the potential of the forces of 

 cohesion V as function of h must, therefore, be asymptote to 

 two straight lines belonging to the potentials of the homo- 

 geneous liquid and vapour phase ; moreover, this curve must 

 have a point of inflexion Q (fig. 5). 



dY 

 The value of jj- is also at this point a maximum. This 



value of the strength of tlie~force may be calculated in the 

 following manner : — 



The hydrostatic pressure perpendicular to the surface of 

 the capillary layer is equal to the vapour-pressure, and can 

 be considered as the difference between the thermic pressure 

 6 and the cohesion in the same direction. Now I have found 



* Phil. Mag. for Dec. 1906, p. 565, equation (11). 



