Theory of Surface Forces. 521 



That is to say : 



Ihe average value p of the pressure p 2 parallel to the surface 

 of the capillary layer may be considered as equal to the pressure 

 of the theoretical isotherm in the point F (fig. 4), where the 

 thermodynamic potential fju is equal to their value in the homo- 

 geneous phases H and K. 



The equation 



6 a 2 ( Pl -p 2 ) 2 _ 15 H 

 DTff H ~ 16^?,— p 



(17) 



may be written 



- 25777 r 



■p= Pi -P=^[- 



H 

 Pi 



P-2. 



(17 a) 



In this formula for the difference between the pressure p x 

 resp. perpendicular and the (average) pressure parallel to the 

 surface of the capillary layer, / denotes the constant of the 

 used potential function : 





while a is the coefficient of the expression ap 2 of Laplace for 

 the so-called molecular pressure or the function a in the 

 equation of state of van der Waals. Although a is a function 

 of the temperature, we will, as a first approximation, consider 

 a and f as constants ; k being a new constant, we have also 



LP1-P2J 



H 

 Pi ~ P 'Pi-Pi 



Now is proportional to the rise of a liquid in a 



Pi — P-2 

 capillary tube and therefore proportional to T* — T *, where 



T/t is the critical temperature and T the temperature of 

 observation. We have also (when the liquid wets the walls 

 of the tube) : 



Pi-P 

 a. being a constant. 



= «(l-£j. ..... (19) 



* E. C. de Vries. Inaugural-dissertation, and Arch. Nierl. Science. 

 xxviii. pp. 210-219' (1894), and J. £. Yerschaffelt, Koninkl. Akad. v. 



Wetensch. at Amsterdam, April 1896. 



