Theory of Surface Forces. 145 



chano-ino; volume and mass, and v the specific volume o£ the 

 capillary layer, we have : 



* +!=£=„, or s^-W', and !-,= !!&&=&. 



Pi P2 Pl~P3 Pi" PS 



Per unit of mass o£ the capillary layer the available 

 energy, which the homogeneous phases have engendered, 

 becomes : 



Pi" P2 Pl"~P2 



=Pi + + *> • 



Pl-P2 "2 — ^! 



As wo mentioned above we have : 



So the last expression becomes 

 ^1- 



P^P*- P v (25) 



P1-P2 



If, consequently, the surface of the capillary layer per 

 unity of mass is S, the available energy of the capillary 

 layer is given by : 



e ^ ^ Pi^JPj -pr + HS. . . • (26) 



Pi~P2 



For, what we usually call the capillary energy per unity 

 of mass and represent by HS is the difference between e— T17 

 and the expression (25). Further we have : 



.. _ €1 + e 2 T Vt + Va p,v x + 



^~ 2 2 T 2 



After a simple deduction (26) becomes therefore : 



e _£l+£?_T^-^) +i ,(t,- c l±^)=HS*. (27) 



For a plane capillary layer p becomes the ordinary vnpour 



pressure, and we find the equation for this case, as I have 



found already in a preceding paper f. 



i 2 

 * pt= f p'dv, where p' denotes the pressure of the theoretical 



isotherm (see above). 



t Ann. d. Phys. xvii. p. 492 (1905). 



Phil May. S. 6. Vol. 20. No. 115. July 1910 L 



