658 Thermodynamic^ Theory of Surface Tension. 



Now take the case of only one independent constituent, 

 so called. Here (29) becomes 



s+-$f*-i(g)',K- ■ w 



Take the same case and let a unit area of the capillary 

 layer be formed from the first liquid under a constant 

 temperature and in a reversible manner, and let W be the 

 work done on the mass spreading over the unit area of the 

 layer, and Q the quantity of the heat absorbed by the same 

 mass during the change, and IT its intrinsic energy. Then 

 it can easily be shown that 



W=T + ^( P -p')dx, .... (31) 



Q dX 1 dp' C. .. , 

 ^—M-fSB-fr-Ad*, (32) 



V=u^pdx + T-0^ + fa -e^fo-tfdx, (33) 



where v! is the intrinsic energy per unit mass of the fluid at 

 P'. As may be expected, equations (31), (32), and (33) 

 have the same form compared with the case in which the 

 electric field in the layer of transition is not taken into 

 account. 



Now consider the case in which there is neither cation nor 

 anion. When the layer is placed in an electric field normal 

 to it, the surface tension T depends upon the electric 

 displacement D' at P'. By using (6) and observing that the 

 right-hand sides of (20) and (21) are even functions of D', 

 it is immediately seen that when D' is infinitely small and 

 the temperature is kept constant during the change, 



dW ' 



which shows that the surface tension is maximum or minimum 

 for a D' = 5 although, perhaps, ^"-^/ does not vanish. 



