Theory of Surface Forces. 425 
We will denote half the sum of the max. and min. value 
of the hydrostatic pressure by p, and write : 
..... (18 a) 
V 2 
dp^<M_lJdJ__cWf 1 VjA 
dv dv -Aa dv dv \ 2 a /' 
In differentiating (18 a) we have : 
dp_d6__'2jdV__d£ 
dv dv -la dv dv 
because dO=—pdV. 
Further we have : 
Y + 2ap=/n 1 -fju*. 
Thus dp _d0/ji 1 — /ju 
dv dv 2ap 
ja is the thermodynamic potential of the homogeneous phase, 
which corresponds with the density of the considered point 
in the capillary layer. When the index 1 corresponds to 
the homogeneous phase of the liquid, we have 
M— A*i=ji vcl P> 
Fiff. 5. 
M 
t/ieor Isoth 
ABC^M -V Carve 
AUWVG=P*--V Curve 
Surf=DAGM = Surf*LEGM 
Surf ^NH AD ~Surf*NKCQ 
H 7C= empirical feoth 
V-AXJ* 
\I£ we construct, therefore, in fig. 5 the curve AEC, which 
presents half the sum of the pressures p x and p 2 (p) as a 
* Phil. Mag. October 1907, p. 516, equation (4). See also above. 
