( 261 ) 
The limits 1 en 2 relate to liquid and vapour. 
In the theory of VAN DER WAALS is: 
Res taro Sordi 
—— 2 die? Fase RN is. 
w a Q 9 dh? 1 4 dh® 
for 
Dg as. tee ais A hete Heb, 
dh 2 dh? 7 4 dh? 
By substitution of the squares of these expressions in (23), making 
use of the expressions : 
Cg C4, “ 
2 — 2 ‘a uw 4 bs 6 oe 
a Cg C4, 
2 n4 
we find easily: 
2 2 2 
3 *7 do \?2 “7 do \2 
S=a fe dh —a kh? J (=) dh +a en, (ae) dh 
1 
As the tension normal to the lines of force per unit of surface 
was equal to the potential energy per unit of volume with reversed 
sign, we have also found this energy. 
We can also easily derive the value of the energy directly from 
the equation for the energy, with which the energy of the unit of 
mass in a point of the surface layer exceeds that in a point within 
the liquid. Prof. vAN DER WAALS finds for this: 
( ca de ca dao 
ME eae 5 eae 
For the whole separating layer we get per unit of surface a potential 
energy : 
2 2 2 2 
a : di Cork do Cg of aarp 
a o* dh Ja af odh— ad Q Whe dh — Di Oort dh — 
1 1 1 
fe dh = mass of the separating layer per unit of surface = 
L9 
Proceedings Royal Acad. Amsterdam. Vol. II. 
