414 
Dr. G. Bakker on the 
the surface of the capillary layer is therefore concave to the 
side of the vapour. Because the forces of attraction between 
Fitr. 1. 
M 
w 
ER G FPK- theor rsoOv. 
my MTYK-p 2 r Curve 
&rfccefirff7lGMA r =S. LFGML 
SF-FW 
TT-AXIS 
the elements of a liquid or vapour are perceivable through 
only a very small range, the potential V of these forces at 
some internal point depends only on the elements embraced 
in the sphere of action of which it is the centre, and should 
be thus for a homogeneous phase proportionate to the 
density p. For the potential function of the forces of attrac- 
tion we have thus : 
*W=-/ — ' 
which gives for the potential V the following differential 
equation : 
yv y-y yv 
(i) 
For our case, and in many others, it is necessary to intro- 
duce instead of the coordinates a, ?/, z more general coordi- 
nates and to transform the left of the equation (1) into a 
more general expression. For this purpose, we divide the 
whole space into elements of volume by a set of orthogonal 
surfaces. Such a system of surfaces cut one another, as 
every one knows, according to the theorem of Dupin, into 
lines of curvature. The differentials of these curves we shall 
* "On the Theory of Surface Forces — I." 
p. 558. 
Phil. Mag. Dec. 1906, 
