( 257 ) 
In exactly the same way as MAXWELL we may conclude from 
this that, when a part of the whole system is enclosed by a poten- 
tial surface, we may consider the action of the other part on the 
enclosed part of the system as a tension (or pressure), normal to 
that potential surface, so in the direction of the lines of force, and 
a pressure (or tension) round the lines of force normal to them. 
The value of the tension is here: 
R? — ¢2 y?2 
82(A + B) Saf 
or if B=0 and A=>—/: DE 
7 
The quantity ¢g is the reciprocal value of 4 in the potential func- 
tion of VAN DER Waats. Hence: 
Er me). | 
‚2 
If > Ta the expression becomes negative and the tension 
becomes a pressure. The value of this expression becomes: 
(5%) ee ee REEL 
If we take the surface element for which the tension or pressure 
is to be determined, normal to the lines of force and represent by 
1, m and n the direction cosines of the normal measured outwards, 
then the z-component of the force, acting on the element (considered 
as a part of a closed surface) is: 
l Pax JM Pyz + 0 Pez « 
Now: 
Sn (A + BEL paz + m pyz + 2 pz} = 
(EEC reren Eran ee 
sane dx dy dz ae (+ de dy 1 dat dz 
and we get the relation: 
