( 258 ) 
By combining these two equations, we find easily : 
82 (A + B){l psx + m py: +2 pee} = 
=H ved 
As we have taken the force as a vector in the direction of the 
normal measured outwards, the above equation indicates the force 
which acts on the element from the inside towards the outside. 
The expression between the braces in the right-hand member is 
always positive, and we get therefore a negative tension or a pos- 
itive pressure: 
R? +- gw? 
82 (A + B) 
For the potential function of van DER Waats B=0, A= —f 
and g= so we find here a (positive) tension : 
gor 
1 
8 7 / 
For an electric system the system of forces may be described as 
a system of tensions in the direction of the lines of force and a 
system of pressures normal to the lines of force, here however we 
sce that we must assume both tensions and pressures in the direction 
of the lines of force. Normal to the lines of force there are only 
tensions, whereas for electric agens the reverse is found. For electric 
agens the numeric value of the tension is equal to that of the 
pressure; in our case the tension is not equal to the pressure, except 
where yw and Z are zero. For the potential energy per unit of 
volume we found: 
from which follows that: 
the absolute value of the potential energy per unit of volume ts 
equal to the tension normal to the lines of force. 
