(255 ) 
4 j . 1 
òf substituting Zi for 9: 
ae (w+ 4.) Ngo Naan akan 
8a f 
If f=1 and ¢=0, we find once more the well-known expression : 
R2 
8a 
This expression is negative, because the constant of the potential 
function is equated to zero. So it represents without the — sign 
the work required to separate different parts of the agens at an 
infinite distance from one another, when the forces are attractive. 
Tension in the medium. 
MAXWELL has proved that the force which two electric systems 
exercise on each other, may be considered as a simple system of 
tensions in the medium. The same applies to the general potential 
Aevt Ber 
Ui 
function p (r) = - aud so also to the potential function 
of VAN DER WAALS. 
Imagine an agens spread over different spaces, for which the 
potential function g(r) holds good, and enclose a certain region by 
a closed surface. Call this region system I and all other space 
system II. The resultant of the X-components of the forces exercised 
by system II on system I is: 
d 
Hef geer 4) 
According to equation (3): 
au — MW 
O~40(A+B) 
SO 
d rrd 
An (A zone f vr a fuga : 
da da 
1) Here the same remark holds good as I have made repeatedly in such like calcu- 
lations. See e.g. Journal de Physique 1899, p. 546. Generally this expression for 
X, is proved in a iengthy way. 
