450 
curve can be obtained. When a layer of fluid covers the surface and 
this is thick enough for us to assign to it the properties of fluid 
in mass, there exists a simple thermodynamical relation; at least 
when there is a discontinous change in density of the layer of fluid 
and of the coexisting vapour (which is ailowed as a first approxi- 
mation) and when we neglect the very small compressibility of 
the water. 
Then there exists for the vapourtension p, coexisting with fluid 
water at a distance / from the solid wall, the relation: 
RT Int =k 
Po 
where & is the potential of the attraction of the solid wall on a 
distance J, p, the maximumtension of water at the absolute tempera- 
ture 7, and R the constant of gases. *) 
If the potential of molecular attraction were known, it would be 
possible to predict how the vapourtension, which is in equilibrium 
with a layer of fluid of the thickness 7, depends on /. And be- 
cause the quantity of adsorbed water 7 (in gr. of water pro 1 gr. 
of dry powder) is related to the surface O according to the formula 
ie 
O 
it would be known at the same time, how the quantity of adsor- 
bed water depends on the vapourtension. 
The potential function of Lord Rayusien and Prof. vaN DER WAALS. 
Prof. van DER Waats proposed, that [ should see how far we 
come with the potential function, which Lord Rarrerem and he had 
adopted in course of their studies about capillarity. They assumed 
1) This relation is easily deduced from the general property (VAN DER WAALS— 
KounstamM, Lehrbuch der Thermodynamik I, p. 197) according to which in a 
system, subjected to the action of external forces, the total potential of a sub- 
stance possesses the same value through the entire system. When is the potential 
of watervapour, p! the density-potential of water in the point / (that means the value 
which the potential of the water would have with the same density but without 
external forces) and k the potential of the molecular forces at a distance / 
at the solid wall, we have. 
wptk=u. 
When the compressibility of water can be negtected, u = RTIn po, while 
= RTInp. It follows from this, that 
eee ec ype 
Po 
