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that the potential on a distance / from a plain surface (pro unit 
of weight of adsorbed fluid) is represented by 
l 
k=—fe 
where f is a large positive constant and À is a number of the order 
of magnitude of the diameter of one molecule. 
This leads to the relation. 
RTin = —— fe * 
Po 
or 
ey a 
Po 
Discussion of this function gives a curve which begins about hori- 
zontally, having its convexity below, then gets a point of inflection; 
with a still larger 7 it has its concavity downward, and finishes 
about horizontally. So exactly what has been found experimentally. 
On the relative values of the coefficients f and 4 it depends how 
large the horizontal beginning will be. One could be in doubt 
for a moment, whether the formula deduced for a plain surface is 
valid for the particles of a powder. But as long as these particles 
are large, compared with the molecular dimensions, an error is 
made, which is not of importance. And how fine the powders of 
Dr. Day may be, the diameter of the particles is always still large 
compared with the diameter of a molecule. 
Is it possible to determine from experiments on surface-adsorption 
how the potential function of molecular forces 
depends on distance ? 
Finally an interesting question. We have seen, that the vapour- 
„tension p and the potential of the molecular forces k (on a distance 
/ from the glass-surface) are related to one another according to the 
formula 
k= RT Inf. 
Po 
If the theory of Prof. van per Waats is really a sufficient approxi- 
mation, then it will be possible to calculate £ from the measured 
vapourtensions. So we get the relation between & and the adsorbed 
quantity of water z. We should like to know the manner in which 
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