( 739 ) 
just as the membrane just now, be pressed downward with a force 
equal to the osmotic pressure defined by us, and transfer this force 
to the underlying layer of the pure solvent, which is pressed outward 
with this force. But this pressing force is evidently equal to the 
difference in pressure which may be measured experimentally *). 
§ 8. Thus it seems to me that van “rT Horr’s law for dilute solu- 
tions is kinetically explained in the same way as the law of Borrr- 
Gay Lussac-Avocapro for dilute gases and that of vaN per W aars 
for liquids and gases, i.e. we have obtained an kinetic insight how 
these laws result from the condition of motion in the homogeneous 
mass, while we have left out of account what happens in the 
eventually (probably always) present unhomogeneous bounding layers. 
It appears from the explanation convincingly, that vaN Laar goes 
too far, when he states *), that we cannot speak of osmotic pressure 
in an isolated solution. Here too this notion has a clear physical 
signification, and the laws which govern it, are to be derived. 
1) This hydrostatic proof may easily be replaced by a purely kinetic one, though 
the latter is somewhat more elaborate. The layer AB, which (in consequence of 
course of the neighbourhood of the membrane) behaves as a layer of water, through 
which the dissolved substance cannot penetrate (Cf. Nernsr’s well-known osmotic 
experiment) imparts to the molecules of the dissolved substance per second a 
quantity of motion equal to the osmotic pressure defined by us, and receives itself 
an equally large quantity in opposite sense, which it transfers to the underlying 
layers, as the kinetic theory teaches. (See e. g. Bourzmans, Zsch. phys. Ch. 6, 480). 
Now the whole mass of water, which is in the neighbourhood of the membrane, (on 
either side, reckoned on one side from B, on the other from a plane, so far from 
the membrane that the latter does not act on it any more), does not move downward, 
so it must receive an equally strong but opposed impulse, which, of course, cannot 
issue from anything but the membrane. Of what nature the forces acting here are 
is quite unknown. It cannot be the ordinary molecular attraction, for then the 
denser liquid found above the membrane would probably be drawn more strongly 
downward than that found ander it upward. We might think of friction in the pores, but 
it would then have to be different in one direction from that in the other; in short I dare 
not venture on any conjecture about this. This alone is certain, such forces must 
exist, at least if the case put by us ever actually occurs. This appears already 
from the fact that the pure solvent aboye the membrane is subjected to a higher 
pressure, so has a greater density than under it. Such an equilibrium occurs for 
all kinds of kinetic questions (liquid vapour, gas under the influence of gravity), 
but the necessary condition is always a force, which at a cursory examination 
seems to have the result, that the velocity of the molecules in one part (so the 
temperature) would be higher than in the other, but in reality only proves to have 
influence on the densiiy. The membrane, which furnishes this impulse, receives an 
equally strong one back from the reaction, and so here too, though indirectly, we 
see a force equal to the osmotic pressure defined by us, exercised on the membrane 
from the inside to the outside. 
*) Chem. Weekblad 1905, N°, 9, § 3. Voordracht Bat. Gen. 3. 
