The experiment is particularly instructive, in the first place, because it illustrates with a gas, subject to the laws of gases, why and how osmosis takes place through a semipermeable membrane—namely as a result of the solubility of the diffusing substance in the membrane, and through the flow of the diffusing substance [p026] from higher to lower concentrations. In the second place, while the increase in total pressure in the inner chamber undoubtedly is ‹brought about› by the ‹osmosis› of ‹hydrogen› into the chamber, the excess pressure when equilibrium has been reached, necessarily measures accurately the partial pressure of the ‹nitrogen›. In other words, the semipermeable membrane is merely a means or ‹device for measuring› the partial pressure of the nitrogen—the membrane is not the ‹cause› of the pressure; the latter is a definite one, whether we know what it is or not, and the osmosis of the hydrogen through the palladium merely gives us a means of ascertaining it. Similarly, it would be wrong to consider that the osmotic pressure of a solution is caused, or brought about, by the flow of the solvent through a semipermeable membrane (osmosis); the latter simply is a ‹device› which enables us to recognize and ‹measure› the pressure that exists in the solution, both in the presence and the absence of such a membrane.
We may consider, then, that the osmosis, or migration of the solvent through a semipermeable membrane into a solution, is the result of the reduced concentration (or ‹partial pressure›) of the solvent in the solution, resulting from the presence of the solute.
Inasmuch as the ‹effect› of the ‹solute› on the solvent can be overcome by a ‹pressure› on the surface of the solution, one is led to the conclusion that the solute acts by exerting, in turn, ‹a force or pressure› against the surfaces of the solvent, in the directions opposite to the hydrostatic pressure required to overcome it. The significant identity of the value of this pressure, as thus measured, with the gas pressure that would be exerted by a gas of the same number of molecules, in the same volume and at the same temperature, leads us to the last of the three questions which have been raised, namely, the question concerning the theory of the intimate relations between gas and osmotic pressures (p. 21).