the solvent in the solution, will be less than the solubility (‹c›) at ‹A›, where the membrane is in contact with the pure solvent, and water will pass into the solution at ‹B›, until the concentration of the water in the membrane at ‹B› has fallen [p024] from (‹c›) to (‹c′›). In such a membrane, as in every solution or gas, there must be a tendency towards the establishing of uniform concentration by diffusion from points of higher concentration to those of lower, and the solvent will, therefore, ‹diffuse from points along the surface A of the membrane to points along the surface B›; the surface ‹A› will become ‹unsaturated› and will take up solvent from the pure liquid bathing it, and the surface ‹B› will be kept continuously supersaturated and will lose solvent continually to the solution. Consequently, the solvent will pass continuously through the membrane from the pure solvent to the solution. Equilibrium will be reached, and the flow will cease, only ‹when the solution has become infinitely dilute›, equal hydrostatic pressure obtaining on solution and solvent, or ‹when the disturbing influence of the solute, which dilutes the solvent in the solution, is exactly counterbalanced by an external hydrostatic pressure, exerted on the solution›. When such a pressure on the surface of the solution balances the force exerted against the solvent by the solute we shall have equilibrium. It is clear, then, that the ‹osmosis›, or passage of the solvent through the membrane, is brought about by the unequal concentrations (or, more exactly, the resulting ‹unequal partial pressures›) ‹of the solvent itself. But this inequality is produced by the presence of the solute, and it is a characteristic and significant fact, that the effect of the latter, in dilute solutions, may be overcome by a hydrostatic pressure, corresponding to the gas pressure which the same number of molecules of a gas in the same volume at the same temperature would exert against this hydrostatic pressure›. [Illustration: FIG. 6.]