The osmotic pressures and the membrane equilibrium of chloride ions have been determined for solutions of gelatin in equilibrium with solutions of magnesium chloride containing from 4.0 to 9.0 equivalents per litre.

The pressures increase more rapidly than the concentration, an effect represented by a high value of the term "b" in van der Waals' equation p (V – b) = RT. Calculations made by a thermodynamical formula which makes allowances for deviations from the ideal solution laws show that the high value of "b" is not due to an unequal distribution of diffusible ions.

The theory that the high values of the hydration estimated from viscosity formulae account for the magnitude of "b" has been examined and the conclusion has been reached that the term "b" for gelatin as well as for haemoglobin is considerably larger than the volume of the protein hydrate.

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