T H E Swelling Pressure of Gelatin and the Mechanism of Swelling in Water and Neutral Salt Solutions. By

The swelling of gelatin when placed in aqueous solution may be readily separated into three types. The swelling in acid or in alkali and the effect of neutral salts on this swelling have been shown by the work of Procter and Wilson, ~ and Loeb ~ to be due to the osmotic pressure of the ions of the electrolyte, in accordance with the Donnan equilibrium. The initial swelling of dry gelatin in water--which evidently is not connected with the Donnan equilibrium--has been carefully studied by Katz, 8 who was able to show that the heat effects, volume, pressure, and vapor pressure changes were strictly analogous to those observed in the formation of concentrated solutions of many substances, and that the system as a whole behaved as an ideal concentrated solution. The large amount of heat liberated indicates strongly that a reaction occurs between the water and gelatin, resulting, presumably, in the formation of a gelatin hydrate. When sufficient water has been added, however, to reduce the gelatin concentration to less than 50 per cent, the heat effects become very small and yet the gelatin may swell, under favorable conditions, until the concentration of gelatin is 5 per cent or less. I t appears improbable that this water is all combined with the gelatin in the [orm of a hydrate, and it seems necessary to consider this as a third distinct type. I t is this type of swelling which is discussed in the present paper. The influence of salts on this tertiary swelling was found by the