SWELLING OF PROTEIN JELLIES 305 



and also, possibly, to internal N.HOC -groups, resulting in the 

 Depolymerization of the protein. Not only osmotic phenomena, but 

 Hydration of the gelatin must, therefore, occur in the process of swelling. 

 Confirmation of this view is afforded by the fact that the Swelling of 

 gelatin is accompanied by an absorption of heat. Evidently the 

 processes of solution and swelling are each composed of two factors, 

 one leading to a disengagement and the other to the absorption of 

 heat. The former process is the chemical binding of water by the 

 protein, the latter the passage of the hydrated protein into solution 

 (or, in swelling, the osmotic imbibition of water). In swelling, the 

 chemical heat-effect predominates, in the dissolving of the gelatin, 

 the heat-effect of solution. 



The degree of swelling which Gelatin plates undergo in water is 

 greatly enhanced by the addition of small amounts of acid or alkali to 

 the water, the minimal imbibition of water being at a reaction very 

 close to the neutral point. The phenomena attending the swelling of 

 plates of gelatin in acidified water have recently been very thoroughly 

 investigated by Procter. This investigator has found that gelatin 

 absorbs both acid and water from acid solutions, but absorbs the acid 

 in excess, so that the proportion of acid in the surrounding fluid dimin- 

 ishes. If the initial concentration of acid in the external fluid lies 

 between 0.01 and 0.25 N, then, assuming that at the end of the process 

 (attainment of maximal swelling) the concentration of free acid is the 

 same within and without the jelly, having been equalized in the course 

 of time by diffusion, the amount of acid which is "bound" by the 

 gelatin is 0.7 to 0.8XlO~ 3 (=70 to 80XlO~ 5 ) equivalents per gram. 

 The equivalence at the attainment of maximal swelling is the same for 

 all strong acids, but falls below this value for weak acids. While the 

 proportion of acid which is "bound" by the gelatin varies but slightly 

 with the concentration of the acid in the surrounding fluid, this is not 

 true of the degree of swelling attained, which in strongly acid solutions 

 attains its maximum at a dilution below that required for complete 

 fixation of the acid by the gelatin, and then falls in a continuous curve 

 with increasing concentration of the external acid solution. The same 

 inhibition of swelling is brought about in varying degree by strong 

 solutions of various inorganic salts, and is attributable to the dehydra- 

 tion of the protein which, in still stronger solutions, culminates in its 

 Coagulation. 



The taking up of water by gelatin from acid solutions is accounted 

 for by Procter in the following way: He pictures the gelatin acid- 

 compound as a coherent mass from which the gelatin molecules cannot 

 diffuse or separate and which, in most respects, behaves like a single 

 enormous complex molecule. It is reasonable, he considers, to visualize 

 it as a felted mass of amino-acid chains held to each other by attrac- 

 tions which possibly attach only their ends, but freely admitting the 

 passage of liquid between them. He assumes, in accordance with our 

 older conception of the mode of formation and ionization of protein 

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