296 PHYSICAL PROPERTIES 



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. Procter interprets this by supposing that the 

 swelling of the gelatin is determined by two opposing forces, the 

 osmotic pressure of the acid-gelatin compound imprisoned by 

 its indiffusibility within the jelly being balanced by the osmotic 

 pressure of the acid outside. That the acid in the outer fluid 

 does exert an osmotic pressure at the surface of the jelly Procter 

 infers from the fact that concentrated neutral salts contract the 

 jelly to a horny consistency and expel the associated (imbibed 

 or combined) water without affecting the acid-gelatin compound. 

 In view of the considerations set forth in Chap. VI we can see, 

 however, that the assumption that the external acid exerts an 

 osmotic pressure is an unnecessary one and, indeed, inconsistent 

 with the fact that the acid freely penetrates the gelatin and 

 combines with it. In view of the known dependence of coagula- 

 tion upon phenomena of dehydration (Cf. Chap. VI) and the fact 

 that protein salts may be coagulated as such and without decom- 

 position, we may infer that the shrinkage of gelatin jellies upon 

 addition of concentrated salts to the external medium is not 

 due to the osmotic pressure of the salts, since gelatin is known 

 to be permeable to inorganic ions, but to the competition between 

 the inorganic salt and the gelatin salt for water. 



Procter has taken advantage of the fact that the acid-gelatin 

 compound is coagulated without decomposition by concentrated 

 salt solutions to determine the quantity of acid "bound" by 

 gelatin in acid solutions of varying concentration. He finds 

 that the proportion of acid which is bound per unit mass of 

 gelatin does not agree with the requirements of the Ostwald 

 dilution-law for a binary electrolyte, no values of the hydrolytic 

 dissociation-constant and the molecular weight yielding calcu- 

 lated results which are consistent with the initial rapid and 

 subsequent slow rise in combining capacity with increasing con- 

 centration of acid in the surrounding medium. Better agree- 

 ment between theory and experiment is attained by assuming 

 that gelatin behaves as a diacid base having a molecular weight 

 of 839, which agrees with the molecular weight estimated by 

 Paal from freezing- and boiling-point measurements (68). 



The taking up of water by gelatin from acid solutions is ac- 



