52 



greatest volume at time of setting absorbed 14-6 times its weight 

 of water, the second sample, 7 • 7 ; and the sample with smallest setting 

 volume only 5 • 8 times its weight of water. 



Gelatin swells to a much greater extent in dilute acid solutions 

 than in water. The parts of solution absorbed by one part of a 

 certain sample of gelatin, at 18°, were in pure water 8, in 0-006 N 

 HCl 42, in 0-05 N HCl 28, in 0-3 N HCl only 17. Stronger solutions 

 of the acid caused the gelatin to soften and finally dissolve. This 

 action appears to be independent of the swelling phenomenon, and 

 at higher temperatures becomes more marked, even with lesser 

 concentrations of acid. By plotting the amount of swelling against 

 the concentration of acid up to 0-3 N, a curve with a maximum is 

 obtained, the explanation of which has given rise to several theories, 

 which ^viU be treated later. 



The most extensive investigation of the hydrochloric acid gelatin 

 equilibrium seems to have been made by Procter, who showed that 

 the concentration of free acid is always less in the solution absorbed 

 by the gelatin than in the remaining external solution, and that the 

 sum of the amounts of free acid in both these solutions is less than 

 the amount in the solution before the introduction of the gelatin. 

 He attributes this difference to chemical combination between the 

 gelatin and some of the acid and regards the product as a hydrolyzable, 

 but highly ionizable chloride of gelatin. This agrees with the electro - 

 metric determinations of Manabe and Matula, who found that in 

 certain hydrochloric acid solutions of gelatin nearly aU hydrogen ions 

 were bound by the protein and nearly aU chloride ions were free. 

 They exammed acid solutions of serum albumin similarly and regarded 

 the behaviour of both proteins as that of weak bases forming 

 hydrolyzable salts. 



Gelatin swells likewise in solutions of other acids, but not to the 

 same extent as in HCl, nor does the point of maximum occur at 

 exactly the same concentration of either total acid or hydrogen ion. 

 With strong acids the maximum in the swelling curve is very 

 pronounced, becoming less so with weaker acids. With acetic acid 

 the maximum is hardly reached at concentrations so great as to cause 

 solution of the gelatin. Extremely weak acids like boric produce 

 very little sweUing. 



If sodium chloride, or other neutral salt, be added to acid-swollen 

 gelatm, the latter contracts and gives up the solution it had absorbed 

 to an extent depending upon the concentration of added salt. If 

 the solution be saturated with salt, the gelatin shrinks to a horny mass. 

 Fischer found that even non-electrolytes, such as sugars, produce 

 this repression of sweUing, although not nearly to the same extent 

 as salts. Repression is produced by the acid itself when present 

 in concentrations greater than that required to produce maximum 

 sweUing. 



Experiments dealing with the swelling of gelatin in alkaUs are 

 generally not so satisfactory as those with acids, because of the more 

 powerful solvent action of alkalis on the swollen gelatin. Nevertheless, 

 by using only very dilute solutions at low temperatures, sufficient 

 data have been collected to show that alkaline swelling is of the same 



