80 



THEORY OF COLLOIDAL BEHAVIOR 



The curve for the swelling of gelatin sulphate, where the anion 

 combining with gelatin is bivalent, is only about half as high as 

 the curve for the salts of gelatin with the anion of weak dibasic 

 acids (Figs. 19 and 20). 



FIG. 20. Influence of citric, tartaric, and acetic acids on swelling of gelatin. 

 The curves for citric and tartaric acids are practically identical with those for 

 HC1 and HNO 3 in Fig. 19. That for acetic acid is a little higher owing possibly 

 to some specific and secondary effect of this acid on the cohesion of the jelly. 



Acetic acid gives an increasing amount of swelling, but it must 

 be remembered that 1 M acetic acid had to be used to bring the 

 pH of the gelatin to 3.0, and it is not impossible that in this case 

 the high concentration of undissociated acid caused a secondary 



