SWELLING 193 



the gelatin. This is, however, as a rule, not the case. Only 

 the valency but not the nature of the ion in combination with 

 gelatin influences the swelling of gelatin. Thus, at the same 

 temperature, at the same pH, and the same concentration 

 of originally isoelectric gelatin, the swelling of gelatin chloride, 

 nitrate, trichloracetate, oxalate, tartrate, phosphate, citrate, 

 etc., is approximately the same, while that of gelatin sulphate is 

 considerably lower. The swelling of Li, Na, K, and NH 4 

 gelatinate is also practically the same at the same pH and the 

 same concentration of originally isoelectric gelatin, but the 

 swelling of Mg, Ca, and Ba gelatinate is considerably less (see 

 Chap. V). 



It was shown in Chap. V that the same valency effect which 

 exists in regard to osmotic pressure exists also in regard to swell- 

 ing, and the theoretical discussion given in the preceding 

 chapter for this valency effect in the case of osmotic pressure 

 covers also the similar effect in the case of swelling. 



In the case of casein-acid salts, which are less soluble than 

 gelatin-acid salts, the nature of the anion is not without influence 

 on the cohesive forces. Thus casein trichloracetate is practically 

 as insoluble as casein sulphate, and neither of the two salts is 

 capable of swelling; while the more soluble casein chloride and 

 casein phosphate are capable of swelling. In the latter case the 

 valency rule also holds since the degree of swelling is practically 

 the same for casein phosphate and casein chloride, at the same 

 pH temperature and concentration of originally isoelectric 

 casein. 1 The valency rule holds wherever colloidal behavior is 

 concerned, since colloidal behavior is only the consequence of the 

 Donnan equilibrium and the equilibrium equation is only con- 

 cerned with the sign and valency of the ion. The problems of 

 solubility and of cohesion have only an indirect connection with 

 colloidal behavior, and the fact that solubility and cohesion depend 

 upon the specific nature of the ion (in addition to its sign of charge 

 and valency) is not in conflict with the other fact that in the truly 

 colloidal phenomena only the sign of charge and valency of an 

 ion are concerned. 



At the isoelectric point gelatin is practically not ionized and 

 there can therefore be no Donnan equilibrium. Yet when dry 



1 LOEB, J., and LOEB, R, F., J, Gen. Physiol., vol. 4, p. 187, 1921-22. 

 13 



