306 PROPERTIES CONFERRED BY COLLOIDAL CONSTITUENTS 



salts, that the compound yields acid anions (for example, chlorine ions 

 when the compound is gelatin hydrochloride), but that these anions, 

 although diffusible, are held within the mass by the electrostatic attrac- 

 tion of the oppositely charged ion which, being colloidal, cannot leave 

 the jelly in the company of its associate. The only way, therefore, in 

 which the osmotic pressure of the anions can take effect is not by their 

 own movement, but by the inward movement of water, resulting in 

 the swelling of the entire jelly-mass and its dilution by admixture with 

 the outside solution. 



Two very serious objections attach to this interpretation of the 

 phenomena. In the first place, as Procter himself has pointed out, 

 were this the actual mechanism of swelling, then the operative force 

 compelling movement of the water would, in ultimate analysis, be the 

 Electrostatic Tension which prevents the issuance of the inorganic 

 anions from the jelly into the solution. There should thus be a 

 measureable difference of electrical potential between the swollen 

 gelatin jelly and the surrounding medium. Such a difference of poten- 

 tial has not been found. 1 In the second place, as Procter also points 

 out, another difficulty lies in the fact that the condition which would 

 thus arise, would offer no equilibrium, since the concentration of the 

 inorganic anions and the free acid itself could never become simul- 

 taneously equal within and without the jelly; no matter what degree of 

 swelling and consequent dilution of the jelly may have occurred there 

 will still, ex hypothesi, be an excess of inorganic anions within the jelly. 



Our more recent views regarding the mode of formation and ionization 

 of Protein Salts reconcile both these difficulties, however, for, since we 

 now know that no inorganic ions, or at most a very small proportion, 

 are yielded by the protein acid compound, the swelling of the jelly 

 must be due, just as it is in the case of gelatin immersed in neutral water, 

 to the osmotic pressure of the colloidal particles themselves, which, being 

 unable to penetrate the colloidal network in which they are entangled, 

 necessarily compel the compensating migration of water. No electro- 

 static tension between the jelly and the external solution need be 

 assumed, because both ionic components of the protein salt are attract- 

 ing water by virtue of their osmotic pressure, and they are necessarily 

 present in the jelly in equimolecular concentration since neither of 

 them can leave it, even in the minimal quantity necessary to create 

 an electrostatic tension. Furthermore, since no inorganic anions are 

 yielded by the protein salt, simultaneous equality of concentration 

 of the uncombined acid and the acid anions within and without the 

 jelly will be simply assured by their equal and unhampered diffusion 

 into the jelly. The increased Swelling-capacity of gelatin in solutions 

 of acids or alkalies is merely the expression of the fact that the ionization 



1 Ehrenberg, R.: Biochem. Ztschr., 1913, 53, p. 356. Even if both of the ions 

 of the protein salt were able to issue from the jelly, a difference of potential would arise 

 from their unequal speeds of diffusion. See F. E. Bartell and C. D. Hocker: Jour. 

 Am. Chem. Soc., 1916, 38, pp. 1029 and 1036. 



