SWELLING 297 



counted for by Procter in much the same manner as that out- 

 lined above (76). He pictures the gelatin acid-compound as a 

 coherent mass from which the gelatin molecules cannot diffuse or 

 separate and which in most respects behaves like a single enor- 

 mous complex molecule. It is reasonable, he considers, to visual- 

 ize it as a felted mass of amino-acid chains held to each other by 

 attractions which possibly attach only their ends, but freely 

 admitting the passage of liquid between them. He assumes, in 

 accordance with our former conceptions of the mode of forma- 

 tion and ionization of protein salts, that the compound yields 

 acid anions, but these anions, although diffusible, are held within 

 the mass by electrostatic forces, since they cannot pass beyond 

 the sphere of attraction of their companion colloid ions which 

 form the jelly and which are therefore immobilized. The only 

 way, therefore, in which the osmotic pressure of the anions can 

 take effect is, not by their movement but by the 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 opera- 

 tive force compelling movement of the water would, in ultimate 

 analysis, be the electrostatic tension which prevents the acid 

 anions from moving outwards into the surrounding solvent. 

 There should thus be a measurable potential difference between 

 the gelatin jelly and the external medium. This potential differ- 

 ence has been sought for by Ehrenberg who was unable to detect 

 any measurable potential between the interior of a jelly and the 

 external medium (23). 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 

 acid anions and the free acid itself could not simultaneously 

 be equal in concentration within and without the jelly. Our 

 more recent views regarding the mode of formation and ionization 

 of protein salts reconcile both these difficulties, however, for, 

 since we may assume 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 gel- 

 atin immersed in neutral water, to the osmotic pressure of the 

 colloid particles themselves, which, being unable to penetrate 



