JACQUES LOEB 593 



and a negative NO3 Ion. On this assumption, the double layer at the 

 boundary of the protein film and a solution would appear to be ionic 

 in character, the positive stratum of the double layer being formed 

 by the protein cations of the surface films of the membrane, while the 

 mobile, watery stratum of the double layer contains the anions of 

 the protein-acid salt. On this assumption it is clear why the hydrogen 

 ion concentration required to make the membrane positive varies in 

 the same sense as the isoelectric point of the protein used and why it 

 is always higher than that of the isoelectric point, since a certain part 

 of the surface of the protein film must be transformed into protein- 

 acid salt before the membrane assumes a positive charge. At a 

 hydrogen ion concentration below that of the isoelectric point the 

 protein film is negatively charged since the protein exists here in the 

 form of metal proteinate dissociating into a protein anion and a 

 metal ion, the latter forming the positive watery stratum of the 

 double layer, while the membrane owes its negative charge to the 

 protein anion. 



It does not follow, however, that this transformation of protein 

 into protein-acid salt is the only, or even the essential, condition 

 for the reversal of the sign of charge of the double layer by acid. 

 Since acid can bring about a reversal in the electrical double layer 

 of water bounded by air (or possibly even in a vacuum) , it is obvious 

 that the modification of the surface layer of the water which is in 

 contact with the membrane may also play a role in the reversal of 

 the sign of charge of the protein film. This possibility has to be 

 considered also in view of the fact that trivalent or tetravalent 

 cations can bring about a reversal in the sign of charge of a mem- 

 brane treated with protein, even when the hydrogen ion concentra- 

 tion excludes a salt formation between protein and trivalent cation. 

 These facts will be discussed in a subsequent paper. 



SUMMARY. 



1. It had been shown in previous papers that when a collodion 

 membrane has been treated with a protein the membrane assumes a 

 positive charge when the hydrogen ion concentration of the solution 



