JACQUES LOEB 221 



A comparison of Figs. 3 and 4 shows that the two sets of curves, 

 that for the p.d. (Fig. 3) and for the values pH inside minus pH out- 

 side (Fig. 4) when plotted over the initial pH as abscissae, have cer- 

 tain features in common. Both sets of curves have a minimum at 

 or near the isoelectric point of gelatin (pH 4.7). They both rise to 

 a maximum at or not far from pH 4.2, and then drop again when the 

 initial pH of the solution drops further. This parallelism suggests 

 that this form of the p.d. curve is determined by the differences in 

 the hydrogen ion concentration produced on the opposite sides of 

 the membrane in conformity with Donnan's theory of membrane 

 equilibrium. 



//. Potential Differences in Collodion Membranes Free from Protein. 



The experiments just described were repeated with collodion mem- 

 branes which had not been treated with a protein. The difference 

 between the p.d. in this case (Fig. 5) and the p.d. observed in the 

 case of collodion membranes coated with a fihn of gelatin (Fig, 3) 

 is striking. The method of experimentation was the same in the 

 case of the collodion bags free from gelatin as in the experiments of 

 the preceding chapter. The initial concentration of HNO3 was 

 always the same inside and outside. The inside solution, however, 

 contained in addition to the acid a solution of m/256 of one of the 

 four salts, CeCls, CaCl2, NaCl, and Na2S04. After 1 hour the p.d. 

 inside and outside was measured. 



We notice that the p.d. is no longer a minimum at or near 

 pH=4.0 and we can therefore be certain that this feature of 

 the curves in Fig. 3 was due to the gelatin. The curves in Figs, 

 3 and 5 have, however, one feature in common; namely, that the 

 P.D. increases with the increasing valency of the cation and dimin- 

 ishes with the increasing valency of the anion. This feature is, 

 therefore, not a specific function of the film of gelatin. 



Since the water is positively charged even in the presence of acid 

 when the membrane consists of collodion free from protein, it is to 

 be expected that only the Na2S04 solution should attract the water, 

 while no such attraction should occur in the case of m,'256 CeCls or 

 m/256 CaCLi. Fig. 6 shows that this is true. 



