MEMBRANE POTENTIALS 145 



equilibrium. The term derived from the equilibrium equation, 

 log (1 -\ ) makes the P.D. a function of z and y, i.e., that ion 

 which has the opposite sign of charge to the protein ion. 

 THE INFLUENCE OF THE CONCENTRATION OF PROTEIN ON THE P.D. 



While the addition of neutral salt depresses the P.D. of protein 

 solutions across a membrane (as it depresses all the other prop- 

 erties) the addition of protein has the opposite effect, increasing 

 the P.D. (as it increases also the other properties). This influ- 

 ence of the concentration of the protein follows mathematically 



from the equilibrium equation. Since P.D. = -=- log (1 + -) 



* y 



millivolts, it is obvious that if y remains constant (i.e., if no salt 

 is present and the pH remains the same) while z increases as a 

 consequence of the increase of the concentration of protein, the 

 P.D. must rise with the concentration, and this was found to be 

 the case. 



Collodion bags, connected with glass manometers in the way 

 described, containing 50 c.c. of different concentrations of origi- 

 nally isoelectric gelatin varying from 0.125 per cent to 2 per cent 

 and containing enough H 3 PO 4 to bring the gelatin solution to a 

 pH of 3.5 were put into beakers containing 350 c.c. H 3 PO 4 

 solution of pH 3.5. In order to prevent dilution of the protein 

 solution through osmosis, the glass manometers were filled at 

 the beginning of the experiment with the same gelatin phosphate 

 solution as that contained in the collodion bag, to that height 

 which the osmotic pressure measured in preceding experiments 

 amounted to. After about 20 hours the pH in the inside and the 

 outside solutions and the P.D. across the membrane were meas- 

 sured. Some of the experiments were made in duplicate (Table 

 XXI). 



It is obvious, first, that the P.D. increases with the concentra- 

 tion of gelatin, and second, that the increase of P.D. observed 

 agrees quantitatively with the increase calculated on the assump- 

 tion of the validity of Donnan's theory. 



THE P.D. OF SOLUTIONS OF CRYSTALLINE EGG ALBUMIN 



The experiments mentioned thus far had all been done on 



gelatin. It was of importance to determine whether or not 

 10 



