JACQUES LOEB 563 



and outside the gel, and that the curves expressing the influence of 

 neutral salts on the value of pH inside minus pH outside the gel, and 

 on the swelling run approximately parallel. 



SUMMARY AND CONCLUSION. 



1. It is well known that neutral salts depress the osmotic pressure, 

 swelhng, and viscosity of protein-acid salts. Measurements of the 

 p.D. between gelatin chloride solutions contained in a- collodion bag 

 and an outside aqueous solution show that the salt depresses the p.d. 

 in the same proportion as it depresses the osmotic pressure of the 

 gelatin chloride solution. 



2. Measurements of the hydrogen ion concentration inside the gela- 

 tin chloride solution and in the outside aqueous solution show that the 

 difference in pH of the two solutions allows us to calculate the p.d. 



HT C 



quantitatively on the basis of the Nernst formula E = In — if 



nF Co 



we assume that the p.d. is due to a difference in the hydrogen ion 



concentration on the two sides of the membrane. 



3. This difference in pH inside minus pH outside solution seems to 

 be the consequence of the Donnan membrane equihbrium, which only 

 supposes that one of the ions in solution cannot diffuse through the 

 membrane. It is immaterial for this equihbrium whether the non- 

 diffusible ion is a crystalloid or a colloid. 



4. When acid is added to isoelectric gelatin the osmotic pressure 

 rises at first with increasing hydrogen ion concentration, reaches a 

 maximum at pH 3.5, and then falls again with further fall of the pH. 

 It is shown that the p.d, of the gelatin chloride solution shows the 

 same variation with the pH (except that it reaches its maximum at 

 pH of about 3.9) and that the p.d. can be calculated from the difference 

 of pH inside minus pH outside on the basis of Nernst's formula. 



5. It was found in preceding papers that the osmotic pressure of 

 gelatin sulfate solutions is only about one-half of that of gelatin chlo- 

 ride or gelatin phosphate solutions of the same pH and the same con- 

 centration of originally isoelectric gelatin; and that the osmotic pres- 

 sure of gelatin oxalate solutions is almost but not quite the same 

 as that of the gelatin chloride solutions of the same pH and concentra- 



