JACQUES LOEB 669 



of equilibrium the relative distribution of HCl inside and outside the 

 solid block of gelatin chloride is determined by the following equation : 



x^ = y {y + z) 



where x is the concentration of H ions (and of CI ions) outside the 

 gelatin, y the concentration of the H (and CI) ions of the free HCl 

 inside the gelatin, and z the concentration of the CI ions in combina- 

 tion with the gelatin ions. Since all quantities are positive, it 

 follows that X must be greater than y; i.e., the concentration of 

 free HCl in the outside solution must be greater than in the gel. 

 The writer has shown that exactly the same happens when we sepa- 

 rate a solution of gelatin chloride from pure water by a collodion 

 membrane. 



This difference in the concentration of acid on the two sides of the 

 membrane leads to a difference in p.d. at the boundary of the mem- 

 brane.^ 



This can be proved by the fact that it is possible to calculate 

 the P.D. of the system 



outside solution 

 (free from gelatin) 

 concentrated HCl 



collodion 

 membrane 



inside solution 



(gelatin chloride) 



dUute HCl 



+ 



with a fair degree of accuracy on the basis of Nernst's well known 

 logarithmic formula from the difference of the hydrogen ion con- 

 centration on the opposite sides of the membrane. On the basis of 

 Nernst's formula for concentration cells the p.d. is at a temperature 



Q 



of 24°C. 0.059 log — , where Ci is the concentration of hydrogen ions 

 C2 



inside the gelatin solution and C2 the hydrogen ion concentration 



outside the gelatin solution. Hence we may substitute the value 



C 



pH inside minus pH outside for the value log — . (The term pH 



C2 



inside means the pH in the gelatin solution and the term pH outside 

 means the pH in the outside solution with which the gelatin solution 

 is in equilibrium.) This paper intends to show that if we multiply 



5 Loeb, J., /. Ge7i. Physiol., 1920-21, iii, 247. 



