670 DONNAN EQUILIBRIUM AND MEMBRANE POTENTIALS 



the value of pH inside minus pH outside by 59 we can calculate with 

 a fair degree of accuracy the p.d. observed at 24°C. in millivolts when 

 equilibrium between the gelatin chloride solution and the outside 

 solution is established. 



II. METHODS. 



The potential differences were determined with the aid of a Comp- 

 ton electrometer giving a deviation of about 2 mm. on the scale for 

 1 millivolt at a distance of about 2 m. The gelatin solution was 

 inside the collodion bag closed with a rubber stopper through which 

 a funnel was introduced ; the funnel was filled high enough with liquid 

 to permit the electrode to dip into the gelatin solution. The col- 

 lodion bag containing the solution of gelatin-acid salt (e.g., gelatin 

 chloride) dipped into the water with which the gelatin solution was 

 in equilibrium. The water, therefore, contained always free acid 

 and in certain experiments also salt solution when the nature of the 

 experiment demanded this. The second electrode was introduced 

 into this outside watery solution. Calomel electrodes with saturated 

 XCl solution were used and the saturated KCl solutions were brought 

 into contact with the outside and inside solutions through glass 

 tubes. The ends of these tubes which dipped into the inside and 

 outside solutions were drawn out into capillaries and bent upwards to 

 prevent the influence of gravity on the diffusion of the saturated 

 KCl solution into the inside and outside solutions. The only poten- 

 tial differences existing in this system were those on the opposite 

 sides of the membrane. 



///. The Influence of Neutral Salts on the Potential Difference between 

 Gelatin Chloride Solutions and Outside Solutions. 



1 gm. of isoelectric gelatin was made into a 1 per cent solution by 

 either dissolving it in H2O or in a solution of NaCl differing in molec- 

 ular concentration from m/4,096 to 1 m. To every solution so 

 much HCl was added that the pH of the solution was 3.5. Collodion 

 bags of a volume of about 50 cc. were filled with this solution, each 

 collodion bag being connected with a glass tube serving as manom- 

 eter, as described in preceding publications. These collodion bags 



