MEMBRANE POTENTIALS 127 



pressure and P.D., the osmotic pressures were observed in all the 

 experiments used for measuring the influence of pH on the P.D., 

 and Fig. 14 gives the osmotic pressures. A comparison of the two 

 figures for P.D. (Fig. 41) and for osmotic pressure (Fig. 14) shows 

 the following similarities: Both sets of curves rise from the 

 isoelectric point with a lowering of the pH until they reach a 

 maximum; this maximum is, however, not identical in the two 

 cases. For the P.D. it varies between 3.6 and 4.0, while for os- 

 motic pressure it lies near 3.5. With a further fall in pH both 

 sets of curves show approximately the same steep drop. 



The second point of similarity is the influence of valency. The 

 curves for the P.D. (Fig. 41) are practically the same for gelatin 

 chloride and gelatin phosphate, and are but slightly lower in the 

 case of gelatin oxalate, while the curve for the P.D. is considerably 

 lower in the case of gelatin sulphate. The same is true for the 

 osmotic pressure curves (Fig. 14). 



If these characteristic curves are exclusively determined by 

 Donnan's membrane equilibrium it should be possible to show 

 that the variation of the observed P.D. with pH is accompanied 

 by a parallel variation of the value pH inside minus pH outside 

 and that the agreement between these two sets of values is as 

 perfect as the accuracy of the measurements permits. Tables 

 XI, XII, and XIII show that this is true. The upper two hori- 

 zontal rows give the pH inside and outside, the third horizontal 

 row gives the difference, pH inside minus pH outside, and the 

 fourth row gives the P.D. calculated in millivolts by multiplying 

 the values pH inside minus pH outside by 59. The last hori- 

 zontal row gives the observed P.D. in millivolts. The agree- 

 ment between observed and calculated P.D. is sufficiently close 

 to permit us to say that the characteristic curves representing the 

 influence of the pH on the P.D. are a consequence of the Donnan 

 equilibrium. 



THE EXPLANATION OF THE P.D. CURVE 



Figure 41 shows that the P.D. of gelatin-acid salts is a minimum 

 at the isoelectric point, that it rises rapidly with the increase in 

 the hydrogen ion concentration until reaching a maximum at pH 

 about 4.0 to 3.8, and then drops again with a further increase of 



