774 



AGGREGATES AND MEMBRANE POTENTIALS 



powdered isoelectric gelatin were made so that 100 cc. always con- 

 tained 0.8 gm. of isoelectric gelatin in all. The proportion of solid 

 and liquid gelatin varied, however, in each case as indicated in 

 Table II. In each 100 cc. of the mixture were contained 8 cc. of 

 0.1 N HCl. The mixtures were kept for 2 hours at 20°C. and 

 frequently agitated to accelerate establishment of equilibrium be- 

 tween granules and solution. The solid powdered gelatin was then 

 separated from the supernatant liquid by filtration. 



TABLE II. 



Donnan Equilibrium between Particles of Powdered Gelatin and Gelatin in Solution. 



Powdered gelatin per 100 cc, gm. 

 Dissolved gelatin per 100 cc, gm. 



pH of powdered gelatin 



pH of supernatant gelatin solution. 

 pH solid minus pH liquid gelatin . . 



P.D. calculated, millivoUs . 

 p.D. observed, millivolts . . 



0.8 

 



26 



62 



0.64 



37.0 

 33.0 



The solid gelatin was melted and poured into the vessels described 

 on page 154 of a recent book by the writer, and the p.d. between 

 the solid and liquid gelatin was measured. The values are found in 

 Table II, showing that there exists a considerable p.d. between the 

 gelatin granules and weak solutions of gelatin, and that this p.d. 

 increases with the relative increase in the concentration of solid 

 gelatin, as was to be expected. 



Measurements of the pH in the solid and liquid gelatin showed 

 that this p.D. was determined by the Donnan equilibrium. ^ There 

 exists, however, a discrepancy between calculated and observed 

 values for the p.d., which requires further investigation. On the 

 whole, however, the figures seem to prove that when a suspension 

 of powdered protein in a weak gelatin solution is put inside a 

 collodion bag, the latter dipped into an aqueous solution free from 

 gelatin, two equilibria are established; namely, one between the solid 



^ The measurements of the P.D. between solid gelatin and solution are not as 

 accurate as the measurements between liquids across a membrane. 



