JACQUES LOEB 677 



Procter's osmotic theory of swelling which assumes that the swelling" 

 of a gel is due to the osmotic pressure inside the gel, the surface of the 

 gel acting as a membrane impermeable to gelatin ions but permeable 

 to the H or CI or salt ions. On this assumption we should expect 

 the establishment of a Donnan equilibrium between the liquid inside 

 the gel and the outside liquid, and Procter has proved that this is the 

 case. We should also expect a p.d. at the surface of the gel between 

 the solution inside and outside the gel, due to the Donnan equilib- 

 rium, and we should, moreover, expect that the p.d. could be calculated 

 from the value of pH inside minus pH outside as described for the 

 experiments on osmotic pressure. This expectation is confirmed 

 except that the results are liable to be less regular than in the case of 

 the osmotic pressure experiments. This is probably due to acci- 

 dental errors, one being possibly that some of the gel dissolves in the 

 outside solution so that the outside solution is no longer free from 

 gelatin. Such an error does not exist when we separate a gelatin 

 solution from the outside solution by a collodion membrane. 



Our method was as follows. 1 gm. of powdered gelatin of pH 7.0 

 was brought to the isoelectric point by treatment with m/128 acetic 

 acid and subsequent washing with cold H2O as described in previous 

 papers. The doses of powdered wet isoelectric gelatin were then put 

 into 200 cc. of m/128 HCl made up in different concentrations of 

 NaCl varying from to m/8, and the degree of swelling was measured 

 in terms of the height of a column of the powdered gelatin in 100 cc. 

 graduates after equilibrium was established (after two hours). From 

 this the volume of the isoelectric gelatin was deducted. Fig. 3 shows 

 that the swelling diminished with the quantity of salt added. The 

 mass was put on a filter and allowed to drain thus separating the 

 gelatin from the supernatant liquid. The gelatin was then melted 

 and its pH determined electrometrically. This gave us the pH 

 inside, and by determining the pH of the outside solution the values 

 of the pH inside minus pH outside were ascertained. The liquid 

 gelatin was then poured into cylinders with a small bent side tube 

 attached, which was also filled with gelatin. The gelatin was then 

 cooled and allowed to set to a jelly. The p.d. between the solid jelly 

 and the outside solution was then determined by pushing one elec- 

 trode into the gel while the other electrode was introduced into the 



