770 AGGREGATES AND MEMBRANE POTENTIALS 



same weight of powdered gelatin (without changing the pH) the 

 osmotic pressure of the solution is lowered, and this lowering of the 

 osmotic pressure increases the more the more of the dissolved gelatin 

 is replaced by powdered gelatin.^ This shows that only that part of 

 the protein which is in true solution, i.e. which does not exist in the 

 form of larger aggregates, influences the osmotic pressure of a 

 solution. The question arose whether or not powdered particles of 

 gelatin would influence the membrane potential. To obtain an 

 answer to this question the following experiment was made. 



Powdered gelatin going through the meshes of sieve 30 but not 

 through 60 was rendered isoelectric in the way previously described. 

 Part of this isolectric gelatin was melted and the melted and powdered 

 isoelectric gelatin were mixed. The total weight of isoelectric 

 gelatin in 100 cc. solution was always the same, but the proportion 

 of powdered to dissolved gelatin varied as indicated in Table I. 

 Thus when the weight of the powdered gelatin was 0.5 gm., the weight 

 of the dissolved gelatin was about 0.3 gm.; when the weight of the 

 powdered gelatin was 0.2 gm., that of the dissolved was 0.6 gm., etc. 

 100 cc. of the mixture contained 8 cc. of 0.1 n HCl, and the pH of 

 the gelatin solution (at the equilibrium condition to be described) 

 was between 3.2 and 3.3. At this pH the osmotic pressure of a gela- 

 tin solution is nearly a maximum. A 1 per cent solution of gelatin 

 chloride has an osmotic pressure of about 450 mm. water at pK 3.4. 

 Only a small part of this osmotic pressure is due to the osmotic 

 pressure of the protein particles themselves; the rest of the observed 

 osmotic pressure of gelatin chloride solutions of pH 3.4 is due to the 

 excess concentration of the crystalloidal ions inside the collodion bag 

 (in which the osmotic pressure of the gelatin solution is measured) 

 over that of the outside aqueous solution free from gelatin, and 

 this quantity is determined by the Donnan equilibrium.'* 



Collodion bags of a content of about 50 cc. were filled with these 

 suspensions and closed with rubber stoppers perforated with glass 

 tubes serving as manometers to measure the osmotic pressure.'* 



' Loeb, J., Proteins and the theory of colloidal behavior, New York and London, 

 1922, 232; /. Gen. Physiol., 1921-22, iv, 97. 



"* Loeb, J., Proteins and the theory of colloidal behavior, New York and Lon- 

 don, 1922, 169; /. Gen. Physiol., 1920-21, iii, 691. 



