JACQUES LOEB 773 



fusion in opposite directions through the membrane. This is the 

 osmotic pressure of the protein solution in Table I. The osmotic 

 pressure inside each particle of solid powdered gelatin results in 

 swelling, i.e. in an increase of the force of cohesion between the 

 molecules of the gel particle, and this effect does not appear in the 

 osmotic pressure of the solution. Only that part of the osmotic 

 forces in a protein solution appears in the form of hydrostatic pres- 

 sure which is directly or indirectly due to the isolated molecules of 

 the protein; and this hydrostatic pressure is diminished when part of 

 the protein in solution is replaced by aggregates or micellae of protein. 



II. 



When a solution of gelatin chloride containing solid granules of 

 gelatin is separated by a collodion membrane from an aqueous solu- 

 tion (free from protein) two different equilibria are established; one 

 across the membrane between the aqueous solution outside and the 

 gelatin solution inside the membrane, and a second one between the 

 solid granules of gelatin and the gelatin solution in which the granules 

 are suspended. At first thought it might seem strange that when solid 

 granules of isoelectric gelatin are suspended in a solution of gelatin 

 and HCl, there should arise a difference in the distribution of the H 

 and CI ions inside the solid granules and the surrounding gelatin solu- 

 tion. Yet this is the case, as Table II shows, and the reason is easily 

 understood. In the solid granules of gelatin the concentration of 

 protein molecules is much higher than in the weak solutions of gelatin 

 surrounding the granules, and if HCl is added the concentration of 

 gelatin ions must be higher inside the solid gelatin granules than in 

 the dilute gelatin solution in which the granules are suspended. It 

 follows from Donnan's theory that this difference in the concentra- 

 tion of protein ions inside the powdered particles and the solution 

 must give rise to a Donnan equilibrium; as a consequence of which 

 a p.D. must exist between the solid particles and the weaker gelatin 

 solution. '^ 



This consequence of the theory was confirmed by the following 

 experiment. Mixtures of a solution of isoelectric gelatin and 



^ Locb, J., Proteins and the theory of colloidal behavior, New York and 

 London. 1922, 145. 



