CHAPTER XI 



SWELLING 



Procter (1914) and Procter and Wilson (1916) applied 

 Donnan's equilibrium theory to the explanation of the swelling 

 of gelatin in acid. According to these authors, the force which 

 causes the entrance of water and hence the increase of volume in 

 a solid block of gelatin in acid is osmotic, and the opposing force 

 which limits the swelling is the force of cohesion between the 

 gelatin molecules or ions constituting the framework inside of 

 which the water is occluded. These cohesive forces thereby play 

 the same role in the swelling equilibrium as does the hydrostatic 

 pressure on the membrane in the experiments on osmotic pressure. 



The protein ions constituting a jelly of gelatin chloride cannot 

 diffuse and hence, according to Procter and Wilson, can exercise 

 no measurable osmotic pressure, while the chlorine anions in 

 combination with them are retained in the jelly by the electro- 

 static attraction of the gelatin ion but exert osmotic pressure. 

 This difference in the diffusibility of the two opposite ions of the 

 gelatin chloride gives rise to the condition leading to the establish- 

 ment of Donnan's membrane equilibrium. It is immaterial 

 for this equilibrium whether the diffusion of dissolved protein 

 ions is prevented by a collodion membrane, or whether it is pre- 

 vented by the forces of cohesion between the gelatin ions of a 

 solid gel. If x be the concentration of the H and Cl ions in the 

 outside solution, y the concentration of the free H and Cl ions in 

 the solid gel, and z the concentration of Cl ions in combination 

 with gelatin, the Donnan equilibrium is expressed by the equation 



x 2 = y(y + z) 

 and the osmotic force e for the absorption of water by the gel is 



e = 2y + z - 2x 



The reader will notice that this is the formula applied later by 

 the writer to osmotic pressure. 



189 



