HISTORICAL INTRODUCTION 23 



brane, and the results were in general agreement with Donnan's 

 theory. They also investigated a liquid membrane, namely, amyl 

 alcohol, and the electrolytes employed were KC1 and LiCl. 



."So far as the preliminary experiments go, the equilibrium concentra- 

 tion of the Li and Cl ions and the undissociated part of the electrolyte 

 agree with Donnan's theory." 



We shall see that Donnan's theory explains the influence of 

 electrolytes on the physical properties of proteins. He foresaw 

 the bearing which his theory was likely to have for colloid chem- 

 istry and physiology, as is shown by the following remarks. 



"In this paper an attempt is made to describe ion equilibria which 

 are bound to occur when certain ions (or their corresponding non- 

 dissociated salt) cannot diffuse through a membrane. Such equilibria 

 possess a great importance for the theory of dialysis and of colloids 

 as well as for the mechanism of the cell and for general physiology." 



As far as the writer is aware, Procter and J. A. Wilson were the 

 only authors who attempted the application of Donnan's theory 

 to colloidal problems. 



Procter 1 proposed in 1914 an ingenious theory of swelling 

 based on Donnan's membrane equilibrium. According to this 

 theory the force which causes the entrance of water into the gel 

 and thus determines the swelling is the osmotic pressure of the 

 excess of crystalloidal ions inside over that outside the gel, this 

 excess being caused by the Donnan equilibrium. The opposing 

 force which limits the swelling is the force of cohesion of the 

 colloidal particles. 



According to Procter, the gelatin ion constituting a jelly of 

 gelatin chloride cannot diffuse and hence can exercise no osmotic 

 pressure, while the chlorine 'anions in 'combination with them are 

 retained in the jelly by the electrostatic attraction of the gelatin 

 ion, but exert osmotic pressure. This difference in the diffusi- 

 bility of the two opposite ions of gelatin chloride gives rise to the 

 establishment of Donnan's membrane equilibrium. 



Procter put solid gelatin chloride into a watery solution of 

 HC1 and determined by titration the distribution of free HC1 

 inside tlie gel and outside at the time of equilibrium. In this 

 case there exists inside the gel free HC1 and gelatin chloride, out- 



1 PROCTER, H. R., J. Chem. Soc., vol. 105, p. 313, 1914. PROCTER, H. R. 

 and WILSON, J, A., J, Chem, Soc., vol. 109, p. 307, 1916. 



