HISTORICAL INTRODUCTION 25 



In the first place, the application of Donnan's theory to the 

 behavior of proteins requires the proof that proteins form true 

 salts with acids and alkalies and that these salts dissociate 

 electrolytically into a protein ion and a crystalloidal cation or 

 anion. Such an assumption was in conflict with the adsorption 

 hypothesis accepted by the colloid chemists. Moreover, the 

 application of the Donnan theory to proteins tacitly implied 

 that only the valency and sign of charge should have an effect 

 on the proteins, while the nature of the ion should have no 

 effect; and this was in conflict with the belief in the Hofmeister 

 ion series. But even authors, like Robertson, who was a cham- 

 pion of the purely chemical conception of the behavior of proteins, 

 refused to accept Procter's theory of swelling. 



" There should be a measurable potential difference between the 

 gelatin jelly and the external medium. This potential difference has 

 been sought for by Ehrenberg who was unable to detect any measurable 

 potential between the interior of a jelly and the external medium." 1 



This gap has been filled by the writer's experiments, which 

 have demonstrated the existence of this potential. The writer 

 has not only been able to furnish support for Procter's theory 

 of swelling but has also been able to show that the potential 

 differences across a membrane separating a solution of a protein 

 salt from pure water fully support Donnan's theory. 2 When we 

 have a solution of a gelatin-acid salt with monovalent anion, 

 e.g., gelatin chloride (or gelatin phosphate) inside a collodion bag 

 which is dipped into pure water, the hydrogen ion concentration 

 as well as the anion concentration on the opposite sides of the 

 membrane are different when osmotic equilibrium is established. 

 The writer was able to show that the potential differences calcu- 

 lated from this difference of the concentration of ions on the 

 basis of Nernst's formula agree with the actually observed P.D., 

 and that the calculated P.D. is the same whether based on a 

 measurement of the difference in the concentration of the hydro- 

 gen ions or of the difference in the concentration of the chlorine 

 ions on the opposite sides of the membrane. This latter fact 



1 ROBERTSON, T. B., "The Physical Chemistry of the Proteins," p. 297, 

 New York, London, Bombay, Calcutta, and Madras, 1918. 

 2 LoEB, J., J. Gen. Physiol, vol. 3, p. 667, 1920-21. 



