HISTORICAL INTRODUCTION 15 



the ion have no influence as long as no constitutional change in 

 the protein molecule occurs. 



This fact established a complete harmony between the results 

 of the titration experiments and the influence of ions on the phys- 

 ical properties of gelatin. In the titration experiments it had 

 been found that at a hydrogen ion concentration of above 2 X 

 10~ 5 N weak dibasic or tribasic acids generally combine with a 

 protein as if they were entirely or chiefly monobasic. Hence, the 

 anions of the protein salts formed with these weak dibasic or 

 tribasic acids, e.g., phosphoric, citric, tartaric, succinic, were 

 monovalent, and it was found that the osmotic pressure or 

 viscosity of solutions of protein phosphates were the same as 

 those of protein chlorides for the same hydrogen ion concentration 

 and the same concentration of originally isoelectric protein. 



On the other hand, the titration experiments showed that the 

 anion of protein sulphate is dibasic and it was found that the os- 

 motic pressure and viscosity of protein sulphate is less than one-half 

 of that of protein chloride or phosphate or succinate, etc., at the 

 same hydrogen ion concentration and the same concentration 

 of originally isoelectric protein. 1 



In this way the influence of ions on the physical properties of 

 proteins, especially in the case of gelatin, turned out to be in 

 harmony with the results of titration experiments. In the case of 

 gelatin and apparently also crystalline egg albumin, only the 

 valency but not the nature of the ion in combination with the 

 protein influences its properties. The statements to the con- 

 trary were due to two errors, first and foremost, the failure to 

 measure the hydrogen ion concentration of the protein solutions, 

 and second, the confusion of phenomena of solubility with phe- 

 nomena of colloidal behavior. 



5. THE AGGREGATION HYPOTHESIS 



It was perhaps not very fortunate for the development of a 

 theory of colloids that the attention of investigators was focussed 

 especially on the phenomena of precipitation. Since precipita- 

 tion is due to an aggregation of particles it over-emphasized the 

 significance of aggregate formation. This led, as we have seen, 

 to the erroneous idea that proteins do not combine stoichiome- 



1 LOEB, J., J. Gen. PhysioL, vol. 3, pp. 85, 247, 1920-21. 



