HISTORICAL INTRODUCTION 17 



of colloids prevailed. The quantitative explanation of the col- 

 loidal behavior of proteins to be given in this book rests on the 

 proof that they form true ionizable salts with acids and alkalies. 



6. PAULI'S HYDRATION THEORY 



Laqueur and Sackur, l in studying the influence of the addition 

 of different quantities of NaOH to a given mass of casein, 

 assumed correctly that the two substances combined to form 

 sodium caseinate. The viscosity of the sodium caseinate solu- 

 tion was high and it varied in a peculiar way with the quantity of 

 NaOH added to the casein. When little NaOH was added, the 

 viscosity increased at first with an increase in the quantity of the 

 NaOH added until a maximum was reached, when the addition of 

 more NaOH diminished the viscosity again. This again is a 

 fundamental fact which has since been confirmed for the influ- 

 ence of acids and alkalies not only upon the viscosity but also 

 upon other properties of proteins and which holds not only for 

 casein but apparently for all proteins. 



Laqueur and Sackur explained their results on the basis of 

 Reyher's 2 experiments on the viscosity of solutions of fatty 

 acids. Reyher had found that the viscosity of solutions of salts 

 of the fatty acids is greater than that of solutions of fatty acids 

 themselves; and since the salts of the fatty acids undergo elec- 

 trolytic dissociation to a much greater extent than the acids it was 

 assumed that the increase in viscosity is determined chiefly by 

 the ionization. Laqueur and Sackur made the same assumption 

 for the casein solutions, attributing the high viscosity of casein 

 solutions to the casein ions, and they support their assumption 

 by the fact that the addition of little NaOH to casein at first 

 increases the viscosity until a maximum is reached and that the 

 addition of more NaOH diminishes the viscosity again. A dimi- 

 nution of viscosity could also be produced by the addition of 

 neutral salt to the solution of Na caseinate. Laqueur and Sackur 

 assume that this drop in the viscosity is caused by a lowering of 

 the degree of electrolytic dissociation of the Na caseinate by the 

 Na ion of the NaOH or NaCl added in excess. 



1 LAQUEUR, E. and SACKUR, O., Beitr. chem. Physiol. u. PaihoL, vol. 3, 

 p. 193, 1903. 



2 REYHER, R., Z. physik. Chem., vol. 2, p. 744, 1888. 



