COLLOIDAL SUBSTANCES 279 



also colloidal behavior in regard to viscosity when the tempera- 

 ture is not too high, and this is due, as we have seen, to the exist- 

 ence in such solutions of submicroscopic particles of gel in which 

 the diffusion of the protein ions is prevented by the forces of 

 cohesion between the protein ions forming the gel. The degree 

 of swelling, and the relative volume occupied by these particles 

 in the solution is regulated by the Donnan equilibrium and, hence 

 the viscosity of a gelatin solution is also regulated by the Donnan 

 equilibrium ; in other words, the viscosity of gelatin solutions has 

 the peculiarities of colloidal behavior. Only in one respect do 

 the aqueous solutions of gelatin behave as if gelatin were in the 

 crystalloidal state, namely in respect to the stability of solutions. 

 It requires high concentrations of salts to precipitate gelatin 

 from its aquoues solutions and the sign of charge of the precipitat- 

 ing ion has no relation to the sign of charge of the protein ion. 

 Of course, it remains still to be proved that gelatin exists in 

 aqueous solution essentially in the form of isolated ions or molecules 

 and almost exclusively so if the temperature is above 35C. This 

 proof can only be furnished if the calculations of the molecular 

 weight from osmotic pressure measurements are supported by 

 other measurements, especially by determinations of the chemical 

 constitution of the gelatin molecule. Dakin's 1 analysis leads 

 to a molecular weight which is quite compatible with the results 

 from the osmotic pressure determinations (see Chap. X). 



We have seen that solutions of gelatin in alcohol-water mixtures 

 behave like suspensions inasmuch as they can be precipitated 

 by low concentrations of salt and inasmuch as the precipitating 

 ion has now the opposite sign of charge to that of the protein ion. 

 When the gelatin solution is in this state, it differs in two respects 

 from a gelatin solution in pure water: it has a comparatively 

 low viscosity, and it no longer" sets to a gel. It is also, as a rule, 

 opalescent. The change in viscosity can be shown in the follow- 

 ing way. 2 



To 1 gm. of isoelectric gelatin enough HC1 is added so that in 

 a 1 per cent solution in water the pH would be about 3.0. This 

 gelatin is dissolved in mixtures of water and alcohol, heated 

 rapidly to 45C., and cooled rapidly to 15C. The time of 



1 DAKIN, H. D., J. Biol. Chem., vol. 44, p. 499, 1920. 



2 The following experiments have not yet been published. 



