OSMOTIC PRESSURE AND VISCOSITY 



241 



had undergone an increase during the hour the solution had been 

 standing at 20C. after having been heated to 45C.; and that the 

 increase caused in the viscosity of the liquid gelatin was a maxi- 

 mum at the isoelectric point, being almost zero at a pH below 3.4; 

 while the addition of acid had the opposite effect on the solid 

 granules of gelatin, since their volume was increased according 

 to the rules of the Donnan equilibrium. 



It is necessary that we convince ourselves that a Donnan equi- 

 librium exists when particles of solid gelatin are suspended in a 

 solution of gelatin. That this is actually true was shown in the 

 following way: 0.5-gm. doses of powdered gelatin were added to 

 100 c.c. of 0.5 per cent gelatin chloride solutions of different pH. 

 The different beakers containing these mixtures were kept for 3^ 

 hours at 20C. The mass was then filtered through cotton wool 

 and the pH of the filtrate (0.5 per cent gelatin solution) and of the 

 solid gelatin granules were determined, that of the latter after 

 they had been melted. It was found that the pH of the gelatin 

 granules was higher than that of the solution and that the differ- 

 ence followed the Donnan equilibrium equation (Table XLIV), 

 though the result was slightly irregular owing to the fact that it 

 is impossible to free the suspended particles of gelatin completely 

 from the supernatant liquid. When we separate a gelatin solu- 

 tion from water by a collodion membrane we have two equilib- 

 ria, one across the membrane caused by the protein ions on one 

 side of the membrane; and a second one inside the protein solu- 

 tion caused by the solid particles of jelly. 



TABLE XLIV 



The reciprocal relation between viscosity and osmotic pressure 

 of protein solutions disposes of the attempt to explain the influ- 

 ence of electrolytes on the physical properties of protein solu- 

 tions on the basis of the micella or aggregate theory. We have 

 seen that both the osmotic pressure of a gelatin chloride solution 

 as well as its viscosity are depressed when a neutral salt is added 



16 



