JACQUES LOEB 105 



Viscosity measurements were made every 5 or 10 minutes. Fig, 5 

 gives the result. The lower the pH the less the viscosity increases 

 on standing. 



There exist, therefore two different and to some extent antagonistic 

 effects of acid on the viscosity of gelatin solution. One effect is due 

 to the swelling of the submicroscopic particles of solid gelatin already 

 present in the solution at the time the acid is added. This effect 

 is regulated by the Donnan equilibrium and increases when (not too 

 much) acid is added to the isoelectric gelatin solution. The second 

 effect consists in retarding the formation of solid particles of jelly 

 on standing. This inhibitory effect of the acid seems to increase steadily 

 with the hydrogen ion concentration of the solution without going 

 through a maximum at a pH above 1.8. 



If we now return to the discussion of the curves in Fig. 2 we may 

 say that the results in that part of the curves which belongs to the 

 abscissae of pH above 3.8 is the expression of the fact that that part of 

 the viscosity which is due to the gelatin in solution had undergone an 

 increase during the hour the solution had been standing at 20° after 

 having been heated to 45°C.; and that the increase caused in the 

 viscosity of the liquid gelatin was a maximum 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 equilibrium 

 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. of powdered gelatin was added to 100 cc. of a 0.5 per cent 

 gelatin chloride solution of different pH. The different beakers 

 containing these mixtures were kept for 3^ hours at 20°C. 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 difference followed the Donnan equilibrium 

 equation (Table I), 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 wc sei)arate 



