98 OSMOTIC PRESSURE AND VISCOSITY 



particle of gel must cause a Donnan equilibrium to be established 

 between the solid particles and the solution inside the bag. 



If we assume that certain protein solutions contain two different 

 constituents, namely, first, isolated ions and molecules, and second, 

 isolated submicroscopic particles capable of occluding water, these 

 two constituents of such solutions must play a different role in osmotic 

 pressure and in viscosity. The characteristic influence of electrolytes 

 on the osmotic pressure of protein solutions must be due to the isolated 

 protein ions since these alone can bring about the Donnan equilibrium 

 across the membrane ; while the similar influence of electrolytes on the 

 viscosity of protein solutions must be due to the solid submicroscopic 

 particles since these alone are capable of swelling and of giving rise to 

 a Donnan equilibrium between each individual particle and solution .^ 

 If this inference is correct, it should be possible to demonstrate the 

 existence of a reciprocal relation between the viscosity and the 

 osmotic pressure of those protein solutions which contain isolated 

 protein ions as well as solid protein particles capable of occluding 

 water; e.g., gelatin or casein solutions. It is the purpose of this 

 paper to show that this reciprocal relation between the osmotic 

 pressure and viscosity of such protein solutions actually exists. 



Fig. 1 shows that the osmotic pressure of a 1 per cent solution of 

 originally isoelectric gelatin diminishes the more, the more we replace 

 the dissolved gelatin by small granules of powdered gelatin. The 

 ordinates of the upper curve represent the values of the osmotic 

 pressure of a 1 per cent solution of originally isoelectric gelatin at 

 different pH, the pH serving as abscissas of the curves. The acid 

 used was HCl, and the curve is the usual one given repeatedly in 

 the writer's preceding publications. At the beginning of the experi- 

 ment the gelatin solution was rapidly heated to a temperature of 

 45''C. and rapidly cooled to 20°C. and then kept at that temperature 

 throughout the entire experiment. The pH is that of the gelatin 

 solution at the end of the experiment. 



The middle curve represents an experiment in which 0.5 gm. of 

 the isoelectric gelatin in solution was replaced by 0.5 gm. of isoelectric 

 powdered gelatin. The latter did not contribute to the osmotic 



2 Locb, J., 7. Gen. Physiol., 1921-22, iv, 73. 



