OSMOTIC PRESSURE AND VISCOSITY 233 



there should exist a reciprocal relation between the viscosity and 

 the osmotic pressure of the solution since, the transformation of 

 the submicroscopic particles of solid jelly should lower the vis- 

 cosity and raise the osmotic pressure of a gelatin solution and 

 vice versa. It can be shown that this conclusion is supported by 

 observations on gelatin solutions. 



It was noticed in the preceding chapter that the viscosity of 

 solutions of gelatin chloride does not always increase on standing 

 but that it diminishes when the temperature exceeds a certain 

 limit. This was shown for a 2 per cent solution of gelatin chloride 

 of pH 2.7 in Fig. 57. The viscosity of such a solution increases 

 very rapidly on standing at 15C., much less rapidly at 25C., 

 but diminishes when kept at a temperature above 35C., and the 

 more rapidly the higher the temperature. This we assume tobe 

 due to the fact that at a temperature above 35C. the rate of 

 melting of submicroscopic particles of solid jelly exceeds the rate 

 of their formation from isolated ions or molecules. 



Several liters of a 0.55 per cent solution of isoelectric gelatin 

 were kept at about 10C. for 48 hours and at 20C. for the next 

 24 hours. Then the stock solution was divided into two parts. 

 The one part was subdivided into doses of 90 c.c. each, and each 

 was brought to a different pH by adding 10 c.c. containing 

 different quantities of HC1. In this way the concentration of 

 originally isoelectric gelatin was, therefore, in every case 0.5 

 per cent. The second portion was treated in the same way 

 except that before adding the acid the gelatin was kept for 1 

 hour at 45C. This was done to melt part of the submicroscopic 

 pieces of jelly assumed to exist in the solution, and thus to 

 increase the concentration of the isolated ions and molecules and 

 to diminish the relative quantity of solid submicroscopic particles 

 responsible for the high viscosity characteristic of gelatin solu- 

 tions. After this second portion of the stock solution of iso- 

 electric gelatin had been kept for 1 hour at 45C. it was rapidly 

 cooled to 20C., the HC1 was added in the way described for the 

 first portion and the solutions were put into collodion bags to 

 measure the osmotic pressure. Each collodion bag contained 

 about 50 c.c. of gelatin solution. The temperature now remained 

 constant at 20C. for both sets of experiments. It was noticeable 

 from the first that the osmotic pressure of the gelatin solution 



