110 OSMOTIC PRESSURE AND VISCOSITY 



These experiments support the idea expressed in the preceding 

 paper that protein sokitions are true solutions which may or may not 

 contain solid particles of protein capable of swelling. In the case 

 of gelatin solutions the formation of submicroscopic particles of solid 

 jelly from isolated molecules or ions is a reversible process and this 

 leads in this case to a reciprocal variation of osmotic pressure and 

 viscosity of such solutions. 



This probably explains a phenomenon which has puzzled the writer 

 for a long time; namely, that the osmotic pressures of gelatin solutions 

 of the same pH and concentration of originally isoelectric gelatin 

 occasionally showed variations for which he could not account. It 

 now becomes probable that this was due to a factor which was not 

 taken into consideration, namely, that on standing at room temp- 

 erature a gradual transformation of isolated molecules or ions into 

 larger aggregates takes place, which must diminish the osmotic 

 pressure but increase the viscosity. This source of variation was 

 eliminated in the viscosity experiments in which the gelatin solution 

 was always heated first to 45°C. and then as soon as this tempera- 

 ture was reached the solution was cooled to the temperature desired 

 for the viscosity measurements. It is probable that the same 

 uniformity of treatment is also required for the osmotic pressure 

 experiments. 



This reciprocal relation between osmotic pressure and viscosity 

 exists probably also in the case of solutions of casein salts. Solutions 

 of Na caseinate are less opaque than those of casein chloride (of the 

 same concentration of originally isoelectric casein) which indicates 

 that the Na caseinate solution contains more isolated casein ions 

 and molecules and less submicroscopic solid particles than the so- 

 lution of casein chloride. 



The writer had already shown in a preceding paper that the maximal 

 viscosity of a 1 per cent solution of casein chloride is higher than 

 the viscosity of solutions of Na caseinate of equal concentration of 

 originally isoelectric casein, while the osmotic pressures of solutions 

 of the two salts show exactly the reverse relation, the maximal osmotic 

 pressure of a 1 per cent solution of Na caseinate being almost 700 mm. 

 H2O while the maximal osmotic pressure of a 1 per cent solution of 

 casein chloride is only about 200 mm. 



