214 THEORY OF COLLOIDAL BEHAVIOR 



When a 0.5 per cent solution of isoelectric gelatin is rapidly 

 heated to 45C., cooled rapidly to 20C., and brought immediately 

 to a different pH by the addition of HC1, at 20C. a viscosity 

 curve like the lowest in Fig. 56 is obtained. When, however, 

 the 0.5 per cent isoelectric gelatin solution is allowed to stand for 

 3 hours at 20C. before the acid is added, a parallel viscosity curve 

 is formed at 20C. but higher than the first one (middle curve, 

 Fig. 56), for the reason that during the 3 hours an additional 

 number of solid jelly particles capable of swelling has beenformed. 

 If the solution of isoelectric gelatin stands for 17 hours at 20C., 

 before the HC1 is added, the curve is still higher though practically 

 parallel with the first curve (upper curve, Fig. 56), except at the 

 summit. It is probable that on standing not only the number 

 but the size of individual particles also increases, and the writer 

 has observed that for the size of granules used in his experiments 

 the greater the size the greater the viscosity, since the viscosity 

 is chiefly but not exclusively a function of the relative volume of 

 the particles. 



Since jelly formation of gelatin is a reversible process we should 

 expect that two opposite processes always take place simultane- 

 ously in a gelatin solution on standing, namely, first, the forma- 

 tion of solid particles of jelly through the aggregation of previously 

 isolated gelatin molecules and ions, and second, the dissolution 

 of such aggregates (micellae) back into isolated molecules and 

 ions. It is easy to show that powdered gelatin of a given pH 

 dissolves the more rapidly the higher the temperature. If, 

 therefore, our assumption is correct that in a solution of gelatin 

 two opposite processes go on constantly, the rate of melting of 

 the micellae should increase if the temperature rises. Hence, 

 at very low temperature the viscosity of a gelatin solution should 

 increase rapidly on standing since the formation of new micellae 

 takes place constantly, while practically no melting of micellae 

 occurs. When, however, the temperature is raised beyond a 

 certain point, the rate of melting of micellae increases more 

 rapidly than the rate of formation of new micellae. Hence, at 

 such a temperature the viscosity of a gelatin solution should not 

 increase but decrease on standing. 



This conclusion was tested experimentally and found to be 

 correct. A 2 per cent solution of gelatin chloride of pH 2.7 was 



