218 THEORY OF COLLOIDAL BEHAVIOR 



the solid jelly particles already existing to dissolve; while the 

 tendency of the isolated gelatin molecules or ions to adhere to 

 each other is not increased. It should follow that on standing the 

 viscosity of a 0.5 per cent solution of gelatin chloride or gelatin 

 sulphate will increase the less at 20 the lower the pH of the solu- 

 tion. Figure 58 shows that this is the case. 



In Chap. XIV we shall see that the rate of solution of 

 powdered gelatin in water is influenced in a different way by 

 different salts. Na2SO 4 diminishes the rate of solution of 

 powdered gelatin chloride when the concentration of Na2SO 4 

 exceeds M/64; and the diminution is the greater the higher the 

 concentration; while CaCl-2 accelerates the rate of solution of 

 powdered gelatin chloride when the concentration of CaC^ 

 exceeds M/4. 



Gelatin chloride solutions of pH 3.4, containing 1 gm. of 

 originally isoelectric gelatin in 100 c.c. solution, were made up in 

 various concentrations of Na 2 SO 4 and CaCl2. The solutions 

 were rapidly heated to 45 and rapidly cooled to 20C. and kept 

 at this temperature for 1 hour. The time of outflow of the 

 solution through a viscometer was measured immediately and 

 in intervals of 5 or 10 minutes. The time of outflow of water 

 through the viscometer at 20 was 61 seconds. 



The viscosity of a gelatin chloride solution of pH 3.4 rises 

 gradually but very slowly (uppermost curve in Fig. 59) and the 

 rate of increase of viscosity on standing is not materially altered 

 in M/512 Na 2 SO 4 and only little in M/128 Na 2 SO 4 . In M/32 

 Na 2 SO 4 the viscosity increases more rapidly on standing, in 

 M/8 Na 2 SO 4 still more rapidly, and in M/2 Na 2 SO 4 very sharply. 

 This is exactly what we should expect, since the Na 2 SO 4 causes a 

 diminution of the rate of solution of gelatin chloride as soon as 

 the concentration of Na 2 SO 4 is above M/64. In such solutions 

 the rate of solution of micellae will be less and less, and since 

 new micellae are constantly formed at 20C. the viscosity will 

 rise more rapidly on standing when the solution contains Na 2 SO 4 

 in concentrations above M/64 than when the solution contains 

 less Na 2 SO 4 or none at all. 



Figure 60 shows that CaCl 2 in concentrations up to M/8 does 

 not alter the increase in viscosity of gelatin chloride solution on 

 standing, but that the viscosity of gelatin chloride of pH 3.4 no 



