212 THEORY OF COLLOIDAL BEHAVIOR 



The values in Table XLIII show that Einstein's formula gives 

 the correct values for viscosity when the volume of the gelatin 

 is small, since in that case c is equal or nearly equal to 2.5, as his 

 formula demands. 



When, however, the volume is larger, the value for c exceeds 

 2.5. The fact that the value for c exceeds 2.5 when the relative 

 volume occupied by the particles in the solution is large, was 

 found also by Hatschek, Smoluchowski, and Arrhenius. Hat- 

 schek replaced the value 2.5 in Einstein's formula by a larger one, 

 namely 4.5. This, however, meets in our case with the difficulty 

 that the value c shows a drift reaching a maximum when the 

 volume of the gelatin particles is a maximum. This difficulty 

 is largely avoided in Arrhenius's formula and we have to change 

 from Einstein's formula to that of Arrhenius whenever the 

 relative volume of the particles in solution or suspension exceeds 

 the limits of the applicability of Einstein's formula, as we shall 

 see in the next chapter. 



The experiments on the viscosity of suspensions of powdered 

 gelatin in water have, therefore, led to the result, first, that the 

 influence of pH, of the valency of ions, and of the concentration 

 of neutral salts on the viscosity of suspensions of finely powdered 

 gelatin in water is similar to the influence of these three agencies 

 on the viscosity of gelatin solutions; second, that the influence of 

 electrolytes on the viscosity of the suspensions is due to the 

 variation of the swelling (or relative volume) of the suspended 

 particles; and third, that this latter fact explains why the Donnan 

 equilibrium determines also the variation of viscosity of these 

 suspensions. If it could be shown that a solution of gelatin 

 contains also some (submicroscopic) particles of solid jelly 

 (capable of swelling), we should understand at once why electro- 

 lytes influence the viscosity of gelatin solutions as they influence 

 the swelling, osmotic pressure, or the P.D. of these solutions. 



3. We have only indirect means of testing the occlusion theory 

 for gelatin solutions but these tests give an unequivocal answer. 

 When a 0.5 per cent solution of isoelectric gelatin is heated rapidly 

 to 45, cooled rapidly to a lower temperature, e.g., 20C., and 

 kept at this temperature, the solution will ultimately set to a 

 continuous gel but will steadily increase its viscosity before this 

 stage is reached. It is natural to assume that the formation 



