230 THEORY OF COLLOIDAL BEHAVIOR 



viscosity of casein chloride solutions never becomes as high as 

 that of gelatin solutions containing equal masses of protein per 

 100 c.c. of solution. 



All the experiments described agree with the occlusion theory 

 but not with the hydration theory. Thus the fact that the 

 viscosity of a 0.5 per cent solution of isoelectric gelatin increases 

 rapidly at a temperature of 20C. or below cannot possibly be 

 explained on the basis of the hydration theory since isoelectric 

 gelatin is not ionized. It might be explained on the basis of 

 another suggestion which attributes to the gelatin solution a 

 similar structure to that possessed by the solid jelly of gelatin. 

 This idea would lead us to the assumption that in addition to the 

 source of viscosity due to the relative volume of the protein 

 solution there exists a second type peculiar to protein solutions 

 which has no connection with the volume. 



"Bearing in mind the possibility that protein solutions may contain a 

 preformed molecular structure analogous to that of the jellies or coagula 

 which they can form, we are strongly impelled towards the belief that the 

 type of viscosity which solutions of proteins exhibit may in some manner 

 owe its existence to this structure, and not to the type of internal friction 

 which hinders molecular and ionic motion. Thus a netlike structure, 

 such as a tennis net, will offer no hindrance to the passage through it of a 

 quickly moving body which is smaller than its meshes, other than that 

 which is due to the fact that the material which composes the net occu- 

 pies a small fraction of the area which the body must traverse, but to 

 any force which involves deformation of the structure, for instance, a 

 force which seeks to drag it through a small tube, it will offer a very 

 considerable resistance." 1 



This theory becomes untenable in the case of suspensions of 

 powdered gelatin and of casein chloride which have no tendency 

 to set to a jelly. It fails, moreover, to account for the fact that 

 the influence of pH on the viscosity resembles that on the osmotic 

 pressure of gelatin solutions. The assumption of a second 

 type of viscosity independent of the relative volume occupied 

 by the solute becomes unnecessary, since the theories of Einstein 

 and of Arrhenius respectively, which derive the viscosity from the 

 relative volume, suffice to account for all the phenomena observed. 



1 ROBERTSON, T. B., "The Physical Chemistry of Proteins," pp. 324-25, 

 New York, London, Bombay, Calcutta, and Madras, 1918. 



