CHAPTER XII 



VISCOSITY 1 



1. We have seen in Chaps. V and VI that the influence of 

 electrolytes on the viscosity of the solutions of certain proteins, 

 e.g., gelatin or casein, is similar to the influence of electrolytes 

 on osmotic pressure, swelling, and potential differences. The 

 explanation given for the influence of electrolytes on the last 

 named properties was based on the theory of Donnan's membrane 

 equilibirum. This theory can only be applied where the diffusion 

 of one type of ions is prevented, while no such block exists for 

 other ions. In the experiments on osmotic pressure or P.D. of 

 protein solutions the collodion membrane permits the diffusion of 

 crystalloidal ions while preventing the diffusion of the protein 

 ions ; and in the case of the solid gel the protein ions are prevented 

 by the forces of cohesion from diffusing into the surrounding solu- 

 tion free from protein. But this raises the problem of how the 

 Donnan equilibrium can be applied to the viscosity of protein 

 solutions. We intend to show that the answer lies in the fact that 

 although protein solutions may be and probably are as a rule true 

 solutions, consisting of isolated protein ions and molecules dis- 

 tributed equally through the water, they contain under certain 

 conditions submicroscopic solid particles of protein. We shall see 

 that the viscosity of protein solutions is only influenced in the 

 same way by electrolytes as is the osmotic pressure, when such 

 solid protein particles are present in considerable numbers. If 

 they are absent, or if they are scarce, electrolytes will not influ- 

 ence the viscosity of protein solutions in the same way as electro- 

 lytes influence the osmotic pressure or the P.D. of protein solu- 

 tions. In the following discussion we shall measure the viscosity 

 of protein solutions by the time of outflow through a capillary 

 tube, as described by Ostwald, and the quotient of this time over 

 the time of outflow of pure water through the same viscometer at 



^OEB, J., J. Gen. Physiol., vol. 3, p. 827, 1920-21; vol. 4, pp. 73, 97, 

 1921-22. LOEB, J., and LOEB, R. F., /. Gen. Physiol., vol. 4, p. 187, 1921-22. 



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