76 DONNAN EQUILIBRIUM AND VISCOSITY 



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 electrolytes influence the viscosity of gelatin solutions in a similar 

 way as they influence the swelling, osmotic pressure, or the p.d. 

 of these solutions. We have only indirect means of testing this 

 occlusion theory of viscosity in the case of gelatin. 



2. If these ideas are correct it would follow that if we melt a suspen- 

 sion of 0.5 gm. of powdered gelatin, in 100 cc. of water the viscosity 

 of the 0.5 per cent solution of gelatin should be considerably lower 

 than the viscosity of the suspension if measured at the same tempera- 

 ture, since in melting, part of the solid particles of the suspension 

 should be transformed into individual molecules or ions or into par- 

 ticles too small to occlude water. In other words, as a consequence 

 of the melting there should be a diminution of the relative volume 

 occupied by the solid gelatin held originally in suspension. 



This experiment was tried in the following way : 0.5 gm. of powdered 

 gelatin was put into 100 cc. of water containing 0, 1, 2, 3, 4, 5, 6, 7, 

 8, 10, 12.5, 15, and 20 cc. of 0.1 n HCl to bring the gelatin to different 

 pH. The suspension was allowed to stand 1 hour at 20° to bring 

 about the swelling of the particles and the viscosity of the suspension 

 was measured in a straight viscometer at 20°C. The time of outflow 

 of water through the viscometer at 20° was 48.5 seconds. The upper 

 curve in Fig. 2 gives the ratio of viscosity of suspensions to that of 

 water at 20°C. (When the viscosity is high, the values obtained 

 are a little too great owing to a gravity effect which causes the 

 solid particles to collect above the upper opening of the capillary 

 tube during a part of the time of the experiment, thus increasing 



