206 THEORY OF COLLOIDAL BEHAVIOR 



protein at not too high a concentration, at low temperatures, and 

 at pH above 1.0. 



2. If this assumption is correct, it would follow that a suspen- 

 sion of powdered gelatin in water should have a greater viscosity 

 at a given temperature than if the same mass of gelatin is dis- 

 solved in water, since in the latter case part of the gelatin at 

 least is in true solution (as we shall see later) and this latter is 

 incapable of increasing its volume by occluding water. It would 

 follow, furthermore, that the influence of electrolytes on the 

 viscosity of suspensions of powdered gelatin would be the same 

 as the influence of electrolytes on the osmotic pressure of gelatin 

 solutions. It can be shown that both expectations are fulfilled. 



Doses of 0.5 gm. of powdered gelatin were put into 100 c.c. of 

 water containing 0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 12.5, 15 and 20 c.c. of 

 0.1 N HC1 to bring the gelatin to different pH. The suspensions 

 were allowed to stand 1 hour at 20 to bring about the swelling 

 of the particles, and the viscosity of the suspensions was measured 

 in a straight viscometer at 20C. The time of outflow of water 

 through the viscometer at 20 was 48.5 seconds. The upper 

 curve in Fig. 52 gives the ratio of viscosity of suspensions to 

 that of water at 20C. (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 temporarily the density of the suspension.) 

 After the viscosity of a suspension was measured the suspension 

 was transformed into a solution by heating the suspension to 45C. 

 for 10 minutes; after that the solution was rapidly cooled to 20C. 

 and the viscosity of the gelatin solution was immediately meas- 

 ured with the same viscometer at 20C. The lower curve in 

 Fig. 52 shows that the viscosity was now considerably diminished. 

 The abscissae are the pH of the gelatin solutions. 



If we measure the volume of the suspended particles we find 

 that it varies in a similar way as the viscosity. Samples of 0.5 

 gm. of Cooper's powdered commercial gelatin of a pH of about 6.0 

 were added to 100-c.c. portions of water containing varying 

 amounts of HC1. The particles had uniform size (going through 

 sieve 100 but not through sieve 120), but their shape was ex- 

 tremely irregular. They were left in the solution several hours 



