238 THEORY OF COLLOIDAL BEHAVIOR 



sure of the isolated gelatin ions and the isolated gelatin molecules. 

 Theoretically, of course, the coarse particles of gelatin also 

 participate in the osmotic pressure but this effect is negligible on 

 account of the small number of such particles. (The gelatin 

 particles used were of grain size slightly above %o of an inch 

 diameter.) At the beginning of the experiment the 0.5 per 

 cent solution of gelatin was rapidly heated to 45C. and rapidly 

 cooled to 20C., and then the powdered gelatin was added. The 

 pH is that of the 0.5 per cent gelatin solution at the end of the 

 experiment. 



The lowest curve represents the osmotic pressure of 1 gm. of 

 powdered isoelectric particles in 100 c.c. of HC1 of different pH. 

 The slight osmotic pressure observed is probably due to the fact 

 that a little of the gelatin went gradually into solution. This 

 apparently happened to a less extent in a repetition of this experi- 

 ment and the osmotic pressures observed were still lower than in 

 the lowest curve in Fig. 70. All these osmotic pressure experi- 

 ments were made in a thermostat at 20C. 



The viscosity is affected in exactly the opposite sense from the 

 osmotic pressure if part of the dissolved gelatin is replaced by 

 solid particles of gelatin. The more dissolved gelatin is replaced 

 by solid particles of gelatin the higher the viscosity, a result 

 to be expected from the experiments and conclusions already 

 stated. 



Solutions of 0.5, 0.625, 0.750, 0.875, and 1.0 gm. of isoelectric 

 gelatin were heated quickly to 45C. and cooled quickly to 20C., 

 and so much powdered gelatin of pH 7.0 was added as to bring 

 the total gelatin in 100 c.c. to 1 gm.;z.e., to a 0.5 per cent solution 

 of gelatin was added 0.5 gm. of powdered gelatin (between mesh 

 sizes 100 and 120), and to a 0.875 per cent solution of liquid 

 gelatin was added 0.125 gm. of powdered gelatin, while no 

 powdered gelatin was added to the 1 per cent solution of liquid 

 gelatin. The different mixtures were brought to different pH 

 through the addition of different quantities of HC1 and the solu- 

 tions were allowed to stand for 1 hour before the viscosities were 

 measured in order to give the powdered gelatin a chance to swell. 

 The results of the measurements are represented in Fig. 71. 

 The reader will see that within the range of the pH between 3.6 

 and 1.4 the viscosity is the greater, the more liquid gelatin is 



