100 OSMOTIC PRESSURE AND VISCOSITY 



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 

 one sixtieth of an inch diameter.) At the beginning of the experiment 

 the 0.5 per cent solution of gelatin was rapidly heated to 45°C. and 

 rapidly cooled to 20°C., 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 pow- 

 dered isoelectric particles in 100 cc. of HCl 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 experiment and the 

 osmotic pressures observed were still lower than in the lowest curve 

 in Fig. 1. All these osmotic pressure experiments were made in a 

 thermostat at 20°C. 



The viscosity is affected in exactly the opposite way 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 experiment and conclusions published in the preceding 

 paper.2 



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

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

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

 gelatin in 100 cc. to 1 gm. ; i.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 HCl and the solutions 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. 2. 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 

 replaced by powdered gelatin. This supports the idea that the 



