368 AMPHOTERIC COLLOIDS. Ill 



outside liquid was in each case HBr of the same concentration as that 

 in which the gelatin was dissolved. The whole experiment was car- 

 ried on in a water bath of a tempei^ature of 24°C. The curve at the 

 top in Fig. 2 is the curve for the osmotic pressure observed after 

 about 20 hours, at which time equilibrium was established. 



The conductivity of the gelatin solution was then ascertained, as 

 well as the amount of Br found in 25 cc. of 1 per cent gelatin solution. 

 The curves are given in Fig. 2. A glance at Fig. 2 will show that the 

 curve for osmotic pressure is not parallel to the curves for conductivity 

 and for the Br number. It is, therefore, impossible to arrive from 

 experiments of this type at a decision whether the influence of HBr 

 (or other electrolytes) upon gelatin is of a stoichiometrical or of a col- 

 loidal character. Yet those familiar with the literature of this sub- 

 ject will remember that the conclusions of most colloid chemists are 

 based on experiments in which the action of the electrolyte upon the 

 protein was measured in the presence of an excess of electrolyte. 

 The second fact which deserves attention becomes clear by a com- 

 parison of Figs. 1 and 2; namely, that the minima which appear in 

 the two sets of curves lie at different acid concentrations: in Fig. 1 

 between m/256 and m/512, in Fig. 2 (for osmotic pressure) between 

 m/2048 and m/4096. Comparing, however, the pH in the two sets of 

 curves we notice that the minimum is at the same pH, namely about 

 4.7, which is the isoelectric point for gelatin. A good deal of the work 

 on which the colloidal theory of the behavior of proteins rests was 

 done without any measurements of pH and by plotting the effect 

 against the concentration of the acid. It is no wonder that work 

 with two such shortcomings in its method did not furnish any proof 

 for the stoichiometrical character of the action of electrolytes on the 

 physical properties of amphoteric colloids. 



///. Efect of Washing. 



Our method consists in removing the excess of HBr (or of any other 

 electrolyte) after it has had a chance to act on the gelatin. 1 gm. of 

 powdered gelatin is put again into each of a series of beakers for 30 

 minutes at about 15° or 20°C., each beaker containing 100 cc. of 

 HBr of a different concentration. Then the gelatin is poured on a 



