370 



AMPHOTERIC COLLOIDS. Ill 



filter and the acid allowed to drain off. The swelling is measured as 

 described. From now on the method of procedure is different from 

 that in the previously mentioned experiment. Instead of melting 

 the gelatin in 100 cc. of the acid solution with which it had been 

 treated, we melt it in 100 cc. of distilled water. The rest of the deter- 

 minations — viscosity, osmotic pressure, conductivity, and titration 

 for Br — are all made with such gelatin. Moreover, the osmotic pres- 



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Figs. 3 and 4. Curves for viscosity and swelling (Fig. 3); bromine number, 

 conductivity, and osmotic pressure (Fig. 4) of 1 per cent gelatin solution treated 

 previously with different concentrations of HBr (abscissae) the acid having been 

 allowed to drain off. A 1 per cent solution of the gelatin in distilled water is then 

 prepared, and the osmotic pressure of this gelatin is measured against distilled 

 water, and conductivity and Br number are determined after 20 hours dialysis 

 against distilled water. The curves for osmotic pressure, swelling, viscosity, and 

 conductivity are parallel to the curve for bromine number from pH = 4.7 to 

 pH = 2.9 or 3.3 respectively. The gelatin is free from bromine for pH > 4.7. 



sure was measured against HoO, thereby allowing more of the free acid 

 not combined with the gelatin which had not drained off to diffuse 

 out during the process. The result of this experiment is represented 

 in Figs. 3 and 4. Fig. 4 contains the measurements for osmotic pres- 

 sure and Br number, and the curves are almost parallel (with the excep- 

 tion of the value for the osmotic pressure for gelatin treated with m/8 

 acid) . This parallelism is the missing link which allows us to decide 

 in favor of the purely chemical and against the colloidal conception 



