364 AMPHOTERIC COLLOIDS. Ill 



It seemed to the writer that experiments on gelatin might per- 

 mit us to satisfy the demand of PauH; namely, to supply the molecular 

 measurements necessary to show that the osmotic pressure and the 

 •other properties of gelatin solutions vary in proportion to the 

 amount of acid combining with a given amount of gelatin. In two 

 previous papers the writer has already shown that this is true for 

 the influence of neutral salts on these properties of gelatin.^ Proc- 

 ter's^ experiments also indicate a purely stoichiometric basis for the 

 influence of acids on the swelling of gelatin. 



According to Werner^ amphoteric electrolytes are characterized by 

 their ability to add H ions or OH ions and not by their ability to give 

 off H and OH ions. It is generally assumed, and probably correctly, 

 that when an acid like HBr combines with an amino-acid or a pro- 

 tein, the reaction occurs in an NH2 group of the amino-acid or pro- 

 tein. According to Werner when NH3 and HBr combine, the posi- 

 tively charged hydrogen ion of HBr is attached by a secondary valency 

 to the N whose three negative charges now hold four positively charged 

 H ions. No other positive ion except H can act in this way. The Br 

 is able to dissociate as freely in the NH4Br as in the free acid. The 

 same assumption is to be made for the way an acid, e.g. HBr, com- 

 bines with amino-acids or proteins. 



/H2 

 NH2 N< 



COOH COOH 



Such a molecule, e.g. gelatin bromide, dissociates into a positively 

 charged gelatin ion and a negatively charged Br ion, the H ion of the 

 HBr now forming a part of the complex and positively charged 

 gelatin ion. 



Gelatin ((^ 



COOH 



4- 



GelatirK^ 



COOH 



-I- Br 



3 Loeb, J., /. Gen. Physiol., 1918, i, 39, 237. 



4 Procter, H. R., and Wilson, J. A., J. Chem. Soc, 1916, cix, 307. Procter, 

 H. R., and Burton, D., /. Soc. Chem. Ind., 1916, xxxv. 



^Werner, A., Neuere Anschauungen auf dem Gebiete der anorganischen 

 Chemie, Braunschweig, 2nd edition, 1909. 



