372 AMPHOTERIC COLLOIDS. Ill 



of the influence of electrolytes upon the osmotic pressure. Since the 

 bromine number of gelatin increases parallel with the osmotic pressure 

 (and also the viscosity and the swelling) of gelatin, these properties 

 must depend upon the same variable ; namely, the number of gelatin 

 bromide molecules formed. 



We are giving the values for the bromine number and for conduc- 

 tivities as actually found by analysis and measurements of resis- 

 tance. These values demand, however, a correction owing to the 

 fact that in all cases a certain amount of free HBr was present. 

 The actually measured quantity for the bromine number is in each 

 case the sum of the Br contained in the form of gelatin bromide and 

 of the Br contained in the form of free HBr. The latter can be cal- 

 culated from the pH. This value for the quantity of free HBr 

 should be deducted from the Br numbers given in the curves. Since 

 the correction thus required would be slight within the limits of pH 

 from 4.7 to 3.6, we have omitted it in this paper. We may state, 

 however, that the parallelism between the corrected curves for the 

 Br number and those for osmotic pressure is even more perfect than 

 that between our uncorrected curves for bromine number and the 

 curves for osmotic pressure. The correction necessary for the con- 

 ductivity curves can be found by measuring the resistance of a HBr 

 solution for each pH in the same measuring cell (with fixed elec- 

 trodes) which served for the measurements of the resistance of the 

 gelatin bromine solutions. These corrections are greater than those 

 required for the bromine number, especially for pH < 3.9. In order 

 to obtain reliable values for conductivity we must use purified gel- 

 atin. Experiments of this kind will be reported in a subsequent 

 paper. The conductivity measurements will not be considered in 

 this paper. 



All three curves for osmotic pressure, viscosity, and bromine num- 

 ber show a drop again after having reached a maximum. This drop 

 exists in a still more pronounced way in the curves for viscosity and 

 swelling than in that for osmotic pressure, because the gelatin con- 

 tained more HBr before than after dialysis. This drop is of great 

 theoretical significance because it shows free HBr is present in excess of 

 the binding capacity of gelatin for HBr. The free HBr represses the 

 ionization of gelatin bromide on account of the common Br ion and 



