366 AMPHOTERIC COLLOIDS. Ill 



While Pauli is right in stating that gelatin treated by acid is more 

 highly ionized than common gelatin, I cannot agree with him that it 

 is this ionization which causes the difference in the osmotic pressure 

 of the gelatin at the isoelectric point and gelatin treated with acid. 

 The increase in osmotic pressure is as our numerical results will show 

 merely the expression of the increase in the number of particles in 

 solution and there is no need or room for the assumption that the 

 hydratation or any other quality except the number of particles in 

 solution has anything to do with this increase in pressure. 



Since viscosity and swelling vary practically parallel with the 

 osmotic pressure, these phenomena must also be a function of the 

 number of particles or ions in solution. 



Hardy® has pointed out that the ionization of a protein increases its 

 viscosity, and the increase of viscosity of gelatin with its ionization 

 might then account for the parallelism between the curves for the 

 bromine number and for the viscosity of the gelatin solution. 



As far as a theory of swelling is concerned, the only one possessing 

 any quantitative basis at present is Procter's.'* 



If we can, therefore, prove that under the influence of HBr the 

 osmotic pressure of gelatin changes in proportion with the gelatin 

 bromide formed, we have no further reason to question the purely 

 chemical or stoichiometrical basis of the influence of acid upon all the 

 physical properties of gelatin. 



//. Measurements without Washing. 



1 gm. of finely powdered gelatin is put for 30 minutes at 15°C. 

 into each of a series of beakers containing 100 cc. of HBr of a different 

 concentration, varying from m/8 to m/8192. As a control 1 gm. of 

 gelatin is put for 30 minutes at 15°C. into 100 cc. of distilled water. 

 The powdered gelatin is then poured into a cylindrical funnel and the 

 acid aflowed to drain off. The diameter of all the funnels being the 

 same, the height of the gelatin gives a measure for the relative amount 

 of swelling. Each gram of gelatin is then put into 100 cc. of HBr 

 of the same concentration with which it had been treated before and 

 is liquefied by heating to 50°C. Immediately after melting the time 



" Hardy, W. B., /. Physiol., 1905-06, xxxiii, 251. 



