JACQUES LOEB 365 



Such a gelatin salt can only exchange the Br with the anion of a 

 neutral salt and it is impossible for a complete molecule of a neutral 

 salt like KCl to combine with the NH2 group as has been assumed. 

 The writer's experiments on the action of neutral salts on gelatin 

 treated previously with acid are in harmony with the ideas of Werner 

 and opposed to the assumption of a pentavalent N atom in the pro- 

 tein molecule capable of adding a whole molecule of a neutral salt. 

 We do not know yet whether only one or more NH2 groups in the 

 gelatin molecule are able to bind a molecule of HBr. 



Gelatin is an amphoteric electrolyte which at the isoelectric point 

 (which for gelatin lies at pH = 4.7) is practically insoluble. When 

 we prepare a gelatin solution and give it a hydrogen ion concentra- 

 tion of 2.10"^ (i.e. pH = 4.7), the solution in less than 24 hours 

 becomes opaque on cooling in as low a concentration as 0.25 per 

 cent and probably at any concentration; except that the opacity 

 due to the insolubility becomes too slight in very low concentrations 

 to be noticeable. This explains why gelatin at the isoelectric point 

 has practically no osmotic pressure, no swelling, a minimal conduc- 

 tivity, viscosity, etc. 



When we treat isoelectric gelatin with a limited quantity of HBr of a 

 low concentration a certain amount of gelatin is transformed into gela- 

 tin bromide, which is soluble and dissociates electrolytically. The 

 higher the concentration of acid used the more gelatin bromide is 

 formed and the more molecules of gelatin go into solution, until at a 

 certain point all the insoluble gelatin molecules are converted into 

 soluble gelatin bromide molecules. Since a 1 per cent gelatin bromide 

 solution should possess the same degree of electrolytic dissociation as 

 the HBr combined with it and since a 1 per cent gelatin bromide 

 solution on account of the high molecular weight of gelatin must be 

 considered as a very dilute solution, we shall commit no great error in 

 assuming a complete electrolytic dissociation of the gelatin bromide. 

 If it is true that the increase in osmotic pressure of gelatin under the 

 influence of HBr is merely due to an increase in the number of sol- 

 uble gelatin molecules, it must be possible to show that the osmotic 

 pressure in this case increases approximately with the number of 

 gelatin bromide molecules formed. This we intend to prove in the 

 present paper. 



