JACQUES LOEB AND ROBERT F. LOEB 205 



Donnan effect. It was of interest to find out whether such a maxi- 

 mum followed by a drop existed in the influence of acid on the solu- 

 bility of gelatin, but this is not the case at least between pH 4.7 and 

 1,0. Measurements of the dry weight of gelatin dissolved in a certain 

 time at different pH, showed that the amount of gelatin dissolved 

 increases with the hydrogen ion concentration. This corroborates the 

 conclusion that the solution (and precipitation) of gelatin in water 

 is not influenced by forces governed by the Donnan equilibrium and 

 does therefore not show the characteristics of colloidal behavior. 



These experiments also contradict the suggestion that the solution 

 of solid gelatin is necessarily preceded by swelling and that swelling 

 and solution are continuous processes. The contradiction lies in 

 the fact that swelling in acid reaches a maximum at pH of about 3.0 

 and then diminishes upon further increase in hydrogen ion concen- 

 tration, while the rate of solution of solid gelatin granules continues 

 to increase steadily when the hydrogen ion concentration increases 

 beyond pH of 3.0. The mechanism of swelling and the mechanism 

 of solution of solid gelatin in water are determined by forces of an 

 entirely different character; the swelling by osmotic pressure, amd 

 the solution by the secondary valency forces responsible for the 

 solution of crystalloids. 



V. Solubility and Viscosity of Gelatin Solutions. 



We assumed in a preceding paper that the increase in the viscosity 

 of gelatin solutions on standing is due to the gradual formation of 

 larger aggregates from originally isolated gelatin molecules or gelatin 

 ions. When we melt a solid gel of gelatin by rapidly heating it to 

 45°C. and cooling it rapidly to 20° there is produced a true solution 

 containing isolated gelatin molecules or gelatin ions side by side with 

 submicroscopic pieces of solid jelly. On standing two opposite 

 processes are constantly going on in such a mixture; i.e., the solution 

 of these aggregates into isolated molecules or ions and the reverse 

 process; namely, that of the formation of new aggregates by the 

 union of formerly isolated gelatin molecules or gelatin ions. When 

 the velocity of aggregate formation prevails over the velocity of the 

 solution of aggregates, the viscosity of the gelatin solution will in- 

 crease on standing; when the velocity of solution of aggregates 



