252 THEORY OF COLLOIDAL BEHAVIOR 



ion in combination with the gelatin is bivalent, comparatively 

 little alcohol suffices for precipitation regardless of the pH. 



On the other hand, the addition of acid with monovalent anion 

 or alkali with monovalent cation to isoelectric gelatin, so that the 

 pH is either 4.4 or below or 5.0 or above increases the power of 

 attraction between gelatin and water to such an extent that now 

 the 1 per cent solution of originally isoelectric gelatin becomes 

 soluble even when comparatively much alcohol is added. 



Isoelectric crystalline egg albumin remains clear in solutions 

 at low temperature, e.g., 2C., for many months even in a con- 

 centration of 8 per cent. When the temperature is raised, a 

 change occurs in the molecule whereby its attraction for mole- 

 cules of water is diminished and a 1 per cent solution precipitates 

 at pH 4.8 at a temperature not far from 60C. (the exact tempera- 

 ture was not ascertained). This precipitation is spoken of as the 

 heat coagulation of egg albumin. If we add slight quantities of 

 HC1 the temperature at which the coagulation occurs is raised. 

 At pH 4.39 the coagulation occurs on rapid heating at about 80; 

 at pH 4.25 or below the forces of attraction between the molecules 

 of albumin and water become so great that heat coagulation no 

 longer occurs even at 95C. ; the solution only becomes opalescent. 

 It was found that the pH at which heat coagulation of a 1 per cent 

 solution of crystalline egg albumin no longer occurs at 95C. is 

 approximately the same for HC1, HBr, HNO 3 , CH 3 COOH, H 3 PO 4 , 

 and succinic acid. For oxalic and tartaric acids it is only slightly 

 lower, probably because at this pH some of the acid anions are 

 bivalent. The main fact is, that for H 2 SO4, whose anions are all 

 bivalent, the pH at which coagulation becomes impossible is 

 markedly lower; namely, 3.42. All this is in harmony with the 

 writer's observations on the effect of different acids on the solu- 

 bility of gelatin in alcohol-water mixtures. 



On the alkaline side from the isoelectric point the critical pH 

 at which heat coagulation disappears is practically identical for 

 KOH and NaOH while the pH is considerably higher for 

 Ba(OH) 2 . 1 



The explanation of these phenomena is a part of the general 

 problem of solubility; they have no direct connection with the 

 theory of colloidal behavior. 



1 Unpublished results. 



