Gelation in Beverdhle Colloidal Systems. 97 



The concentration of the gelatine in the mixture exerts a very 

 remarkable influence upon the configuration of the hydrogel. When 

 it is present in large quantity the internal phase is less viscous and of 

 smaller gelatine content than the external phase, and on cooling it is 

 the external phase which becomes a solid solution. The effect of 

 increasing the proportion of gelatine above a certain amount is 

 therefore very striking — it, so to speak, turns the system inside out, 

 so that the gel is composed of a continuous framework of solid solution, 

 out of which are hollowed spherical spaces filled with fluid. The 

 general mechanical properties of the gel, built on this plan, naturally 

 diff'er very much from those of a gel with a small proportion of gelatine, 

 which consists of an open framework of solid holding fluid in its 

 interstices. 



A mixture of gelatine, water, and alcohol is a ternary mixture 

 which resembles a mixture of benzene, acetic acid, and water. In 

 each there are two immiscible substances and a common solvent. The 

 immiscible substances are gelatine and alcohol in the one case, and 

 benzene and water in the other, while the common solvent is water in 

 the former and acetic acid in the latter case. In both systems the 

 solubility of the immiscible substances in the common solvent varies 

 widely. Thus acetic acid and water, and water and alcohol, mix 

 readily with rise of temperature ; while acetic acid and benzene and 

 water and gelatine mix freely only when the temperature is above 15° 

 in the former case, and above 40° in the latter case. Duclaux's 

 researches'^ show that in ternary mixtures having this last charac- 

 teristic the distribution of the constituents in the two phases varies 

 widely with variations in the composition of the whole mass. 



These different characters are illustrated by the following figures, 

 which give the amount of gelatine present in grammes per 100 c.c. 

 They are, however, only approximate for the solid phase, owing to 

 the difficulty in separating it completely from the fluid phase. 



Total mixture. Internal phase. External phase. 



T. 15° 6-7 17-0 2-0 



13-5 18-0 5-5 



36-5 8-5 40-0 



The temperature at which the internal and external phases in this 

 ternary system mix was found to be altered by altering the ratio 

 of the masses of the components. Increasing the proportion of either 

 of the two immiscible components, alcohol or gelatine, was found to 

 raise the temperature, while an increase in the proportion of the 

 common solvent water was found to lower it. 



The curvature of the surface which separates the phases was found 



* * 'Ann. de Chim. et de Phys.,' serie 5, tome 7, 1876, p. 264. 



