302 



PHYSICAL PROPERTIES 



the remarkable hysteresis observed in reversible gels. . . . When 

 water is added to a ternary mixture so as to considerably swell 

 the droplets the system is unstable and the two phases mix at once 

 when it is mechanically agitated" (34). 



In gels of this type which are dilute with respect to the colloid, 

 therefore, the structure is that of an open sponge-work; the 

 meshes being filled with water or a water-rich solution of the sub- 

 stance forming the gel, while the frame-work of the sponge con- 

 sists of anastomosing threads composed of linearly arranged 

 globules of the water-poor phase. In such gels, therefore, the sur- 

 face of the water-poor phase is convex while that of the water-rich 

 phase is concave; in other words, the water-poor phase is internal 

 to the water-rich or external phase. If, however, to a ternary 

 mixture of gelatin, alcohol and water which forms such a gel 

 as that described above, more gelatin be added, the character 

 of the gel changes entirely, the water-poor phase becomes con- 

 cave and the water-rich phase instead of being, as formerly, 

 concave becomes convex to it. On cooling such a mixture to a 

 temperature below that at which it forms an optically homo- 

 geneous solution, droplets separate out which are poor in gelatin, 

 while the interstitial portion of the system, which is rich in 

 gelatin, solidifies. Thus the gel comes to possess a honeycomb 

 structure, the droplets being poor in gelatin, rich in water. This 

 is very clearly shown in the following determinations of Hardy. 



TEMPERATURE OF THE MIXTURE 15 DEGREES 

 (Equal parts of water and alcohol) 



From these determinations it is also clear, as I have said, 

 that the two phases are not of constant composition, but may, 

 under different conditions of total concentration, etc., vary widely 

 in their relative and absolute gelatin and water content. This 

 system differs, therefore, from the system phenol-water not only 



