JELLIES AND GELATINIZATION 299 



76 per cent, of phenol, separates, at temperatures below 80 C., into 

 two phases, the one a solution of phenol in water, the other a solution of 

 water in phenol. According to the view developed by Hardy the two 

 cases differ only in the fact that upon separation of the two phases in 

 the agar-water system the system retains a structure, while in the 

 phenol-water system no structure is retained and the components 

 separate into two clearly demarcated layers. Essentially, the dif- 

 ference between the two systems consists in this : that when the phenol- 

 water system separates into two phases, the phases become separated 

 by the minimal possible surface, namely a plane; while when the agar- 

 water system separates into two phases they remain in contact over an 

 area far larger than the minimum. In the latter case it would seem 

 that the surface-tension at the interface of the two phases is very low, 

 so that the force leading to the diminution of surface is small. The 

 resistance to the diminution of the interface is also very large because 

 of the high viscosity of the gel. 



The manner in which the structure of a gel is built up can be readily 

 observed in the ternary mixture, alcohol, gelatin and water. If 13.5 

 grams of Gelatin are mixed with 50 c.c. of water and 50 c.c. of absolute 

 alcohol, a mixture is formed which is optically homogeneous at 17 to 

 20 C., but which separates into two phases at temperatures below 

 this. Hardy thus describes the sequence of events on cooling this 

 mixture below the temperature of gelation: "As the temperature falls 

 below the limit a clouding occurs which I find to be due to the appear- 

 ance of fluid droplets which gradually increase in size until they 

 measure 3 /z.ju. On cooling further, these fluid droplets become 

 solid and they begin to adhere to one another. The framework is 

 therefore an open structure which holds the fluid phase in its inter- 

 stices." "When once formed the phases have considerable stability. 

 If the droplets are composed of a solid solution one may, by the addi- 

 tion of water, cause them to increase to relatively vast dimensions 

 without their being destroyed; as they increase in size their refractive 

 index approximates more and more to that of the external phase until 

 they are finally lost sight of. The addition of alcohol, however, once 

 more brings them into view and causes them to shrink. Owing to 

 this stability, once a configuration has been established, one has to far 

 overstep the conditions of its formation in order to destroy it. This 

 would account for 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." 



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

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

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

 substance forming the gel while the framework of the sponge consists 

 of anastomosing threads composed of linearly arranged globules of the 

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



