96 



Mr. W. B. Hardy. On the Mechanism of 



at considerable length ; and initial experiments were made with the 

 ternary systems, gelatine-water-alcohol, gelatine-water-mercuric chlor- 

 ide, and agar-water-alcohol. These mixtures are homogeneous when 

 heated, but, on cooling, there occurs a division into two fluid phases. 

 In the binary systems and in the ternary system agar-water-alcohol, 

 the conjugate phases have approximately the same refractive index. 

 In the ternary system, gelatine-water-alcohol, the refractive index of 

 the one phase differs so much from that of the other as to permit of 

 direct microscopical investigation of the form of the surface which 

 separates the phases. For this reason I propose to treat this ternary 

 system first. 



The Ternary System — Gelatine-water-alcohol. 



When 13*5 grammes of dry gelatine are dissolved in 100 c.c. of a 

 mixture of equal volumes of absolute alcohol and water, a system is 

 produced which is clear and homogeneous at temperatures above 20°. 

 As the temperature falls below this limit a clouding occurs, which I find 

 to be due to the appearance of fluid droplets which gradually increase 

 in size until they measure 3/x. On cooling further these fluid droplets 

 become solid, and they begin to adhere to one another. In this 

 way a framework is built up, composed of spherical masses hanging- 

 together in linear rows which anastomose with one another. The 

 framework, therefore, is an open structure, which holds the fluid phase 

 in its interstices. The macroscopical result of the change is the con- 

 version, with falling temperature, of the fluid into a loose gel. The 

 droplets can be readily separated from the interstitial fluid by the help 

 of a centrifugal machine. 



The phenomena above described undoubtedly depend upon the 

 separation of a homogeneous mixture into two phases owing to a fall of 

 temperature. Each phase contains water, alcohol, and gelatine, and 

 the system may be described as a conjugate composed of a fluid solu- 

 tion of gelatine in a mixture of water and alcohol, and a solid solution 

 of water with a trace of alcohol in gelatine. Both phases, however, 

 are fluid within a small range of temperature. The surface of separa- 

 tion of the two phases is curved, and at first discontinuous, and owing 

 to the small size of the droplets it is very large. 



When the gel is heated the two phases again mix to form a clear 

 fluid. Owing however to the fact that the droplets adhere to one 

 another, they tend to fuse as temperature rises, so as to form irregular 

 masses of viscous fluid, which are separated from the other phase by a 

 surface of no particular shape. The irregular form of this surface, and 

 the ease with which it is modified by any chance slight currents, show 

 that at this stage the surface tension between the two phases must be 

 exceedingly slight. In order to simplify the description I shall call that 

 phase which separates as small spheres the internal phase." 



