Theory of Gel Structure. 157 



that the liquid phase of the soap gel has been replaced wholly by 

 ether (containing, of course, some water in solution). The gel now 

 exudes ether on compression, and if lit will burn violently. The 

 discharged fluid, however, has some of the solid phase in solution, 

 for it contains more than double the amount of solid matter present 

 in the fluid expressible from the original gel, and moreover, if 

 exposed to the air, and so allowed to lose its dissolved ether, will 

 revert to a moderately firm jelly. Indeed this seems to me to be 

 an ideal method for studying stages of gel formation, as the 

 process can be made slow or rapid as desired. I had at first attri- 

 buted this partial solution of the solid phase to the presence of 

 alcohol, but repeated washing with water failed to remove this 

 faculty from the ether. The ether layer above the soap jelly in these 

 experiments was found to contain some solid matter in solution, 

 and from the qualitative tests I employed this seemed to be fatty, 

 acid, but I have been unable to pursue this particular investiga- 

 tion. 



The discharged fluid will set, therefore, on-'exposure to the air, 

 and the gel thus formed can be once more subjected to ether ; the 

 liquid phase and a portion of the solid dissolved in it will be discharged 

 and the remainder of the solid phase left behind impregnated with 

 ether. This procedure can be repeated three or four times. 



The fact that the liquid phase of the soap can be discharged and 

 be replaced by ether is obviously similar to that obtained with wet 

 sand. It certainly indicates that the liquid phase of the soap gel 

 is held by capillarity in the open meshwork of the solid phase. 



If, however, a 5% gelatine gel is submitted to ether immersion 

 no fluid, or only an exceedingly small amount is extruded. "Where- 

 as a 2% gelatine will show the Struve-Baumstark phenomenon 

 clearly. That there exists a profound physical distinction between 

 these two is evident to the sense of touch alone — the 5% gelatine 

 is dry, the 2% is wet. In the 5%, and in greater concentration, the 

 fluid, we may assume, is not merely held by capillarity, but exists 

 in solid solution in the substance of the framework, whereas in the 

 2% gel the water-logged lattice holds by capillarity a fluid with 

 a small amount of solid matter in solution. This is at any rate 

 an explanation that falls in with modern physico-chemical theories 

 of gelation. Another possible hypothesis is that the liquid phase 

 in the more rigid gels is formed of small vesicles completely en- 

 closed by walls of solid phase in honey-comb fashion. This expla- 

 nation is, however, rendered most unlikely by the results of ultra- 



