Gelation in Reversible Colloidal Systems, 



10? 



energy at the surface of separation of fluid and solid to a minimum, 

 which manifests itself in the spontaneous shrinkage of some of these 

 hydrogels, would act so as to raise the vapour pressure with which the 

 gel is in equilibrium, Kut the operation of this factor would diminish 

 as the surface was diminished by decrease of the water content. These 

 two forces operating simultaneously would alone produce the character- 

 istic gradual diminution in the vapour pressure of the gel as the fluid 

 component is diminished. The break in the direction of the curve 



Fig. 2 (reproduced from van Bemmelen, 'Zeits. f. Anorg. Chem.,' toI. 13, 1896, 

 p. 233). — Equilibrium between a Hydrogel of Silica and Water Vapour. 



Ciu^e tbe rate of removal of water very slow. In curve much 



more rapid. The arrows indicate whether the curve shows the removal or 

 the reabsorption of water. 



when dehydration has been relatively rapid and is nearly complete, is 

 what must occur when the capijlary spaces in the framework become 

 commensurate with the masses (small spheres for instance) of solid 

 out of which the framework is built, and when, therefore, any further 

 diminution in the capillary spaces involves deformation of those masses,, 

 unless the removal of water is so slow that the very slow rate of 

 readjustment in the solid phase is not exceeded. Lastly, the very 

 linjited powers of reabsorption of fluid by ♦completely .dried irrever- 

 sible gels would, on this view, again not necessarily represent a 

 reformation of the phases, that is to say, a real interaction between 

 silica and water, but the refilling of capillary spaces by water due to 

 the. excessive capillary tension of these very minute capillaries. The 

 capacity for reabsorption would therefore be diminished by any agent 

 which facilitates the annealing of the dried gel and so destroys the 

 capillary interspaces. Such an agent is heat, and van Bemmelen found 



