106 



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



beyond which the equilibrium is no longer between a fluid solution and 

 a solid solution, but between two fluid solutions. 



The first worker to regard gelation as being due to the formation of 

 two phases, one fluid and the other solid, was van Bemmelen.* He 

 has given a suggestive discussion of the formation and structure of 

 gels, based chiefly upon the manner in which amorphous material is 

 precipitated from a solution, and he is led to the conclusion that 

 " coagulation or the precipitation of a gel from a solution seems to be a 

 similar phenomenon ; a desolution (Entmischung) which forms, not two 

 layers completely separated from one another, but 



" 1. A framework of a material which is in a more or less transitional 

 state between fluid and solid, and which presents those special 

 properties to which the term colloid is applied. 



" 2. A fluid which is enclosed within this framework." 



Van Bemmelen, however, does not consider that these two parts can 

 be considered as two phases in the sense of the phase rule, since there 

 is no sharp line between them,! and he therefore concludes that the 

 phase rule cannot be employed to elucidate the phenomena. This 

 opinion is based upon a study of the equilibrium between the water 

 content of various gels and the vapour pressure, so patent and 

 thorough as to give it very great weight. The curves of the equi- 

 librium points are gradually bending lines if the dehydration of the 

 gels is sufficiently slow, but if dehydration is relatively rapid there is a 

 sudden change of direction (fig. 2) when the water content is very 

 much diminished (1 to 2H2O to IS1O.2). 



It is possible that the form of these curves does not necessarily 

 depend upon the absence of a clear separation between the fluid and 

 the solid portions of the gel. When one considers how small is the 

 mutual solubility of silica and water and how slight therefore the in- 

 fluence which a given mass of silica is likely to exert upon the vapour 

 pressure of even a relatively small mass of water, it is probable that 

 the form of the curve is determined more by the operation of secondary 

 influences, such as capillary tension, which depend on the structure of 

 the gel, than upon the direct interaction of silica and water. Capillary 

 tension would tend to lower the vapour pressure J with which the gel 

 is in equilibrium to a greater and greater extent as the spaces in the 

 solid framework of the gel became smaller and smaller with the 

 decrease in the water content. The tendency to reduce the surface 



* ' Zeits. f . Anorg. Chem.,' vol. 18, 1898, p. 20. 

 t ' Zeits. f. Auorg. Cliem.,' vol. 18, 1898, p. 121. 



J The vapour pressure which van Bemmelen measured is that of the free surface 

 of the gel. It is analogous to that at the ojpen ends of a niimber of capillary tubes 

 filled with fluid. . 



