On the Mechanism of Gelation in Reversible Colloidal Systems. 95 



" On the Mechanism of Gelation in Eeversible Colloidal Systems." 

 By W. B. Hardy, Fellow of Gonville and Caius College, 

 Cambridge. Communicated by F. H. Neville, F.E.S. 

 Keceived January 12, — Eead January 25, 1900. 



Speaking generally, colloidal matter occurs in three conditions : — 



(1) As fluid mixture, colloidal solutions, or sols, as Graham called 



them ; 



(2) Solid mixtures of fluid and solid, the gels ; and 



(3) Solids, such as dry silica or dry glass. 



The property of forming gels is not possessed by all those mix- 

 tures which have been classed as colloids. Some only form slimes, 

 which even to the point of actual drying retain the fluid property of 

 flowing. Serum albumen and water is an instance. 



Those which form gels fall into two well-defined classes, according 

 to whether the change from the sol to the gel is, or is not, reversible 

 by a reversal of" the conditions which produce it. Silica and water 

 may be taken as the type of the latter, gelatine and water of the 

 former. When a hydrosol of silica forms a hydrogel, the latter is 

 *' insoluble." To this class belong hydrosols of metallic hydrosulphides 

 and oxides. A hydrosol of gelatine sets to a hydrogel by lowering the 

 temperature ; the process is however reversed when the temperature is 

 again raised. As the inner mechanism of the gelation of the hydrosols 

 must diff'er in the two cases, since in the one irreversible, in the other 

 reversible molecular aggregates are formed, I propose to distinguish 

 the processes by diff'erent names. The production of an insoluble gel 

 I will call " coagulation," of a soluble gel " setting." This nomen- 

 clature is in accordance with general usage. 



Temperature is the most potent factor in determining whether a 

 mixture which forms reversible gels is in the sol or gel state. There 

 is also a limiting concentration of the solid below which the gel state 

 is impossible at any temperature. 



" Setting," as a rule, follows on a fall of temperature. Caseine, the 

 chief proteid of milk, furnishes, I believe, the only known exception. 

 In the presence of a small quantity of free alkali it forms a hydrosol. 

 When a small quantity of a solution of calcium chloride or nitrate is 

 added to this, a mixture is produced which forms a hydrogel on 

 warming, and which reforms the hydrosol on again cooling.* 



Part I. BeversiblelColloids. 



Systems containing two or three components occur, that is, binary 

 or ternary mixtures. The binary system, agar-and-water, was studied 



* Sydney Einger, * Journal of Physiology,' vol. 11, p. 464, 1890. 



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