388 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



When not strained they resemble liquids. If stretched, (i) they 

 contract on warming and rapid cooling produces expansion, (2) they 

 become doubly refracting. 



They are deformed without change of volume (cross-section x 

 length) ; on stretching a cylinder of gelatin its cross-section diminishes 

 as the length increases. 



(3) Nature of Gels. In composition gels resemble the organic 

 material of plants ; in these there is a cell structure. Liquid is enclosed 

 in cells formed by a solid phase. Gels consist of a solid x continuous 

 phase enclosing a liquid phase. 



The reversibility is accompanied by a distribution of water between 

 the phases and is affected by the presence of salts. 



(4) Diffusion of Substances and Reactions in Gels. In dilute gels 

 diffusion takes place as in water. The rate is slower with strong gels. 

 The rate of diffusion is affected by various substances : urea, iodides, 

 chlorides accelerate diffusion; sodium sulphate, glucose, alcohol, 

 glycerol retard diffusion. 



These substances affect the distribution of water between the two- 

 phases and probably also the relative volumes of the gel wall and the 

 free liquid. Diffusion takes place chiefly in the liquid. The reaction 

 does not proceed continuously, but the product if insoluble is deposited 

 in strata. Many substances can thus be obtained in the form of large 

 crystals and often spherolites are formed. 



Some organic compounds on separation from hot solvents on cool- 

 ing first form transient gels which gradually crystallise. Crystalline 

 minerals may be formed from gels in a similar way. 



(5) Structure of Gels. Gels resemble the organic matter of plants 

 and animals in composition that is solid matter containing 80-90 per 

 cent of water. Cell structure in animals and plants is visible with a 

 microscope. Though apparent structure can be seen in gels with a 

 microscope it is not real, but the presence of a structure in gels is 

 indicated by the diffusion and reaction of substances in gels. 



The Phenomenon of Adsorption by Colloids. 



The phenomenon of adsorption by colloids is due partly to the 

 large boundary surface between the particles and partly to the electrical 

 charge upon the particles. In some cases the first cause may pre- 

 dominate, in other cases the second cause. 



1 In this connection a solid is a substance less delormable than a liquid, but not non- 

 deiormab.e. 



