5 STRUCTURE OF GELS 75 



some of the imbibing medium is simply pressed out of the gel (syn- 

 eresis). As the gels do not contain individual particles, ultrafiltration 

 cannot be used as a method to discover whether they contain sub- 

 microscopic or amicroscopic structural elements. It gives some in- 

 formation, however, about the approximate pore size of the network 

 structure, since on account of its structure each reticular colloid re- 

 presents an ultrafilter, provided it possesses the firmness needed to 

 resist the fihration pressures applied. In all other methods of research 

 mentioned in Table X the contrast between the movable particles of 

 sols and the immovable frame of the gels finds expression. In dialysis 

 and in the study of Donnan equilibria, amicroscopic particles are 

 removed by diffusion through a membrane which is impermeable 

 to colloid particles. In the case of insoluble gels no membrane is 

 needed, because the colloid portion is itself immovable (see p. 202). 

 For the same reason, the osmotic laws are not applicable to gels, 

 whereas in true sols, where the individual particles are completely in- 

 dependent, they allow of a determination of the number (and therefore 

 the weight) of the particles. Finally, when concentration gradients or 

 potential gradients are applied to gels, the amicroscopic particles 

 diffuse through the gel frame (permeation), or the imbibition liquid 

 migrates through the electrically charged network (electrosmosis). 



Similarly when the equilibrium in a colloid system is disturbed, 

 the behaviour of gels and sols is fundamentally different. Sols can be 

 diluted by the solvating liquid, whereas in true gels only limited swell- 

 ing occurs. In sols the disturbance of stability factors (hydration and 

 charge) may lead to flocculation or coagulation. In contrast to what 

 is commonly asserted, gels do not coagulate, they are "hardened". 

 In technology this is denoted by tanning and in cytology by fixation. 

 Separation of sols results in two microscopically uniform "phases" 

 (Fig. 15, 21, p. .17, 22), whereas in gels the separated drops usually 

 cannot unite and give rise to vacuolization in the originally micro- 

 scopically uniform system (Fig. 23, p. 23). The concepts of limited 

 swelling, fixation and vacuolization, which are mentioned at the bot- 

 tom of the last column in Table X, are familiar to all cytologists and 

 we need waste no time on the question as to which colloids are of the 

 first importance in microscopic and submicroscopic morphology. 



Indeed, the number of colloid systems in biology, whose nature has 

 been ascertained successfully by means of the methods of research 



