394 THE COLLOIDAL STATE 



According to Oden * the humic acid of the soil exerts a 

 protective action on clay, preventing its coagulation by elec- 

 trolytes. 



Electric Endosmose. — This term is applied to a phenome- 

 non which in a sense may be regarded as the inverse of 

 kataphoresis. Whereas in the latter case it is the disperse 

 phase which wanders in the electric field, while the solvent, 

 or continuous phase, remains at rest, the reverse conditions 

 hold in the case of Endosmose. This is effected by placing 

 the colloidal sol in a vessel the walls of which are impermeable 

 to the colloid but permit of the free passage of the continuous 

 phase. When this is submitted to a high difference of poten- 

 tial the effect is to draw the solvent out of the containing 

 vessel and thereby to dehydrate the sol. Attempts have been 

 made to apply this principle to the problem of the economical 

 dehydrating of peat for the purpose of obtaining fuel but so 

 far they have not been very successful. 



It has also been proposed to dry timber by electrosmotic 

 removal of the cell sap and to preserve the timber by replacing 

 the sap removed by a suitable preservative. The principle 

 of kataphoresis has also been applied to the dehydration and 

 purification of clay.f 



EMULSOIDS. 



The emulsoids form the second great group of colloids 

 and from a biological point of view they are the more im- 

 portant of the two. 



Substances such as albumen, gelatine, gums, starch, agar, 



etc., which belong to this group, tend to swell up in contact 



* Oden : " J. Landw.," 1919, 67, 177. 



t For a fuDer account of these processes, see " Second Report on 

 Colloid Chemistry," etc., British Ass. Reports, 1918. 



