THE COLLOIDAL MATTEE OP THE SOIL 131 



tion.^ A very good example is afforded by treating a colloidal 

 clay suspension with a little calcium hydroxide. The tiny 

 particles almost immediately coalesce into floceules, and be- 

 cause of their combined weight, sink to the bottom of the 

 containing vessel, leaving the supernatant liquid clear. The 

 same action will take place in the soil itself, but of course with 

 less rapidity and under conditions less noticeable to the eye. 

 Some dispersed materials, when thus separated from their 

 dispersive medium, will reassume the colloidal state with 

 ease when an opportunity is offered. In other cases, the col- 

 loidal condition is difficult to restore. Gelatin is an example 

 of the first group and is called a reversible colloid. Ferric 

 hydrate is an example of the more or less irreversible type. 



Just why this phenomenon of flocculation or agglutination 

 takes place is rather difficult to state. It is found that cer- 

 tain colloids, when subjected to the proper electric current, 

 will migrate to either the positive (anode) or the negative 

 (cathode) pole. These particles evidently carry a charge of 

 electricity. Hydrated ferric oxide, aluminium hydrate, and 

 basic dyes, for example, move toward the cathode and carry 

 a positive charge ; while arsenious sulphide, silicic acid, gold, 

 silver, humus and acid dyes move toward the anode and are 

 negative. It is assumed that as long as the colloidal particles 

 remain charged, they repel each other and the colloidal state 

 persists. When an electrolyte is added, which develops by 

 ionization a dominant opposite charge, it is supposed to cause 

 a neutralization of the repellent electricity carried by the 

 colloidal particles, and flocculation occurs. 



Certain colloids may flocculate certain others, as the gela- 



tinization of silic acid by hydrated ferric oxide. At times 



one colloid may protect another, probably by surrounding it 



former carrying a positive and the latter a negative charge of electricity 

 (H-*- 4- C- ) . KNOg gives K-*-+ NO3-. The ionization varies with the 

 substance, the dilution and certain other conditions. 



*See Wolkoff, M. I., Flocculation of Soil Colloidal SoluUo%s; Soil 

 Sci., Vol. I, No, 6, pp. 585-601, June, 1916. A good bibliography is 

 appended. 



