THE COLLOIDAL MATTER OF THE SOIL 129 



maintaining a considerable gap between the two states of 

 matter. 1 



70. The phases of a colloidal state. — As already empha- 

 sized, two phases are necessary for a colloidal state — a dis- 

 persive medium and a material that will heterogeneously 

 disperse therein. Threee materials may function as a dis- 

 persive medium — a liquid, a solid, or a gas. In the same way, 

 with each dispersed material there are three possibilities — 

 a liquid, a solid, or a gas. This gives eight general phases 

 to be considered in colloidal chemistry. 2 



The liquid-solid and the liquid-liquid phases are by far 

 the most important as far as soil materials are concerned. 

 The dispersed materials of soil colloids are the minerals either 

 in a hydrous or non-hydrous condition and the organic mat- 

 ter in various stages of decay. The dispersive medium is of 

 course the soil solution. 



71. Colloids vs. crystalloids. — It must not be inferred, 

 because the colloidal state is often wrongly contrasted with 

 the crystalloidal, that material in a colloidal condition is al- 

 ways amorphous. It is often crystalline. Moreover, it may be 

 animate, as some bacteria are minute enough to function col- 

 loidally. It is obvious also that the same chemical material 

 may exist either in the colloidal or non-colloidal state. For 

 example, silicic acid, hydrated ferric oxide, gold, carbon black, 



1 Bancroft, W. T>., Applied Colloidal Chemistry, p. 183 ; New York 

 1921. 



2 The eight phases with examples are : 



Solid in solid Carbon in steel. 



Liquid in solid water of crystallization 



Gas in solid gases in minerals 



Solid in liquid colloidal solution of metals 



Liquid in liquid emulsions of oil in water 



Gas in liquid air in water, foam 



Solid in gas smoke in air 



Liquid in gas clouds 



Gas in gas noncolloidal, merely a mixture of 



molecules. 

 After Burton, E. F., The Physical Properties of Colloidal Solutions, 

 p. 10; London, 1916. 



