THE ORIGIN OF GUMBOTIL 109 



Ferrous silicates upon hydrolysis give ferrous ions. These 

 may react with other ions of the soil solution to form the slightly 

 soluble ferrous hydroxide or carbonate, or, what is more probable 

 still, they may be immediately oxidized to ferric ions by the dis- 

 solved oxygen and then precipitated as the insoluble hydrated 

 ferric oxide or basic carbonate. 



Some may raise the question as to the possibility of the oxida- 

 tion of the iron below the surface and out of contact with the air 

 above. It must not be forgotten that by virtue of diffusion not 

 only oxygen but also other dissolved gases tend to go just as far 

 as the water does. Except at points where organic matter is 

 undergoing decay, it is very doubtful if more than minimal traces 

 of secondary ferrous compounds exist within or beneath the soil. 



Crystalloids and colloids. — The resulting so-called weathering 

 products may be grouped into two great general classes, namely, 

 crystalloids and colloids. Crystalloids include all of the soluble 

 acids, bases, and salts. They are simple in structure, that is, they 

 exist in the dissolved state either as molecules or as ions. They are 

 characterized by a relatively high diffusion speed and by the 

 power to pass, though more or less slowly, through colloidal mem- 

 branes. Nor is the colloidal membrane lacking in the soil; the 

 clay itself may be considered as such a membrane. 



Generally speaking, the term colloid is applied to all glue- 

 like, gelatinous, amorphous substances. Strictly speaking, colloids 

 represent suspensions of matter in an extremely fine state of 

 subdivision, the suspended particles having diameters varying 

 from 1 ix to 100 fMfx. The properties of colloids are primarily surface 

 properties. The extent of surface development, and hence the 

 magnification of specific properties, such as solubility, adsorption 

 power, etc., may be seen from the following illustration: A centi- 

 meter cube of any solid substance, say platinum, exposes a surface 

 of 6 square centimeters. Let this cube be divided successively 

 and decimally to the dimensions of colloid particles. The total 

 surface then exposed by the platinum will vary between 60 and 

 6,000 square meters. 



Some colloidal properties involved in this problem. — Since colloids 

 play an important role in the formation of soil strata and since they 



