THE VARIOUS ROCKS AS SOIL-FORMERS. 



59 



" Colloidal " Clay. 1 



In connection with soils, clay may be defined, in the most 

 general terms, as being the substance which imparts plasticity 

 and adhesiveness to soils when wetted and kneaded, and 

 which, when heated to redness, loses this property completely 

 and permanently, becoming hard and coherent in proportion 

 to the degree of heat to which it is exposed. 



In common life, however, the name is applied to the whole 

 of any naturally occurring earth which on wetting and knead- 

 ing assumes a reasonable degree of plasticity and adhesiveness. 

 When the latter property becomes nearly or quite insensible, 

 the earth is designated as a " loam," more or less " clayey " 

 according to the amount of the pure, plastic and adhesive ma- 

 terial associated with the mineral powders and sand that form 

 the bulk of most soils. 



Chemically, the pure clay substance 2 probably consists (as 

 has been stated above) of silica and alumina in the proportion 

 of nearly 46 to 40, the rest (14%) being water of hydration, 

 which is lost on burning the clayey material. But while it is 

 true that such is the composition of the plastic substance of 

 clays, plasticity and adhesiveness are by no means invariable 

 properties of this compound. In its purest state, as kaolinite, 

 it is readily mistaken for chalk, ( and is sometimes used as 

 such), being powdery to the touch and entirely devoid of plas- 

 ticity 3 when wetted and kneaded. The microscope shows this 



1 This term was first employed by Th. Schloesing, in communications to the 

 French Academy of Sciences, and reported in the Comptes Kendus of that body ; 

 first in 1870. Unaware of Schloesing's work, the writer began a full investiga- 

 tion of the subject of mechanical soil analysis in 1871, and published the results 

 in iSjj (Am. Jour. Sci., Oct. 1873). Up to that time the limited resources of the 

 library of the University of Mississippi had not given him an opportunity to see 

 Schloesing's publication. The two independent investigations, chough conducted 

 on somewhat different lines, gave of course practically the same results, and com- 

 plement each other. 



2 There is still some discussion as to the chemical identity of colloidal clay with 

 Kaolinite; but the objections are not convincing. 



1 It has of late been attempted to extend the meaning of this word to the be- 

 havior of all powders when wetted with water. Hut the adhesive plasticity of 

 clay stands almost alone, in that (aside from contraction) it preserves in drying 

 the form into which it may have l>een molded while wet, even when struck^ 

 whereas other powdery substances similarly treated at once collapse back into the 

 original powder. The exclusive use of clay in modeling offers the typical example 

 of plasticity as generally understood. The addition of any powdery substance, 

 however fine, diminishes the plasticity ot clay. 



