60 SOILS. 



chalky kaolinite to consist of minute, mostly rounded, origin- 

 ally six-sided, thin plates, which when pure resemble to the 

 touch powdered talc (soapstone) or even black-lead, rather 

 than any clay known to common life. But being exceedingly 

 soft, the kaolinite substance is easily ground or triturated into 

 an extremely fine powder ; and Johnson and Blake 1 succeeded 

 in producing sensible plasticity and adhesiveness by long-con- 

 tinued trituration of kaolinite with water in a mortar. A 

 similar process, but continued much longer by the mechanical 

 agencies concerned in soil-formation (see chapt. i), is un- 

 questionably the chief factor concerned in the formation of 

 natural plastic clays; but whether this is the only process by 

 which the powdery kaolinite may be transformed into plastic 

 clay, is a question not definitely settled. It is at least possible 

 that repeated freezing and thawing, as well as the action of 

 hot water, may take a part in the transformation, beyond that 

 by which they destroy the crumbly (flocculated) structure of 

 soils and clays, and render them plastic ; as is done in the ma- 

 turing of clays by potters. 



Causes of Plasticity. In any case the property of plasticity 

 and adhesiveness is restricted to the particles so fine that they 

 fail to settle, in the course of 24 hours, through a column of 

 pure water eight inches (200 m) high, while some are so ex- 

 tremely minute that they will not settle for many months, and 

 even for several years. 2 Such turbid " clay water " may 



1 American Journal of Science, 2d Ser., Vol. 43, p. 357. 



2 Williams (Forsch. Agr. Phys. Vol. 18, p. 225 ff.) claims that the diameter of 

 the minutest clay particles is one-thousandth of a millimeter, their form being that 

 of scales showing continual (Brownian) motion in water. He maintains that the 

 plasticity of clay is due to this minute size, and this view has gained wide accept- 

 ance in late works on the subject. But this assumption cannot be maintained in 

 the face of the fact that nothing like the adhesive plasticity of clay can be attained 

 even by the finest powders of other substances, least of all by those having the 

 closest mineralogical resemblance to kaolinite, such as graphite and talc. Above 

 all, the most persistent trituration with water utterly falls to restore plasticity to 

 clay once baked so as to expel its water of hydration, although the fineness of the 

 particles is thereby not only not diminished, but actually increased, by contraction 

 in heating. No powders however fine can replace the functions of clay in soils, 

 viz. the maintenance of floccules, and tilth dependent thereupon ; and they dis- 

 tinctly impair the plasticity of clay. The fine " slickens " of quartz mills merely 

 render soils containing them more close and impervious, and more difficult to 

 flocculate. Even gelatinous masses like hydrated ferric and aluminic oxids fail to 

 replace clay in its adhesive functions. 



