THE VARIOUS ROCKS AS SOIL-FORMERS. 6 1 



sometimes be found existing in nature, in moist, secluded 

 places, for weeks after the subsidence of the overflows of 

 rivers whose water is exceptionally free from dissolved mineral 

 matter. 



Separation of Colloidal Clay. This property of the plastic clay 

 substance, of diffusing in pure water, furnishes the means of separating 

 from it the coarser, sandy and silty portions of soils and natural clays, 

 and observing its characteristic properties, so far as the almost un- 

 avoidable admixture of some other substances, presently to be considered, 

 permits. 



In natural soils the clay particles usually incrust the powdery ingredi- 

 ents, cementing them together ; or themselves form complex aggregates 

 (floccules) of large numbers of individual particles. These may be 

 loosened from their adhesion or cohesion either by prolonged, gentle 

 kneading of the wet clay, or by more or less prolonged digestion (soak- 

 ing) in hot water, or more expeditiously, by lively boiling with water. 

 The boiling should not, however, be prolonged beyond the time actually 

 required for disintegration, since (as Osborne ' has shown) long-pro- 

 tracted boiling tends to render the clay permanently less diffusible. 



From the turbid clay-water the diffused clay may be obtained either 

 by evaporating the water (which as the bulk is very large, is usually 

 inconvenient), or, more conveniently, by throwing it down from its 

 suspension by the action of certain substances which possess the prop- 

 erty of curdling (coagulating) the clay substance into flocculent masses 

 that settle quickly. Of all known substances, lime, in the form of 

 lime-water, acts most energetically in producing this change ; but other 

 solutions of lime, as well as most salts and mineral acids, produce the 

 same effects when used in sufficient quantity. Common salt is among 

 the most convenient, because it can most readily be leached out of the 

 clay precipitate thus thrown down. This when white, resembles boiled 

 starch, but being usually colored by iron might be easily mistaken for 

 the mixed precipitate of ferric hydrate and alumina so commonly 

 obtained by chemists in soil analysis. When separated from the water 

 and dried, the jelly-like substance ( " colloidal clay " ) shrinks as 

 extravagantly as would so much boiled starch, into hard, shiny crusts or 

 flakes, which when struck in mass are sometimes even resonant, and 

 bear more resemblance to glue than to the clay of everyday life. Like 

 glue, too, but much more quickly and tenaciously, the dried colloidal 



1 Rep. Conn. Agr. Expt. Stn., 1886, 1887. 



