AGRICULTURE AND RURAL ECONOMY. 539 



self. " It is highly probable that all clays contain kaolinite as 

 the chief chemical ingredient of their clayey portion, but more 

 or less in admixture with other hydrous aluminous silicates, 

 and with silicates of iron as well as with alumina and iron hy- 

 drates." The clay which pure water takes np from clayey 

 soils "is a mixture of all the very finest kinds of matter 

 which the earth may contain, and its properties vary accord- 

 ing to the kinds and states of that matter." The character- 

 istic ingredient of clays, whatever it may be, (1) is so fine 

 that much of it cannot be defined by the microscope ; (2) re- 

 mains for weeks and months suspended in water from which 

 sand, silt, and rock-dust have settled out ; (3) can be coagu- 

 lated and precipitated out, as Schloessing has shown, by very 

 small quantities of common salt, and still smaller proportions 

 of salt, potash, lime, and magnesia ; or, as Professor Johnson 

 has lately noticed, by freezing. As Hilgard has shown, (4) 

 it becomes, " on drying, a hard, often horny, mass, difficult to 

 break, and somewhat resonant;" (5) "when dry, adheres to 

 the tongue so tenaciously as to render separation painful ; 

 (6) but imbibes water with great energy, and swells to many 

 times the bulk it has when dry; (7) swells less readily in 

 presence of much iron ; and, finally, (8) when moistened, can 

 be worked into a plastic condition, and becomes " exceeding- 

 ly tenacious and sticky." In brief, clay " belongs to the class 

 of bodies designated by Graham as colloids (glue-like), and 

 confers on the soil peculiar and valuable properties." 



Influence of Clay in the Soil. 



"When a dry clay soil is copiously rained upon, or ex- 

 posed to the abundant pure water of melting snow, its 'clay' 

 swells, assumes a gummy or clayey consistency, and, by en- 

 veloping the sand and silt grains, confers upon the whole 

 mass its own sticky qualities. When soluble salts of the sur- 

 face-soil are completely washed out of it, then the clay en- 

 ters into suspension, and is carried down into the pores of the 

 soil and clogs them up. . . . Thus it comes that clay lands 

 are so muddy and impervious to rain in spring aud autumn, 

 and that the water they contain dries out slowly, as it does 

 from dissolved glue, gum, or jelly. The sprinkling of the 

 wet and sticky clay with a weak brine or with a much 

 weaker solution of a lime-salt, either sulphate of lime (gyp- 



