_Amount of Colloidal Clay in Soils._—Any and all of the kinds of clay mentioned (p. 57) as occurring naturally may, of course, enter into and form part of soils. But as the amount of true, plastic clay substance contained in them is very indefinite, it becomes necessary, in order to classify soils in respect to their tillableness, to ascertain more definitely the amount of pure, or nearly pure, colloidal clay substance contained in the several classes of soils ordinarily recognized and mentioned in farming practice. That this determination can at best be only approximate, is obvious from the fact mentioned above (chapt. 4, p. 59), that pure kaolinite itself is not plastic, and only becomes so by the indefinite comminution and hydration it experiences in the processes of soil-formation. As the progress of this process is also indefinite, the same soil containing particles ranging from the finest to the chalky scales of pure kaolinite, the drawing of a line must be more or less arbitrary and empirical. From numerous experiments and comparisons made, the writer has been led to place the limits of “plastic clay” at and below such grain sizes as will remain suspended (afloat) in a water column eight inches high, during 24 hours. To go beyond this point in the examination of soils for practical purposes, would render such examinations so laborious and hence so rare, that this kind of work would be practically excluded from ordinary practice. According to this view the following percentages of such “clay” correspond approximately to the designations placed opposite: Very sandy soils .5 to 3% clay Ordinary sandy lands 3.0 to 10% “ Sandy loams 10.0 to 15% “ Clay loams 15.0 to 25% “ Clay soils 25.0 to 35% “ Heavy clay soils 35.0 to 45% and over. It must be distinctly understood, however, that these figures make no claim to accuracy or invariability. For, the tilling qualities of a soil containing one and the same amount of such “clay” may be very materially modified according to the kind and amount of each of the several grain-sizes of rock powder or sand they contain. _Influence of fine powders on plasticity and adhesiveness._—An admixture of a large amount of fine powders diminishes materially the adhesiveness of a clay soil, even though it may render it even more “heavy” in tillage; while the admixture of coarse sand, even in very considerable proportions, does not greatly influence the adhesiveness of the clay. The latter alone cannot therefore serve as a proper guide or basis for the classification of soils in respect to tillage; we must also take into consideration the nature and amount of the several granular sediments mixed with it.