collected as above will not diffuse in water it may be washed with water and its weight directly obtained. An excess of iron in clay will usually allow of the above treatment. =228. Properties of Pure Clay.=—The percentage of pure clay as obtained by the procedure described is about seventy-five in the finest natural clays, forty-five in heavy clay soils, and fifteen in ordinary loamy soils. When freshly precipitated by brine it is gelatinous resembling a mixed precipitate of iron and aluminum oxids. Its volume greatly contracts on drying, clinging tenaciously to the filter, from which it may be freed by moistening. On drying, it becomes hard, infriable, and often resonant. It usually possesses a dark brown tint due to iron oxid. Under the action of water it swells up like glue, the more slowly as the percentage of iron is greater. In the dry state it adheres to the tongue with great tenacity. According to Whitney the finest particles of colloidal clay have a diameter of 0.0001 millimeter. With a magnifying power of 350 diameters, however, Hilgard states that no particles can be discerned. =229. Chemical Nature of the Fine Clay.=—The fine particles separated as above consist essentially of hydrous aluminum silicate or kaolinite. It doubtless contains, however, other colloids or hydrogels whose absorptive powers are similar to those of clay. It appears also to contain sometimes free aluminum hydroxid, and colloidal ferric hydroxid, and amorphous zeolitic compounds. While the most careful mechanical separation can give at best only approximately the really plastic kaolinite substance, yet it is far closer than that attained by determination of total alumina with boiling sulfuric acid. By the latter treatment all the lime-kaolinite particles are decomposed and the method does not lead to even an approximate estimate of the soil’s plasticity. =230. Separation of the Fine Sediments.=—The sediments remaining after the separation of the clay and fine silt are ready for separation in the churn elutriator. The apparatus mounted, as already described, is brought into use by beginning with a low velocity of the water in the upright tube. The rate of flow should be set at from 0.25 millimeter to 0.50 millimeter per second, and the churn put in motion.