“Silt”, like most insoluble substances, when suspended in water is most easily flocculated by calcium salts when the suspension is neutral. The addition of alkali stabilizes the suspension and renders flocculation more difficult. Soil “clay,” however, behaves in an opposite manner and is precipitated from alkaline suspensions more readily than from neutral suspensions. In this behaviour clay resembles silicic acid and some other members of the so-called “emulsoid” colloids, and it is suggested that the clay particles are protected by such colloids and thus behave as an “emulsoid” and not as a “suspensoid.”

If this is true then the action of lime, which being alkaline nevertheless flocculates clay, is seen to be in accordance with the facts of colloid chemistry.

The view is advanced and some experimental support of it is described, that clay, as an “emulsoid,” protects the larger particles which by themselves are “suspensoid.” The soil aggregates are conceived as having large nuclei surrounded by particles which become smaller from the centre of the aggregate outwards, the clay ultimately imposing its “emulsoid” nature on the whole aggregate, and on the whole soil in normal cases. Fine silt soils are not flocculated by calcium hydroxide on account of the inefficiency of the relatively small amount of “emulsoid” clay to protect the large “suspensoid” surface exposed by the fine silt.