It has long been recognised that a trace of soluble salt will bring about the flocculation of the material suspended in a turbid liquid; and because of its bearing upon such practical matters as the clearing of drinking water, the deposition of silt at the mouths of estuaries, and the improvement of the texture of heavy soils, the process has received a considerable amount of investigation. It is probably related to the flocculation of colloids by similar reagents; the fine particles of clay which chiefly cause the turbidity of natural waters being composed of a hydrated silicate of alumina and other bases possessing properties akin to those of the colloids. Indeed Schloesing has proposed to call that portion of clay material which remains obstinately suspended in water, sometimes for days together, ‘colloid clay,’ as something distinct in kind from the other particles in the soil. However it is reasonable to suppose that the distinction between the ‘colloid’ clay and the rest is in the main one of size, the colloid particles being so small as to be beyond the limits of microscopic vision, just as the trae colloids may be regarded as consisting of very large molecules, lying between the molecules which go into solution and the molecular aggregates that remain in water as suspended solids.