Theory of the Settling of Fine Particles.

in density due to particle content, the new elements which are continuously brought to the top of the cold wall are drawn from the top portion of the liquid and are of nearly uniform density. When A reaches its equilibrium depth, its downward flow is decelerated and it turns into the interior of the liquid, this motion being followed by the elements behind A. The equilibrium depth for A thus acts approximately as a boundary limiting the flow, and a local convection is established in the upper portion of liquid, this portion being mixed so that' its particle content becomes uniform, equal to the average content of the portion before mixing. Thus a definite upper layer is formed, with an internal convection system, the density being uniform and less than that of the liquid below. The bottom of this layer acts for the fluid below it, as did the top surface of the liquid in the formation of the top layer. A second layer establishes itself, similar in character to the top layer, and in this manner the entire volume of liquid is divided into layers. The layer depth is characterized by the fact that the thermal contraction of the fluid elements originally at its top is sufficient to increase the density to that originally obtaining at its bottom, due to the increased content of suspended particles. A little consideration shows that this conception of the role of convection in the formation of strata fits at least qualitatively all of the observations which have been described above, namely, the effect of changing the amount of suspended matter, changing the temperature gradients, and aging or preliminary settling.