to rest at the bottom of the beaker, becomes more and more rounded, and finally spherical. Then, unaided, it rises to the top and mingles itself with the aniline which remained on the surface. After a brief interval a second drop falls, imitating the performance of the first one; and, like its predecessor, rises to the surface, after remaining for a short time at the bottom of the vessel. And so long as we keep the temperature a few degrees above that of equi-density, the process of partition and reunion goes on indefinitely. The action is automatic and continuous, and the large size of the drop and of the neck, and the slowness of the procedure, enables us to follow with ease every stage in the formation of a parting drop. [Illustration: __Fig._ 21._] [Illustration: __Figs._ 21 and 22.—Automatically formed aniline drops, showing the formation of droplets from the neck._] And now as to the explanation of this curious performance. When the aniline reaches the surface, and spreads out, it cools by contact with the air more rapidly than the water below. As it cools, its density increases, and soon becomes greater than that of the water, in which it then attempts to sink. The forces of surface tension prevent the whole of the aniline from falling—the water surface can sustain a certain weight of the liquid—but the surplus weight cannot be held, and therefore breaks away. But when the detached drop reaches the bottom of the vessel, it is warmed up again; and when its temperature rises above that of equi-density it floats up to the top. And so the cycle of operations becomes continuous, owing to cooling taking place at the top and heating at the bottom. Perpetual motion, you might suggest. Nothing of the kind. Perpetual motion means the continuous performance of work without any supply of energy; it does not mean merely continuous movement. A steam-engine works so long as it is provided with steam, and an electric motor so long as it is fed with electricity; but both stop when the supply of energy is withdrawn. So with our aniline drop, which derives its energy from the heat of the water, and which comes to rest immediately the temperature falls below 147° F. or 64° C. But in order that the process of separation and reunion may continue, the cooling at the top is quite as necessary as the heating at the bottom. Our aniline drop is in essence a heat-engine—although it does no external work—and like all heat-engines possesses a source from which heat is derived, and a sink into which heat at a lower temperature is rejected. We might, with certain stipulations, work out an indicator diagram for our liquid engine, but that would be straying too far from our present subject.