Figs. 14 to 20.—Formation of a drop of orthotoluidine, showing the droplet. Seven stages.
Ascending or Inverted Drops.—If we discharge orthotoluidine into water when both are hotter than 75° F., the former liquid will rise, as its density is now less than that of water. If, therefore, I take a funnel with the stem bent into a parallel branch, so as to discharge upwards (A, [Fig. 13]) and raise the temperature of both liquids above 75° F., we see that the drop gradually grows towards the top of the water, finally breaking away and giving rise to the droplet. Everything, [pg 32] in fact, was the same as in the case of a falling drop, except that the direction was reversed. A slight rise in temperature has thus turned the whole process topsy-turvy, but the action is really the same in both cases. When, on heating, the water acquired the greater density, its buoyancy overcame the pull of gravitation on the orthotoluidine, and accordingly the drop was pushed upwards, the result being the same as when it was pulled downwards. An inverted drop may always be obtained by discharging a light liquid into a heavier one, e.g. olive oil into water, or water into any of the liquids mentioned on p. 19, below the equi-density temperature.
[pg 33]
LECTURE II
Automatic Aniline Drops.—In the foregoing experiments the drop was enlarged until it broke away by feeding it with liquid; but it is possible to arrange that the formation shall be quite automatic. The experiment, as we shall see, is extremely simple, and yet it contains an element of surprise. Into a beaker containing water nearly boiling I pour a considerable quantity of aniline, which at first breaks up into a large number of drops. After a short time, however, all the aniline floats to the surface, having been warmed by contact with the water to a temperature higher than that of equi-density (147° F., or 64° C.)—which is exactly what we should expect to happen. There it remains for a brief period in the form of a large mass with the lower portion curved in outline. Soon, however, we observe the centre of the mass sinking in the water, and taking on the now familiar outline of a falling drop. Gradually, it narrows at the neck and breaks away; but as aniline is a viscous liquid, the neck in this case is long and therefore easily seen. The large drop breaks away and falls to the bottom of the beaker, its upper surface rising and falling for some time owing to the recoil of its skin after separation, [pg 34] finally becoming permanently convex. Immediately after the large drop has parted, the upper mass shrinks upwards, spreading out further on the surface of the water, with the result that the long neck is severed at the top, its own weight assisting the breakage. Now follows the resolution of the detached neck into two or more spheres, usually a large and a small ([Fig. 22]). And now, to those who view the experiment for the first time, comes the surprise. The large drop, which [pg 35] was more or less flattened when it came 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, [pg 36] 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.
Fig. 21.
