forced down tight into the middle of the tube. This instrument gives at each expulsion of the colored ring a corresponding return ring, a kind of negative ring, that descends clear in the interior of the colored liquid, or vice versa, if it is the liquid in the upper part that has been colored. The same phenomena appear when smoke is used, as in Fig. 4. We may dispense with all apparatus and Let fall from the height of about an inch a lightly tinted drop into a still colorless mass. From this experiment chemists learned, even before Trowbridge explained the theoretical reason for it, that it was best to employ liquids differing but little from each other, or at least such as were easily diffusible in each other. Reusch, however, has described some very unstable rings of oil in water that may be produced with an apparatus similar to ours. It is interesting, in view of the cosmic examples to which we have referred, that after the oil of the upper compartment has been replaced with water, each jet carries along from the borders of the tubing myriads of little oily drops, the drift of which renders visible in the interior of the clear water the annular constitution of the vortex formed by the irruption of a mass of absolutely identical liquid. The generality of the fact may be verified with an emulsion of ginger, or simply with superficial dust on the brightly lighted bottom of a white porcelain plate.
Imperceptible grains of coloring substance, put upon the surface of the water, immediately give rise to fine descending trains, which permit us to detect in the phenomena of solution the same character of intermittence and discontinuity that marks that of osmose. A penful of ink, a bit of sugar, a thousand simple means of observing the development of vortices, may be suggested. A piece of thread, hanging from a glass filled to the brim, makes an excellent capillary siphon, and furnishes perfect continuous movements, so that, unless the liquid is superficially stirred, or the lower glass is shaken, we obtain trains of provoking fixity, that follow their determined route without giving a pulsation to betray their interior motion. Of this character are the upright columns of mist that rise from the calm plain of the desert. So a flexible thread, carried by a balloon, revolves in the breath of the wind without breaking. The direction of a straight line is not an essential one; and, if the density of the liquid happens to be variable, the thread takes the form of an elongated spiral, the curves of which are alternately narrower and wider, and end in scrolls resembling foliage or florescence. These phenomena are not transient, but may last for days, although the rings we have been considering continue only for a few moments, and, while they present a real elasticity of form as long as they are in motion, never survive the loss of the impulse that produced them. Most frequently denser nuclei appear in their interior, which take the lead, and descend as if suspended from elastic arches, of which they draw out long branches (Fig. 2, B). Each of these nuclei appears to undergo, in its interior, the different transformations
