STUDIES OF VORTEX-RINGS. 181 



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 differ- 

 ing 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 per- 

 mit us to detect in the phenomena of solution the same character of 

 intermittence and discontinuity that marks that of osmose. A pen- 

 ful of ink, a bit of sugar, a thousand simple means of observing the 

 development of vortices, may be suggested. A piece of thread, hang- 

 ing 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 foli- 

 age 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 



