All who have learned a little of chemistry doubtless remember the experiment with vortex rings produced by phosphorus trihydride mixed with a little phosphide of hydrogen. As this curious phenomenon evidently does not depend upon the peculiar properties of this gas, I have been trying for some time to reproduce it by means of tobacco smoke, and even with chemical precipitates, which are, in a way, liquid smoke. After a few tentatives made at different times, my experiment succeeded perfectly. The following is, in brief, the mode of operating:
Take up a little hydrochloric acid in a pipette and put a few drops of it into a very dilute solution of nitrate of mercury, and you will obtain rings of mercurial chloride that will, in their descent, take on the same whirling motion that characterizes the aureolas of phosphureted hydrogen.
The drops of acid should be allowed to fall slowly, and from a feeble height, to the surface of the liquid contained in the vessel. It is unnecessary to say that the result may be obtained through the use of other solutions, provided that a precipitate is produced that is not very thick, for in the latter case the rings do not form. If need be, we may have recourse to milk, and carefully pour a few drops of it into a glass of water.
As regards smoke rings, it is easy to produce these by puffing cigar smoke through a tube (Fig. 1). But, in order to insure success, a few precautions are necessary. The least current of air must be avoided, and this requires the closing of the windows and doors. Moreover, in order to interrupt the ascending currents that are formed in proximity to the body, the operation should be performed over a table, as shown in the figure. The rings that pass beyond the table are not perceptibly influenced by currents of hot air. A tube ¾ inch in diameter, made by rolling up a sheet of common letter paper, suffices for making very beautiful rings of one inch or more in diameter. In order to observe the rings well, it is well to project them toward the darkest part of the room, or toward the black table, if the operator is seated. The first puffs will not produce any rings if the tube has not previously been filled with smoke. The whirling motion is perfectly visible on the exit of the ring from the tube, and even far beyond.
As for the aspect of the rings projected with more or less velocity to different distances from the tube, Figs. 2, 3, and 4 give quite a clear idea of that. Figs. 3 and 6 show the mode of destruction of the rings when the air is still. There are always filaments of smoke that fall after being preceded by a sort of cup. These capricious forms of smoke, in spreading through a calm atmosphere, are especially very apparent when the rays of the sun enter the room. Very similar ones may be obtained in a liquid whose transparency is interfered with by producing a precipitate or rings in it. - La Nature.