Why do the light particles, instead of scattering promiscuously in the liquid or gaseous medium, take this special state of mobile equilibrium and regular grouping? Why this form so fragile in appearance, instead of any other more simple and possibly more stable? Is it by pure chance or by pure direction? And, if the latter is the case, what is the secret of it? It will not be difficult to detect the secret in a short time, and a few words will show that we have here, not an exceptional and rare case, but a general law of nature, common examples of which exist under our eyes.
Fig. 1. Wreaths of Tobacco-Smoke. (From Brauwer's picture in the Lacaze Gallery, Louvre.)
Wreaths of the same shape are often seen to issue from the mouths of cannon when they are fired off; I have seen them following the locomotive of an express-train. Any one who has pursued a course in chemistry may remember the beautiful experiments in which bubbles of phosphoretted hydrogen take fire spontaneously as they rise to the surface of the basin, and develop superb wreaths of white fumes.
Nature herself sometimes produces this phenomenon without any human intervention, and shows us the vortices of the ignis fatuus, to which the middle ages attached so many poetical legends, rising on summer evenings from the stagnant waters of marshes. The craters of volcanoes also frequently give off smoke in the form of magnificent ring-clouds.
Huygens observed, with one of the first telescopes, the annular form of those curious satellites of Saturn, the singular equilibrium of which is still a subject of discussion. The periodical November meteors form a real ring of planetary fragments around the earth; everything seems to prove that the milky way is nothing but a gigantic ring of cosmic dust, of which our sun and its satellites are only a few grains; the question of the form of the zodiacal light is hardly doubtful; and, finally, among the infant worlds that we call nebula?, the annular figure recurs with such frequency that it can no longer be considered exceptional; and the inspiration of genius which caused Laplace to see in the fracture of such a ring the whole origin of our solar system is reflected in the broken rings which are found among some of these nebulæ.
The laws of nature, however, frequently reveal themselves best in the infinitely little; and I have made my first observations toward a new study in watching the filiform currents produced during the osmotic interchange of two liquids through the pores of a membrane.[2]
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