Page:Popular Science Monthly Volume 24.djvu/620

602 bright emerald, as pure and brilliant as ever gem that glistened; again we lose it, and again an opening shows it to us in its own golden light; and then once more it is the bright green; and now it rises higher, clears the ridge, and is once more the golden orb." The Rev. G. H. Hopkins, of Cornwall, England, has observed that in a clear sky, as the disk of the sun sinks down beneath the horizontal line of the ocean, the parting ray is of a deep emerald green. The effect is not produced if there are clouds around the sun. Dr. F. A. Forel, of Morges, Switzerland, mentions as a fact confirmatory of the opinion that meteorological factors alone can not furnish a sufficient explanation of the phenomenon, that in Switzerland the glow, after having decreased subsequently to the 3d of December, attained a second maximum on the 24th and 25th of the month, when the atmospheric conditions were quite different from those which prevailed in the country at the time of the first maximum.

The hypothesis that the spectacle was caused by the presence in the atmosphere of a cloud of "cosmic dust," which the earth has encountered in its travels, has been advanced by several observers, and is supported by Mr. Proctor. Mr. Nordenskiöld and other men eminent in science have taught us to believe that a meteoric dust falling upon the earth from space plays a much more important part in terrestrial economy than we have been accustomed to suppose; and they have collected, in uninhabited countries and far away from any volcano, quantities of dust—little rounded particles of metallic compounds unlike anything the earth is known to produce, and strikingly like what meteors of that size would be. Investigating whether an unusual quantity of such dust is now falling upon us, Mr. W. Mattieu Williams has found it in carefully selected snow from his garden. M. Émile Yung, of Geneva, has also found an extraordinary quantity of a similar dust in fresh snow that fell in the latter part of November and early in December on the steeple of the cathedral of Saint-Pierre, at "les Treize-Arbres," Mont Salève.

Numerous suggestions have been made that the phenomena are the result of the diffusion through the whole atmosphere of the entire earth of ashes and cinders from the eruption of the volcano of Krakatoa, in the Straits of Sunda, which took place on the 26th of August last. This theory has the support of Professor Lockyer and other eminent men of science, and there is much to be said in favor of it. The principal objections to it are summarized in a remark by Mr. Proctor, "that we should have to explain two incongruous circumstances: first, how the exceedingly fine matter ejected from Krakatoa could have so quickly reached the enormous height at which the matter producing the after-glow certainly was; and, secondly, how, having been able to traverse still air so readily one way, that matter failed to return as readily earthward under the attraction of gravity." It will not do to limit our ideas of the effect that may have followed the eruption of