Page:Popular Science Monthly Volume 23.djvu/79

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ON THE COLORS OF WATER.

nature—the seas, lakes, and rivers—we shall receive a different impression. In these, the water not only appears colored, but of various colors, and of a rich diversity of shades. The Mediterranean is of a beautiful indigo, the ocean is sky-blue, the Lake of Geneva is celebrated for its lovely and transparent azure waters; the Lake of Constance and the Rhine, the Lake of Zurich and the Lake of Lucerne, have waters quite as transparent, but rather green than blue; and the green waters of the little Lake of Kloenthal, near Glaris, can hardly be distinguished from the surrounding meadows. Other waters are of a darker color, like those of the Lake of Staffel, at the foot of the Bavarian Alps, which was quite black the day I saw it, though clear in shallow places.

These facts start the questions whether water, after all, has not a color; if it has, what the color is, and what causes the varied tints under which it is seen. The solution of these questions has long occupied the minds of scientific inquirers, and it can not yet be said that they have been answered. Disagreement still prevails respecting them.

M. Durocher, in his "Studies of the Glaciers of Northern and Central Europe," has expressed the opinion that the blue color of some waters is of glacial origin, and that it is so peculiar to water from snow-fields and glaciers as to constitute a mark by which to distinguish whence it has proceeded. "If the color of water is really blue," he adds, "the substitution for it of gray or greenish tints proceeds in the majority of cases from organic substances, chiefly vegetable rather than animal."

M. Durocher's view is disputed by M. Th. Martins, who points to the snow-fed Lakes of Sioron and the Bachalpsee, as one azure blue, the other yellowish green, and the Lake of Brienz, whose yellowish green waters, after crossing the Isthmus of Interlachen, become blue in the Lake of Thun.

Bunsen was the first one to deny, with any real knowledge, the absence of color in water. Struck with the green-blue color of the hot water of the Icelandic geysers, he examined pure water in a tube, found it blue, and concluded that that was the true color of the liquid, while other colors observed were derived from foreign matters or by reflection from a colored bottom.

Tyndall, Soret, and Hagenbach took up the question about twenty years after Bunsen. Tyndall found by experiments on polarization that the blue of the atmosphere was caused by reflection of the shorter blue light-waves at the expense of the longer waves, from particles of aqueous vapor in an extreme state of division, which he called nascent cloud. If the particles were larger, longer waves would be reflected, and the color would approach white. Soret, seeking to learn if the blue color of the Lake of Geneva had not an analogous origin, applied the polarization experiments to it, and concluded that it contained