Page:Popular Science Monthly Volume 23.djvu/80

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minute particles similar to those found by Tyndall in the atmosphere. Hagenbach repeated the experiments, with like results, in the Lake of Lucerne; and Tyndall, a year later, with water of the Mediterranean and the Lake of Geneva, sent to him in London. Mr. Hayes examined the water of the Lake of Geneva, to see if it did not contain a coloring substance, and found none.

The later of these experiments indicate that, contrary to Bunsen's belief, water by itself may be colorless, but nothing is less certain. M. Soret says that the lake was still blue in cloudy weather, when he could not get a trace of polarization effects. Is this not enough to prove that reflection is not the only cause of color in water? Moreover, if the blue in water were wholly of the same origin as that of the sky, the light transmitted by water should be crimson, as that which is shown on the tops of high mountains, or which is transmitted through the clouds at the rising and setting of the sun; but nothing of the kind is the case. Professor Tyndall states this, and Father Secchi has established the absence of the red and yellow from the absorption spectrum of water. It is also well known to those who have had occasion to make submarine excursions, or who have visited the glacial grottoes in Switzerland, that the transmitted light has a blue tone, and the red is so weak that the figures have a livid aspect.

These facts show that the question is still waiting a definite solution. We now turn to the explanations which have been offered of the diversities in the colors of natural waters.

According to Arago, water has two colors, "a color of transmission and a color of reflection, wholly different from the other. It appears blue by reflection, and its transmitted color is green." This supposition can not be reconciled with optical laws, but Arago used it to explain the variations of tint in the water of a shallow, white-bottomed sea. "When the sea is deep, light is reflected from the water and appears blue; but, if it is not very deep, the sand at the bottom receives the light through a stratum of water. The light then reaches the bottom, already green, and, in returning from the sand to the air, the green color is deepened, frequently so much as to predominate, on coming out, over the blue. This, probably, is the whole secret of those shades which are in calm weather the sure and valuable index to the experienced sailor of the depth of the bottom. This explanation fails when it is applied to other quarters than those for which Arago conceived it. The Swiss lakes are green, or blue, independently of their depth. Arago suggests, after Davy, that the change from blue to green may be caused by the presence of vegetable, M. Durocher of colored, matters. These suppositions are gratuitous, and supported by no evidence. H. Sainte-Claire Deville, in 1848, analyzed a number of natural waters, and found that the blue ones gave hardly perceptible colored residues, while the green ones yielded such considerable quantities of organic matter that the soluble salts became yellow after