may, then, be thus explained: Absolutely pure water, viewed in masses of sufficient thickness, is of a beautiful blue color. If it holds in complete solution colorless salts in small mass, its color is not changed; but, in proportion as it may contain matter on the verge of precipitation, the light traversing it will be of a yellow or darker color, until a stage is reached when the liquid will let no light through, and becomes opaque or black. The yellow light will combine with the blue light of the water, and thus will be produced green-blue, bluish-green, and green tints, according to the strength of the yellow. If the latter is very strong, the dark blue will be wholly smothered, and the water will appear yellow, brown, or of a still darker color.
In nature, generally, the feebly soluble substances contained in natural waters, and appearing, perhaps, in the state of nascent precipitation, are carbonate of lime or magnesia, silica, silicate of aluminum, and alumina. A blue water should contain carbonate of lime more completely dissolved in proportion as it is more distinctly blue, and should consequently have in it enough carbonic anhydride to produce the acid carbonate of lime. A green water, on the other hand, should contain carbonate of lime in less complete solution, as would be the case if there were a less relative proportion of carbonic anhydride in it. The blue waters of the Rhône and the green waters of the Rhine, as analyzed by Sainte-Claire Deville, illustrate and confirm this rule. It may also be presumed that a blue water, containing limestone in full solution, should become green when lime is added to it. This is illustrated on the north shore of the Lake of Achen, where the blue waters of the deep lake become chrome-green when they break over the limestone pebbles of the strand, and, generally, in the greener color of the bottom and shore waters of seas and lakes. Other substances than lime, particularly silica and alumina, may produce the same effects, but their action is more complicated. These substances, without being really soluble in water, are pseudo-soluble, or form an emulsion with it; and water which has taken them up from the ground over which it flows does not become perfectly clear on standing. If, however, it meets a solution of chloride of sodium, alumina, or silicate of alumina, it is precipitated rapidly; and this is what takes place at the mouths of rivers, and is the immediate cause of the deposits out of which deltas are built up. The changes in the color of the sea-water observed by M. Schleinitz, on board the Gazelle, may be accounted for by reference to this fact.—Translated for the Popular Science Monthly from the Revue Scientifique.