7 4 THE POPULAR SCIENCE MONTHLY. 



may, then, be thus explained : Absolutely pure water, viewed in masses 

 of sufficient thickness, is of a beautiful blue color. If it holds in com- 

 plete solution colorless salts in small mass, its color is not changed ; 

 but, in proportion as it may contain matter on the verge of precipita- 

 tion, 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 precipi- 

 tation, 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 Rhone 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 sub- 

 stances, 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 so- 

 dium, 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 Scientijique. 



