evaporation. According to this, green waters, and, a fortiori, yellow or brown waters, owe their color to the presence of a small quantity of yellow mud. If pure water is really blue, the presence of a small quantity of yellow matter would be enough to turn its color to green or yellow. The same idea was advanced some time ago by M. Wiltsthein, who believed he had proved, by analyses of the waters of a number of Bavarian streams and lakes, that brown and yellow waters contained more organic matter than green ones, and that they were less hard than the latter. He thought that mineral substances of themselves had no effect on the color of the water, but that organic substances, naturally brown, existing in it as humic acids, were held in solution through the presence of alkaline matters with them, and that they made the water, according to their abundance, green, yellow, brown, or black. His views are not supported by the results of my analyses, which indicated that the colors of different waters on which they were made bore no relation to the quantity either of organic matter or of alkalies held in them. I have also not been able to find any relation between the color of water and its hardness or softness. It is, however, probable that very dark water may owe its color to dark organic matter dissolved in it.
M. Schleinitz attributes the diversities in the color of sea-water to variations in the quantity of salt dissolved in it. During a voyage in the Gazelle, from Ascension to the Congo, he observed that the blue water had a higher specific gravity than that which was of a greenish tinge. This observation leads to an erroneous conclusion, but affords a confirmation of some results which I have reached.
M. J. Brun has noticed in the water and the ice of the Lake of Neufchâtel an alga which is green, orange, red, or brown, according to the stage of growth it has reached, and black after it is dead. Its presence would not be without influence on the color of the lake.
This review shows that the problem of the color of water still calls for more investigation. It may be useful to speak of a few researches that I have made. My object was to determine the color of pure water, and to observe the variations in color produced by the presence of different substances. I used glass tubes, five metres long and four centimetres in interior diameter, closed at the end with glass plates, and passing through a black sheathing that intercepted the side-light. They abutted against a ground-glass pane in the window of my laboratory, so as to receive diffused light in the direction of their axes.
M. V. Meyer, who used a similar arrangement, found the color of distilled water to be a blue-green. I found it pure blue. In my first experiment, I also found distilled water of a blue-green color, like that of a diluted solution of ferric sulphate. A second experiment, with freshly distilled water, gave a pure sky-blue, which in the course of seventy hours became blue-green like the former water, without losing any of its transparency. This indicated that the distilled water of