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84 LIGHT AND WATER amount of colour in the Mediterranean water, and in the bluish well waters, was as near as could be judged the same as in pure water, it does not seem necessary to call in the aid of impurities to account for the blue colour seen in lakes and seas, the colour being principally due to the water itself; and the different substances in solution, instead of making the water blue, tend to change its proper colour and make it green or yellow.” Prof. Threlfall, in “Nature,” vol. lix, page 461, describes some observations that he made during a voyage from Sydney to Marseilles in 1898. He was able exactly to match the colour of the sea-water, as seen endwise in a tube 736 cm. (about 8 yards) long, by viewing mixtures of definite substances in a second tube 18 cm. (about 7 inches) long, placed alongside the first. The colour was seen in each case by looking through the tube against a white background. A formula is given by means of which the colour of a considerable depth of Mediterranean water can in this way easily be reproduced in a short tube. “Make up the following solution:—Water, 500 cc.; soluble Prussian blue, ‘ooI grm.; saturated lime water just precipitated by the smallest excess of bicarbonate of soda, 5 cc. This mixture, when viewed through a tube 18 cm. long, will show with considerable precision the colour of a sample of water from the Mediterranean, lat. 36° 27 N; long. 17° 51 E of Paris. By using various lengths of tubes I found that when a match has once been made, it can be preserved (within the limits tested) by increasing the amount of Prussian blue proportionally to the length of the column of water under investigation. . . . The majority of the samples of water examined by me took 25°/, less blue to match than the example quoted; and when the water was soiled by the tube, and perhaps at other times, it was necessary to add an amount of picric acid rising to a large proportion of the Prussian blue, and, of course, giving a green solution. Ihe transparency of the water is estimated by the amount of precipitated chalk it is necessary to add.”
