Stranica 146 [146]
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, plac¬
ed 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 con¬
siderable 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 Mediter¬
ranean, 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.”