Blue water also takes on another tint when objects lying on the ground are seen through it, and this mixed color tone depends on the color of the ground. We can easily verify this by the simple experiments described above with blue colored water in a cylinder glass. White bodies, pieces of porcelain for instance, appear light-blue, yellow-green, red-violet, and, the deeper they sink, the more is this shading from blue washed out, till it is destroyed. The red shades vanish first of all.
The depth to which no trace of bottom-colors reaches us is certainly not little, and may, under favorable conditions, be estimated at several hundred metres. But the question is a large one, and we will consider a little more carefully to what extent the more or less favorable conditions I have mentioned have been determined.
I have already said that pure water does not exist in Nature. It always must contain dissolved or floating substances which will change its colors. Peat waters contain brown and blackish organic matters in solution. They may be perfectly clear and transparent, but the colors which the humus acids and similar substances lend them will always produce a certain effect upon them, which will be re-enforced by the dark-brown or black colors of the bottom of the peat lakes. It has also been observed that filtered water from a blue lake on evaporation leaves a white or light gray, and that from green lakes a yellow sediment; and that thus blue lakes contain white matters and green lakes yellow ones in solution, whose colors produce with those of the water mixed tints. The difference in the colors of the Lake of Geneva and of the Bodensee is explained on this principle, but the results of the experiments on which the conclusion rests have been disputed, and there is much room for doubt on the subject. Whatever may be thought of this, it is certain that no water in Nature is perfectly clear and transparent, but is more or less turbid by the presence of other substances floating in it. That this turbidity is of greater or less importance, that we can distinguish at greater or less depths objects swimming in the water, like fishes, or lying on the bottom, are taught by daily experience as well as by experiments which have been made by sinking solid bodies in sunlight and on cloudy days and at different seasons, or by letting down sources of light, such as burning lamps and incandescent electric lights, and ascertaining the depth at which a perceptible glimpse of them can be obtained. It is to be regretted that these as well as other experiments upon the penetrating power of light have been made only in waters not quite clear, as in a few Swiss lakes and the Mediterranean Sea. Whoever has traveled on the coasts of Norway must have been astonished at the transparency of the water in many of the fiords; it is also affirmed that in some
