Of course, he admitted that the water in the Indian and Pacific Oceans has frequent patches of red, brown, or white colour, from the myriads of animalcules suspended in the water. Ehrenberg found that it was vegetable matter which gave to the Red Sea its characteristic name. But these, and similar waters, are not pure. It is to Dr. Aitken that the final discovery of the real colour of water is due. When on a visit to several towns on the shores of the Mediterranean, he set about making some very interesting experiments, which the reader will follow with pleasure. It is a well-known fact that colour transmitted through different bodies differs considerably from colour reflected by them. In his first experiment he took a long empty metal tube, open at one end, and closed at the other end by a clear-glass plate. This was let down vertically into the water, near to a fixed object, which appeared of most beautiful deep and delicate blue at a depth of 20 feet. Scientific men know that, if the colour of water is due to the light reflected by extremely small particles of matter suspended in the water, then the object looked at through it would have been illuminated with yellow (the complementary colour of blue). A blackened tube was then filled with water (which had a clear-glass plate fixed to the bottom), and white, red, yellow, and purple objects were sunk in the water, and these colours were found to change in the same way as if they were looked at through a piece of pale-blue glass. The white object appeared blue, the red darkened very rapidly as it sank, and soon lost its colour; at the depth of seven feet a very brilliant red was so darkened as to appear dark brick-red. The yellow object changed to green, and the purple to dark blue. But, still further to satisfy himself that water is really blue in itself, even without any particles suspended in it, he tested the colour of _distilled_ water. He filled a darkened tube with this water (clear-glass plates being at the ends of the tube), and looked through it at a white surface. The effect was the same as before, the colour was blue, almost exactly of the same hue as a solution of Prussian blue. This is corroborated by the fact that, the purer the water is in nature, the bluer is the tint when a large quantity is looked through. Some Highland lochs have crystal waters of the most extraordinary blue. Of course, some cling to the old idea that this is accounted for by the reflected blue of the clear heavens above. No doubt, if the sky be deep blue, then this blue light, when reflected by the surface of the water, will enrich and deepen the hue. But the water itself is _really_ blue.