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Meteorology; or Weather Explained

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<h2>CHAPTER XXII</h2> THE BLUE SKY If we look at the sky overhead, when cloudless in the sunshine, we wonder what gives the air such a deep-blue colour. We are not looking, as children seem to do, into vacancy, away into the far unknown. And even, if that were the case, would not the space be quite colourless? What, then, produces the blueness? Some of the very fine dust-particles, even when clothed with an exceedingly thin coating of water-vapour, are carried very high; and, looking through a vast accumulation of these, we find the effect of a deep-blue colour. Why so? Because these particles are so small that they can only reflect the rays of the blue end of the spectrum; and the higher we ascend, the smaller are the particles and the deeper is the blue. But it is also because water, even in its very finest and purest form, is blue in colour. For long this was disputed. Even Sir Robert Christison concluded, after years of experimenting on Highland streams, that water was colourless. 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. At the same time, the dust-particles suspended in the water have a great effect in making the water appear more beautiful, brilliant, and varied in its colouring; because little or no light is reflected by the interior of a mass of water itself. If a dark metal vessel be filled with a weak solution of Prussian blue, the liquid will appear quite dark and void of colour. But throw in some fine white powder, and the liquid will at once become of a brilliant blue colour. This accounts for the change of depth and brilliancy of colour in the several shores of the Mediterranean. When, then, you look at the face of a deep-blue lake on a summer evening—the heavens all aglow with the unrivalled display of colour from the zenith, stretching in lighter hues of glory to the horizon—though to you the calm water appears like a lake of molten metal glowing with sky-reflected light, so powerful and brilliant as entirely to overpower the light which is internally reflected, yet blue is the normal colour of the water: blueness is its inherent hue. Looking upwards, we observe three distinct kinds of blue in the sky from the horizon to the zenith. All painters in water-colours know that. Newton thought that the colour of the sky was produced in the same way as the colours in thin plates, the order of succession of the colours gradually increasing in intensity.     <hr style="width: 50%;" /> <div style="break-after:column;"></div>

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