Secrets of Earth and Sea

by Sir E. Ray Lankester

CHAPTER V

BLUE WATER

MOST people know and admire the splendid expanse of blue colour offered by the clear sea water on many parts of our coasts, and by that of lakes at home and abroad. I find that there is still a sort of a fixed determination not to believe that this colour is due (as it is) to the actual blue colour of pure water. Pure, transparent water is blue. Those who think they know better will point to a glass of pure water, hold it up to the light, and affirm that it is colourless. But this apparent colourlessness is due to the small breadth of water in the glass through which the light passes. It is definitely ascertained that if water as pure and as free from either dissolved or suspended matter as it is possible to make it (by distillation and the use of vessels not acted upon by water) be made to fill an opaque tube 15 ft. long, closed at each end by a transparent plate, and then a beam of light be made to traverse the length of the tube, so that the eye receives the light after it has passed through this length of 15 ft. of water, the colour of the light is a strong blue. Water is blue in virtue of its own molecular character, just as sulphate of copper is. Liquid oxygen, prepared by the use of intense cold, is also transparent blue, and the peculiar condensed form of oxygen known as "ozone" is, when liquefied, of a darker or stronger blue than oxygen.

At one time (some thirty years ago) there was still some doubt as to whether water was self-coloured blue, or whether its blue colour was due to the action on light of excessively minute solid white particles of chalk suspended in the water. Such fine suspended particles in some cases act on the light which falls on to them so as to reflect the blue rays. This occurs in certain natural objects which have a blue colour. But these can be distinguished from transparent self-coloured blue substances by the fact that whilst the light reflected from their surface is blue, the light which is made to traverse them (when they are held up to the light so that they come between one's eye and the sun's rays) is brown. This is the case with very hot smoke, and can be well seen when a cigar is smoked in the sunlight. The smoke which comes off from the lighted end of the cigar is very hot, and its particles are more minute than those of cooler smoke. The hot smoke shows a bright blue colour when the sunlight falls on it and is reflected, but when you look through the smoke-cloud at a surface reflecting the sunlight, the cloud has a reddish-brown tint. As the smoke cools its particles adhere to one another and form larger particles, and the light reflected from the cloud is no longer blue but grey, and even white. Thus the smoke which the smoker keeps for half a minute in his mouth is cooled and condensed, and reflects white light—is, in fact, a white cloud—when he puffs it out, and contrasts strongly with the blue cloud coming off from the burning tobacco at the lighted end of the cigar. The blue colour of the sky is held by many physicists (though other views have been of late advanced) to be due to the same action on the part of the very finest particles of watery vapour, which are diffused through vast regions of our atmosphere above the condensed white-looking clouds consisting of larger floating particles of water.

Vapours are given off by many liquids, and even by some solids, varying in their production according to the heat applied in different cases. They are gases, and quite transparent and invisible at the proper temperature, like the atmospheric air. Thus water is always giving off "water-vapour," which is quite invisible. When water is heated to the boiling point it is rapidly converted into transparent invisible vapour. Steam, as this vapour is called, is invisible, and we all habitually make a misleading use of the word "steam" when we apply it both to this and to the slightly cooled and condensed cloud which we can see issuing from the spout of a kettle or from a railway engine. It seems that the fault lies with the scientific writers, who have applied the word "steam" to the invisible water vapour or gas before it has condensed to form a cloud. The old English word "steam" certainly means a visible cloudy emanation, and not a transparent invisible gas. A cloud is not a vapour, but is produced by the coming together or condensation of the minute invisible particles of a vapour to form larger particles, which float and hang together, and reflect the light, and thus are visible.

By the examination of other vapours or gases than that which is gaseous water, namely, the vapours of bodies like chloroform and ether, it has been shown that "cloud" forms in a vapour not merely in consequence of the cooling of the vapour, but in consequence of the presence in the air (or in the tube in which the vapour is enclosed for observation) of very fine floating dust particles. They act as centres of attraction and condensation for the vapour particles. When there are no dust particles present clouds do not form readily in cooling vapours, or only at lower temperatures, and in larger mass. Tyndall made some beautiful experiments on this subject, obtaining clouds of great tenuity in vapours enclosed in tubes, which reflected the most vivid blue tints when illuminated by the electric arc-lamp. Later Aitken, of Edinburgh, showed that the "fog" which forms in smoke-ridden towns is due to the condensation of previously invisible watery vapour as "cloud" around the solid floating particles of carbon of the smoke. Aitken further used this property of solid floating particles, namely, that they cause the formation of fog and cloud in vapours—to test the question as to whether the excessively minute odoriferous particles which affect our noses as "smell" are distinct solid floating particles as often supposed, or are of the nature of gas and vapour. He admitted strong perfumes such as musk into tubes containing watery vapour, at such a temperature that the vapour was in a "critical" state—just ready to condense and precipitate as "cloud." If he had admitted fine solid particles such as a minute whiff of smoke, or some "dusty" air—the cloud would have formed. But the admission of the perfume had no such effect Therefore, he concluded that the odoriferous emanations used by him are not distinct particles like those of smoke or dust, but are gaseous.

The beautiful blue tint of the semi-transparent "white" of a boiled plover's egg is due to a fine-particled cloud dispersed in the clear albumen. London milk used to be "sky-blue" for a similar reason, before the recent legislation against the adulteration of food. The blue eyes of our fair-haired race and of young foxes are not due to any "pigment"—that is to say, a separable self-coloured substance—but to a fine cloud floating in a transparent medium which reflects blue rays of light as blue smoke does. The iris of the eye can and often does develop a pigment, but it is a brown one. When present in small quantity it produces a green-coloured iris, the pale yellow-brown being added to the blue cloud-caused colouring. When present in larger quantity the same pigment gives us brown and so-called "black" eyes. The blue colours in birds' feathers and insects' wings are produced without blue pigment by special effects of reflection, and where green is the colour it is often due to the addition of a small quantity of yellow pigment to what would otherwise look blue: though some caterpillars and grasshoppers have a real green pigment in their skin. Flowers, on the other hand, have true soluble blue "pigments," and green ones too, notably that called leaf-green or chlorophyll. The little green tree frog has no blue or green pigment in its skin; only a yellow pigment. Sometimes rare specimens are found in which the yellow pigment is absent altogether, and then the little frog is turquoise-blue in colour. But there is no blue pigment in the skin; only a finely-clouded translucent film overlying a dead-black deep layer of the skin, and the result is that the frog is of a wonderful pure blue. Sometimes the commoner large edible frog is found with a similar absence of yellow pigment (I found some in a garden near Geneva six years ago), and then all the parts of its skin which usually are green show as brilliant blue.

It is at first difficult to believe that such fine, smoothly-spread turquoise blue as that of the blue frog is due merely to a "reflection effect," and that there is no blue pigment present which would show as blue if light were transmitted through it, or could be separated and dissolved in some medium. Yet this is undoubtedly the case. The nearest experimental production of such a blue surface without blue pigment is obtained by first varnishing a black board, and when the varnish is nearly dry passing a sponge wetted with water over it. Some of the varnish is precipitated from its solution in the spirit (or it may be turpentine) as a fine cloud, and until the water has evaporated it looks like blue paint, as the poet Goethe found when cleaning a picture. It would be interesting to know more precisely the precautions to be taken in order to get the blue colour in this way in fullest degree.