It appears that when light is reflected from a cloud of fine colourless particles so as to give a predominant blue colour, the light so reflected is affected in that special way which physicists describe as being "polarized." It is possible by the use of certain apparatus (the polariscope) to distinguish polarized from non-polarized light, so that it should be possible to decide (or at any rate to gain evidence) whether blue water—a sheet of blue water—owes its colour to fine particles suspended in it or to the self-colour of the water. An admirable case for making this simple experiment is presented by the great tanks—some 20 ft. cube—which are used by the water companies which draw their water supply from the chalk, for the purpose of precipitating the dissolved chalk—"Clarking" the water, as it is called, after the inventor of the process—and so getting rid of its excessive "hardness." Such tanks are to be seen by the side of the railway near Caterham. The water in these tanks is of such a brilliant turquoise blue that many people suppose that copper has been added to the water to free it from microbes! Such, at any rate, was the conviction expressed by a friend in conversation with me only a few weeks ago. The water in these tanks, when seen from the railway, looks like a magnificent blue dye, and a very important point for those (not a few) who believe that the blue colour of seas and lakes is due to the reflection of the blue colour of the sky overhead is that the water in the tanks looks just as blue when the sky is overcast with cloud as when there is blue sky. The blue colour of water has, as a rule, nothing to do with the reflection of the sky, though it is the fact that a shallow film of water may at a certain angle reflect the sky to our eyes, just as a mirror may. The effect is quite unlike that due to light passing through deep water when reflected from below it. If we examine the tanks in question we find that they have been filled with water pumped from the chalk, and that then lime has been added to the water in order to combine with the carbonic acid dissolved in it and form chalk or carbonate of lime—which is insoluble in pure water and falls as an excessively fine white powder to the bottom of the tank. But the important fact is that water having carbonic acid dissolved in it can dissolve carbonate of lime or chalk to a certain amount: and this water pumped from the chalk, having carbonic acid naturally dissolved in it, has consequently also dissolved a quantity of chalk. It is this which gives the chalk-spring water the objectionable quality of "hardness." When lime is added to the chalk-spring water as pumped into the tanks, the carbonic acid in it is taken up by the lime, and the chalk previously dissolved by the carbonic-acid-holding water is, so to speak, "undissolved," and thrown down as a very fine white powder, together with the chalk newly formed by the union of the lime and the carbonic acid. These large tanks are used to allow the fine powder of chalk to settle down and leave the water clear. The brilliantly white chalk sediment accumulates not only on the floor of the tank, but on its sides. Any light which falls on the tank is refracted and reflected from side to floor and from floor to side, and eventually emerges from the tank, a great deal of it having traversed the 20 ft. breadth and depth many times. Most of its red, yellow, and green rays are quenched by the many feet of blue water through which it has passed, and it issues as predominantly blue. This is largely due to the fine reflecting surface furnished by the "white-washed" or chalk-coated floor and sides and the great purity of the white reflecting material—no yellow or brown matter being present to give a greenish tinge to the result I remember being taken to see "Clark's process" in use, and the splendid blue colour of the water in the "softening" tanks at Plumstead, when the process was first used by the North Kent Water Company, sixty-four years ago.
It is, I think, still a possible question as to whether the fine floating particles of precipitating chalk act in any way as a "cloud"—in short, as the blue clouds of smoke, egg-white, milk, and varnish. There is no evidence that they do, but no one, so far as I know, has ever taken the trouble to settle the question. It could be done by examining the blue light from the tanks with a polariscope, and also by sinking a black tarpaulin into the tank to cover the white floor and hanging others at the sides. Then if the blue colour were due to light reflected from the white floor and sides traversing repeatedly the clear self-coloured blue water, the blue colour should no longer be visible, for the reflecting surfaces would be covered by the black tarpaulin and little light sent up through the water. But if it were due to a cloud of greatest delicacy in the water—like fine smoke reflecting the blue light rather than the other rays—then the colour should be as intense or more intense when the black background is introduced. I am surprised that some inquirer, younger and more active than I am, does not put the matter to the test of experiment.
On the whole, practically all the facts which we know about "blue water" are consistent with the blue self-colour of water, and not with that of a "blue cloud" in the water. Now that we have porcelain baths of the purest white and of large size, one may often see the strong blue colour of water of great purity in the bath, especially where waves or ripples send to our eyes those rays of light which have taken a more or less horizontal course from side to side of the bath, and have thus been through a large thickness of the pale-coloured fluid. Great masses of clear ice, such as one may study in glaciers, are blue; the "crevasses" which transmit light which has passed through a considerable thickness of ice (as, for instance, in an ice cave), are deep blue; there is no question of a reflection from suspended particles. The green colour which some glaciers show at a little distance is due to the yellow rust—iron oxide—blown on to the surface of the ice and dissolved. Many glaciers or parts of glaciers are quite free from it, and of a splendid indigo blue in their deeper fissures. So, too, as to the sea and lakes. The Blue Grotto or Cavern of the island of Capri, near Naples, is a case in point. All the light which enters it comes through the sea-water and is blue. I was taken to it in a boat rowed by two men. As the boat enters the low mouth of the cavern you have to bend down to avoid knocking your head against the rock. Then you find yourself floating in a vast and lofty chamber the white rocky floor of which is some twenty feet below the surface of the clear water. No light enters the cavern by the low part of the entrance above water. Below the surface it widens and the strong Southern sun shines through the clear water and its light is reflected up into the cave from the bottom. It is blue, and everything in the cave above as well as below the water is suffused with a blue glow—a truly wonderful and fascinating spectacle. In order to get the best effect you must choose an hour when the sun is in a favourable position. Where there is a white bottom at a depth of fifty or a hundred feet, the sea has a fine ultra-marine colour, so long as it is clear. It is often made green by yellow-coloured impurities, either fine iron-stained sediment or by minute living things in the water. The colour of the water of either sea or lakes, when it is clear and overlying great depths (200 fathoms and more), tends to be dark indigo owing to the deficiency of reflected light. But there are enough white particles as a rule to send some of the light, which penetrates the water, upwards again. Even the great ocean has a dark purplish-blue colour, but never the bright blue of clear water in shallow seas with light-coloured or white bottom.
One of the most beautiful exhibitions of the colour of clear water in various thicknesses which I know, is at the entrance of the Rhone into the Lake of Geneva. The thick pale-coloured brownish-white sediment of the river shoots out for a quarter of a mile or more into the dark blue waters of the deep lake, and on a bright sunny day as it subsides reflects the light upwards from different depths through the clear water. Where it has sunk but little the colour is green, owing to the influence of the yellow mud. Farther on it is ultra-marine blue, and then, where it has sunk deeper, we get full indigo tints. The movement of the water and its churning up by the steamers' paddles add to the variety of effects, since the foam of air-bubbles submerged throws up the light through the water. It is not possible to doubt as one watches the admixture of the river and the lake, and the eddies and hanging walls of sediment, that one is floating over a vast depth of magnificent blue self-coloured fluid which is traversed by the sunlight in ways and degrees varying according to its depth and the volume of the pale mud of the in-rushing Rhone and the abundance of fine air-bubbles "churned" into the water by the paddle-wheels of the steamer.
CHAPTER VI
THE BIGGEST BEAST
THERE is a prevalent notion, encouraged by the fanciful exaggerations of newspaper gossips, that the animals of past ages, whose bones are from time to time dug out of rocks and sand quarries, were many of them much bigger than any at present existing, and that we are living in an age of degeneracy. It is true that the mammoth and the mastodon were enormous creatures, but they were not bigger than their living representatives, the great elephants of Africa and India. The African elephant often stands 11 ft. high at the shoulder, and occasionally attains 12 ft.
Some eighty years ago Dr. Gideon Mantell became celebrated by his discovery of the bones of huge reptiles—far bigger than any existing crocodile or lizard—nearly as big as elephants, in the Wealden rocks of Tilgate Forest in Sussex. He and Sir Richard Owen distinguished several kinds—the Iguanodon, the Megalosaurus, the Hylæosaurus, and others. Models of these creatures as they appeared when clothed in flesh and hide were carefully made, and placed picturesquely among the ponds and islands of the gardens of the Crystal Palace at Sydenham when it was first opened to an enchanted public in the fifties. As a small boy I, at that time, fell under their spell.