It is a curious fact, that in many parts of the ocean, fresh-water springs burst from the bottom of the sea. Thus, in the Gulf of Spezzia, and in the port of Syracuse, large jets of fresh water mingle with the brine; and Humboldt mentions a still more remarkable submarine fountain on the southern coast of Cuba, in the Gulf of Xagua, a couple of sea miles from the shore, which gushes through the salt water with such vehemence, that boats approaching the spot are obliged to use great caution. Trading vessels are said sometimes to visit this spring, in order to provide themselves in the midst of the ocean with a supply of fresh water.
The sea is not colourless; its crystal mirror not only reflects the bright sky or the passing cloud, but naturally possesses a pure bluish tint, which is only rendered visible to the eye when the light penetrates through a stratum of water of considerable depth. This may be easily ascertained by experiment. Take a glass tube, two inches wide and two yards long, blacken it internally with lamp-black and wax to within half an inch of the end, the latter being closed by a cork. Throw a few pieces of white porcelain into this tube, which, after being filled with pure sea-water, must be set vertically on a white plate, and then, looking through the open end, you will see the white of the porcelain changed into a light blue tint.
In the Gulf of Naples, we find the inherent colour of the water exhibited to us by Nature on a most magnificent scale. The splendid "Azure cave," at Capri, might almost be said to have been created for the purpose. For many centuries its beauties had been veiled from man, as the narrow entrance is only a few feet above the level of the sea, and it was only discovered in the year 1826, by two Prussian artists accidentally swimming in the neighbourhood. Having passed the portal, the cave widens to grand proportions, 125 feet long, and 145 feet broad, and except a small landing place on a projecting rock at the farther end, its precipitous walls are on all sides bathed by the influx of the waters, which in that sea are most remarkably clear, so that the smallest objects may be distinctly seen on the light bottom at a depth of several hundred feet. All the light that enters the grotto must penetrate the whole depth of the waters, probably several hundred feet, before it can be reflected into the cave from the clear bottom, and it thus acquires so deep a tinge from the vast body of water through which it has passed, that the dark walls of the cavern are illumined by a radiance of the purest azure, and the most differently coloured objects below the surface of the water are made to appear bright blue. Had Byron known of the existence of this magic cave, Childe Harold would surely have sung its beauties in some of his most brilliant stanzas.
All profound and clear seas are more or less of a deep blue colour, while, according to seamen, a green colour indicates soundings. The bright blue of the Mediterranean, so often vaunted by poets, is found all over the deep pure ocean, not only in the tropical and temperate zones, but also in the regions of eternal frost. Scoresby speaks with enthusiasm of the splendid blue of the Greenland seas, and all along the great ice-barrier which under 77° S. lat. obstructed the progress of Sir James Ross towards the pole, that illustrious navigator found the waters of as deep a blue as in the classical Mediterranean. The North Sea is green, partly from its water not being so clear, and partly from the reflection of its sandy bottom mixing with the essentially blue tint of the water. In the Bay of Loanga the sea has the colour of blood, and Captain Tuckey discovered that this results from the reflection of the red ground-soil.
But the essential colour of the sea undergoes much more frequent changes over large spaces, from enormous masses of minute algæ, and countless hosts of small sea-worms, floating or swimming on its surface.
"A few days after leaving Bahia," says Mr. Darwin, "not far from the Abrolhos islets, the whole surface of the water, as it appeared under a weak lens, seemed as if covered by chipped bits of hay with their ends jagged. Each bundle consisted of from twenty to sixty filaments, divided at regular intervals by transverse septa, containing a brownish-green flocculent matter. The ship passed several bands of them, one of which was about ten yards wide, and, judging from the mud-like colour of the water, at least two and a half miles long. Similar masses of floating vegetable matter are a very common appearance near Australia. During two days preceding our arrival at the Keeling Islands, I saw in many parts masses of flocculent matter of a brownish green colour, floating in the ocean. They were from half to three inches square, and consisted of two kinds of microscopical confervæ. Minute cylindrical bodies, conical at each extremity, were involved in large numbers in a mass of fine threads."
"On the coast of Chili," says the same author, "a few leagues north of Conception, the 'Beagle' one day passed through great bands of muddy water; and again, a degree south of Valparaiso, the same appearance was still more extensive. Mr. Sulivan, having drawn up some water in a glass, distinguished by the aid of a lens moving points. The water was slightly stained, as if by red dust, and after leaving it for sometime quiet, a cloud collected at the bottom. With a slightly magnifying lens, small hyaline points could be seen darting about with great rapidity, and frequently exploding. Examined with a much higher power, their shape was found to be oval, and contracted by a ring round the middle, from which line curved little setæ proceeded on all sides, and these were the organs of motion. Their minuteness was such that they were individually quite invisible to the naked eye, each covering a space equal only to the one-thousandth of an inch, and their number was infinite, for the smallest drop of water contained very many. In one day we passed through two spaces of water thus stained, one of which alone must have extended over several square miles. The colour of the water was like that of a river which has flowed through a red clay district, and a strictly defined line separated the red stream from the blue water."
In the neighbourhood of Callao, the Pacific has an olive-green colour, owing to a greenish matter which is also found at the bottom of the sea, in a depth of 800 feet. In its natural state it has no smell, but when cast on the fire, it emits the odour of burnt animal substances.
Near Cape Palmas, on the coast of Guinea, Captain Tuckey's ship seemed to sail through milk, a phenomenon which was owing to an immense number of little white animals swimming on the surface, and concealing the natural tint of the water.