The specific weight of sea water is a little above that of fresh water, the proportion being as a thousand to a thousand and twenty-seven. The Dead Sea, which receives no fresh water into its bosom to maintain itself at the same level as other seas, acquires a higher degree of saltness, and is equal to a thousand and twenty-eight. The specific gravity of sea water is about the same as the milk of a healthy woman.
The colour of the sea is continually varying, and is chiefly caused by filtration of the solar rays. According to the testimony of the majority of observers, the ocean, seen by reflection, presents a fine azure blue or ultramarine (cæruleum mare). When the air is pure and the surface calm this tint softens insensibly, until it is lost and blended with the blue of the heavens. Near the shore it becomes more of a green or glaucus, and more or less brilliant, according to circumstances. There are some days when the ocean assumes a livid aspect, and others when it becomes a very pure green; at other times, the green is sombre and sad. When the sea is agitated, the green takes a brownish hue. At sunset, the surface of the sea is illumined with tints of every hue of purple and emerald. Placed in a vase, sea water appears perfectly transparent and colourless. According to Scoresby, the Polar seas are of brilliant ultramarine blue. Castaz says of the Mediterranean, that it is celestial blue, and Tuckey describes the equinoctial Atlantic as being of a vivid blue.
Many local causes influence the colours of marine waters, and give them certain decided and constant shades. A bottom of white sand will communicate a greyish or apple-green colour to the water, if not very deep; when the sand is yellow, the green appears more sombre; the presence of rocks is often announced by the deep colour which the sea takes in their vicinity. In the Bay of Loango the waters appear of a deep red, because the bottom is there naturally red. It appears white in the Gulf of Guinea, yellow on the coast of Japan, green to the west of the Canaries, and black round the Maldive group of islands. The Mediterranean, towards the Archipelago, sometimes becomes more or less red. The White and Black Seas appear to be named after the ice of the one and the tempests to which the other is subject.
At other times, coloured animalcules give to the water a particular tint. The Red Sea owes its colour to a delicate microscopic algæ (Trychodesmium erythræum), which was subjected to the microscope by Ehrenberg; but other causes of colouration are suggested. Some microscopists maintain that it is imparted by the shells and other remains of infusoria; others ascribe the colour to the evaporation which goes on unceasingly in that riverless district, producing salt rocks on a great scale all round its shores. In the same manner sea water, concentrated by the action of the solar rays in the salt marshes of the south of France, when they arrive at a certain stage of concentration take a fine red colour, which is due to the presence of some red-shelled animalcules which only appear in sea water of this strength. The saline lakes on the Great Thibetian water sheds are due to this cause. Strangely enough, these minute creatures die when the waters attain greater density by further concentration, and also if it becomes weaker from the effects of rain.
Navigators often traverse long patches of green, red, white, or yellow coloured water, all of which are due to the presence of microscopic crustaceans, medusæ, zoophytes, and marine plants; the Vermilion Sea on the Californian coast is entirely due to the latter cause.
The phenomenon known as Phosphorescence of the Sea is due to analogous causes. This wonderful sight is observable in all seas, but is most frequent in the Indian Ocean, the Arabian Gulf, and other tropical seas. In the Indian Ocean, Captain Kingman, of the American ship Shooting Star, traversed a zone twenty-three miles in length so filled with phosphorescent animalcules that at seven hours forty-five minutes the water was rapidly assuming a white, milky appearance, and during the night it presented the appearance of a vast field of snow. "There was scarcely a cloud in the heavens," he continues, "yet the sky, for about ten degrees above the horizon, appeared as black as if a storm were raging; stars of the first magnitude shone with a feeble light, and the 'Milky Way' of the heavens was almost entirely eclipsed by that through which we were sailing." The animals which produced this appearance were about six inches long, and formed of a gelatinous and translucent matter. At times, the sea was one blaze of light, produced by countless millions of minute globular creatures, called Noctilucæ. The motion of a vessel or the plash of an oar will often excite their lucidity, and sometimes, after the ebb of tide, the rocks and seaweed of the coast are glowing with them. Various other tribes of animals there are which contribute to this luminous appearance of the sea. M. Peron thus describes the effect produced by Pyrosoma Atlanticum, on his voyage to the Isle of France: "The wind was blowing with great violence, the night was dark, and the vessel was making rapid way, when what appeared to be a vast sheet of phosphorus presented itself floating on the waves, and occupying a great space ahead of the ship. The vessel having passed through this fiery mass, it was discovered that the light was occasioned by animalcules swimming about in the sea at various depths round the ship. Those which were deepest in the water looked like red-hot balls, while those on the surface resembled cylinders of red-hot iron. Some of the latter were caught: they were found to vary in size from three to seven inches. All the exterior of the creatures bristled with long thick tubercles, shining like so many diamonds, and these seemed to be the principal seat of their luminosity. Inside also there appeared to be a multitude of oblong narrow glands, exhibiting a high degree of phosphoric power. The colour of these animals when in repose is an opal yellow, mixed with green; but, on the slightest movement, the animal exhibits a spontaneous contractile power, and assumes a luminous brilliancy, passing through various shades of deep red, orange, green, and azure blue."
The phosphorescence of the sea is a spectacle at once imposing and magnificent. The ship, in plunging through the waves, seems to advance through a sea of red and blue flame, which is thrown off by the keel like so much lightning. Myriads of creatures float and play on the surface of the waves, dividing, multiplying, and reuniting, so as to form one vast field of fire. In stormy weather the luminous waves roll and break in a silvery foam. Glittering bodies, which might be taken for fire-fishes, seem to pursue and catch each other—lose their hold, and dart after each other anew. From time immemorial, the phosphorescence of the sea has been observed by navigators. The luminous appearance presents itself on the crest of the waves, which in falling scatters it in all directions. It attaches itself to the rudder and dashes against the bows of the vessel. It plays round the reefs and rocks against which the waves beat, and on silent nights, in the tropics, its effects are truly magical. This phosphorescence is due chiefly to the presence of a multitude of mollusks and zoophytes which seem to shine by their own light; they emit a fluid so susceptible of expansion, that in the zigzag movement pursued they leave a luminous train upon the water, which spreads with immense rapidity. One of the most remarkable of these minute mollusks is a species of Pyrosoma, a sort of mucous sac of an inch long, which, thrown upon the deck of a ship, emits a light like a rod of iron heated to a white heat. Sir John Herschel noted on the surface of calm water a very curious form of this phosphorescence; it was a polygon of rectilinear shape, covering many square feet of surface, and it illuminated the whole region for some moments with a vivid light, which traversed it with great rapidity.
The phosphorescence of the sea may also result from another cause. When animal matter is decomposed, it becomes phosphorescent. The bodies of certain fishes, when they become a prey to putrefaction, emit an intense light. MM. Becquerel and Breschet have noted fine phosphorescent effects from this cause in the waters of the Brenta at Venice. Animal matter in a state of decomposition, proceeding from dead fish which floats on the surface of ponds, is capable of producing large patches of oleaginous matter, which, piled upon the water, communicates to a considerable extent the phosphorescent aspect.
Whatever may be the case elsewhere, there are local causes which affect the colour of the waters in certain rivers, and even originate their names. The Guaïnia, which with the Casiquaire forms the Rio Negro, is of a deep brown, which scarcely interferes with the limpidity of its waters. The waters of the Orinoco and the Casiquaire have also a brownish colour. The Ganges is of a muddy brown, while the Djumna, which it receives, is green or blue. The whitish colour belongs to the Rio Bianco, or White River, and to many other rivers. The Ohio in America, the Torgedale, the Goetha, the Traun at Ischl, and most of the Norwegian rivers, are of a delicate limpid green. The Yellow River and the Blue River in China are distinguished by the characteristic tint of their waters. The Arkansas, the Red River, and the Lobregat in Catalonia, are remarkable for their red colour, which, like the Dart and other English rivers, they owe to the earth over which they flow, or which their waters hold in suspension.