The purest spring is not more limpid than the water of the ocean; it absorbs all the prismatic colours, except that of ultramarine, which, being reflected in every direction, imparts a hue approaching the azure of the sky. The colour of the sea varies with every gleam of sunshine or passing cloud, although its true tint is always the same when seen sheltered from atmospheric influence. The reflection of a boat on the shady side is often of the clearest blue, while the surface of the water exposed to the sun is bright as burnished gold. The waters of the ocean also derive their colour from animalcules of the infusorial kind, vegetable substances, and minute particles of matter. It is white in the Gulf of Guinea, black round the Maldives; off California the Vermilion Sea is so called on account of the red colour of the infusoria it contains; the same red colour was observed by Magellan near the mouth of the river Plate. The Persian Gulf is called the Green Sea by eastern geographers, and there is a trail of green water off the Arabian coast so distinct that a ship has been seen in green and blue water at the same time. Rapid transitions take place in the Arctic Sea, from ultramarine to olive-green, from purity to opacity. These appearances are not delusive, but constant as to place and colour; the green is produced by myriads of minute insects, which devour one another and are a prey to the whale. The colour of clearer shallow water depends upon that of its bed; over chalk or white-sand it is apple-green, over yellow sand dark-green, brown or black over dark ground, and grey over mud. The sea is supposed to have acquired its saline principle when the globe was in the act of subsiding from a gaseous state. The density of sea-water depends upon the quantity of saline matter it contains: the proportion is generally a little above 3 per cent., though it varies in different places; the ocean contains more salt in the southern than in the northern hemisphere, the Atlantic more than the Pacific. The greatest proportion of salt in the Pacific is in the parallels of 22° N. lat. and 17° S. lat.; near the equator it is less, and in the Polar Seas it is least, from the melting of the ice. The saltness varies with the seasons in these regions, and the fresh water, being lightest, is uppermost. Rain makes the surface of the sea fresher than the interior parts, and the influx of rivers renders the ocean less salt at their estuaries; the Atlantic is brackish 300 miles from the mouth of the Amazons. Deep seas are more saline than those that are shallow, and inland seas communicating with the ocean are less salt, from the rivers that flow into them; to this, however, the Mediterranean is an exception, occasioned by the great evaporation, and the influx of salt currents from the Atlantic. The water in the Straits of Gibraltar at the depth of 670 fathoms is four times as salt as that at the surface.