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. * * * * * The water of the sea is essentially _salt_, of a peculiar flavour, slightly acrid and bitter, and a little nauseous. It has an odour perfectly _sui generis_, and is slightly viscous. In short, it includes a great number of mineral salts and some other compounds, which give it a very disagreeable taste, and render it unfit for domestic use. It contains nearly all the soluble substances which exist on the globe, but principally chloride of sodium, or marine salt, and sulphate of magnesia, of potassium, and of lime. Pure water is produced by a combination of one volume of oxygen and of two volumes of hydrogen, or in weight, 100 oxygen and 12·50 hydrogen. Sea water is composed of the same; but we find there, besides, other elements, the presence of which chemistry reveals to us. In 1000 grains of sea water the following ingredients are found:-- Water 962·0 Chloride of sodium 27·1 Chloride of magnesium 5·4 Chloride of potassium 0·4 Bromide of magnesia 0·1 Sulphate of magnesia 1·2 Sulphate of lime 0·8 Carbonate of lime 0·1 Leaving a residuum of 2·9 ----- 1000 =====