OXIDES, METALLIC, are the most important of all the compounds of the metals, and in many cases occur naturally as abundant and valuable ores. They are divided by chemists into three classes—viz., (1) basic oxides or bases, (2) saline or indifferent oxides, and (3) acid oxides or metallic acids. The different oxides of the same metal usually afford illustrations of two, and not mdrequently of all three of these classes. Thus (to take the case of manganese referred to in the last article) the protoxide (MnO) is a powerful base, the red oxide is a saline or indifferent oxide, showing little tendency to combine either with acids or alkalies, while permanganic acid pre sents all the properties of an acid. "As a general rule, the greater the number of atoms of oxygen which an oxide contains, the less is it disposed to unite with the acids; on the contrary, it frequently possesses acid properties, and then unites with bases to form salts. Protoxides generally are strong saliflable bases; they require one equivalent of a monobasic acid to form neutral salts. Sesquioxides are weaker bases; their salts are usually unstable; they require three atoms or equivalents of a monobasic acid to form a salt which is neutral in composition, though it may not be neutral to test-paper;' and in general, all oxides require as many equivalents of acid as they contain atoms of oxygen in their composition. Some of the metallic acids, like the stannic and titanic, contain
two atoms of oxygen to one atom of metal, but most of them contain three atoms of oxygen—such, for example, as the manganic, ferric, chromic, tungstic, molybdic, and vanadic acids; whilst in a few cases. such as the arsenic, antimonic, and permanganic, the proportion of oxygen is still higher."—Miller's Inomanic Chemistry, 2d edit. p. 314. Of the basic oxides, which form by far the most important class, it may be observed that they arc devoid of all metallic appearance, and present the characters of earthy 'natters, and that six only of them are soluble in water to any considerable 'extent—viz., the three alkalies and baryta, strontia, and lime. All the oxides are solid at ordinary, temperatures, and as a general rule, the addition of oxygen to a metal renders it much less fusible and soluble; the protoxide of iron, the sesquioxide of chromium, and molybdic acid being the only oxides that melt more readily than the metal,