The violet colour given to porcelain is obtained from manganese. This substance is also used for the glazing of black earthen ware, as a paint, and an ingredient in printers’ ink. As a discharger of colour it is applied in small quantities, and, by the oxygen which it gives out, it is said completely to destroy any tinge left in the glass, by the presence of iron, and some other colouring matters. This property has obtained for it the appellation of the _soap of glass_. It is from manganese that all the oxygen gas (21) used by chemists is obtained. By the application of a red heat this is yielded in such abundance that an ounce of the oxide of this metal will yield about two quarts of gas. The consumption of manganese has, of late years, become very considerable by the discovery of the oxygenated muriatic acid, which is now extensively used in the bleaching of linen and cotton; that liquor being made by the distillation of the oxide of manganese with spirit of salt (muriatic acid, 29). ──────────────────────────────────────────────────────────────────────── COMPOUND MINERALS, OR ROCKS. ─────── 249. There exist considerable masses of minerals in a state of combination, or aggregation with each other. These constitute the rocks and soil of which the globe of the earth is composed; and the study of them is called GEOLOGY. The opinions of learned men relative to their structure, and original formation, have produced various systems denominated _theories of the earth_; but, when we consider that the greatest depth beneath the surface to which the art and industry of man have been able to penetrate, does not exceed 1/35000 part of the earth’s diameter, we must confess that this is very insufficient to allow of any correct opinion being thereby formed concerning the structure of the whole. Modern geologists, for the more convenient arrangement of the compound minerals, have divided them into four classes, which they denominate _primitive rocks_, _secondary rocks_, _alluvial depositions_, and _volcanic rocks_. I. PRIMITIVE ROCKS. 250. _These are so called from their being considered by geologists, to belong to the first formed parts of the globe._ Rocks of this description are of a nature extremely hard. They contain no vestiges whatever of animal or organic remains; and the substances of which they are composed are crystallized. They rise through other rocks at various elevations, in every quarter of the globe; and never either alternate with, or rest upon rocks that enclose organic remains, though they are themselves frequently covered by such. The following are the principal kinds.