Page:The fairy tales of science.djvu/69

 isolated as a dark-brown powder. Sand, flint, and quartz consist almost entirely of silica; so do the granitic and siliceous rocks which form, so large a portion of the earth's crust.

The highest and most extensive mountain ranges are huge masses of silica, and the deserts of Africa and Asia are vast plains of the same abundant substance. Silica forms the sand and shingle of the sea-shore, and enters into the composition of every soil; it is the chief ingredient of some of our most precious jewels; of the invaluable transparent glass; and of the stones with which we pave our streets and build our temples.

Alumina is a compound of oxygen with a very extraordinary metal named aluminium, of which we shall have to speak in another of our fairy tales. Alumina is the basis of every kind of clay, and is only second in importance to silica. It is also a constituent of our rocks and soils, of our gems, and our building materials; and we make use of it to form earthenware, a substance which rivals glass in usefulness.

Lime is another abundant metallic oxide or rust, its base being calcium, a beautiful silver-white metal, which burns brilliantly when heated in the air. In nature, lime is generally found in combination with carbonic acid, one of the constituents of the atmosphere. The well-known substance, chalk, which forms our far-famed white cliffs, the compact limestones used in architecture, and all