~Silicon dioxide~ (_silica_) (SiO_{2}). This substance is found in a great variety of forms in nature, both in the amorphous and in the crystalline condition. In the form of quartz it is found in beautifully formed six-sided prisms, sometimes of great size. When pure it is perfectly transparent and colorless. Some colored varieties are given special names, as amethyst (violet), rose quartz (pale pink), smoky or milky quartz (colored and opaque). Other varieties of silicon dioxide, some of which also contain water, are chalcedony, onyx, jasper, opal, agate, and flint. Sand and sandstone are largely silicon dioxide. ~Properties.~ As obtained by chemical processes silicon dioxide is an amorphous white powder. In the crystallized state it is very hard and has a density of 2.6. It is insoluble in water and in most chemical reagents, and requires the hottest oxyhydrogen flame for fusion. Acids, excepting hydrofluoric acid, have little action on it, and it requires the most energetic reducing agents to deprive it of oxygen. It is the anhydride of an acid, and consequently it dissolves in fused alkalis to form silicates. Being nonvolatile, it will drive out most other anhydrides when heated to a high temperature with their salts, especially when the silicates so formed are fusible. The following equations illustrate this property: Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2}, Na_{2}SO_{4} + SiO_{2} = Na_{2}SiO_{3} + SO_{3}. ~Silicic acids.~ Silicon forms two simple acids, orthosilicic acid (H_{4}SiO_{4}) and metasilicic acid (H_{2}SiO_{3}). Orthosilicic acid is formed as a jelly-like mass when orthosilicates are treated with strong acids such as hydrochloric. On attempting to dry this acid it loses water, passing into metasilicic or common silicic acid: H_{4}SiO_{4} = H_{2}SiO_{3} + H_{2}O. Metasilicic acid when heated breaks up into silica and water, thus: H_{2}SiO_{3} = H_{2}O + SiO_{2}. ~Salts of silicic acids,--silicates.~ A number of salts of the orthosilicic and metasilicic acids occur in nature. Thus mica (KAlSiO_{4}) is a salt of orthosilicic acid. ~Polysilicic acids.~ Silicon has the power to form a great many complex acids which may be regarded as derived from the union of several molecules of the orthosilicic acid, with the loss of water. Thus we have 3H_{4}SiO_{4} = H_{4}Si_{3}O_{8} + 4H_{2}O. These acids cannot be prepared in the pure state, but their salts form many of the crystalline rocks in nature. Feldspar, for example, has the formula KAlSi_{3}O_{8}, and is a mixed salt of the acid H_{4}Si_{3}O_{8}, whose formation is represented in the equation above. Kaolin has the formula Al_{2}Si_{2}O_{7}.2H_{2}O. Many other examples will be met in the study of the metals.