Quartz

QUARTZ, a compound of a silicon atom with two oxygen atoms, SiO2, crystallizes in the hexagonal system in the form of transparent hexagonal prisms with pyramidal ends; specific gravity 2.6; is very widespread in nature, entering into the composition of very many rocks. Quartz includes various sorts of sand, white sand being almost pure quartz. Yellow sand is quartz colored by iron oxides. To quartz also belong some colored crystals, for example amethyst, topaz, agate, carnelian. Quartz is a body possessing a very high melting point (1,625°), and the characteristic peculiarity of the melting of quartz is the fact that it becomes viscous, tacky long before the melting point and transformation into a liquid, which allows various objects to be prepared from it in the same way as they are prepared from glass. Molten quartz at present has great technical significance, mainly in laboratories, since, firstly, it does not dissolve in water and thus allows distilled water to be kept without change, and, secondly, quartz vessels, possessing a very small coefficient of expansion, are capable of withstanding sharp fluctuations in temperature. A red-hot quartz flask can be immersed in cold water without the risk of its breaking. Quartz in the crystalline state is capable of rotating the plane of polarization of light, and two modifications of quartz are encountered: one rotates the plane of polarization to the right, the other to the left. Rotating quartz has enormous significance in physical instruments and, above all, in polarizing instruments intended for the physical determination of the amount of sugar, in the so-called saccharimeters. Molten quartz does not possess the ability to rotate the plane of polarization. Both crystalline quartz and molten quartz possess the ability to transmit a considerable part of ultraviolet rays. A mercury arc placed in a quartz tube gives a long ultraviolet spectrum, which is used at present also for medical purposes (the treatment of rickets, lupus, and a whole series of other diseases; see Bakh mercury-quartz lamp).