SELENITD (Gr. Selene, the moon), a transparent and beautiful variety of gypsum (q.v.), white, or tinged with green, gray, or yellow. It receives its name from its pecul iar moon-like luster. It is often crystallized in six-sided prisms, sometimes in leases, and twin crystals and quadruple crystals occur. It is found in common gypsum, in rock-salt, in the blue clay of the s. of England, etc. There is in the British museum a splendid group of crystals of selenite presented by the late prince Albert. Selenite js easily cut, and is capable of being split into extremely thin plates, which are flexible, although not elastic. It was used by the ancients for some of the purposes for which we use glass. The Romans imported it from Spain, Cyprus, Cappadocia, and Africa. The hot-houses of Tiberius were covered with it, and Pliny mentions that it was used in the construction of beehives by those who wished to watch the operation of the bees. It is used for making the finest kind of stucco, and the 'most delicate pastel colors. When burned, and perfectly dry, its powder is used for cleansing and polishing articles of gold and silver, precious stones, and pearls.
SELEITII/Id (synals. Se, equiv. 39.5—new system, 79—and sp. gr. 4.28) is one of the metalloid elements. At ordinary temperature, it occurs as a solid of a dark-brown color, and when broken, presents a conchoidal vitreous fracture; thin splinters of it are, how ever, of a dark-red tint when seen by transmitted light. It is tasteless and inedorous, a non-conductor of electicity; and like sulphur, to which it presents a remarkable anal ogy, it may be obtained in all three forms of atomic aggregation, being solid up to 39r, when it fuses into a fluid, which boils at 65V, emitting an inodorous vapor of a deep yellow tint. When heated in the air, selenium does not very readily take fire, but it is
combustible, and burns with a blue flame, while a portion of it is volatilized in red fumes emit an odor resembling that of bisulphide of carbon or garlic. The products of combustion are oxide of selenium and selenious acid, the peculiar odor being prob ably due to the former.
Selenium is of rare occurrence in nature; it is chiefly found as a selentde to combina tion with lead, silver, copper, or iron; but it has also been discovered in the sulphur from the Lipari isles, and in certain sulphides of iron, which accounts for its detection In sulphuric acid. It is unnecessary to enter into any description of the mode of iselst ing 'it; nor need we do more than simply mention that it forms three compounds with cxygen—Se0, oxide of selenium; Se0,, selenious acid; and selenic acid; while with hydrogen it forms HSe, seleninreted hydrogen, or hydroselenic acid, a colorless, 'gas, which resembles, but is more offensive than sulphureted hydrogen. Berzelhis found that by the application of the nose to a bubble of it not larger than a pea. he was deprived of the sense of smell for severid hours._ It is prepared in the same way as the Corresponding sulphur gas. As it is soluble in water, it should be collected over mer Cury.
Selenium was discovered in 1817 by Berzelins, who named it from Selene, the Gr. for "the moon." because it was associated with tellurium, which is named from Tellus, the Lat. for " the earth."