SILUBIAN SYSTEM. 174 SILVEB. from those of the Ordovician. The most im- portant were Atrypa, Spirifera, and Pentamerus. The bivalve mollusks were similar to those of the Ordoviciaii, but other orders sho^^ed more or less change. Among the pteropods a very abundant form is the Tentaculites, whose remains occur in great numbers in certain strata of the lower Helderberg series. Tlie only vertebrates that are known to have existed were fishes such as ostracoderms and sharks, "but their remains are rather fragmentary. The economic minerals of the Silurian are fairly diversified. In the rocks of the Clinton age we find a very persistent bed of the hema- tite iron ore known as the Clinton or fossilifcrous iron ore. Wherever the Clinton rocks are found this ore is known to occur and forms the basis of the iron industry at Birmingham, Ala., Avhere a deposit four miles long and from 12 to 20 feet thick is worked. In the rocks of the Salina group we find the deposits of g^'psum and rock salt, the latter material being of great economic value in the State of New York. Many of the Silurian rocks are also excellently adapted for building purposes, and of these the Medina sandstone, named from its occurrence at Medina, New York, is specially well known. Bibliography. Murchison, Siluria (London, 18.59) ; Geikie, Text-Book of Geology (ib., 1893) ; Dana, Manual of Geology (4th ed.. New York, 1890) ; Scott, Introduction to the Study of Geology (ib., 1902) ; and the following trea- tises: Darton, "Notes on the Stratigraphy of a Portion of Central Appalachian, Virginia," American Geologist, vol, x. (Rochester, 1892) ; Prosser, "The Thickness of the Devonian and Silurian Rocks of Central New Y'ork," Geological Society of America Bulletin, vol. iv. (ib., 1893) ; Weller, "The Silurian Fauna Interpreted on the Epicontinental Basis," in Journal of Geology, vol. vi. (Chicago, 1898) ; Clarke, "Note on the Silura-Devonic Boundary," in Science, new series, vol. xii. (New Y'ork, 1900) ; Schuehert, "Lower Devonic Aspect of the Lower Helderberg and Oriskany Formations," Geological Society of American Bulletin, vol. xi. (Rochester, 1900). See Clinton Stage; Salina Stage, etc. SILTJRID.ffi. A very large family of soft- rayed fishes. See Catfish. SILVA, sel'vd, Antonio Jo.si; da (1705-39). A Portuguese plaj'wright who became a victim of religious fanaticism and was burnt at the stake by the Obscurantists, October 18, 1739. Silva was the son of a converted -Jew. His Operas are often coarse and rough, but full of wit and humor of a popular kind. Consult "Portu- giesisehe Litteratur," in Grober, Grundriss der romanisclien Philologie (Strassburg, 1897). SILVA'NUS. In Latin mythology, a divinity of the fields and forests, the protector of the boundaries of fields and of cattle. He is by later writers identified with Pan. Faunus, and other divinities, and is represented by the poets and in art as an old man, in love with Pomona. He is especially associated with the cypress and the pine. His sacrifices consisted of grapes, com, meat, milk, wine, and pigs. SILVANUS. A leader of the primitive Chris- tian Church in Jerusalem. See Sila.s. SILVELA, sel-va'la, Francisco (1843—). A Spanish statesman, born at Madrid. He studied law, and in 1869 was elected to the Cortes as a Conservative. In 1879 he became Minister of the Interior (under Campos), and from 1883 to 1884 he was Minister of Justice. After the death of Ciinova.s, Silvela, as the head of tlie reorganized Conservative Party, became Prime ^Minister in February, 1899. He resigned in October, 1900, but after the fall of Sagasta resumed ofliee, in December, 1902. SILVEB (AS. seolfor, seolubr, Goth. st7»6r, OHG. silabar, silbar, Gcr. Silber, silver; probably from the Pontic city of 'AXi5/35;, Alybr, where silver abounded). A metallic element that was known to the ancients, and when first mentioned is re- ferred to as a medium of exchange. It is de- scribed in early Hebrew writings under the name Kcseph, the root of which signifies 'to be pale,' while among the Greeks it was known as Apyvpos, argyros, signifying 'shining.' The alchemists called it luna or Diana, and referred to it in their writings b^' the crescent symbol. It occurs native, and specimens weighing several hundred ]iounds have been found, although it usually occurs in combination, as given below in the table of ores. It also occurs in lead ores, which form one of the main sources of its production. It is found in sea water, and small quantities of it, in the form of chloride, have been detected in volcanic dust. The metal may be readily pre- pared by heating silver sulphide with litharge or lead sulphate, the lead being separated from the resulting alloy by cupellation. Metallic silver may also be obtained by reducing silver chloride W'ith zinc, or by fusion with carbon and sodium carbonate. Silver (symbol Ag. ; atomic weight, 107.92) is a white lustrous metal that is very ductile and malleable, with a specific gravitv of 10.57 and a melting-point of 1040° C. (about 1900° F.). When in the liquid state it possesses the power of absorbing oxygen from the air, which it gives up on solidification. When a mass of the metal is rapidly cooled, the silver solidifies before the oxygen has escaped from the interior, and this gas then bursts through the crusts, driving out part of the fused silver in globular masses and excrescences — a phenomenon known as 'spitting.' Metallic silver finds extensive use in coinage, and, owing to the high polish it takes, for table- ware and decorative articles; for silver plating, the silvering of mirrors, and to a slight extent for laboratory purposes. Silver forms alloys (q.v.) -with many metals, and that consisting of 9 parts of silver to 1 part of copper is the standard alloy used for the United States coins, while 83.5 parts silver to 105 parts copper is the standard employed in the Latin LTnion. An alloy of 100 parts of aluminum with 5 parts of silver is used' for making pans of balances, etc., as it is harder and more easily polished than alumi- num. With oxygen silver forms three oxides, an ar- gentous oxide or sub-oxide, a protoxide or normal oxide, and a peroxide or dioxide. Of these the protoxide is the most important. It is obtained as a hroMm pulverulent precipitate when silver nitrate is treated with potassium or sodiimi hy- droxide. This compound is used to give a yellow color to glass, and finds some employment in medicine as a substitute for silver nitrate. Silver nitrate, or 'lunar caustic' is prepared by dissolv- ing silver in nitric acid and evaporating to erys-
