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LEFT SILVER 432 SILVER son in 1839, though he had begun to use it as early as 1835. It implied that, speaking broadly, the rocks so described were well developed in the country of the old Silures. The term has been univer- sally adopted. Murchison divided his Si- lurian system into Upper and Lower Silurian, contending that the Cambrian system of Sedgwick was not independent, but simply Lower Silurian. Whatever be the case with the Cambrian, the Lau- rentian system, since established, is un- equivocally older than the Silurian. Under the Upper Silurian were ranged in descending order: (1) the tilestone of Brecon and Caermarthen; (2) the Lud- low ironstone and shale, and (3) the Wenlock or Dudley limestone. Under the Lower Silurian: (1) the Caradoc sand- stone; (2) the Llandeilo flags and schists. Now the Table of Strata stands thus: I. Upper Silurian: 1. Ludlow formation; 2. Wenlock formation; 3. Llandovery for- mation or group. II. Lower Silurian: 1. Bala and Caradoc beds; 2. Llandeilo flags; 3. Arenig or Stiper Stone Group. There is a great break between the Upper and Lower Silurian, which are uncomformable, and a greater break be- tween the Upper Silurian and the Devo- nian. During the early part of the Silurian the land was sinking ; during the deposition of the Llandeilo the sea was moderately deep. Algae, corals, brachio- pods, trilobites, and other Crustacea, and, in the upper strata, fishes, are the charac- teristic fossils. The higher vertebrates had not yet appeared. Vast areas in Russia, etc., are covered by Silurian rocks. In America, the Canadian and the Trenton formations are believed to be Lower Silurian; the Niagara forma- tion to be Upper Llandovery and Wen- lock, and the Salina, the Lower Helder- berg, and the Oriskany to be formations of Ludlow age. SILVER, a precious metal. It appears to have been known almost as early as gold, and, without doubt, for the same reason, because it occurs very frequently in a state of purity in the earth and requires but an ordinary heat for its fu- sion. Pure silver is of a fine white color. It is softer than copper but harder than gold. When melted its sp. gr. is 10.47; when hammered, 10.510. _ Its chemical symbol is Ag. It is next in malleability to gold, having been beaten into leaves only .00001 of an inch in thickness. It may be drawn out into a wire much finer than a human hair, and a wire of silver 0.078 of an inch in diameter is capable of supporting a weight of 187.13 pounds avoirdupois. It excels all other metals as a conductor of heat and electricity. Silver melts when heated completely red hot, and may be boiled and volatilized by a very strong and long-continued heat. When cooled slowly crystals of silver may be obtained. Silver is not oxidized by exposure to the air, or affected by water, but it is blackened or tarnished by sul- phuretted hydrogen. The at. wt. of silver is 107.9. Oxide of silver (Ag 2 0) is pro- duced by dissolving silver in a solution of nitric acid and precipitating with an alkali. Its sp. gr. is 7.14. The compound called horn silver or chloride of silver (AgCl) is obtained by dissolving silver in nitric acid and mixing the solution with a solution of common salt. Its sp. gr. is 5.550. When exposed to the light it turns to a blackish color, hence its great use in photography. Bromide of silver is the most sensitive to light of any known solid. It is used for coating the "dry plates" employed in photography. When silver is long exposed to the air it acquires a covering of a violet color, which deprives it of its luster; this coat- ing is sulphide of silver. Sulphide of silver occurs native as silver glance. Sil- ver readily forms alloys with iron, steel, lead, tin, and mercury. Of the combina- tions of acid and silver the most impor- tant is nitrate of silver (AgNOs), obtained by dissolving silver in nitric acid. If the silver and acid are pure the solution of silver nitrate is colorless, very heavy, and caustic; it stains the skin and all animal substances an indelible black; after evaporation it deposits, on cooling, transparent crystals of Nitrate op Sil- ver (q. v.). There are five important silver ores, native silver, vitreous silver (or silver glance), black silver, red silver, and horn silver. The first is usually found in den- tiform, filiform, and capillary shapes, also in plates formed in fissures and in superficial coatings ; luster metallic ; color silver-white, more or less subject to tar- nish; ductile; hardness between gypsum and calcareous spar; sp. gr. 10.47. Na- tive silver occurs principally in veins, traversing gneiss, clay-slate, and other palaeozoic rocks, but not usually in great quantity. It often forms a natural alloy with gold. Vitreous silver presents itself in various shapes, and is of a blackish lead-gray color with a metallic luster. It is malleable, about as hard as gypsum, and subject to tarnish; sp. gr., 7.19. It is more or less pure silver sulphide, and has been found almost exclusively in veins along with ores of lead, antimony, and zinc. It occurs in Saxony, Bohemia, Hungary, Mexico, and Peru; and is an important species for the extraction of silver. Black silver generally occurs in granular masses of an iron-black color.