Page:EB1911 - Volume 25.djvu/128

 that same day one of his popular operettas was given at a Lisbon theatre.

SILVANUS (Lat. silva, wood), a deity or spirit of Italian woodland; not, however, of the wholly wild woodland, but of that which borders the clearings in a country not entirely reclaimed. Thus he is partly wild and partly civilized, and reflects the experience of the earliest settlers in Italy, whose descendants took him with them to the farthest limits of the empire, even to Britain, where we have many votive inscriptions to him, always as the friendly deity dwelling outside the new clearing, benevolent towards the settler in a strange land. This leading characteristic of Silvanus is shown clearly in Roman literature: Horace writes of the “horridi dumeta Silvani” (Odes, iii. 29) but he also calls him “tutor finium” (Epod. ii. 22) while for Virgil he is “arvorum pecorisque deus” (Aen. viii. 600). A writer on land measurement (Script. gromatici, i. 302) tells us that each holding had three Silvan—domesticus (of the holding itself), agrestis (of the wilder pasture-land) and orientalis (of the boundaries). It is plain that in him the Italians had a very useful deity, and in all these capacities he became extremely popular, as the extraordinary number of his inscriptions shows. Unlike Mars, from whom he was probably in origin an offshoot (cf. the Mars Silvanus of Cato, De re rustica, 141; see ), he never made his way into the towns, but is almost the only Roman deity who from first to last retained the same perfectly intelligible rustic character. His double nature as deity of woodland and cultivated land is seen well in the artistic representations of him; he carries a young tree in one hand, a pruning-hook in the other.

 SILVER (symbol Ag, from the Latin argentum, atomic weight 107·88 (O＝16)), a metallic chemical element, known from the earliest times and of great importance as a “noble” metal for articles of value—coinage, ornamentation and jewelry. Etymologically the word “silver” probably refers to the shining appearance or brightness of the metal. The Latin argentum is cognate with the Greek ixpyvpos, silver, which in turn is derived from apyos, shining. The Hebrew Keseph is connected with a root meaning “to be pale.” The alchemists named it Luna or Diana, and denoted it by the crescent moon; the first name has survived in lunar caustic, silver nitrate. Silver is widely diffused throughout nature, occurring in minute amount in sea-water, and in the mineral kingdom as the free metal, as an amalgam with mercury and as alloys with gold, platinum, copper and other metals. Native silver is occasionally met with in metalliferous veins, where it has been formed by the alteration of silver-bearing minerals. It crystallizes in the cubic system, but the crystals are usually distorted and indistinctly developed: twisted wire-like forms are much more common. The best crystallized specimens have been obtained from Kongsberg in Norway, large masses, weighing as much as 5 cwts., having been found. It is also found in other silver mines, especially those of Mexico, Peru and Chile; in the Lake Superior copper mining region it occurs in association with native copper. The element is a constituent of many mineral sulphides, some of which are of sufficiently frequent occurrence to rank as ores of silver. Of these the more important are noticed under Metallurgy; here we may notice the rarer minerals. Silver sulphide, Ag2S, occurs naturally as the orthorhombic acanthite, and the cubic argentite; the telluride, Ag2Te, named hessite, assumes cubic forms; other tellurides containing silver are petzite, (Ag,Au)2Te, and sylvanite, AuAgTe4. In association with antimonious and arsenious sulphides, silver sulphide forms many important minerals, which sometimes present dimorphous forms, reflecting the dimorphism of silver sulphide; moreover, the corresponding arsenious and antimonious compounds are frequently isomorphous. This is illustrated by the hexagonal pyrargyrite 3Ag2S·Sb2S3, and proustite, 3Ag2S·As2S3, and the monoclinic pyrostilpnite, isomeric with pyrargyrite, and xanthoconite, isomeric with proustite. Other pairs of isomorphous argentiferous minerals are: the cubic polybasite, 9Ag2S·Sb2S3, and pearceite, 9Ag2S·As2S3; and the germanium minerals argyrodite, 4Ag2S·GeS2, and canfieldite, 4Ag2S·(Sn,Ge)S2.

Physical Properties.—In appearance silver presents a pure white colour with a perfect metallic lustre. It is the most malleable and ductile of all metals with the exception of gold: one gramme can be drawn out into a wire 180 metres long, and the leaf can be beaten out to a thickness of 0·00025 mm.; traces of arsenic, antimony, bismuth and lead, however, make it brittle. In hardness it is superior to gold, but inferior to copper. Its specific gravity, according to G. Rose, lies between 10·514 and 10·619 at 14°; an average value is 10·57. Its specific heat is 0·05701 (Regnault) or 0·0559 (Bunsen); its coefficient of linear expansion is 0·00001921. Its thermal conductivity is, according to Wiedemann and Franz, superior to that of other metals, being in the ratio of 100 : 74 as compared with copper and 100 : 54 with gold; it is the most perfect conductor of electricity, standing to copper in the ratio 100:75, and to gold 100:73. Silver melts at about 1000°C; recent determinations give 960·7° (Heycock and Neville) and 962 (Becquerel); at higher temperatures it volatilizes with the formation of a pale blue vapour (Stas). Its vapour density has been determined at 2000°, and corresponds to a monatomic molecule. When molten, silver occludes the oxygen of the atmosphere, absorbing 20 times its own volume of the gas; the oxygen, however, is not permanently retained, for on cooling it is expelled with great violence; this phenomenon is known as the “spitting” of silver. It is prevented by preserving the molten metal from contact with air by covering the surface with non-oxidizing agents, or by traces of copper, bismuth or zinc.

Chemical Properties.—Silver is not oxidized by oxygen, but resembles mercury in being oxidized by ozone. It has no action on water. It is readily soluble in dilute nitric acid, nitric oxide and silver nitrate being formed; it also dissolves in hot, strong sulphuric acid, sulphur dioxide being evolved. Hydrochloric acid forms a surface film of silver chloride; hydriodic acid