silicone or polysiloxane Any of a diverse class of polymers manu¬ factured as fluids, resins, or elastomers. They are partially organic com¬ pounds, but, unlike most polymers, they have a backbone containing no carbon, composed instead of alternating silicon and oxygen atoms. In most silicones, two organic groups (see functional group), usually methyl or phenyl, are attached to each silicon atom. Silicones in general are excep¬ tionally stable and inert. Silicone fluids are used in hydraulic fluids, emulsion-breaking compositions, and breast implants and as adhesives, lubricants, water repellents, and protective coatings. Silicone rubbers are used as electrical insulators in encapsulations, coatings, and varnishes; as gaskets and caulking material; in specialized tubing; as automobile engine components; as flexible windows in face masks and air locks; for lami¬ nating glass cloth; and as surgical membranes and implants.
silicosis \,si-b-'ko-s9s\ Common pneumoconiosis caused by long-term inhalation of silica mineral dust. Known since the 18th century, it usually occurs after 10-20 years of exposure in jobs such as mining, stonecutting, grinding, or polishing. The smallest particles do the most damage, kill¬ ing macrophages (see reticuloendothelial system) that engulf them in the pulmonary alveoli. Dead cells accumulate, forming fibrous masses that reduce lung elasticity. Decreased lung volume and poor gas exchange lead to shortness of breath and then to coughing, difficulty in breathing, and weakness. Patients are vulnerable to tuberculosis, emphysema, and pneumo¬ nia. In the absence of effective treatment, control of silicosis depends on prevention with face masks, proper ventilation, and X-ray monitoring of workers’ lungs.
Siljan Vsil-,yan\, Lake Lake, central Sweden. Covering 112 sq mi (290 sq km), it is Sweden’s third largest lake. It is fed by the Osterdal River and extends into two bays. Its wooded shores are interspersed with mead¬ ows and quaint villages, making it an attractive tourist destination.
silk Animal fibre produced by certain insects as building material for cocoons and webs. In commercial use it refers almost entirely to filament from cocoons produced by the caterpillars of several moth species of the genus Bombyx, commonly called silkworms. Silk is a continuous filament around each cocoon. It is freed by softening the cocoon in water and then locating the filament end; the filaments from several cocoons are unwound at the same time, sometimes with a slight twist, to form a single strand. In the process called throwing, several very thin strands are twisted together to make thicker, stronger yarn. Silk was discovered in China sometime before 2700 bc, and the secret of its production was closely guarded for millennia. Along with jade and spices, silk was the primary commodity traded along the Silk Road beginning about 100 bc. Since World War II, nylon and other synthetic fibres have replaced silk in many applications (e.g., parachutes, hosiery, dental floss), but silk remains an important luxury material for clothing and home furnishings. More than 50% of the world’s silk is still produced in China.
Silk Road Ancient trade route that linked China with Europe. Origi¬ nally a caravan route and used from c. 100 bc, the 4,000-mi (6,400-km) road started in Xi'an, China, followed the Great Wall to the northwest, climbed the Pamir Mtns., crossed Afghanistan, and went on to the east¬ ern Mediterranean Sea, where goods were taken by boat to Rome. Silk was carried westward, while wool, gold, and silver were carried eastward. With the fall of Rome, the route became unsafe; it was revived under the Mongols, and Marco Polo used it in the 13th century.
silkscreen or serigraphy \s3-'ri-gr3-fe\ Sophisticated stenciling technique for surface printing, in which a design is cut out of paper or another thin, strong material and then printed by rubbing, rolling, or spray¬ ing paint or ink through the cutout areas. It was developed c. 1900 and originally used in advertising and display work. In the 1950s fine artists began to use the process. It got its name from the fine-mesh silk that, when tacked to a wooden frame, serves as a support for the cut-paper stencil, which is glued to it. To make a silkscreen print, the wooden frame holding the screen is hinged to a slightly larger wooden board, the print¬ ing paper is placed on the board under the screen, and the paint is pressed through the screen with a squeegee (rubber blade) the same width as the screen. Many colours can be used, with a separate screen for each colour.
silkworm moth Any moth in the genus Bombyx (family Bombycidae). The Chinese silkworm ( B. mori ) has been used in commercial silk produc¬ tion for centuries. The adult, which has a wingspan of about 2 in. (50 mm) and a thick, hairy body, lives only two or three days. The female lays 300-500 eggs. The pale, naked larvae are fed mulberry leaves until pupation begins, when they are about 3 in. (75 mm) long. They spin a cocoon of one continuous white or yellow silken thread, about 1,000 yards (900 m) long. The pupa is killed with hot air or steam to pre¬ serve the thread intact. See also sat- URNIID MOTH.
Sill or sheet In geology, a tabular igneous intrusion emplaced parallel to the bedding of the enclosing rock. Although they may have inclined orientations, nearly horizontal sills are most common. Sills may range from a few inches to hundreds of feet thick and up to hundreds of miles long. They include rock compositions of all types.
Silla Kingdom of ancient Korea that in ad 668 consolidated other polities on the Korean peninsula under the Unified Silla dynasty (668-935). Tra¬ ditionally believed to have been founded by Hyokkose in 57 bc, Silla emerged as a full-fledged kingdom in the 6th century. In the reign of King Chinhung (540-576), a unique military corps, the hwarang (see Hwarangdo), was organized; allied with Tang-dynasty China, it defeated the Korean state of Paekche in 660 and Koguryo in 668, then expelled the Tang forces to create a unified and independent Korean state. It adopted a Chinese bureaucratic structure, but its aristocracy was never replaced by a
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Sillanpda ► silver salmon I 1753
bureaucratic class based on merit. Silla art before unification shows a ten¬ dency toward abstraction; postunification art reflects Tang naturalism.
Sillanpda Vsel-lan-.paV Frans Eemil (b. Sept. 16, 1888, Hameen- kyro, Fin., Russian Empire—d. June 3, 1964, Helsinki, Fin.) Finnish nov¬ elist. The son of a farmer, he studied natural science but returned to the country to write. Shocked by the Finnish civil war of 1918, he produced his most substantial novel, Meek Heritage (1919), relating how a humble cottager becomes involved with the Red Guards. After several collections of short stories in the late 1920s, he published his best-known work, The Maid Silja (1931), about an old peasant family. People in the Summer Night (1934) is his most polished and poetic novel. In 1939 he became the first Finnish writer to win the Nobel Prize for Literature.
sillimanite Vsi-b-mo-.nhA or fibrolite Vfi-bro-JlA Brown, pale green, or white glassy silicate mineral that often occurs in long, slender, needle¬ like crystals frequently found in fibrous aggregates. An aluminum sili¬ cate, Al 2 0Si0 4 , it occurs in high-temperature regionally metamorphosed clay-rich rocks (e.g., schists and gneisses). Sillimanite is found at many localities in France, Madagascar, and the eastern U.S.; a pale sapphire- blue gem variety occurs in the gravels of Sri Lanka.
Sillitoe Ysi-li-.toV Alan (b. March 4, 1928, Nottingham, Nottingham¬ shire, Eng.) English writer. The son of a tannery worker, he worked in factories from age 14. Many of his later novels and stories are brash and angry accounts of working-class life, beginning with his successful first novel, Saturday Night and Sunday Morning (1958; film, 1960). Perhaps his best-known work is the title story in the collection The Loneliness of the Long-Distance Runner (1959; film, 1962). His other works include the novels The Death of William Posters (1965), The Widower’s Son (1976), and The Open Door (1989) and the story collections The Rag¬ man’s Daughter (1963; film, 1974) and Second Chance (1981).