, 1789, Erlangen, Bavaria—d. July
Ohm, detail of a lithograph
HISTORIA-PHOTO
© 2006 Encyclopaedia Britannica, Inc.
1402 I oil ► O'Keeffe
oil Any greasy substance liquid at room temperature and insoluble in water. It may be a fixed (nonvolatile) oil, an essential oil, or a mineral oil (see petroleum). Fixed oils and fats (derived from animals and plants) have the same chemical composition—both are esters of glycerol and fatty acids. These oils have a variety of industrial and food uses. Linseed, tung, and other drying oils are highly unsaturated (see saturation); these and large quantities of soybean, sunflower, and safflower oils (also constituents of foods) are used in paints and varnishes. When exposed to air they absorb oxygen and polymerize (see polymerization), forming a tough coating. Some specialty oils and oil derivatives are also used in leather dressing and textile manufacture.
oil-drop experiment See Millikan oil-drop experiment
oil painting Painting in oil colours, a medium consisting of pigments suspended in drying oils. Oil paint enables both fusion of tones and crisp effects and is unsurpassed for textural variation. The standard consistency of oil paint is a smooth, buttery paste. It is applied with brushes or a thin palette knife, usually onto a stretched linen canvas. Finished oil paintings are often coated with varnish. Oil as a painting medium is recorded as early as the 11th century, though the practice of easel painting with oil colours stems directly from 15th-century techniques of painting with tem¬ pera (see tempera painting). In the 16th century oil colour emerged as the basic painting material in Venice; it has been the most widespread medium for easel paintings ever since.
oil seal or shaft seal In machines, a device that prevents the passage of fluids along a rotating shaft. Seals are necessary when a shaft extends from a housing (enclosure) containing oil, such as a pump or a gearbox. Leather, synthetic rubber, and silicones are among the materials used for the sealing ring. See also bearing.
oil shale Any fine-grained sedimentary rock that contains solid organic matter (kerogen) and yields significant quantities of oil when heated. This shale oil is a potentially valuable fossil fuel, but the present methods of mining and refining it are expensive, damage the land, pollute the water, and produce carcinogenic wastes. Thus, oil shale will probably not be exploited on a wide scale until other petroleum resources have been nearly depleted. Estonia, China, and Brazil have facilities for producing rela¬ tively limited quantities, and the U.S. government operates an experimen¬ tal plant in Colorado.
Oise Vwaz\ River River, northern France. It is formed by the conflu¬ ence of two streams, one rising near Chimay in Belgium and the other near Rocroi in France. Flowing southwest into the Seine, the Oise is 188 mi (302 km) long. It is a link in the canal system between the Seine and the canals of northern France.
Oisin See Ossian
Oistrakh V6i-,strak\, David (Fyodorovich) (b. Sept. 30, 1908, Odessa, Ukraine, Russian Empire—d. Oct. 24, 1974, Amsterdam, Neth.) Russian violinist. After his debut in 1928, he entered and won a number of prestigious competitions. He performed for Soviet troops at the front during World War II and made debuts in the West in the 1950s. Dmitry Shostakovich dedicated both his violin concertos to him. Oistrakh taught at the Moscow Conservatory from 1934; one of his most distinguished students was his son Igor (b. 1931), who himself joined the conservato¬ ry’s faculty in 1958.
Ojibwa \o-'jib-wa\ or Chippewa Vchi-po-.woX North American Plains Indian people living mostly in southern Canada and the north-central U.S. Ojibwa is an Algonquian language. The people’s name, spelled Ojibwe in Canada and given as Chippewa in official U.S. documents, is derived from an Algonquian word ojib-ubway, meaning “puckering,” probably referring to a type of moccasin. They call themselves Anishinaabe or Anishinabek, meaning “Spontaneously Created” or “Original Man.” They formerly inhabited a region north of the Great Lakes but during the 17th— 18th centuries moved west to what is now northern Minnesota. Each Ojibwa tribe was divided into migratory bands. In the autumn, bands separated into family units for hunting; in summer, families gathered at fishing sites. They grew corn and collected wild rice. The Midewiwin, or Grand Medicine Society, was the major Ojibwa religious organization. The Ojibwa are one of the largest Native American groups in North America today, numbering about 50,000 in the U.S. and more than 100,000 in Canada. They are closely related to the Ottawa and Pota- watomi.
Ojukwu \o-'juk-wii\, Odumegwu (b. Nov. 4, 1933, Nnewi, Nigeria) Governor of Eastern Region, Nigeria (1966-67), and head of the seces¬ sionist state of Biafra (1967-70). A member of the Igbo, Ojukwu was edu¬ cated at Oxford University. He was appointed head of the traditional Igbo homelands in the east following the overthrow of Nigeria’s civilian gov¬ ernment by Igbo military leaders. He stayed on in that capacity even after members of the opposition Hausa and Yoruba staged a successful coun¬ tercoup. Mounting secessionist pressures compelled him to declare the Eastern Region an independent state in 1967. Following the Biafra con¬ flict he fled to Cote d’Ivoire; he returned to Nigeria in 1982.
Oka \o-'ka\ River River, western Russia. The largest right-bank tribu¬ tary of the Volga River, it flows 932 mi (1,500 km) north to Kaluga, then east to join the Volga at Nizhny Novgorod. Navigable for most of its length, except during winter, it is an important trade artery for lumber and grain.
okapi \o-'ka-pe\ Ruminant species (Okapia johnstoni ) in the giraffe fam¬ ily that lives alone in Congo rain for¬ ests, eating leaves and fruit. Its neck and legs are proportionately shorter than the giraffe’s, and females, which are larger than males, stand about 5 ft (1.5 m) at the shoulder.
The sleek coat is deep brown on the front of the body; the upper legs are black-and-white-striped, and the lower legs are white, with black rings above the hooves. The male’s short horns are covered with skin except at the tips.
Okavango \,o-ka-'vai]-g6\ River in Angola Cubango River River, southwest-central Africa. It is the fourth longest river system in the region. Rising in central Angola, it forms a section of the boundary between Angola and Namibia. It empties into the Okavango Swamp, a large wet¬ land north of Lake Ngami in northern Botswana, after a course of about 1,000 mi (1,600 km). It is unnavigable except for small craft, and its banks are sparsely settled; a major problem for inhabitants is control of the tsetse fly. The Moremi Wildlife Reserve in the northeastern corner of the swamp teems with wildlife.
Okeechobee X.o-ko-'cho-beN, Lake Lake, southeastern Florida, U.S. It is the third largest freshwater lake wholly within the country. It drains to the sea through the Everglades. It is a remnant of the prehistoric Pam¬ lico Sea, and its name means “Big Water” in the language of the Hitchiti Indians. There is a Seminole reservation on the lake’s northwestern shore.
O'Keeffe, Georgia (b. Nov. 15, 1887, near Sun Prairie, Wis., U.S.—d. March 6, 1986, Santa Fe, N.M.) U.S. painter. She studied ait in Chicago and New York City, where she met and married the photogra¬ pher Alfred Stieglitz. By the early 1920s, her highly individualistic paint¬ ing style had emerged, as typified by such works as Black Iris (1926). Her subjects were often enlarged views of the skulls and other bones of ani¬ mals, flowers and plant organs, shells, rocks, mountains, and other natu¬ ral forms. Her mysteriously suggestive images of bones and flowers set against a perspectiveless space have inspired a variety of erotic, psycho¬ logical, and symbolic interpretations. Her later works celebrate the clear skies and desert landscapes of New Mexico, where she moved after her