Upper Canada See Canada West Upper Volta See Burkina Faso
Uppsala \ , up-,sa- l la\ City (pop., 2001 est.: 191,110), Sweden. Located north of Stockholm, it lies near a village which was originally the capital of the ancient pre-Christian kingdom of Svea. By the 13th century it was an important commercial centre. Relinquishing its political primacy to Stockholm, it remained the seat of the archbishop of Sweden; its Gothic cathedral (erected from the 13th—15th centuries) is Sweden’s largest. It is also an educational centre, the site of Sweden’s oldest university, Upp¬ sala University (founded 1477). It is now an industrial city and transpor¬ tation hub. Carolus Linnaeus lived there.
'Uqab, Battle of Al- See Battle of Las Navas de Tolosa
Uqsur, Al- See Luxor
Ur Vor, ur\ Ancient city and district, Sumer, southern Mesopotamia. It was situated on a former channel of the Euphrates River in what is now south¬ ern Iraq. One of the oldest cities of Mesopotamia, it was settled some¬ time in the 4th millennium bc. In the 25th century bc it was the capital of
© 2006 Encyclopaedia Britannica, Inc.
1974 I uracil ► Urban II
southern Mesopotamia under its first dynasty. Though it later declined, it again became important around the 22nd century bc. It is mentioned in the Bible (as Ur of the Chaldees) as the early home of Hebrew patriarch Abraham (c. 2000 bc). In subsequent centuries it was captured and destroyed by many groups, including the Elamites and Babylonians. Neb¬ uchadrezzar II restored it in the 6th century bc. Excavations, especially in the 1920s and ’30s, uncovered remains of great archaeological value.
Northeastern facade (the ascents partly restored) of the ziggurat at Ur, southern Iraq.
HIRMER FOTOARCHIV, MUNCHEN
uracil \'yur-3-,sil\ Organic compound of the pyrimidine family, often called a base, consisting of a ring containing both nitrogen and carbon atoms. It occurs in combined form in many important biological molecules, includ¬ ing RNA and several coenzymes active in carbohydrate metabolism. Dur¬ ing synthesis of an RNA strand from DNA, uracil pairs with adenine. It or its corresponding nucleoside or nucleotide may be prepared from RNA by selective techniques of hydrolysis.
Ural \'yur-3l\ Mountains Mountain range, Russia and Kazakhstan. Generally held to constitute the boundary between Europe and Asia, the range extends north-south for some 1,550 mi (2,500 km) from just south of the Kara Sea to the Ural River. The mountains average 3,000^4,000 ft (915-1,220 m) in elevation; the highest peak is Mount Narodnaya at 6,217 ft (1,895 m). The Central and Southern Urals contain one of the largest industrial regions of Russia, producing metal goods, chemicals, and machinery; that region developed rapidly during World War II (1939-45), when many industrial plants were moved from the western part of the Soviet Union to prevent their destruction by the Germans.
Ural River River, Russia and Kazakhstan. Rising at the southern end of the Ural Mountains, it flows southwest to cross through western Kaza¬ khstan to the Caspian Sea at Atyrau. It is 1,509 mi (2,428 km) long and drains an area of 91,500 sq mi (237,000 sq km). Its lower course is navi¬ gable.
Uralic \yu-'ra-lik\ languages Family of more than 30 languages spo¬ ken by some 25 million people in central and northern Eurasia. A primary division is between the Finno-Ugric languages, which account for most of the languages and speakers, and the Samoyedic languages. The latter lan¬ guages have historically been spoken in the forest region of northern Siberia and in the tundra and coastal zones from the Ob to the White Sea and east into the Taymyr Peninsula. The known languages are Nganasan, Enets, Nenets, Selkup, Kamas (which became extinct in the late 20th cen¬ tury), and Mator (which became extinct in the 19th century). Of the other languages that are still spoken,
Nenets, which has 25,000 speakers and is still being learned by children, is the most viable. The Samoyedic languages share little vocabulary with Finno-Ugric. At its very earliest stages Uralic most probably included the ancestors of the Yukaghir lan¬ guages, still sometimes included with the Paleo-Siberian languages.
uraninite Vyu-'ra-no-.nltV Uranium dioxide (U0 2 ), a major oxide mineral of uranium. Uraninite is radioactive and usually forms black, gray, or brown crystals that are moderately hard and generally opaque. The ele¬ ments uranium and radium were first
Uraninite in pitchblende from Great Bear Lake, Northwest Territories, embedded (for display) in a larger mass of feldspar from Grafton Center, N.H.
COURTESY OF THE FIELD MUSEUM OF NATURAL HISTORY, CHICAGO; PHOTOGRAPH, JOHN H. GERARD
extracted from uraninite ore from what is now the Czech Republic. It has also been mined in Germany and Canada, and in the Colorado Plateau (U.S.). See also pitchblende.
uranium Chemical element of the actinide series (with many transition element properties), chemical symbol U, atomic number 92. A dense, hard, silvery white metal that tarnishes in air, it is isolated from such ores as pitchblende. Until the discovery of the first transuranium element in 1940, uranium was believed to be the heaviest element. Radioactivity was dis¬ covered in uranium by A.-H. Becquerel. All its isotopes are radioactive; several have half-lives long enough to permit determination of the age of the Earth by uranium-thorium-lead dating and uranium-234-uranium-238 dating. Nuclear fission was discovered in 1938 in uranium bombarded with neutrons, and the self-sustaining nuclear chain reaction, the atomic bomb, and the generation of nuclear power followed. Uranium has various valences in compounds, some of which have been used as colours in ceramic glazes, in lightbulb filaments, in photography, and as dyes and mordants.
uranium-234-uranium-238 dating Method of age determina¬ tion that makes use of the radioactive decay of uranium-238 to uranium- 234; the method can be used for dating sediments from either a marine or a playa lake environment. Because this method is useful for the period c. 100,000-1.2 million years before the present, it helps in bridging the gap between the carbon- 14 dating method and the potassium-argon dat¬ ing method.
uranium-thorium-lead dating or common-lead dating
Method of dating very old rocks by means of the amount of common lead they contain. Common lead is any lead from a rock or mineral that con¬ tains a large amount of lead and a small amount of the radioactive pre¬ cursors of lead (i.e., the isotopes uranium-235, uranium-238, and thorium- 232). By this method, the age of the Earth has been estimated to be c. 4.6 billion years. This figure is in good agreement with the age of meteorites and the age of the Moon as determined independently.
Uranus or Ouranus Vyur-s-nos, yu-'ra-nosN Ancient Greek personifi¬ cation of heaven. When Gaea emerged from Chaos, she produced Uranus, the mountains, and the sea. Her subsequent union with Uranus produced the Titans, the Cyclopes, and the Hecatoncheires. Uranus despised his off¬ spring and hid them in Gaea’s body. In response to her appeal for ven¬ geance, Cronus castrated Uranus. From the drops of blood that fell on Earth were born the Furies, the Giants, and the ash-tree nymphs called Meliai. His severed genitals floated on the sea, producing a white foam from which sprang Aphrodite. Uranus also consorted with Clymene, Hem- era, Hestia, and Nyx.
Uranus Seventh planet from the Sun. It was discovered in 1781 by Wil¬ liam Herschel (see Herschel family) and named for the Greek god personi¬ fying heaven. A blue-green gas giant, it has almost 15 times the mass of Earth and over 50 times its volume. It is less dense than Earth; the grav¬ ity at the top of its atmosphere is 11 % weaker. Its equatorial diameter is 31,800 mi (51,100 km). Uranus has 10 sharply defined narrow, dark rings, with broad dust bands between them; the rings consist mainly of boulder- size chunks of dark material. Uranus also has at least 27 moons (most named after Shakespearean characters) and a magnetic field about as strong as Earth’s. The planet rotates once every 17 hours around an axis that, unusually, is almost parallel to the ecliptic; from Earth it appears to spin on its side. It takes 84 years to orbit the Sun, at a mean distance of 1.78 billion mi (2.87 billion km). It has no solid surface; its fluid interior is thought to consist of a mixture of rock, ices, and gas, with little or no rocky core. Its upper atmosphere is mostly hydrogen and helium; the blue-green colour comes from absorption of red light by the small amount of methane present.