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 contains a large amount of lead and a small amount of the radioactive precursors 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 circa 4.6 billion years. This figure is in good agreement with the age of meteorites and the age of the Moon as determined independently.
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Metallic chemical element, chemical symbol Th, atomic number 90. One of the actinide series of elements, natural thorium is a mixture of radioactive isotopes, predominantly thorium-232 (half-life of more than 10 billion years). It is a dense metal that is silver-white in pure form but turns gray or black on prolonged exposure to air. Although not a nuclear reactor fuel itself, thorium-232 can be used in breeder reactors because, on capturing slow-moving neutrons, it decays into fissionable uranium-233. Thorium is added to magnesium and its alloys to improve their high-temperature strength. Added to glass, it yields glasses with a high refractive index, useful for specialized optical applications. It was formerly in great demand as a component of mantles for gas and kerosene lamps and has been used in the manufacture of tungsten filaments for lightbulbs and vacuum tubes.
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(born Dec. 9, 1934, York, Yorkshire, Eng.) British actress. She made her stage debut in 1957 as Ophelia in Hamlet, and Shakespearean works became her specialty. She also performed in musical roles, starring in the London premiere of Cabaret in 1968. Among her many other notable credits were the 1981–84 TV series A Fine Romance and the films 84 Charing Cross Road (1986), Mrs. Brown (1997)—in which she starred as Queen Victoria—and Shakespeare in Love (1998, Academy Award).
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In Islam, the tradition or collection of traditions attributed to the Prophet Muhammad that include his sayings, acts, and approval or disapproval of things. Hsubdotadīth is revered by Muslims as a major source of religious law and moral guidance. It consists of two parts: the oral law itself and the isnād, or chain of authorities who passed it down to posterity. The various collections of Hsubdotadīth provide the major source for studying the development of Islam in its first few centuries.
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