Wythe Vwith\, George (b. 1726, Elizabeth City county, Va.—d. June 8, 1806, Richmond, Va., U.S.) U.S. jurist and statesman. Admitted to the bar in 1746, he was a member (1754-55, 1758-68) and clerk (1769-75) of the Virginia House of Burgesses. He practiced law in Williamsburg, Va., where he taught Thomas Jefferson. At the College of William and Mary (1779-89) he became the first professor of law in the U.S.; among his pupils was John Marshall. A delegate to the Continental Congress, he signed the Declaration of Independence. In 1776 he was appointed, with Jefferson and two others, to revise the laws of Virginia. As a chancery judge (1778-1806), he asserted, in Commonwealth v. Caton (1782), the power of courts to refuse to enforce unconstitutional laws. He was a member of the Constitutional Convention (1787) and of the Virginia con¬ vention (1788) that ratified the Constitution of the United States.
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X-ray ► Xenophon I 2085
X-ray Electromagnetic radiation of extremely short wavelength (100 nanometres to 0.001 nanometre) produced by the deceleration of charged particles or the transitions of electrons in atoms. X-rays travel at the speed of light and exhibit phenomena associated with waves, but experiments indicate that they can also behave like particles (see wave-particle duality). On the electromagnetic spectrum, they lie between gamma rays and ultra¬ violet radiation. They were discovered in 1895 by Wilhelm Conrad Ront- gen, who named them X-rays for their unknown nature. They are used in medicine to diagnose bone fractures, dental cavities, and cancer; to locate foreign objects in the body; and to stop the spread of malignant tumours. In industry, they are used to analyze and detect flaws in structures. See illustration below.
X-ray astronomy Study of astronomical objects and phenomena that emit radiation at X-ray wavelengths. Because Earth’s atmosphere absorbs most X-rays, X-ray telescopes and detectors are taken to high altitudes or into space by balloons and spacecraft. In 1949 detectors aboard sounding rockets showed that the Sun gives off X-rays, but it is a weak source; it took 30 more years to clearly detect X-rays from other ordinary stars. Beginning with the Uhuru X-ray satellite (launched 1970), a succession of space observatories carried increasingly sophisticated instruments into Earth orbit. Astronomers discovered that most types of stars emit X-rays but usually as a tiny fraction of their energy output.
Supernova remnants are more powerful X-ray sources; the strongest sources known in the Milky Way Galaxy are certain binary stars in which one star is probably a black hole. In addition to myriad point sources, astronomers have found a diffuse background of X-ray radiation ema¬ nating from all directions; unlike cosmic background radiation, it appears to have many distant individual sources. The Chandra X-Ray Observatory and XMM-Newton X-ray satellite (both launched 1999) have made numerous discoveries relating to the nature and quantity of black holes in the universe, the evolution of stars and galaxies, and the composition and activity of supernova remnants.
X-ray diffraction Phenomenon in which the atoms of a crystal, by virtue of their uniform spacing, cause an interference pattern of the waves in an incident beam of X-rays. The crystal’s atomic planes act on the X-rays in the same way a uniformly ruled grating acts on a beam of light (see polarization). The interference pattern is specific to each substance and gives information on the structure of the atoms or molecules in the crystal. See also William Henry Bragg.
Xanthus Vzan-thssX or Xanthos Ancient city of Lycia, Anatolia. Situ¬ ated near the mouth of the Xanthus (modem Esen Cayi) River in present- day southwestern Turkey, its ruins include a theatre, temples, and tombs. A number of pieces were taken for the British Museum. The ancient city
electrons
X-rays
An X-ray tube. Electrons "boil" off the cathode when the filament is heated by a current. A high voltage between cathode and anode causes the electrons to acceler¬ ate toward the anode, which rotates to avoid overheating of the target. When the electrons strike the anode's target area, X-rays are emitted.
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was twice besieged and destroyed: in 540 bc by the Persians of the Achae- menian dynasty and in 42 bc by the Romans. The ruins were designated a UNESCO World Heritage site in 1988.
Xavier Vza-ve-orV, Saint Francis (b. April 7, 1506, Xavier Castle, near Sangiiesa, Navarre—d. Dec. 3, 1552, Sancian Island, China; canon¬ ized March 12, 1622; feast day December 3) Spanish-born French mis¬ sionary to the Far East. Born into a noble Basque family, he was educated at the University of Paris, where he met Ignatius of Loyola and became one of the first seven members of the Jesuits. He was ordained in 1537, and in 1542 he embarked on a three-year mission to India. In 1545 he established missions in the Malay Archipelago, and in 1549 he traveled to Japan, where he was the first to introduce Chris¬ tianity systematically. He returned to India in 1551 and died the following year while attempting to secure entrance to China. He is believed to have bap¬ tized about 30,000 converts; his success was partly due to adaptation to local cultures. In 1927 he was named patron of all missions.
Xenakis \zen-a-‘kes,\ English \ze-'na-kis\, Iannis (b. May 29, 1922, Braila, Rom.—d. Feb. 4, 2001, Paris, France) Romanian- born Greek-French composer. His Greek family returned to Greece in 1932, and he studied engineering. A wartime resis¬ tance fighter, he was forced to flee Greece in 1947 when he was denounced as a communist. He worked closely with the architect Le Corbusier (1947-59), while studying com¬ position with Olivier Messiaen and others. Interested in expressing mathematical structures in music, he used the term “stochastic music” to refer to situa¬ tions in which the number of elements precludes prediction of what each element will do but the overall behaviour of the group is determinate (see stochastic process). He often based his compositions on standard concepts of mathematics and physics, such as the Fibonacci sequence ( Metastasis , 1954), the Boltzmann constant (. Pithoprakta , 1956), and Markov chains {Analogique A and Analogique B, 1958-59).
Xenocrates \zi-'nak-r3-,tez\ (d. 314 bc, Athens) Greek philosopher. A pupil of Plato, he succeeded Speusippus (d. 339/338 bc) as head of Pla¬ to’s Academy. His writings are lost except for fragments, but his doc¬ trines, as reported by Aristotle, appear to resemble those of Plato. He divided all of reality into three realms: the sensibles, or objects of sen¬ sation; the intelligibles, or objects of true knowledge, such as Plato’s forms (see form); and the bodies of the heavens, which mediate between the sensibles and the intelligibles and are therefore objects of “opinion.” A second threefold division separated gods, men, and “demons.” He main¬ tained that the origin of philosophy lies in mankind’s desire to be happy, happiness being defined as the acquisition of the perfection that is pecu¬ liar and proper to mankind (see eudaemonism).
xenon \'ze-,nan\ Chemical element, chemical symbol Xe, atomic num¬ ber 54. One of the noble gases, it is colourless, odourless, tasteless, and nearly inert, combining only with fluorine and oxygen and in complexes (see bonding). Xenon occurs in slight traces in Earth’s atmosphere and in rocks. Obtained by fractional distillation of liquefied air, it is used in lumi¬ nescent tubes (see electric discharge lamp), flash lamps, lasers, and tracer studies and as an anesthetic.