product conditions and theorize that the activation energy is the amount of energy required to boost the initial materials “uphill” to the transition state; the reaction then proceeds “downhill” to form the product materi¬ als. Catalysts (including enzymes) lower the activation energy by altering the transition state. Activation energies are determined by experiments that measure them as the constant of proportionality in the equation describing the dependence of reaction rate on temperature, proposed by Svante Arrhenius. See also entropy, heat of reaction.
active galactic nucleus Small region at the centre of a galaxy that emits a prodigious amount of energy in the form of radio, optical. X-ray, or gamma radiation or high-speed particle jets. Many classes of “active galaxies” have been identified; they appear similar in many respects to quasars. Astronomers suspect that the observed energy is generated as matter accretes onto a supermassive black hole with a mass millions or even billions of times that of the Sun. The accreting matter can outshine the rest of the galaxy as it is heated in very high-speed collisions outside the black hole’s event horizon. It is believed that many galaxies harbour these central black holes and that they might have been quasars in their early history, although they now appear to be dormant unless orbiting matter is accreting onto the black hole.
activity coefficient Number expressing the ratio of a substance’s chemical activity to its molar concentration (see mole). The measured concentration of a substance may not be an accurate indicator of its chemi¬ cal effectiveness as represented by the equation for a particular reaction; in such cases, the activity is calculated by multiplying the concentration by the activity coefficient. In solutions, the activity coefficient is a mea¬ sure of how much the solution differs from an ideal solution.
Acton (of Aldenham), John Emerich Edward Dahlberg Acton, 1st Baron (b. Jan.10, 1834, Naples, Kingdom of Naples—d. June 19, 1902, Tegernsee, Bavaria, German Empire) English historian. He served in the House of Commons (1859-65). Editor of the Roman Catholic monthly The Rambler (1859-64), he resigned because of papal criticism of his scientific approach to history. An adviser to William E. Gladstone from 1865, he was raised to the peerage in 1869. In 1895 he was appointed Regius Professor of Modern History at the University of Cambridge. He later coordinated the massive publication project of The Cambridge Mod¬ ern History. A critic of nationalism, he coined the familial - aphorism “Power tends to corrupt, and absolute power corrupts absolutely.”
actor-manager system Method of theatrical production prevalent in 19th-century England and the U.S. In this system, an actor formed a theatre company, chose the plays he wanted to produce, played the lead¬ ing roles in them, and managed the company’s business arrangements. The first actor-managers emerged in the 17th century, and in the 18th cen¬ tury actor-managers such as Colley Cibber and David Garrick gained promi¬ nence. The system produced high performance standards, typified by 19th-century figures such as William Macready, Henry Irving, and Herbert Tree. It waned as actor-managers were replaced first by stage managers and later by directors.
Actors Studio Professional actors’ workshop based in New York City. Founded in 1947 by the directors Cheryl Crawford, Eua Kazan, and Rob¬ ert Lewis as a leading centre of the Stanislavsky method, it was directed by Lee Strasberg from 1948 to 1982. In 1962 it launched a company, and a workshop was established in Los Angeles in 1966. Actors work together without the pressures of commercial production. Membership is by invi¬ tation; six or seven new members are chosen yearly from 1,000 auditions. Members have included Marlon Brando, AAarilyn Monroe, Paul Newman, and Robert De Niro.
actuary One who calculates insurance risks and premiums. Actuaries compute the probability of the occurrence of such events as birth, mar¬ riage, illness, accidents, and death. They also evaluate the hazards of property damage or loss and the legal liability for the safety and well¬ being of others. Usually employed by insurance companies, actuaries set premium rates based on statistical studies, establish underwriting proce¬ dures, and determine the amounts of money required to assure the pay¬ ment of benefits.
Acuff \'a-kof\, Roy (Claxton) (b. Sept. 15, 1903, Maynardsville, Tenn., U.S.—d. Nov. 23, 1992, Nashville, Tenn.) U.S. singer, songwriter, and fiddler. He turned to music after an aborted baseball career and gained immediate popularity with his recordings of “The Great Speckled Bird” and “The Wabash Cannonball.” Reasserting the mournful musical tradi-
© 2006 Encyclopaedia Britannica, Inc.
14 I acupressure ► Adams
tions of Southeastern rural whites, he became a national radio star on the “Grand Ole Opry” broadcasts. In 1942 he and songwriter Fred Rose founded Acuff-Rose Publishing, the first publishing house exclusively for country music. In 1962 Acuff was elected the first living member of the Country Music Hall of Fame.
acupressure or shiatsu \she-'at-su\ Alternative-medicine practice in which pressure is applied to points on the body aligned along 12 main meridians (pathways), usually for a short time, to improve the flow of vital force (Qi). Though often referred to by its Japanese name, shiatsu, it originated in China thousands of years ago. A single point may be pressed to relieve a specific symptom or condition, or a series of points can be worked on to promote overall well-being. Some studies suggest that acu¬ pressure can be effective for certain health problems, including nausea, pain, and stroke-related weakness. Risks are minimal with cautious use. See also acupuncture.
acupuncture Medical technique in which needles are inserted into the skin and underlying tissues, devised in China before 2500 bc. One or more small metal needles are inserted at precise points along 12 meridians (pathways) in the body, through which the vital life force (qi) is believed to flow, in order to restore yin-yang balance and treat disease caused by yin-yang imbalance. Acupuncture appears to relieve pain and is used as an anesthetic for surgery. Theories to explain its effects include stimula¬ tion of release of natural opiates, blockage of pain-signal transmission, and a placebo effect. See also acupressure.
Ada Va -do\ High-level computer programming language whose develop¬ ment was initiated in 1975 by the U.S. Dept, of Defense and standard¬ ized in 1983. Ada (named for the countess of Lovelace) was intended to be a common language for use on the department’s computers, which were produced by many different manufacturers. It is similar to Pascal but contains many additional features convenient for the development of large-scale, multiplatform programs. The 1995 revision, called Ada 95, Supports OBJECT-ORIENTED PROGRAMMING.
Adad \a-'dad\ Babylonian and Assyrian god of weather, the son of Anu ( sometimes called the son of Bel). He was known as the Lord of Abun¬ dance for rains that made the land bloom, but he sent death-dealing storms to his enemies. He was also the god of oracles and divination. Though widely worshiped, he was a minor god and appears to have had no cult center of his own.
Ada I Va-doL Historical Islamic state, eastern Africa, southwest of the Gulf of Aden, with its capital at Harer (now in Ethiopia). Its rivalry with Christian Ethiopia began in the 14th century. In the early 16th century Adal launched a series of attacks. By 1533 Adal, now led by Ahmed Gran, succeeded in gaining control of most of central Ethiopia. Gran was killed in battle in 1543, and the Oromo invasions of the later 16th century ended Adal’s power.
Adalbert Yad-^-.bartV, Saint orig. Vojtech (b. 956, Libice?, Bohemia—d. April 23, 997, near Gdansk, Pol.; feast day April 23) Czech prelate. Descended from the princes of Bohemia, he was trained in the¬ ology at Magdeburg. Elected the first native bishop of Prague in 982, he promoted the political aims of the Bohemian prince by extending the influence of the church beyond the Czech kingdom. Failing to convert his people, he retired in 988 to a monastery near Rome. On papal orders, he returned in 992 to find little change. Disillusioned, he left Bohemia in 994 to become a missionary along the Baltic coast, where he was martyred in 997. An account of his life was written by his friend and disciple St. Bruno of Querfurt. He was recognized as a saint soon after his death.