acrylic \9-'kri-lik\ painting Painting executed in the medium of acrylic resins—synthetic resins that dry rapidly, are water-soluble, and serve as a vehicle for any pigment. Its effects may range from the transparent bril¬ liance of watercolour to the density of oil paint. Acrylics are less affected by heat and deterioration than oils. They were first used by artists in the 1940s but became popular with Pop artists when they were produced commercially in the 1960s.
Act of Union See Act of Union (1707), Act of Union (1800)
Acta Latin "Acts" In ancient Rome, the daily minutes of public busi¬ ness and a record of political and social events. Julius Caesar in 59 bc ordered that the Senate’s daily doings ( acta diurna, commentaria Sena- tus) be made public; Caesar Augustus later prohibited publication, though the Senate’s acts continued to be recorded and could be read with special permission. There were also public registers (acta diurna urbis, “daily minutes of the city”) of the acts of the popular assemblies and the courts as well as births, deaths, marriages, and divorces. These constituted a daily gazette, a prototype of the modern newspaper.
ACTH in full adrenocorticotropic \3- , dre-no- l kor-ti-ko- , tro-pik\ hormone Polypeptide hormone made in the pituitary gland. It regulates the activity of part of the adrenal cortex (see adrenal glands), the produc¬ tion site of important steroid hormones that affect electrolyte and water balance and the metabolism of fats, carbohydrates, and proteins. ACTH is found in vertebrates (except jawless fishes); in mammals it contains 39 amino acids. Overproduction of ACTH is one cause of Cushing syndrome.
actin One of two proteins responsible for contraction of muscle cells and the motility of other cells. It occurs as a monomer, G-actin, a globular protein, and in living cells as a polymer, F-actin, which resembles two strings of beads twisted around each other into thin filaments. The fila¬ ments occur in regular structures, alternated and interwoven with thick filaments that contain myosin, the other major muscle protein. The thick and thin filaments slide past each other, under the control of calcium ions, resulting in contraction (shortening) and relaxation (lengthening) of the muscle cells.
acting Art of representing a character on a stage or before a camera by means of movement, gesture, and intonation. Acting in the Western tra¬ dition originated in Greece in the 6th century bc; the tragedian Thespis is traditionally regarded as founder of the profession. Aristotle defined act¬ ing as “the right management of the voice to express various emotions” and declared it a natural gift that he doubted could be taught. Acting declined as an art in the Middle Ages, when Christian liturgical drama was performed by craft guilds and amateurs. Modern professional acting emerged in the 16th century with Italy’s commedia dell'arte troupes. It
The Acropolis, Athens.
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actinide ► Acuff I 13
flourished during the era of William Shakespeare. Not until the 18th cen¬ tury, however, was acting considered a profession to be taken seriously, through the efforts in England of the actor-manager David Garrick and the talents of actors such as Sarah Siddons, Edmund Kean, and Henry Irving. Modern acting styles have been influenced by Konstantin Stanislavsky’s emphasis on the actor’s identification with his role and by Bertolt Brecht’s insistence on the objectivity and discipline of the actor. The Stanislavsky method was adopted in the U.S. by Lee Strasberg and Stella Adler (1901— 92) and is the basis of most contemporary training, which features the cultivation of emotional and sense memory, physical and vocal training, and improvisation.
actinide \ l ak-t9-,nld\ Any of the series of 15 consecutive chemical ele¬ ments in the periodic table from actinium to lawrencium (atomic numbers 89-103). All are radioactive heavy metals; and only the first four (actinium, thorium, protactinium, and uranium) occur in nature in appre¬ ciable quantities. The other 11 (the transuranium elements) are unstable and are produced only artificially. Actinides are transition elements, so their atoms have similar configurations and similar physical and chemical behaviour; the most usual valences are 3 and 4.
actinolite \ak- , ti-n 3 l-,It\ Colourless to green amphibole mineral, darken¬ ing with increased iron content from green to black. It has a prismatic and splintery texture and is abundant in regionally metamorphosed (see meta¬ morphism) rocks such as schists. Actinolite has a monoclinic crystal struc¬ ture, and it may alter to chlorite. See also asbestos.
actinomycete \,ak-ti-no-'mI-set\ Any of a group of generally low- oxygen-utilizing bacteria identified by a branching growth pattern that results in large threadlike structures. The filaments may break apart to form rods or spheroidal shapes. Some actinomycetes can form spores. Many species occur in soil and are harmless to animals and higher plants; others are important disease-causing agents. For diagnostic purposes in human infections, the presence of sulfur granules is used.
Action Francaise \ak-se-'o n -fra n -'sez\ (“French Action”) Influential right-wing antirepublican group in early 20th-century France, whose views were promoted in a newspaper of the same name. The Action Francaise movement, led by Charles Maurras, espoused the antiparlia¬ mentarian, anti-Semitic, and strongly nationalist views inspired by the Alfred Dreyfus Affair. The movement peaked after World War I, when nationalist feeling was strong. It was denounced by the papacy in 1926, and ceased to exist after World War II because of its association with the collaborationist Vichy government (see Vichy France).
action painting Direct, instinctual, dynamic style of painting that involves the spontaneous application of vigorous, sweeping brush strokes and the chance effects of dripping and spilling paint onto the canvas. The term characterizes the work of Jackson Pollock, Willem de Kooning, and Franz Kune. The “automatic” techniques developed in Europe by the Sur¬ realists in the 1920s and ’30s had great influence on U.S. artists, who regarded a picture not merely as a finished product but as a record of the process of its creation. It was a major force in Abstract Expressionism in the 1950s. See also automatism, Tachism.
action potential Brief (about one-thousandth of a second) reversal of electric polarization of the membrane of a nerve or muscle cell. Stimu¬ lation of the cell by certain chemicals or by sensory receptor cells causes depolarization of the membrane, permitting an impulse to move along the nerve fibre (in nerve cells) or causing the cell to contract (in muscle cells).
Actium Vak-she-omV Battle of (31 bc) Naval battle off Acarnania, Greece, between Octavian (later Augustus) and Mark Antony. With 500 ships and 70,000 infantry, Antony camped at Actium, between the Ionian Sea and the Ambracian Gulf. Octavian, with 400 ships and 80,000 infan¬ try, cut Antony’s line of communication from the north. Desertion by allies and a lack of supplies forced Antony to act. Outmaneuvered on land, he followed Cleopatra’s advice to attack Octavian at sea. Antony’s larger fleet included his own and Cleopatra’s ships. In the heat of battle Cleo¬ patra fled with her galleys, and Antony followed with a few ships. His fleet surrendered immediately, his army a week later. Octavian’s victory left him undisputed ruler of the Roman world.
activation energy Minimum amount of energy (heat, electromagnetic radiation, or electrical energy) required to activate atoms or molecules to a condition in which it is equally likely that they will undergo chemical reaction or transport as it is that they will return to their original state. Chemists posit a transition state between the initial conditions and the