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Accra
Capital and largest city (pop., 1988 est.: 949,000) of Ghana, on the Gulf of Guinea. When the Portuguese first settled on the coast in 1482, the site was occupied by villages of the Ga tribe. In 1650-80 the Europeans built three fortified trading posts, sponsored by the English, the Danes, and the Dutch. The Danes and Dutch left the region in 1850 and 1872, respectively, and in 1877 Accra became the capital of the British Gold Coast colony. In the later 20th cent. it became Ghana's administrative, economic, and educational center. Tema, 17 mi (27 km) east, has taken over Accra's former port functions.


acid
Any substance that in water solution tastes sour, changes the color of acid-base indicators (e.g., litmus), reacts with some metals (e.g., iron) to yield hydrogen gas, reacts with bases to form salts, and promotes certain chemical reactions (e.g., acid catalysis). Acids contain one or more hydrogen atoms that, in solution, dissociate as positively charged hydrogen ions. Inorganic, or mineral, acids include sulfuric acid, nitric acid, hydrochloric acid, and phosphoric acid. Organic acids include carboxylic acids, phenols, and sulfonic acids. Broader definitions of acids cover situations in which water is not present. See also acid-base theory.


acne
Any of some 50 inflammatory diseases of the oil glands of the skin. Acne vulgaris, probably the most frequent chronic skin disorder, results from an interplay of hereditary factors, hormones, and bacteria, beginning in the teen years when overactive sebaceous glands are stimulated by high levels of androgens. Its primary lesion, the blackhead, may be open or closed; it consists of a plug of skin oil (sebum), cell debris, and microorganisms in a hair follicle. Acne has four grades of severity, with increasing degrees of spread, inflammation, pustule formation, and scarring. Lower grades generally respond to treatment, which may include skin medication, sunlight, antibiotics, and hormones; many cases eventually resolve spontaneously.


Acoma
Indian pueblo, W central New Mexico. It is located on a reservation west of Albuquerque and is known as the "Sky City." Its people live in terraced dwellings made of stone and adobe atop a sandstone butte 357 ft (109 m) high. Settled in the 10th cent., it is believed to be the oldest continuously inhabited place in the U.S. In 1540 Spanish explorer F. Coronado described it as the strongest defensive position in the world.


acorn
Nut of the oak. Acorns are usually seated in or surrounded by a woody cupule. They mature within one to two seasons, and their appearance varies depending on the species of oak. Acorns provide food for small game animals and are used to fatten swine and poultry.


Acre
Seaport city (pop., 1993 est.: 44,000), NW Israel on the Mediterranean coast. First mentioned in an Egyptian text from the 19th cent. BC, it was ruled by ...

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