placer mining Oldest method of recovering gold from alluvial depos¬ its. It takes advantage of gold’s high density, which causes it to sink more rapidly from moving water than the lighter siliceous materials with which it is found. Panning, used by 19th-century miners, employed a pan in which a few handfuls of the gold-bearing soil or gravel and a large amount of water were placed; by swirling the pan’s contents, the miner washed the siliceous material over the side, leaving the gold and heavy materials behind. Dredging is the most important placer-mining method today. Used worldwide is the bucket-ladder dredge, with its continuous chain of buck¬ ets rotating around a rigid adjustable frame called the ladder. In sluicing, a slightly sloping wooden trough called a box sluice, or a ditch cut in hard gravel or rock called a ground sluice, is used as a channel along which gold-bearing gravel is carried by a stream of water. Riffles placed trans¬ versely along the bottom of the sluice cause the water to eddy into small basins, retarding the current so that gold may settle.
Placid, Lake See Lake Placid
plagioclase \ , pla-je-o- l klas\ Any member of the series of abundant feldspar minerals that usually occur as light- to medium-grey-coloured,
transparent to translucent grains or crystals. Plagioclase ranges in com¬ position from albite to anorthite. It is used in the manufacture of glass and ceramics; iridescent varieties are valued as gemstones. The primary importance of plagioclase, however, derives from its role as a rock¬ forming mineral.
plague \'plag\ Infectious fever caused by the bacterium Yersinia pestis , carried by the rat flea. It usually spreads to humans only when the flea runs out of rodent hosts. It takes three forms. Bubonic, the mildest, has characteristic swollen lymph nodes (buboes) and is spread only by the flea. It accounts for three-fourths of plague cases. Pneumonic plague has exten¬ sive lung involvement and is spread in droplets from the lungs; it is often fatal in three or four days without treatment. In septicemic plague, bac¬ teria overwhelm the bloodstream and often cause death within 24 hours, before other symptoms have a chance to develop. In the 14th century, plague ravaged Europe and Asia and was called the Back Death. Plague does not respond to penicillin, but other antibiotics are effective. Sanitary measures against fleas and rodents, quarantine, and extreme caution in handling infectious materials help to suppress epidemics. A vaccine can prevent plague.
plaice Commercially valuable European fatfish ( Pleuronectes platessa). At most 36 in. (90 cm) long, the plaice normally has both eyes on the right side of the head and four to seven bony bumps near its eyes. It is brown with red or orange spots. The American plaice ( Hippoglossoides platessoides) is found in both Europe (where it is called the rough dab) and the U.S. It is reddish or brownish and grows to about 24 in. (60 cm) long.
plain Any relatively level area of the Earth’s surface that exhibits gentle slopes and small local relief (differences in elevation). Occupying slightly more than one-third of the terrestrial surface, plains are found on all continents except Antarctica. Some are tree-covered, and others are grassy. Still others support scrub brush and bunch grass, and a few are nearly waterless deserts. With certain exceptions, plains have become the sites of major centres of population, industry, commerce, and transportation.
Plain, the French la Plaine In the French Revolution, the centrist deputies in the National Convention. They formed the majority of the assembly’s members and were essential to the passage of any measures. Their name derived from their place on the floor of the assembly; above them sat the members of the Mountain, or Montagnard. Led by Emmanuel- Joseph Sieyes, the Plain initially voted with the moderate Girondins but later joined the Montagnards in voting for the execution of Louis XVI. In 1794 they helped overthrow Maximilien de Robespierre and other extreme Jacobins (see Jacobin Club).
Plain of Esdraelon See Plain of Esdraelon Plain of Reeds See Plain of Reeds Plain of Sharon See Plain of Sharon
Plains Indian Any member of various Native American tribes that for¬ merly inhabited the Great Pains of the U.S. and southern Canada. Plains Indians are popularly regarded as the typical American Indians. They were essentially big-game hunters, the buffalo being a primary source of food and equally important as a source of materials for clothing, shelter, and tools. Until supplanted by the white settlers from the 16th century onward, the Plains Indians occupied the area between the Mississippi River and the Rocky Mountains, which includes portions of both the United States and Canada. It is a vast grassland stretching from northern Alberta and Saskatchewan in Canada to the Rio Grande border of Texas. The Arapaho, Assiniboin, Backfoot, Cheyenne, Comanche, Plains Cree, Crow, Hidatsa, Kiowa, Mandan, Osage, Pawnee, and Sioux are Plains Indians.
Plains of Abraham See Plains of Abraham
planarian \pta-'nar-e-3n\ Any of about 3,000 species of widely distrib¬ uted, mostly free-living fatworms of the family Planariidae and related families (class Turbellaria), usually found in freshwater but also in marine and terrestrial environments. The soft, ciliated body is leaf-shaped when elongated. The spade-shaped head has two eyes and sometimes tentacles. The tail is pointed. The mouth is on the lower side, often more than halfway
Plaice (Pleuronectes platessa)
JACQUES SIX
© 2006 Encyclopaedia Britannica, Inc.
1508 I Planck ► plant virus
toward the tail. Most species are 0.1-0.6 in. (3-15 mm) long; some grow to about 1 ft (30 cm). Planarians swim with an undulating motion or creep like slugs. Most feed at night on protozoans, snails, and other worms.
Planck \'plaqk\, Max (Karl Ernst Ludwig) (b. April 23, 1858, Kiel, Schleswig—d. Oct. 4, 1947, Gottingen, W.Ger.) German physicist. He studied at the Universities of Munich and Kiel, then became professor of theoretical physics at the University of Berlin (1889-1928). His work on the second law of thermodynamics and blackbody radiation led him to formulate the revolutionary quantum theory of radiation, for which he received a Nobel Prize in 1918. He also discovered the quantum of action, now known as Planck’s constant, h. He championed Albert Einstein’s spe¬ cial theory of relativity, but he opposed the indeterministic, statistical worldview introduced by Niels Bohr, Max Born, and Werner Heisenberg after the advent of quantum mechanics. As the influential president of the Kaiser Wilhelm Society (later the Max Planck Society) until his resigna¬ tion in 1937, he appealed to Adolf Hitler to reverse his devastating racial policies. His son was later implicated in the July Plot against Hitler and was executed.
plane tree Any of ten species of large trees that make up the genus Plcitanus, sole genus of the family Platanaceae, native to North America, eastern Europe, and Asia. Plane trees are planted widely in cities for their resistance to diseases and to air pollution and because they grow rapidly and furnish quick shade. They are characterized by scaling bark; large, deciduous, usually lobed leaves; and globular heads of flower and seed. Ball-shaped smooth or bristly seed clusters, which dangle singly and often persist after leaf fall, are key identifiers. Winter bark is patchy and pic¬ turesque; as the outer bark flakes off, inner bark shows shades of white, gray, green, and yellow.
planer Metal-cutting machine tool in which the workpiece is firmly attached to a horizontal table that moves back and forth under a single¬ point cutting tool. The tool-holding device is mounted on a crossrail so that the tool can be moved across the table in small sideward movements. Since the cutting tool can be moved at almost any angle, a wide variety of grooves and surfaces can be generated. Mechanical planers, or surfacers, are also used to smooth wood to an even thickness. Planers perform the same opera¬ tions as shapers but can machine workpieces up to 50 ft (15 m) long.