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PLANTIGRADA 270 PLATO PLANTIGRADA, in zoology, a section of the Carnivora, embracing those which apply the whole or nearly the whole of the sole ^ of the foot to the ground in progressive motion. Example, the bears, the badgers. PLASENCIA, a town of Spain, in Es- tremadura, 130 miles W. by S. of Madrid and 43 N. E. of Caceres, surrounded with double walls (1197), has a fine Gothic cathedral (1498). The monas- tery of San Yuste, to which Charles V. retired after his abdication, lies 24 miles to the E. of Plasencia. PLASMA, the viscous material of a cell from which the new developments take place; formless, elementary matter. ^ PLASMA, a bright to leek-green va- riety of chalcedony, sometimes almost emerald-green; feebly translucent; lus- ter, somewhat oily; fracture, sub-vitre- ous, probably due to a small amount of opal-silica present. It is rather rare, and was much esteemed by the ancients for engraving on. PLASSEY, a battlefield on the Bhagi- rathi river, 96 miles N. of Calcutta. The river has now eaten away the scene of the struggle. Plassey is celebrated in the history of India for the great victory gained by Clive over Suraj ud Dowlah, subahdar of Bengal, June 23, 1757, a victory which really laid the foundation of British supremacy in India. PLASTER, calcined gypsum or sul- phate of lime, used, when mixed with water, for finishing walls, for molds, ornaments, casts, luting, cement, etc. Also a composition of lime, sand, and water, with or without hair as a bond, and used to cover walls and ceilings. In pharmacy, an unctuous compound, united either to a powder or some met- allic oxide, and spread on linen, silk, or leather, for convenience of external ap- plication. PLASTERING, the art of covering the surface of masonry or wood work with a plastic material in order to give it a smooth and uniform surface, and gen- erally in interiors to fit them for paint- ing or decoration. PLASTER OF PARIS, the name given to gypsum when ground and used for taking casts, etc. PLATA, RIO DE LA, River of Silver; a body of water which extends for more than 200 miles between the Argentine Republic and Uruguay, and is not, strictly speaking, a river, but rather an estuary, formed by the junction of the great rivers Parana and Uruguay. It flows into the Atlantic between Cape St. Antonio and Cape St. Mary, and has here a width of 170 miles. On its banks are the cities and ports of Montevideo and Buenos Ayres. It was discovered in 1515 by Juan Diaz de Solis, and called Rio de Solis; it owes its present name to the famous navigator Cabot. PLAT^A, a city in the W. part of Boeotia, on the borders of Attica, and at the foot of Mount Cithseron, 6 miles from Thebes. In 480 B. C. it was de- stroyed by the Persians, because the in- habitants had taken part with Athens in the battle of Marathon; but in the fol- lowing year it was the scene of the glorious victory won by the Lacedse- monian Greeks, under Pausanias and Aristides, over the Persian hordes com- manded by Mardonius. In the third year of the Peloponnesian war (429) it was attacked by a Theban-Lacedaemon- ian force, and heroically defended itself for more than two years, when the little garrison of about 200 men were put to the sword, and the city demolished. Some Plataeans escaped to Athens and by the treaty of Antalcidas (387) their de- scendants were allowed to return and rebuild their city. They were driven forth again by the Thebans, and half a century passed before Philip of Mac- edon's victory at Chaeronea enabled the Plataeans to return home where they re- mained until the 6th century A. D. PLATINUM, in chemistry, symbol, Pt.; at. wt., 194.3; sp. gr.= 21.6, .a tetrad metallic element discovered first in the United States; and still produced there; also found in the Ural chain, and in copper ore from the Alps. Pure forged platinum takes a high luster, is nearly as white as silver, and very ductile and malleable. It resists the strongest heat of the forge fire, but can be fused by the electric current; is the heaviest known substance excepting osmium and iridium, is unalterable in the air, dis- solves slowly in nitromuriatic acid, but is not attacked by any single acid. It is used extensively in the manufacture of expensive jewelry, and delicate in- struments. During and after the World War (1914-1918) it ranged in value from $120 to $150 per ounce. PLATO, a Greek philosopher; born in Athens, or in ^gina, in May, 429 B, C. He was son of Ariston and Perictione, and was named Aristocles. The name Plato was afterward applied to him in allusion to his broad brow, broad chest, or fluent speech. Endowed with an im- aginative and emotional nature, he early began to write poems, and studied phil- osophy, and at 20 became the disciple of Socrates. He burnt his poems, remained