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PLATEAU

1502

PLATO

480 B. C. it was destroyed by the Persians because the inhabitants had taken part with Athens in the battle of Marathon; but in the following year it was the scene of the victory won by Pausanias and Aristides over the Persians under Mardonius. During the third year of the Peloponnesian War, 429 B. C., it was besieged by a Theban and Spartan force, and heroically defended itself for more than two years until it was starved into surrender, the garrison of 200 men being put to the sword and the city razed to the ground. Such of the Plataeans as escaped were hospitably received at Athens, and by the treaty of Antalcidas, forty years later, their children were allowed to go back and rebuild their city; they were again driven out by the Thebans; and half a century elapsed before the victory of Philip at Chaeronea enabled the Plataeans finally to return to their homes.

Pla'teau, Joseph Antoine Ferdinand, a distinguished Belgian physicist, was born at Brussels in 1801, and died at Ghent in 1883. He was educated at the University of Liege, and was professor of physics at the University of Ghent from 1835 to 1883. His most important contributions to science are along the two wholly different lines of subjective visual phenomena and capillarity. It was while engaged in the former study that he looked directly at the midday sun for 20 seconds in order that he might study its after-effects. One of these after-effects was that he became permanently blind in 1843. His work on surface-tension was carried on, under his direction, by his wife, son and distinguished son-in-law, Van der Mensbrug-ghe. These researches are contained in his Isiatics of Liquids, which has been translated into English by Smithsonian Institution. A more beautiful and ingenious series of experiments on surface-tension than those here described it would be impossible to find. Plateau is to be remembered also as the inventor of the thaumatrope.

Plat'ing consists in covering the surface of a metal with a coating of a more valuable metal. Many metals and alloys are plated with gold or silver, and iron is frequently nickel-plated. The operation is performed most frequently by placing the object to be plated in an appropriate solution and causing the metal to be deposited by means of an electric current. In silver-plating a bath of silver cyanide dissolved in potassium cyanide is commonly used, while an anode of silver supplies this metal as fast as it is deposited upon the objects forming the cathode. Copper and zinc may be deposited at the same time upon iron objects, thus producing brass plating. In most cases electroplated articles require rubbing or burnishing in order that they may acquire a brilliant luster. H. L. WELLS.

Plat'inum is one of the "noble metals." It is generally found in small granules mixed

with other metals, but sometimes in masses as large as a pigeon's egg. In rare cases pieces have been found weighing ten or more pounds. It is chiefly obtained from the Ural Mountains, although it has been found in Brazil, Colombia, California, Canada and Borneo. Platinum is the heaviest form of matter known, except iridium and osmium. It expands less by heat than any other metal, and, as it expands to about the same extent as glass, it is easy to fuse a wire of this metal into glass without causing the latter to break subsequently. Electric currents are thus led into the ordinary incandescent-light bulbs. On account of its power of resisting the action of acids it is of great service in chemical experiments, platinum capsules, crucibles and similar articles being found in every laboratory. Platinum is exceedingly malleable and ductile, but it melts only when subjected to the very highest heat. On this account it is in great demand for electrical as well as chemical apparatus, and the recent introduction of the platinotype process in photography has advanced the price very materially.

Pla'to, a distinguished Grecian philosopher, was born during the early years of the Peloponnesian War, most probably about 425 B. C. A vast amount of detail has come to us respecting his life, but most of it is very doubtful. According to one account Plato was born in Athens; according to another in ^Egina. He came of an aristocratic family, his father boasting descent from the last king of Athens. In his youth Plato indulged in poetry, but, when he compared his compositions with Homer, he abandoned the muse entirely. Having, when about twenty, become acquainted with Socrates (q. v.), he devoted himself to philosophy. His companionship with Socrates continued until the death of the latter. Plato made no attempt to enter on a political career. He went to Megara, where he remained some time, and afterward visited Cyrene, Egypt, Italy and Sicily. On his way back to Athens Plato is said to have been sold as a slave in .^Egina, but to have been ransomed by Anniceris of Cyrene. On his return to Athens about 388 B. C., he began to teach in the Academy, a grove in the western suburb of the city. There he gathered disciples, teaching mainly by questions and conversations, after the manner of Socrates. He twice visited Sicily, where he spent some time. Returning to Athens, he continued teaching and writing until 347 B. C., when he died.

The distinctive principles of the teaching of Socrates are the inductive method and the effort to get general definitions. When people spoke about persons or acts as just or beautiful, Socrates would ask: "What is justice?" "What is beauty?" and would test every definition by applying it to particular instances, content to remove misconception