600 PLATINUM PLATO for blowpipe experiments, and for other chem- ical and philosophical apparatus. Large pla- tinum stills, sometimes weighing 2,000 oz., are used for the concentration of sulphuric acid. It was made into coin by the Russians to the amount of $2,500,000 between 1826 and 1864, when the coinage was discontinued. It is sometimes used for the touch-holes of fowl- ing pieces. Platinum forms alloys with most other metals, which are generally more fusi- ble than the pure metal, the exceptions being with iridium and rhodium. Care must there- fore be taken not to use platinum crucibles for melting other metals. Most of the pla- tinum of commerce contains iridium, which increases its hardness and durability, with- out impairing its power of resisting chemical agents. Although when pure it is perfectly insoluble in nitric acid, when alloyed with 10 or 12 times its weight of silver it is with the latter metal easily dissolved by this acid. Hot oil of vitriol will dissolve out the silver. Platinum forms two series of compounds, the platinous, in which it is bivalent, and the platinic, in which it is quadrivalent, being similar to palladium. The dichloride, or pla- tinous chloride, Pt01 2 , is formed when pla- tinic chloride, Pt01 4 , formerly called bichlo- ride, is exposed in a dried and powdered con- dition to a heat of about 392. F., by which half of the chlorine is expelled. It is an olive- colored powder, insoluble in water, slightly soluble in nitric or sulphuric acid, but readily in hydrochloric acid when warmed. It is solu- ble in caustic potash, and in platinic chloride, forming with the latter a double salt. The so- lution in hydrochloric acid, when treated with an alkaline chloride, deposits a double salt in fine red prismatic crystals, the potassium salt having the formula 2K01,Pt01 2 . These dou- ble salts are called chloroplatinites or plati- nosochlorides. The tetrachloride, or platinic chloride, PtCl 4 , is formed by the action of ni- tromuriatic acid on metallic platinum in which the nascent chlorine has sufficient affinity to combine with the metal. The acid solution when evaporated to dryness yields a deliques- cent red-brown residue, which is very soluble in water and in alcohol, the aqueous solution having a pure orange-yellow tint. Platinic chlo- ride unites with many metallic chlorides, form- ing double salts, called chloroplatinates or pla- tinochlorides, the most important being those formed with the alkaline metals and ammoni- um. Potassium platinochloride, 2K01,Pt01 4 , is formed whenever solutions of chlorides of potassium and platinum are mixed, as a bright yellow precipitate, and on this account platinic chloride is used in the laboratory as a test for potassium compounds. The other salts of pla- tinum, as the iodides, bromides, and cyanides, are of less importance. There are two oxides : a protoxide, prepared by the action of caustic potash on platinous chloride; and a dioxide, obtained by adding sodic carbonate to platinic nitrate. The quantity of platinum in a com- pound or mixture is estimated by causing the formation of double salts of chloride of plati- num with potassium or ammonium. The addi- tion of alcohol favors the formation of the precipitate. PLATO, a Greek philosopher, born in Athens (or according to some authorities in ^Egina) about 429 B. C., died about 348. His father Ariston traced his descent to Codrus, and his mother Perictione reckoned Solon among her ancestors. His original name was Aristocles, derived from his grandfather ; but it was altered to Plato (Gr. 7r/lari>f, broad), whether from the breadth of his forehead, his shoulders, or his diction, is not determined. Owing to his sub- sequent renown a parentage from Apollo was attributed to him, and bees settling on his infant lips were said to have betokened the honeyed sweetness of his style. Besides the ordinary training in gymnastics, grammar, and music, he was initiated by Cratylus into the doctrines of Heraclitus, and the study of An- axagoras gave him the results of the pre-So- cratic physics. The exuberant fancy which he subsequently lavished on dialectics at first over- flowed in poetical compositions, epic, lyric, and dramatic. But he burned his epics on compar- ing them with Homer, and having in his 20th year fallen under the influence of Socrates, he thenceforth devoted himself to philosophy as that essence and soul of harmony of which rhythmical numbers are but the sensuous and shadowy embodiment. He was a pupil of Soc- rates during the last eight or nine years of that great reformer's life, and became thoroughly imbued with his profound ethical spirit, and master of his searching and potent dialectics. Plato alone, of all the disciples of Socrates, seems fully to have appreciated the intellectual greatness and seized the profound scientific conceptions of his master; and hence, while others, looking at single aspects of the Socra- tic teaching, framed one-sided systems which rather caricatured than adequately represented it, Plato developed its germs in all their fulness and fruitf ulness ; and his works are not more a product of his own genius than a tribute to the memory of his master. After the death of Socrates, Plato repaired to Megara, where Euclid, a former fellow disciple, had opened a school in which he sought to engraft the Socratic ethics on the stock of Eleatic ideal- ism. To the ideas arid impulses here acquired we owe very probably that group of dialogues in which Plato seeks to establish, against the Heraclitan doctrine of absolute multiplicity and the Eleatic assumption of absolute unity, the true idea of science. From Megara he visited Oyrene, Egypt, Magna Grsecia, and Sicily. Of alleged journeys to Palestine, Babylon, Persia, India, &c., there is not the slightest evidence; and even of any philosophical fruits of his sojourn in Egypt his writings indicate but the faintest trace. In the Greek cities of lower Italy, however, where Pythagoreanism had its native home and still mainly flourished, he
