PL ATI N A PLATO. 



583 



soluble in water, alcohol, and ether. The muriate 

 of platina is the protochloride, and is resolved, on 

 heating, into platina and chlorine. According to 

 Mr E. Davy, there are two phosphurets and three 

 sulphurets of platina. The salts of platina have the 

 following general characters ; their solution in water 

 is yellowish-brown ; sulphureted hydrogen throws 

 down the metal in a black powder ; hydriodic acid 

 produces a thin film of platina upon the surface of 

 the solution, after a few hours' standing. The sul- 

 phate of platina is formed by the action of nitric 

 acid upon the sulphuret, or hydro-sulphuret. It is 

 soluble in water, alcohol, and elher. When a strong 

 aqueous solution of it is mingled with alcohol, in 

 equal volumes, and heated, it will deposit a black 

 powder, which, on being washed and dried, and 

 heated to the temperature of '400, explodes with 

 violence. This detonating compound is analogous to 

 those which ammonia forms with the oxides of gold 

 and silver. 



The useful alloys of platina are not numerous. 

 With silver it forms a tolerably fusible white alloy, 

 malleable and brilliant when polished ; but it scales 

 and blackens by working. Gold, by a forge heat, 

 combines with platina, and the alloys, in all propor- 

 tions, are more fusible than platina. In the propor- 

 tion of 38 grs. to 1 oz., it forms a yellowish-white, 

 ductile, hard alloy ; it is so elastic after hammering, 

 that it has been proposed to use it for watch-springs. 

 Mercury, by trituration with spongy platina, forms 

 an amalgam at first soft, but which soon becomes 

 considerably firm, and has been much used in obtain- 

 ing malleable platina. A coating of platina can be 

 given to copper and other metals, by applying to 

 them an amalgam of spongy platina, and five parts 

 of mercury ; the latter metal is then volatilized by 

 heat. Lead combines with platina readily ; and 

 iron and copper in like manner : the last mentioned, 

 when added in the proportion of 7 to 1 6 of platina 

 and 1 of zinc, and fused in a crucible, under char- 

 coal powder, forms the alloy called artificial gold. 

 Steel unites with platina in all proportions, and, espe- 

 cially in the proportion of from 1 to 3 per cent, of 

 platina, forms a tough and tenacious alloy, well 

 adapted for cutting instruments. Arsenic unites 

 easily with platina, and was once employed for ren- 

 dering the latter metal fusible. Platina is a metal 

 of great utility. It is used for the construction of 

 large boilers for the concentration of sulphuric acid, 

 the larger ones of which cost upwards of 300 guin- 

 eas. Its uses in chemistry are numerous, and very 

 important. It is also employed for staining pottery ; 

 and has lately been coined in Russia, and forms a 

 part of the circulating medium of that country. 



PLATO (from TA.OSTW, broad). The celebrated 

 Greek philosopher of this name was so called on 

 account of the breadth of his chest and forehead. 

 His original name was Aristocles. He was the son 

 of Ariston and Perictione, and was born about 429 

 B. C. Among his ancestors were king Codrus and 

 the lawgiver Solon. Circumstances were favourable 

 to his early education ; his body and mind were 

 equally developed. In gymnastics, he was instruct- 

 ed by Ariston, in the rudiments of letters by the 

 grammarian Dionysius, in painting and music by 

 Draco and Metellus, in philosophy at first by Craty- 

 lus. Until his twentieth year, the gifted youth tried 

 his creative power in epic, lyric, and dramatic poetry 

 with much ardour. Nothing satisfied him. We now 

 see him introduced by his father to the wise Socra- 

 tes, who, the story says, was just relating to his 

 pupils a dream, in which he had seen a young swan 

 fly from the altar of Cupid into the academy, and 

 alight in his lap, whence he soared, with an enchant- 

 ing song, into the air : " See, here, the academic 



swan !" he exclaims, perceiving Plato. He culti- 

 vated the mind of his new pupil for eight years, and 

 the acute and profound views of the scholar tasked 

 his faculties, and awakened in him admiration and 

 love. After the sage accused of deriding the gods 

 of his country had drank the hemlock, Plato, thirst- 

 ing for the highest knowledge which his times could 

 afford, left Athens. He first visited Megara, where 

 he staid for some time with Euclid (the head of the 

 Megarean school, and formerly also the disciple of 

 Socrates), exchanging opinions, and calling to mind 

 the lessons of their teacher. He then visited Magna 

 Graecia, where he held intercourse with the Pytha- 

 goreans ; Cyrene, a Greek colony in Africa, where 

 he became acquainted with Theodorus ; also Egypt, 

 the land of ancient wisdom, and was prevented only 

 by the breaking out of a war from visiting Syria and 

 Persia. At the age of about forty years, he return- 

 ed, and arrived at Syracuse, then ruled by the tyrant 

 Dionysius. He formed a friendship with Dion, a 

 relative, indeed, of Dionysius, but hostile to the ef- 

 feminate 1'jxury of his countrymen, and inspired with 

 a noble zeal for the improvement of his country. 

 Dion introduced him to the tyrant, to try whether 

 intercourse with his friend might not dispose the 

 ruler more favourably towards liberty. But Plato's 

 philosophic and independent spirit offended the 

 tyrant, who became suspicious of him, and, when 

 Plato embarked, in consequence of the representa- 

 tions of Dion and Aristomenes, he bribed the latter 

 to kill or sell him. Aristomenes did the latter, but 

 Anniceris ransomed him ; and Plato at last returned 

 to Athens, where, about the 99th Olympiad, he 

 taught philosophy in the academy a gymnasium 

 situated in the suburbs. In the second year of the 

 10?,d Olympiad (368 B. C.), Dionysius I. died, and 

 Dionysius II. succeeded him. Dion now entertained 

 new hopes, and Dionysius, who was desirous of 

 having learned men at his court, joined him in an 

 invitation to Plato to visit Syracuse again. The 

 philosopher was fully conscious of the danger and 

 probable failure of his visit ; but the hope of being 

 useful made him once more resolve to leave the 

 shores of Attica. Speusippus accompanied him. 

 The beginning promised well: his entrance into 

 Syracuse was solemnly celebrated. He instructed 

 the prince in mathematics: but he soon became 

 again the object of jealousy and intrigue. Philistins, 

 the historian, who had been banished under the late 

 reign, was recalled, in order, as was pretended, to 

 support the tottering throne. lie fanned the suspi- 

 cions of the tyrant against Dion, and hardly had three 

 months elapsed from the time of Plato's arrival in 

 Sicily, when his friend was carried to the coast of 

 Italy. Plato was obliged, by repeated and urgent 

 invitations, to take up his residence in the royal 

 palace, where he was closely observed ; and if it had 

 not been for the breaking out of a war, his request 

 for dismission would probably not have been compli- 

 ed with. It was granted at length, but on condition 

 that he should return after the conclusion of peace. 

 He now went back to Athens, where he tried to con- 

 sole Dion. Peace was concluded, and Plato was re- 

 minded of his promise. He, however, resisted all the 

 flattering assurances which were made him for some 

 time. At last Dionysius sent a vessel bearing letters 

 and personal friends of Plato to prevail on him to 

 repair to Syracuse. He yielded at last with the 

 hope of doing something for his friend Dion. But 

 his situation became worse than it had been on his 

 previous visit. Dion's income was kept bsick, and 

 Plato's life was even endangered. Archytas, how- 

 ever, persuaded the tyrant that a politifiil measure 

 required the return of Plato : he was allowed to de- 

 part, and even his travelling expenses were pa : d. 



