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 product, K2Pt(CN)4.Cl2.2H2O. It combines directly with iodine. Barium platinocyanide, BaPt(CN)4.4H2O, is prepared by the action of baryta water on the copper salt, by dissolving platinum in barium cyanide under the influence of an alternating current; by the addition of barium cyanide to platinum bichloride, or by the simultaneous action of hydrocyanic and sulphurous acids on a mixture of baryta and chlorplatinic acid (P. Bergsoe, Zeit. anorg. Chem., 1899, 19, p. 318) it crystallizes in yellow monoclinic prisms and is soluble in hot water. It is employed for the manufacture of fluorescent screens used for the detection of X-rays.

The platinum salts combine with ammonia to form numerous derivatives which can be considered as salts of characteristic bases. The first compound of this type was isolated in 1828 by Magnus, who obtained a green salt by the action of ammonia on platinum bichloride. Two series of these salts are known, one in which the metal corresponds to bivalent platinum, the other in which it corresponds to tetravalent platinum. The general formulae of the groups in each series are shown below, the method of classification being that due to Werner

In the above table X represents a monovalent acid radical and R a monovalent basic radical. For methods of preparation of salts of these series see P. T. Cleve, ''Bull. soc. chim.'' 1867 et seq; S. M. Jörgensen, ''Journ. prak. Chem.'' 1877 et seq; C. W. Blomstrand, Ber. 1871 et seq, and A. Werner, ''Zeit. anorg. Chem.'' 1893 et seq. A very complete account of the method of classification and the general theory of the metal ammonia compounds is given by A. Werner, Ber. 1907, 40, p. 15.

Platinum also forms a series of complex phosphorus compounds. At 250° finely divided platinum and phosphorus pentachloride combine to form PtCl2.PCl3, as dark claret-coloured crystals. With chlorine this substance gives PtCl3.PCl4 as a yellow powder, and with water it yields phosphoplatinic acid, PtCl2.P(OH)3, which may be obtained as orange-red deliquescent prisms.

The atomic weight of platinum was determined by K. Seubert (Ann. 1888, 207, p. 1; Ber. 1888, 21, p. 2179) by analyses of ammonium and potassium platinochlorides, the value 194.86 being obtained.

 PLATO, the great Athenian philosopher, was born in 427, and lived to the age of eighty. His literary activity may be roughly said to have extended over the first half of the 4th century His father's name was Ariston, said to have been a descendant of Codrus; and his mother's family, which claimed descent from Solon, included Critias, one of the thirty tyrants, and other well-known Athenians of the early 4th century  That throughout his early manhood he was the devoted friend of Socrates, that in middle life he taught those who resorted to him in the grove named Academus, near the Cephisus, and there founded the first great philosophical school, that (with alleged interruptions) he continued to preside over the Academy until his death, are matters of established fact. It is said by Aristotle that he was at one time intimate with Cratylus the Heraclitean. Beyond this we have no authentic record of his outward life. That his name was at first Aristocles, and was changed to Plato because of the breadth of his shoulders or of his style or of his forehead, that he wrestled well, that he wrote poetry which he burnt on hearing Socrates, fought in three great battles, that he had a thin voice, that (as is told of other Greek philosophers) he travelled to Cyrene and conversed with priests in Egypt, are statements of Diogenes Laertius, which rest on more or less uncertain tradition. The express assertion—which this author attributes to Hermodorus—that after the death of Socrates Plato and other Socratics took refuge with Euclides in Megara, has a somewhat stronger claim to authenticity. But the fact cannot be regarded as certain, still less the elaborate inferences which have been drawn from it. The romantic legend of Plato's journeys to Sicily, and of his relations there with the younger Dionysius and the princely but unfortunate Dion, had obtained some degree

of consistency before the age of Cicero, and at an unknown but probably early time was worked up into the so-called Epistles of Plato, now all but universally discredited. Nor is there sufficient ground for supposing, as some have done, that an authentic tradition is perceptible behind the myth.

The later years of the Peloponnesian War witnessed much mental disturbance and restlessness at Athens. More than at any time since the age of Cleisthenes, the city was divided, and a man's foes were often men of his own tribe or deme. Contention in the law courts and rivalries in the assembly had for many men a more absorbing interest than questions of peace and war. Hereditary traditions had relaxed their hold, and political principles were not yet formulated. Yet there was not less scope on this account for personal ambition, while the progress of democracy, the necessity of conciliating the people, and the apportionment of public offices by lot had a distracting and, to reflecting persons, often a discouraging effect. For those amongst whom Plato was brought up this effect was aggravated by the sequel of the oligarchical revolution, while, on the other hand, for some years after the restoration of the democracy, a new stimulus had been imparted, which, though of short duration, was universally felt.

These events appear in two ways to have encouraged the diffusion of ideas. The ambitious seem to have welcomed them as a means of influence, while those who turned from public life were the more stimulated to speculative disputation. However this may have been, it is manifest that before the beginning of the 4th century the intellectual atmosphere was already charged with a new force, which although essentially one may be differently described, according to the mode of its development, as (1) rhetorical and (2) theoretical and “sophistical.” This last word indicates the channel through which the current influences were mostly derived. A new want, in the shape both of interested and of disinterested curiosity, had insensibly created a new profession. Men of various fatherlands, some native Athenians, but more from other parts of Hellas, had set themselves to supplement the deficiencies of ordinary education, and to train men for the requirements of civic life. More or less consciously they based their teachings on the philosophical dogmas of an earlier time, when the speculations of Xenophanes, Heraclitus or Parmenides had interested only a few “wise men.” Those great thoughts were now to be expounded, so that “even cobblers might understand.” The self-appointed teachers found a rich field and abundant harvest among the wealthier youth, to the chagrin of the old-fashioned Athenian, who sighed with Aristophanes for the good old days when men knew less and listened to their elders and obeyed the customs of their fathers. And such distrust was not wholly unfounded. For, amidst much that was graceful and improving, these novel questionings had an influence that, besides being unsettling, was aimless and unreal. A later criticism may discern in them the two great tendencies of naturalism and humanism. But it may be doubted if the sophist was himself aware of the direction of his own thoughts. For, although Prodicus or Hippias could debate a thesis and moralize with effect, they do not appear to have been capable of speculative reasoning. What passed for such was often either verbal quibbling or the pushing to an extreme of some isolated abstract notion. That prudens quaestio which is dimidium scientiae had not yet been put. And yet the hour for putting it concerning human life was fully come. For the sea on which men were drifting was profoundly troubled, and would not sink back into its former calm. Conservative reaction was not less hopeless than the dreams of theorists were mischievously wild. In random talk, with gay, irresponsible energy, the youth were debating problems which have exercised great minds in Europe through all after time.

Men's thoughts had begun to be thus disturbed and eager when (q.v.) arose. To understand him is the most necessary preliminary to the study of Plato. There is no reason to doubt