PLATINUM 



PLATO 



227 



except in association with platinum. Sometimes, 

 however, it is found in masses of the size of a 

 pigeon's egg, and pieces weighing ten or even 

 twenty pounds have occasionally been found. It 

 was originally found in the Spanish gold-mines of 

 Darien, but is now chiefly obtained from the Ural 

 Mountains, though it has also been found in Brazil, 

 Colombia, San Domingo, California, Oregon, 

 Canada, and liorneo. 



There are two modes of obtaining platinum in the 

 form of ingots from the crude native metal, both 

 of which require notice. The earlier method 

 Cocks's, usually attributed to Wollaston (c. 1800- 

 28) was to precipitate a solution in aqua regia 

 by a solution of sal ammoniac ; the precipitate on 

 incineration gave spongy platinum, which was 

 pressed into form and hammered hot, so as to 

 form an ingot, which it happens to be able to do, 

 since platinum welds like wrought-iron when not 

 too highly polished. 



I'' -ville and Debray's method is first to form a 

 fusible alloy of this metal with lead, by exposing 

 the platinum ore 2 cwt. being used in a single 

 experiment, with equal weights of galena and 

 litharge gradually added, and a little glass to act 

 as a flux to full redness in a reverl>eratory furnace 

 lined with clay. The sulphur of the galena is oxid- 

 ised and expelled, and the liquid alloy of lead and 

 platinum is allowed to rest for some time, to allow 

 the osmide of indium, which is not affected by the 

 preceding operations, to sink to the bottom. The 

 upper portions of the alloy are then decanted, and 

 ca-t into ingot-moulds, which are submitted to 

 cupellation : and the metallic platinum which is 

 left after the cupellation is melted and refined in a 

 furnace of lime which is employed in consequence 

 of its being a very bad conductor of heat by 

 means of the oxyhvdrogen blowpipe. The platinum 

 obtained in this manner is nearly pure, and very 

 ductile and malleable. 



Platinum exhibits a bluish-white metallic lustre ; 

 it is exceedingly malleable and ductile, and is very 

 infusible, melting only before the oxyhydrogen 

 blowpipe, or in a very powerful blast-furnace, such 

 as that used by Deville and liebray (fusing-point, 

 1779 C. ). It expands less by heat than any other 

 metal, and it is the heaviest form of matter known, 

 with the exception of iridium (sp. gr. = 22-42) and 

 osmium (22'48). It is unaffected by atmospheric 

 action, and does not undergo oxidation in the air 

 at even the highest temperatures. It is not acted 

 on by nitric, hydrochloric, sulphuric, or hydro- 

 fluoric acid, or, in short, by any single acid ; but 

 in anna regia it slowly dissolves, and forms a 

 soluble tetrachloride. In consequence of its power 

 of resisting the action of acids it is of great service 

 in experimental and manufacturing chemical pro- 

 cesses, platinum spatulas, capsules, crucibles, &c. 

 being employed in every laboratory. Platinum is, 

 however, corroded if heated with the alkalies or 

 alkaline earths, and especially with a mixture of 

 nitrate of potash and hydrated potash, an oxide 

 lieing formed which combines with the alkaline 

 bases. In consequence of its infusibility and its 

 non-oxidisahility by atmospheric action, it is in 

 great demand for electrical as well as for chemical 

 apparatus ; and the recent introduction of platino- 

 type processes in Photography (q.v. ) has caused 

 such a run upon platinum that its price has gone 

 up from about 35s. per oz. in 1880 to 55s. in 1889, 

 and HOs. in 1890. 



The form of the metal known as sjionqy platinum 

 has been already noticed. The metal may, how- 

 ever, be obtained in a state of subdivision much 

 finer than that in which it is left on heating the 

 double chloride of platinum and ammonium 

 viz. in the state known as Platinum Bind,-. In 

 this form it resembles soot. It may be prepared in 



various ways, of which one of the simplest is to 

 boil a solution of bichloride of platinum, to which 

 an excess of carbonate of soda and a quantity of 

 sugar have been added, until the precipitate formed 

 after a little time becomes perfectly black, and the 

 supernatant liquid colourless. The black powder is 

 then collected on a filter, washed, and dried by a 

 gentle heat. In its finely comminuted state, either 

 as spongy platinum or platinum black, it possesses 

 a remarkable power of condensing and absorbing 

 gases, one volume of platinum black being able to 

 absorb more than 100 volumes of oxygen. This 

 absorption appears to be accompanied by a conver- 

 sion of some or all of the oxygen into the modifica- 

 tion known as Ozone (q.v. ), since the metal becomes 

 capable of exerting the most energetic oxidising 

 action, even at ordinary temperatures. For ex- 

 ample, it can kindle a jet of hydrogen, can oxidise 

 sulphurous acid into sulphuric acid, ammonia into 

 nitric acid, and alcohol into acetic acid, the rise of 

 temperature in the last case being often sufficiently 

 great to cause inflammation. Platinum in the 

 compact form, as foil or wire, possesses similar 

 powers, but in a far lower degree. 



Platinum may he easily alloyed with most of the 

 metals, the alloys lieing in general much more 

 fusible than pure platinum. Hence care must be 

 taken not to boat the oxides of metals of easy 

 reduction, such as lead and bismuth, in platinum 

 crucibles, as, if any reduction took place, the 

 crucible would be destroyed by the fusion of the 

 resulting alloy. An alloy of platinum, iridium, and 

 rhodium is found, by the investigations of Deville 

 and Debray, to be harder and capable of resisting 

 a higher temperature than the pure metal, and 

 hence is admirably adapted for the formation of 

 crucibles, &c. ; but it is not now in the market. 



There are two oxides of platinum, a protoxide, 

 PtO, and a binoxide, PtO~ The sulphides and 

 chlorides correspond in number and composition to 

 the oxides. Of these compounds the tetrachloride, 

 PtCl., alone requires notice. It is formed by 

 dissolving platinum in aqua regia, and evaporating 

 the solution to dryness ; and it is obtained as a 

 deliquescent, reddish-brown mass, which forms an 

 orange-coloured solution in water, from which, on 

 evaporation, it crystallises in prisms. It in also 

 freely soluble in alcohol and ether. A solution of 

 this salt is much used for the recognition and 

 determination of potash and ammonia. 



I'lato. the great Athenian philosopher, was 

 born during the early years of the Peloponnesian 

 war, most probably in 427 B.C. Diogenes Laertius 

 (q.v.), in his gossiping Lives of the Philosophers, 

 and other writers of the Christian era have 

 handed down a considerable amount of detail 

 respecting his life, but most of it is of very 

 doubtful value. As time went on, legends 

 gathered round a famous name ; and many of the 

 authorities used by Diogenes were in all probability, 

 like Diogenes himself, almost entirely wanting in 

 critical capacity. According to one account Plato 

 was born in Athens itself, according to another in 

 the island of JKgiuA. He came of an aristocratic 

 family, his father Ariston boasting descent from 

 Codrus the last king of Athens, who was said to be 

 descended from the god Poseidon ; whilst the family 

 of his mother, Perictione, claimed kinship with 

 Solon, and through Solon with Neleus, a son of 

 Poseidon. Even this double claim to superhuman 

 ancestry was not sufficient for the admirers of 

 the 'divine' Plato. Diogenes tells us on the 

 alleged authority of (among others) Speusippus, 

 Plato's own nephew and successor in the academy, 

 that the story was current at Athens that his 

 real father was Apollo, and that the god ap- 

 peared in a vision to Ariston, who thereupon kept 

 away from his wife till her child was born. PlatoV 



