682 PLATINUM. 



the condition in which the metal is left on igniting the double 

 chloride of platinum and ammonium. Platinum precipitated 

 from solution by zinc, causes the combustion of alcohol vapour. 

 The black powder of platinum is the form in which that metal is 

 most active. This is prepared by dissolving the protochloride 

 of platinum in a hot and concentrated solution of potash, and 

 pouring alcohol into it while still hot, by small quantities at a 

 time ; a violent effervescence occurs from the escape of carbonic 

 acid gas, by which the contents of the vessel, unless capacious, 

 may be thrown out. The liquor is decanted from the black 

 powder which appears, and the latter boiled successively with 

 alcohol, hydrochloric acid and potash, and finally four or five 

 times with water, to divest it of all foreign matters. The powder, 

 when dried, resembles lamp black, and soils the fingers, but still 

 it is only metallic platinum extremely divided, and may be 

 heated to full redness without any change of appearance or pro- 

 perties. It loses these, however, by the effect of a white heat, 

 and assumes a metallic aspect. The powder of platinum, like 

 wood charcoal, absorbs and condenses gases, in its pores, with 

 the evolution of heat, a property which must assist its action on 

 oxygen and hydrogen, although not essential to that action. 

 When moistened with alcohol, it determines the oxidation of that 

 substance in air, and the formation of acetic acid. 



Platinum is insoluble in all acids, except aqua regia. It may 

 be oxidated in the dry way by fusing it with hydrate of potash 

 or nitre. Palladium, osmium and iridium resemble platinum in 

 their chemical relations, the corresponding compounds of these 

 four metals being isomorphous ; platinum and iridium have also 

 the same atomic weight. Of platinum, only two degrees of 

 oxidation are known with certainty, the protoxide, Pt O, and 

 peroxide Pt O 2 



Protoxide of platinum, Platinous oxide, Pt 0, 1333.5 or 106.84. 

 It is obtained by digesting the corresponding chloride of plati- 

 num with potash, as a black powder, which is a hydrate. This 

 oxide is dissolved by the excess of alkali, and forms a green solu- 

 tion, which may become black like ink with a large quantity of ox- 

 ide. Protoxide of platinum forms the platinous class of salts, 

 which have a greenish, or sometimes red colour, and are distin- 

 guished from the platinic salts by not being precipitated by 

 sal ammoniac. 



Sulphuret of platinum, Pt S, is thrown down as a black pre- 



