PLUMBAGO 537 



laminate it very thin, anneal somewhat strongly, boil it half an hour in the first aqua- 

 fortis, and at least a quarter of an hour in the second, in order that the acid may dis- 

 solve the whole of the platinum. 



"Were it required to determine exactly the proportions of platinum contained in an 

 alloy of copper, silver, gold, and platinum, the amount of the copper may be found in 

 the first place by cupellation ; then the respective quantities of the three other metals 

 may be learned by the processes founded, 1, upon the property possessed by sulphuric 

 acid of dissolving silver without affecting gold or platinum ; and, 2, upon the property 

 of platinum being soluble in nitric acid, when it is alloyed with a certain quantity 

 of gold and silver. 



Platina, wrought and tmwrought, imported in 1873. 



Ounces troy Value 



From Russia 39,284 51,642 



Germany 11,540 11,542 



Other countries . . . .6,157 9,580 



Total ~56^981 72,764 



PLATINUM, ALLOYS OF. This metal will alloy with iron; the alloy is 

 malleable and possesses much lustre. Copper and platinum in certain proportions 

 form a brilliant alloy. Silver is much hardened by platinum : although platinum is 

 not soluble in nitric acid, it will, when alloyed with silver, dissolve in that acid. An 

 alloy of platinum and iridium is harder than platinum, and withstands the action of 

 nitro-hydrochloric acid. 



Some other alloys are known, but none of them are employed in the arts. 



PLATINUM BLACK. This interesting preparation, which so rapidly oxidises 

 alcohol into acetic acid, &c., by what has been called catalytic or contact action, is 

 most easily prepared by the following process devised by M. Bcettger : The insoluble 

 powder of potassio-chloride or ammonio-chloride of platinum is to be moistened with 

 sulphuric acid (oil of vitriol), and a bit of zinc is to be laid in the mixture. The 

 platinum becomes reduced to a black powder, which is to be washed first with hydro- 

 chloric acid, and then with water. The fineness of this powder depends upon that of 

 the saline powders employed to make it ; so that if these be previously finely ground, 

 the platinum black will be also very fine, and proportionally powerful as a chemical 

 agent. 



The following method of preparing igniferous black platinum, proposed by Descotil, 

 has been recommended by M. Dobereiner : 



Melt platinum ore with double its weight of zinc ; reduce the alloy to powder, and treat 

 it first with dilute sulphuric acid, and next with dilute nitric acid, to oxidise and dis- 

 solve out all the zinc, which is somewhat difficult to do, even at a boiling-heat. The 

 insoluble black-grey powder contains some osmide of iridium, united with the crude 

 platinum. This compound acts like simple platinum black, after it has been purified 

 by digestion in potash-lye, and washing with water. Its oxidising power is so great 

 as to transform not only formic acid into the carbonic, and alcohol into vinegar, but 

 even some osniic acid, from the metallic osmium. The above powder explodes by heat 

 like gunpowder. 



When the platinum black prepared by means of zinc is moistened with alcohol, it be- 

 comes incandescent, and emits osmic acid ; but if it be mixed with alcohol into a paste 

 and spread upon a watch-glass, nothing but acetic acid will be disengaged ; afibrding 

 an elegant means of diffusing the odour of vinegar in an apartment. 



A yet more simple method of preparing the platinum black than either of those 

 is the following : Protochloride of platinum is dissolved in a concentrated solution 

 of potash with the aid of heat ; then alcohol is added by degrees, constantly stirring the 

 solution. The platinum is precipitated as a black powder, which is boiled successively 

 with alcohol, hydrochloric acid, and potash-water. 



PLATINUM, FULMINATING. An explosive compound, obtained by adding 

 acetic acid, in excess, to a solution of chloride of platinum and ammonium in caustic 

 soda. It may also be prepared by precipitating sulphate of platinum with excess of 

 ammonia. 



PLATINUM, SALTS OP. The salts of platinum being rarely employed in 

 the arts or manufactures, the reader is referred for them to Watts's ' Dictionary of 

 Chemistry.' 



PLATINUM, SPONGY. A loose porous form of metallic platinum, obtained 

 by heating the double chloride of platimim and ammonium. 



PLATINUM YELLOW. A pigment prepared from platinum, by oxidation 

 with acids, is sold under this name. 



PLUMBAGO, commonly called BLACK LEAD ; the name plumbago, aud its com- 



