PL A
P L A
( 231 )
As all the platina -which comes to Europe hasprevioufly undergone the procefs of amalgama-tion in S. America , it generally happens that afmall variable quantity of mercury remains in it,fometimes in very fmall diftinft globules, butmore generally combined with gold into anamalgam. The eafielt way of feparating themercury is to drive it off by heat, either in anopen ladle or in an earthen retort, according asthis fubftance is or is not to be retained.When the mercury is thus got rid of, theremaining platina has generally a much yellowercall than before, on account of the particlesof gold difperfcd through it having now ac-quired their charadteriftic colour. The ore isnow to be fpread thin on a fmooth table, andby the dextrous application of a common pairof bellows, the lighter particles maybe fepa-rated with very confiderable accuracy from theheavier ones. The former confift of very minutecryftals and fragments of quartz, and of twokinds of iron ore in fragments and in fmalloctohedrons: of which fome are compleatlyattraffible by the magnet, (being the magneticiron fand) while the others are not in the leaftfo, and give out when roafted, a flight fulphu-reoue- odour.
The heavier particles are now to be treatedwith a fipall quantity of a fomewhat dilutednitromuriatic acid, by which the whole of thegold will be taken up, with fome iron and avery fmall proportion of platina and the otheringredients. From this folution the gold maybe thrown down by means of green fulphat ofiron, and purified by fubfequent fufion witha mixture of nitre and borax. The proportionof gold contained in crude platina is generallypretty confiderable, fo as to render it wellworth the labour of the chemift to feparate itif he is poffeffed of a confiderable quantity.From one parcel confifling of ioo ounces,Prouft a obtained 7 oz. of gold, and fromanother like quantity he procured no lefs than13 oz. The whitell platina is the richeft ingold, the black varieties containing little ornone of it.
After the feparation of the gold, the platina
to be digefted in nitromuriatic acid, by whichit will be diffolved, with the exception of ablack matter, which was at fir ft taken forP nmbago, but which, from Mr. Tennant’srecent analyfis appears to be a compound oftwo new metallic bodies, that have obtained thenames of ofnrium and iridium. By; the addition
* Aon. Chim.
of muriat of ammonia to the mtro-folution, the greateft part of the platina isthrown down in the form of a yellow powder,which is a nearly infoiuble ammoniaco-muriatof platina. The folution is now to be treatedwith zinc, by which all its metallic contentsexcept the iron, will he precipitated, and thisprecipitate when wafhed is to be digefted invery dilute nitric acid: by this menftruum thecopper and lead ufually contained in crudeplatina will be got rid of, and the remainder isto be diffolved in nitro-muriatic acid. To thislatter folution common fait is to be added, andthe whole evaporated to drynefs. This refiduai-falt contains the foda-muriats of platina, palla-dium, and rhodium, of which the latter aloneis infoiuble in alcohol, and may therefore befeparated from the former by means of thisfluid. The alcoholic folution now containsplatina and palladium, from which nearly thewhole of the former is to be feparated by falammoniac. The folution being now diluted,the addition of pruffiat of potafh will throwdown the palladium in the form of a deeporange precipitate, and from the remainingliquor when concentrated, the platina may be.precipitated by muriat cf ammonia.
§ 3. Methods of working Platina .
The great infufibility of platina, added to theftrong refiftance which it oppofes to commonmenftrua, long excited the attention of chemiftsand artifts, and has given bir.h to various in-genious precedes for condenfing this refradftorymetal into malleable mafl’es, and forming of itcrucibles and other inftvuments of materialfervice to the accuracy and fimplicity of chem-ical analyfis. If the largeft and whiteft grainsare carefully fele£ted from a parcel of crudeplatina, it will be found that thefe are confider-ably malleable even when cold, and ftill morefo when hot: alfo if two grains are laid incontaft with each other and then brought tothe higheft poffible white heat, they may bemade to adhere more or lefs perfectly by aftroke with a hammer, and in this way, bygreat patience and great dexterity, it may bepradticable to form a few grains into a mafs.This however is by much too imperfeft andtedious a method to be employed with anypractical advantage.
It was early diicovered that arfenic combined,very readily with platina, forming an alloy of.eafy fufibility, and from the volatility of theformer of thefe metals, efpecially when in con—tact with charcoal, it was expected that by
ixriii, p. 148.