P L A (23 2 ) PLA

proper management nearly the whole of it there will be obtained a cake of metal well

might be driven oft', leaving the platma behindin a mat's, and pofl'efl'ed of its charadherifticproperties. Willis, Marggraaf, A chard andothers fucceeded to a certain degree, and falhion-ed of this alloy crucibles and other chemicalutenfils, lefs fufible than filver, and capable ofrefilling many of the common menftrua. M.Jeanety of Paris , a working filverfmith, thenturned his attention to the fame objedft, andafter long practice and many failures, difcoveredby far the bell method of preparing and workingthis alloy. The procefs of this artift as reportedby Berthollet and Pelletier, is the following : b

Having firft ground the crude platina inwater, and walhed it over in order to feparatethe earthy matters with which it is mixed;take three half pounds of the metal, threepounds of white arfenic, and one pound ofpearlafli: mix the whole well together, andthen place in a furnace of any convenient con-ftruttion, a crucible capable of holding 20 lbs.of the above mixture. As foon as the crucibleis thoroughly red hot, pour in one third of themixture, and keep flirring it with a rod ofplatina till it comes into a {fate of quiet fufion;Then add another third, carefully flirring it asbefore, and after a while add the remainingthird, and give the whole a good heat, fo as torender it very fluid. Then withdraw the cru-cible, and after it has cooled, gradually breakit up : there will be found a well formed metallicbutton, covered by blackifh brown fcorisc, whichacts pretty powerfully on the magnetic needle.This button being broken to pieces, (which isreadily done on account of its great brittlenefs,)is to be again fufed with white arfenic andpearlafli as before, ar.d the metallic mafs refus-ing from this fecond fufion is generally incapableof adting on the magnetic needle: if however,this fliould not be the cafe, a third fufion witharfenic and alkali mud be had recourfe to.

The firft ftep of the procefs being thusfiniftied, a flat bottomed cylindrical crucible,about 3} inches in diameter is to be madethoroughly hot in a furnace, and is then to becharged with three half pounds of the arfeni-cated platina mixed with an equal weight of•white arfenic, and half a pound of potafh:■when thefe are well mingled and entirely fluid,the crucible is to be removed from the fire, andplaced to cool in a horizontal pofition, in orderthat the cake of metal may be of an uniformthicknefs. The crucible when cold is to be*Srefully broken, and having removed the fcorisc,

k An. Chim

formed and fonorous, weighing about threeounces more than the arfenicated platina em-ployed, and now quite faturated with arfenic:there is no danger of incorporating too much ofthis latter ingredient, it being conftantly obferv-ed that the compleatnefs and rapidity of the fub-fequent purification is exactly in proportion tothe quantity of arfenic which it has previouflybeen made to imbibe.

The metallic mafs thus procured is to beplaced in a muffle, and the heat is to begradually increafed till the arfenic begins toevaporate; the temperature muft then be keptup as nearly as poffible the fame for fix hours,obferving efpecially not to increafe it left thecake melt. At the end of this period the cakewill have become confiderably porous, and isnow to be withdrawn and extinguifhed incommon oil; it is then to be returned to themuffle, by which a further quantity of arfenicwill be evaporated, and this alternate applicationof oil and heating, is to be continued till nomore arfenic makes its appearance. The fufi-bility of the mafs diminishes as the arfenic isgot rid of, fo that a much higher temperaturemay be employed in the latter than in theformer part of this procefs. Having carefullyburnt off at a high heat all the charcoalproduced by the decompofition of the oil, thefpungy cake of metal is to be digefted innitrous acid, and then edulcorated by repeatedboiling in water. Three or more of the cakesare then to be put into a cylindrical crucible,and heated to the higheft poffible degree in apowerful furnace: while they are thus renderedfoft, an iron peftle let down upon therft willmake them cohere: they are then to be with-drawn from the crucible, heated to the utmoftin a fmith’s fire, and carefully forged like ironon the anvil into compact bars.

The advantage of this procefs of M. Jeanetyis its cheapnefs, not requiring the platina to beprevioufly difiolved in nitro-muriatic acid; buton the other hand, the metal, though ap-proaching a ftate of purity, is by no meansabfolutely pure: it contains a fmall quantity ofarfenic and iron, together with, probably thewhole of the lead and copper that may havebeen cafually mingled with the ore, as well asthe palladium, ofmium, iridium and rhodium,and in confequence of this mixture is by nomeans capable of fuftaining the afition ofalkalies and a high heat with fo little injury aswhen more accurately purified.

. xir. p. zp.