PL A
PIT ( 23 0 )
PITCHSTONE. Pierre de Poix, Fr.Pechjiein, Germ.
The colour of this mineral is very various,being grey and green of various {hades, alfohoney yellow, liver brown, blood and brickxed. r l he colours are generally pale, and notunfrequently arranged in bands. It occurs inmafs. Internally it is ufually fhining, with•rather a faint greafy luftre. Its fracture isimperfectly conchoidal, palling into fplintery,and in proportion as it does fo its luftre isfainter, and it approaches to hornftone. Itsfragments are indeterminately angular, fli'arp-.edged. Sometimes it occurs in lmooth granularAiftmct concretions. It is ufually tranllucentonly at the edges, the black varieties are opake.It is conficlerably hard, brittle, and eafily frang-ible. Sp. gr. 2.3.
It is fufible before the blow-pipe into awhite, fomewhat porous enamel. According.to Wiegleb, the pitchftone of Meiffen inSaxony is compofed of
64.58 Silex15.41 Alumine5. Oxyd of Iron
84.9915.01 lofs
Pitchftone occurs in mountain mafies, andeven conftitutes entire mountains, and it formsthe bafe of a particular kind of porphyry.
It abounds in Saxony, Hungary, Siberia ,and other mountainous diftrifts.
It is not unfrequently confounded withhornftone, and femi-opal.
PLASMA. Wern.
The colour of this mineral is green of various{hades mixed with brown, and the whole ar-ranged in dots, clouds and bands. It occurs inmafs, and in angular and rounded fragmentsinclofed in talc or fteatite. It has a feeblegreafy luftre both internally and externally.Its fraifture is conchoidal paffing to fine fplintery.Its fragments are indeterminate and fliarp-edged.It is tranflucent, and in thin pieces femitranf-parent. Its hardnefs is nearly equal to that ofChalcedony; it is brittle, and eafily frangible.
Its proper geological fituation has not as yetbeen afeertained; neither has it been fubmittedto chemical analyfis. It is not uncommon inItaly and the Levant: it occurs near Bojanovitz,in ivioravia, imbedded in ferpentine, and nearToeltfva in Upper Hungary , in rounded pieceswith hornftone. Sometimes it combines withheliotrope and chalcedony, forming agate.
PLATINA. Le Platine , Fr. Der Platin ,Germ. Aurum album, Waller.
§ t. Ores.
This metal has hitherto been found only inone ftate in which according to moft mineral-ogifts and chemifts it is confidered as native,though Prouft is inclined to connder it as in theftate of lulphuret. From this latter opinionwe {hall take the liberty to diffent, and {hallaccordingly deferibe this fubftance as
Native Platina.
The colour of this mineral is between Giverwhite and fteel grey. It comes to Europe onlyin the form of flat and more or lefs roundedgrains, from the Gze of a pea, which is rare,to that of fine fand. The furface of the grainsis moderately fmooth, and they poflefs rathera low degree of metallic luftre: by fri&ion tinsbrightnefs approaches to that of polifhed iron :its hardnefs is greater than that of copper: it isconfiderably ductile, and very flexible when inthin plates. its fp. gr. is variable, but isfeldom lefs than 16.5, or greater than 17.2.
It is infufible before the blow-pipe, and isinfoluble in acids, except the oxymuriatic ornitromuriatic; from its folution in this latter itis precipitable by muriat of ammonia, but notby green vitriol.
Nothing is as yet known of the geologicalfituation of this fubftance except that it accom-panies gold. It is found in the Rio del Pinto,and near Choco in the Viceroyalty of Peru, andnear Carthagena in New Granada.
A Variety of the above in fmaller grains andof a darker colour, is alfo met with.
Platina was firft brought into Europe in 1749,by Mr. Chas. Wood, aflay-mafter of Jamaica ,and in the fucceeding year, fpecimens wereprefented by Dr. Brownrigg, to the RoyalSociety . Thefe came from Carthagena, fromwhich place alfo were difperfed through theSpanifti Weft Indies, various toys and trinketsconfifting of this metal alloyed with fome other,probably filver, as the price of the alloy wasnearly equal to that of this latter metal.
Mr. Wood and Lewis began the inveftigationof the properties of this new fubftance in thevery year in which it was imported: it tHenemployed the abilities of Schaeffer, Marggraaf,Bergman, Macquer , and Beaume; and morerecently of Deliile, Morveau, Jeanety, Vauquelin and Fourcroy , Defcotils, Muffin Pufchkin,Prouft, Tennant, Chenevix and Wollafton.Many difficulties however yet remain to beovercome before the chemical hiftory of thisfubftance can be compleated. 7