PL A

PL A

( 239 )

forms a yellowilh alloy, harder than wroughtiron, and fomewhat brittle. Sp. gr. 10.3.

With an equal weight of tin, a greyifli alloyis produced, inferior in hardnefs to wroughtiron, and extremely brittle: its texture is corn-pad and fine-grained. Sp. gr. 8.1.

With an equal weight of bifmuth it gives agrey alloy, very brittle, and nearly as hard asReel. Sp. gr. 12.5.

Arfenic increafes the fufibility of this metal,and renders it very brittle.

Sulphuric acid, when boiled upon palladium,diffolves a fmall portion, and acquires a brightred colour. Muriatic acid alfo by long ebulli-tion takes up 'a portion, and becomes of a redcolour. Nitric acid, efpecially when yellow andfuming, ads with vigour on this metal, con-verting it firft into a red oxyd, and then diffolvingit. Nitro-muriatic acid is however the bedfolvent of palladium, and forms with it a beauti-ful red folution. From thefe acid folutions thealkalies and earths throw down precipitates,moftly of a bright orange colour, which arepartly rediffolved by the alkalies if added inexcefs : the fupernatant liquor of the precipitateby ammonia, is often of a greenifh-blue. Thefulphats, nitrats, and muriats of potafh or am-monia, produce an orange precipitate, as theydo with the falts of platina. All the metals',except gold, platina, and filver, occafion acopious precipitate of palladium in the metallicdate. Recent muriat of tin produces a darkorange or brown precipitate in the neutralizedfalts of palladium ; but if added to excefs, theliquor remains tranfparent and of a beautifulemerald green. Green fulphat of iron throwsdown the palladium in the metallic date. Withprufliat of potafh it forms an olive-green preci-pitate, and with fulphuretted hydrogen a dark-brown one.

Thefe characters are fufficient to didinguifhpalladium from platina, and every other of theknown metals. It was fufpefted by Mr. Chene-vix to be a compound of mercury and platina,but the experiments of this gentleman havingtotally failed in the hands of other chemids, itmud be confidered, for the prefent at lead, asa didinCt metallic fpecies.

PLUMBAGO. See Coal. Sp. gr. 9.PNEUMATIC APPARATUS. See Ap-pendix .

POLIERSCHIEFER. Polidiing date,ts colour is yellowilh-grey, approaching tocream-colour; it is always driped.

R occurs in mafs : it is dull internally. Itslongitudinal fradture is flaty ; the crofs fracture

is earthy. Its fragments are tabular. Itisfoft;adheres to the tongue ; is fine but meagre to thetouch; and is fometimes fo light as to float onwater.

It occurs in the vicinity of pfeudo-volcanoesin Bohemia .

PONDEROUS EARTH See Barytes.

PORCELAIN. See Pottery.

P ; IRCELAIN, Reaumur’s. See Glass.

PORPHYRY.

A large and important clafs of primitive rocksis didinguidted by modem mineralogifls by thename of porphyry. It is effential to porphyryto confid for the mod part of a compact pade,in which are implanted grains or folitary crydalsof felfpar, of the fame formation as the principalpart of the rock. There are however feveralfpecies of rock agreeing with this definition,which are however arranged among the primi-tive or fecondary traps, and will therefore betreated of hereafter.

There are five fpecies of rocks belonging tothe proper porphyritic formation, namely,

1. Horndone porphyry. The hornftonewhich ferves as the bafe of this fubdance isgenerally red or green, with a conchoidal orfplintery fradture ; and it indofes crydals ofquartz and felfpar.

2. Felfpar porphyry. The bafe of this iscommonly red compadt felfpar, inclofing cryf-tals of felfpar and quartz.

3. Sienitic porphyry. This differs from thepreceding in containing crydals of hornblendein addition to-the other ingredients. .

4 . Pitchdone porphyry. The bafe of this ispitchdone, either red, green, brown, or black.

5. Clay porphyry, of which the bafe is indu-rated clay palling into horndone; it is generallyof a reddilh colour, and contains crydals ofquartz and felfpar.

Mountains of porphyry are not dratified andnever inciofe beds of other lubllances. Itstexture is commonly compact but it occafion-al'y occurs in fchiftone. It is not very rich inmineral veins; the clay porphyry is the modfo; the rich mines of Schenmitz in Hungary being in this fpecies of rock.

The porphyritic formation is not very dif-tindlly feparated from the other rock' formati- •ons which accompany it, nor is its rank amongthe primitive mountains, with regard to anti-quity, very clearly afeertained.

Horndone porphyry feems to be the oldedof the clafs, and clay porphyry the mod recent.The red porphyries are employed in ornamental;