OP A

O P A

( IS8 )

weather, are dull and opake, probably in confe-quence of having loft part of their water ; butby immerfion in this fluid, their minute poresbecome again filled, and they then exhibit theufual play of colours. Such ftones are calledhydrophanes, and are in high eftimation.

The mod beautiful opals known are in theImperial cabinet of Vienna : one is a piece aboutfive inches long, and two and a half in diameter:another is of the fhape and fize of a hen’s egg.Both are remarkably rich and fplendid.

2. Subfp. Common Opal. Gememer Opal,Wern. JJOpale commune , Broch.

Its colour is grcyifh, greenilh, yellowifh ormilk-white, hence it paffes into apple green,honey and wax-yellow, and fometimes, thoughrarely, into tile red, or flelh red. The milkwhite variety appears by tranfmitted light of awine yellow. It occurs maflive, diffeminated,and in angular pieces. Its luftre internally isbetween vitreous and refinous, and is brightfhining. Its frafture is conchoidaV Its frag-ments are indeterminately angular and fharp-edged. It varies from tranflucent to tranf-parent. It is moderately hard, brittle, andeafily frangible. Sp. gr. 1.95 to 2.01.

It is infufible before the blow-pipe withoutaddition. Two varieties, one from Kofemutzand the other from Telkobanya, have beenanalyfed by Klaproth with the following refults.

Kofem. Telkoban.

Silex _ - v 98.75 — 93.5

Alumine - 0.1 — o

Oxyd of iron 0.1 — r.

Water - - o. — 5.

98.95 99.5

It occurs chiefly in porphyry ; but alfo ingranite, gneifs, amygdaloid, and ferpentine.

The green variety is found at Kofemutz inSilefia, where it accompanys chryfoprafe in abed of ferpentine. In. Iceland it alternates withchalcedony. It is alfo met with in variousparts of Saxony and Bohemia, near Florence in Italy, Telkobanya in Hungary , in the Northof Iceland, in Cornwall , in Brittany, and inthe Faroe iilands.

Several of the varieties of common opal aswell as of the noble opal, are hydrophanous.

3. Subfp. Semi-Opal. HalbopaL Wern.La Demi-Opale. Broch.

Its ufual colours are yellowifh, greenifh,and milk-white, alfo afh-grey, and greyifh-llack, apple-green, olive-green, honey-yellow,yellowilh-brown, and cheinut-brown. Its co-

lours are more dull and muddy than thofe ofcommon opal, and they often occur in fpots,bands, and clouds. It occurs maflive, difiemi-nated, tuberous, reniform, botryoidal and fta-ladfitic. Internally it is gliftening or fhining.Its fradture is imperfectly flat conchoidal, verg-ing on even. Its fragments are indetermi-nately angular fharp-edged. It is more or lefstranflucent, is moderately hard, brittle, andeafily frangible. Sp. gr. 2.54.

It is infufible before the blow-pipe withoutaddition. Two varieties, the one from Telko-banya, and the other from Menil Mon tan t,have been analyfed by Klaproth , with thefollowing refults.

Telkob. Menil.

Silex - -

43-50 —

85.5

Alumine -

0. —

1.

Oxyd of iron

47. —

0.5

Lime

0 —

°-5

Water

7-5 —

11.0

98.0 98-5

It occurs in veins in porphyry and amygdaloid,and in metalliferous veins in granite and gneifs.It ufually accompanies common opal, and isfound in the fame places with it. Foffil teethalfo are occafionally found penetrated by thismineral.

4. Subfp. Wood Opal. _ Halzopal. Wern.Opal Ligmforme. Broch.

Its colours are greyifh and yellowifh-white,alfo ochre-yellow, yellowifh-brown, afh-grey,and greyifh-black. It occurs in large pieces,which have the form and texture of wood..Internally it has a gliftening fomewhat refinousluftre. Its crofs fradture is conchoidal, butits longitudinal fradture is broad fplintery. Itsfragments are indeterminately angular fharp-edged. It is tranflucent, moderately hard, andeafily frangible. Sp. gr. 2.6.

It' appears to be wood penetrated with opal,and is found near Schemnitz , and at Telkobanyain Hungary .

ORPIMENT. See Arsenic.

ORPIMENT NATIVE. See Arsenic,Ores of.

ORES.

An ore is a metal in the ftate in which it exiftsin the earth. It may.be either native, that is,pure and uncombined with any other fubftance,or alloyed with another metal, or in the ftateof an oxyd, or a fulphuret, or a carburet, or ofa metallic fait. It is alfo mixed in almoft everycafe with various earthy minerals. For a par-