C O A

CO A.

( 300 )

Var. 2. Columnar glance coal. Stangenkohle ,Werner. Houilk fcapiforme, Broch.

Its colour is dark greyilh black, fometimesmerging to brownilh black, the fides of its natu-ral divifions are often covered with a yellowferruginous earth. It occurs in mafs. It pof-feffes a fhining luftre, between refinous and me-tallic. Its frafture is more or lefs perfectlyconchoidal. It always occurs in thick, curved,parallel, columnar, diftinct concretions, havingfmooth glimmering furfaces. It is foft, veryeafily frangible and light.

It burns without flame or fmell, and leaves agreyifh white afh. It has hitherto been onlyfound at the Meiffner in Heffe, where it occurstogether with other coal in the neweft floetz-trap-formation.

Var. 3. Slaty glance coal. Schiefrige glanz-kohle, Werner. Kohlenblende of Broch. Emmerl.and Wiedenm. Anthracite , Hauy and Dolomieu .Kilkenny coal. Weljh culm.

Its colour is dark iron black, verging on fteel-grey. It occurs in mafs : has a bright metallicluftre : its principal fracture is more or lefscurved flaty, its crofs fradture is fmall and im-perfectly conchoidal. It breaks eafily into in-determinate angular fragments. Sp.gr. 1.5 to1.8.

When pulverized and heated on a muffle itbecomes red, and flowly confumes, with a verylight lambent flame, without fmoak; it does notcake, and when pure emits no fulphureous orbituminous odour: it leaves a variable propor-tion of reddilh afhes, and the unconfumed par-ticles retain their original colour and luftre.

The Kilkenny coal is fomewhat harder thanis cuftomary with this variety, it poffeffes a verybrilliant luftre, which by long keeping is im-paired, in confequence of a light olive tint whichoverfpreads it from decompofing pyrites. Somekinds being nearly pure burn without any fmell,but it often gives out a ftrong fulphureous odour.Thofe fpecimens that abound in pyrites alfocontain mineral charcoal.

Slaty glance coal confifts of carbon, with from3 to 30 per cent, of earth and iron : the follow-ing are the principal analyfes which have beenmade of this fubftance.

Vauquelin

Dolomieu

Panzenberg

Carbon - - 68 —

72.05 —

90 90

Silex - - 3 ° —

—

• 2 4

Alumine - — —

3-29 “

5 4

Oxyd of iron 2 —

3-47 “

3 2

xoo

92.00

loo 100

1" ■

y 1 ■ ' " j . ' —

This mineral occurs in imbedded maffes, bedsand veins in primitive, tranfition, and floetzrocks. It is found in gneifs, in micaceous andargillaceous fchiftus, in mineral veins, with cal-careous fpar, native filver, mineral pitch, andred iron ore, and has been difcovered by Jamefonin the independent coal-formation in the Ifle ofArran .

Sp. IX. Plumbago. Graphit , Werner. Gra-phite, Broch. Fer carbure, Hauy. Black-lead.

Its colour is dark iron black, palling intofteel-grey. It occurs in mafs, in kidney-fhapedlumps, and diffeminated. It has a glifteningmetallic luftre: its fradture is fmall fomewhatcurved foliated approaching to fcaly, or gra-nular uneven; in the great it is flaty. It oc-curs generally in granular or fcaly diftinct con-cretions : it takes a polilh by cutting or rubbing,gives a dark lead-grey ftreak, is undhious tothe feel, foft, and not very brittle. Sp. gr. 1.98to 2.26.

When heated it does not flame, nor can itfupport combuftion by itfelf. After long ex-pofure to a high heat on a muffle its carbon isburnt off, and its earthy and metallic part re-mains behind.

The purer kind of plumbago, according toScheele and Berthollet , confifts of about 90 percent, of carbon and 10 of iron. An impurekind from Pluffier afforded Vauquelin

23 carbon2 iron

37 alumine

38 Silex

100

It is found in primitive and tranfition rocks inEngland, Scotland, France, Spain, Germany ,&c.

Its mod important ufe is as a material forpencils to write or draw with, for which pur-pofe none that has yet been difcovered is com-parable to that from Borrowdale in Cumber-land : it is fometimes ufed to lubricate ma-chinery with inftead of oil; alfo to protect ironfrom ruft, and, when mixed with clay, as a ma-terial for crucibles.

Modern mineralogifts diftinguifh three dif-ferent kinds of coal-formation, or three feriesof rocks entirely different from each other,which contain coal; and as a knowledge ofthefe is important both in an economical pointof view, and to enable us to form a juft theoryconcerning the origin of this interefting mineral,