C o A

C O A

( 298 )

blackifli brown. It occurs in mafs: its parallelfra&ure is fibrous lamellar, palling into flaty,and is lliglitly glimmering; its crofs fracture ismore or lefs conchoidal with a fhining refinousluftre. It acquires a polilh by friction ; is mo-derately hard. Sp. gr. 1.4 to 1.55; the heavieltvarieties contain pyrites.

It burns readily with a weak flame like half-charred wood and a difagreeable bituminousodour, and when heated in clofe veffels it yieldsmuch charcoal; when ignited in an open fire itleaves behind a fmall quantity of white afhes.2©o grains, according to Hatchett, b yield bydiftillation

60 grains acidulous water,

21 thick brown oily bitumen,

90 charcoal,

29 hydrogen, carburettcd hydrogen, and- carbonic acid.

200

In England it is found at Bovey near Exeter,and is called Bovey coal: at this place there arefeventeen ftrata within the depth of feventy-four feet from the furface, alternating withalluvial clay : the greateft thicknefs of the feamsis between fix and eight feet. Brown coal isalfo found in the territory of Heffe, and variousother parts of Germany, in Denmark and Green-land, and in Italy .

It is ufed like the preceding fpecies, for fuel.It pafies into bituminous wood and moor-coal,fometimes into pitch-coal.

Sp. IV. Moor coal. Moorkohle , Werner.Houille limoneufe, Broch.

Its colour is dark blackilh brown. It occursin mafs forming very thick beds, and is charac-terized by being always full of rifts and crevices.Internally it difplays a conliderably refinousluftre. Its longitudinal fra&ure is imperfectlyflaty, its crofs frafture is even, approaching toflat conchoidal. Its fragments are trapezoidalor rliomboidal. It is tender, remarkably frang-ible, and light. In its chemical characters itrefembles the preceding fpecies.

It occurs in alluvial land and in the neweftfioetz-trap-formation. It is met with very fre-quently in Bohemia , and is alfo found in Tran-fylvama and other parts of the Auftrian do-minions, in Denmark and the Faroe illands.

** Black Coal . Perfectly bituminous; of ablack colour.

Sp V. Slate coal. Schiefsr Kohle, Werner.Houille fchifteufe ) Broch.

Its colour is perfeCt black palling into greyilh-black, and often prefents more or lefs of anirridefeent tarnilh. It occurs in mafs and gene-rally pofleffes a high refinous luftre. Its prin-cipal fraCture is ftrait flaty, the crofs fraCtureis fmall-grained uneven, palling into flat con-choidal. It breaks into indeterminately angularor trapezoidal fragments. It is foft and eafilyfrangible. Sp. gr. 1.25 to 1.4. It containsfrom 57 to 64 per cent, of carbon, from 33 to43 of bitumen, being a mixture of maltha andafphalt, and from 3 to 6 per cent, of earth andoxyd of iron. It runs into the two followingvarieties.

Var. r. Foliated coal. Blatterkohle, Werner-Charbon latnelleux, Broch.

This differs from the preceding in having afomewhat higher luftre, and a ftrait foliatedlongitudinal, and flaty crofs frafiture. It is alfofofter and more fubjeCt to decompofition.

Var. 2. Coarfe coal. Grobkohle , Werner..

Houille grofjiere , Broch.

The crofs fraCture of this difplays a coarfergranular texture than common flate-coal; itscolour is not fo pure a black, it poffeffes lefsluftre, is conliderably harder, and its fp. gr. onaccount of the greater quantity of earth that itcontains, amounts to about 1.5..

Moll of the common coals belong to thisfpecies, and from the different phenomena whichthey exhibit during combuftion, a great numberof varieties are known in the market, and arefufficiently obvious to an experienced eye, whichyet cannot be deferibed.

The two points which are principally to beattended to with regard to common coals, in aneconomical point of view, are the intenfity ofthe heat and the duration of the combuftion,and thefe are chiefly influenced by the propor-tion of afphalt which they contain. Coals inwhich the bituminous part is chiefly maltha,with only a fmall quantity of afphalt, kindlevery eafily, burn brilkly and quickly with abright blaze, cake but little, require no ftirring,and by a Angle combuftion are reduced to loofe-alhes, fuch are the Lancalhire coals, the Scotchand moft of thofe which are raifed on the wef-tern fide of England. Thofe on the other hand'in which afphalt prevails kindle difficultly, andafter lying fome time on the fi re become foftand in a ftate of almoft femifufion; they thencohere or cake, fwell confiderably, and throwout on every fide tuoercular fcorite accompaniedby bright jets of flame. In confequence of thecohefion and tumcfacKon of the coals, the paf-fage of the air is interrupted, the fire burns.

b Pkil.Tranf. for 1804.