MINERAL COAL. 85 



pellucid, which unfits them forfhis purpose, and such lenses 

 therefore are seldom made. / L. -^ 



MINERAL COAL. ' 



Massive. Color black or brown, opaque. Brittle 01 

 sectile. H = 1—2-5. Gr = l-2-~- 1-75. 



Composition. Carbon, with usually a few per cent, of '\ 4 

 silica and alumina, and sometimes oxyd of iron ; often con- 

 tains a large proportion of bitumen. The bituminous varie 

 lies burn with a bright flame and bituminous odor ; while 

 those destitute of bitumen afford only a pale blue flame, 

 arising from the decomposition of the water present and the 

 formation of the gas called carbonic oxyd. 



Varieties. — 1. Without bitumen. <w 



Anthracite. Anthracite (called also glance coal and stone 

 coal) has a high luster, and is often iridescent. It is quite 

 compact and hard, and has a specific gravity from 1*3 to 

 1*75. It 'dually contains 80 to 90 percent, of carbon, with 

 4 to 7 of water, the rest consisting of earthy impurities. 

 There is often some bitumen present, in which case it burns 

 with considerable flame. 



Besides the use of anthracite for fuel, it is often made into 

 inkstands, small boxes, and other articles, which have a higb 

 polish, and fine specimens of this kind of ware may be ob- 

 tained in Philadelphia. 



2. Bituminous varieties. 



Bituminous coal varies much and indefinitely in the 

 amount of bitumen it contains, and there is a gradual pas- 

 sage in its varieties into varieties of anthracite. It is softer 

 than anthracite and less lustrous. The specific gravity does 

 not exceed 1*5. 



Pitching or caking coal, as it is distinguished in England, 

 at first breaks when heated, into small pieces, which, on 

 raising the heat, again unite into a solid mass. Its color is 

 velvet or grayish black. It burns readily with a lively yel- 

 low flame, but requires frequent stirring to prevent its caking, 

 and so clogging the fire. The principal beds at Newcastle, 

 England, afford this kind of coal. Clierrp-^oal resembles 

 pitch coal in appearance, but does not soften asul cfclte. It j 



y>4X)ai <resembles 

 en ia^d c^ke. It 



Of what does mineral coal consist ? How does anthracite diffc? 

 from other varieties 1 





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