MINERAL COAL. 327 



MINERAL COAL. 



Massive. Color black or brown ; opaque. Brittle or im- 

 perfectly sectile. H. =0-5-2-5. G. =1 -2-1 '80. 



Composition. Carbon, with some oxygen and hydrogen, 

 more or less moisture, and traces also of nitrogen, besides 

 some earthy mineral which constitutes the ash. The car- 

 bon, or part of it, is in chemical combination with the 

 hydrogen and oxygen. 



Coals differ in the amount of volatile ingredients given 

 off when heated. These ingredients are moisture, and hy- 

 drocarbon oils and gas, derived from the same class of 

 insoluble hydrocarbons that is the source of the oil of shales 

 and other rocks. 



VARIETIES. 



1. Anthracite. Anthracite (called also glance coal and 

 stone coal) has a high lustre, and is often iridescent. It is 

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

 to 1*75. It usually contains 80 to 93 per cent, of carbon, 

 with 4 to 7 of volatile matter ; the rest consisting of earthy 

 impurities. Burns with a feeble blue flame. 



Those yielding the most volatile ingredients are called 

 free-burning anthracite. 



2. Bituminous Coal. Bituminous coal varies much in the 

 amount of oil, coal-tar, or gas it yields when heated; and 

 there is a gradual passage in its varieties through semi- 

 anthracite to anthracite. It is of a black color, Avith the 

 powder black, but it is softer than anthracite, and less 

 lustrous. The specific gravity does not exceed 1*5. The 

 volatile ingredients constitute usually between 20 and 40 

 per cent. 



Caking Coal includes that part of bituminous coal which 

 softens when heated and becomes viscid, so that adjoining 

 pieces unite into a solid mass. It burns readily with a 

 lively yellow flame, but requires frequent stirring to prevent 

 its agglutinating, and so clogging the fire. Non-caking coal 

 resembles the caking in appearance, but does not soften and 

 cake. The chemical difference between caking and non- 

 caking coal is not understood. 



3. Cannel Coal is very compact and even in texture, with 

 little lustre, and breaks with a large conchoidal fracture. It 

 takes fire readily, and burns without melting to a clear yel- 



