large amount of '^ residual carbon." It burns with a glow, but 

 with little flame, and cannot be used for manufacturing gas. 



When a bituminous coal is heated in the retorts of a gas-works 

 and the volatile ingredients removed, it is reduced to the fa- 

 miliar substance known as coke. This is a spongy mass, essen- 

 tially the same in composition as anthracite, only produced 

 rapidly and by great heat, instead of by the slow process and 

 at the moderate temperatures of nature, combined with enor- 

 mous pressure in the rocks. In the gas-retorts the coal softens 

 under the great heat, and the escajjing gas leaves behind a 

 porous mass of residual carbon. It is of great interest to know 

 that in some instances, where molten igneous rocks have broken 

 through a bed of bituminous coal, the latter has been locally 

 altered into a natural coke, much like the artificial product, 

 by the influence of the extreme heat. 



If the process goes so far, however, that all the gaseous com- 

 pounds are expelled and only carbon remains, the material that 

 is left is not available as a fuel. Pure carbon is practically 

 almost incombustible; and the last stage of the process de- 

 scribed is the production of graphite or plumbago, often erro- 

 neously called ''black-lead," from its metalHc luster. This is 

 the material used for pencils, for stove-polish, and for refractory 

 cmcibles. Between the hard anthracites and this substance, 

 there are intervening grades known in nature as "graphitic 

 anthracite ; ' ' and a similar stage of artificial reduction furnishes 

 the ' ' carbon ' ' points and rods used in electric lighting. 



All these substances, so different in their uses and properties, 

 are nevertheless simply products formed at different stages in 

 the series of changes wrought by the general process above de- 

 scribed. There are no sharp lines of division between them, 

 but they pass one into another, as the volatile elements diminish 

 and the percentage of "residual carbon" increases. The names 



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