CARBON AND ITS COMPOUNDS. 27 



beginning of its combustion, which comes from the car- 

 bonic oxyd gas as it unites with the oxygen of the air to 

 form carbonic acid. The formation of this gas in the an- 

 thracite fire is fully explained in 67, Part II. It is from 

 the bituminous coal that the illuminating gas is produced, 

 there being no hydrogen in the anthracite for its forma- 

 tion. The explanation of the process is this : By the ap- 

 plication of heat the hydrogen is driven off, and it takes 

 with it enough of the carbon to make it carbureted hy- 

 drogen ; but most of the carbon is left behind in a light, 

 porous condition, to which is given the name of coke. 



39. Anthracite. This is the common name given to 

 coal which is either mostly or wholly free from bitumen. 

 It is supposed that it was subjected to more heat than 

 the bituminous was after its formation, by which the bi- 

 tuminous elements were driven off. The coke, which is 

 left after the making of illuminating gas, is very much 

 the same thing as anthracite, except that, as it has not 

 been subjected to great pressure, it has not the same 

 closeness of structure. Anthracite is hard, and so is often 

 called very appropriately stone coal. From 80 to 90 per 

 cent, of it is carbon. Then there is from 4 to 7 per cent, 

 of water, and there are some impurities, as silica, alumi- 

 na, lime, magnesia, etc. The redness of the ash of some 

 kinds of coal comes from the presence of oxyd of iron. 

 The slag, or glassy substance which you find in the refuse 

 of the combustion, is composed of silicates. These do 

 not form with the ordinary burning of coal in grates, be- 

 cause the heat is not intense enough. The clinker which 

 collects on the inside of stoves is slag. The reason that 

 oyster-shells put into the fire clear off the clinker is that 

 the lime makes the silicates more fusible, and so dislodges 

 the clinker, mingling it with the burning coal. Anthra- 

 cite has a fine lustre, and often an iridescent play of col- 

 ors. It is capable of a high polish, and has sometimes 

 been made into inkstands and other articles. 



40. Bituminous Coal. This is softer than anthracite, 



