300 INDUSTRIAL PLANTS 



long period of time, the material is so transformed as to be 

 like a soft, brown coal. In regions where wood is scarce peat 

 is highh^ valued as a fuel. It is commonly more bulky than 

 wood, and has from 5 to 15 times as much ash. Its heating 

 power is about the same. 



Coal, like peat, consists of the decomposed and compacted 

 remains of plants. It differs from peat principally in being 

 harder and more completely reduced to carbon. But peat 

 passes into coal by insensible gradations so that none but an 

 arbitrary line can separate them. The coal with which we 

 are most familiar may be regarded as a peat-like material 

 of very great antiquity, — so ancient that the plants from 

 which it was formed have been extinct for many ages. Some 

 idea of the appearance of certain of these coal plants may be 

 gained from Figs. 277, 278. In comparison with wood and peat 

 as a fuel, coal has the advantage of possessing greater com- 

 pactness and more poAver of heating. It will convert into 

 steam about 7 to 9 times its own weight of water. The 

 most objectionable features of coal are its large amount of 

 troublesome ash, which often interferes with good combus- 

 tion, and its offensive smoke, which is excessive from soft 

 coal. 



Charcoal burns without flame or smoke, and has over 

 twice the heating power of wood, or as much as the average 

 coal. It is produced mostly by smothered combustion of 

 billets of wood, commonly arranged in conical piles, and cov- 

 ered with earth. When wood is subjected to dry distillation 

 creosote, wood-alcohol, and other volatile compounds pass 

 into the condenser, leaving charcoal in the retort. The 

 charcoal produced at the highest temperature yields most 

 heat when burned, and is therefore of most use in metallurgy; 

 that produced at as low a temperature as possible is the 

 most inflammable and thus the most suitable for mixing 

 with niter and sulphur to make gunpowder. 



Coke bears somewhat the same relation to coal that char- 

 coal does to wood. It is similarly obtained by smothered 

 combustion in covered piles, or by heating in special ovens or 

 retorts. Like charcoal it is nearly pure carbon, and is used 

 extensively in metallurgy and for other purposes where a 



