300 INDUSTRIAL PLANTS 



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

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

 is highly 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 decomposefl and compacted 

 remains of plants. It tliffcrs from peat principally in being 

 harder and more completely reduced to carbon. But peat 

 passes into coal by insensililc gradations so that none but an 

 arlntrarj' line can separate them. The coal with which we 

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

 of very great antifiuity, — so ancient that th(> plants from 

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

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

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

 as a fuel, (;oal has the advantage of possessing greater com- 

 pactness and more power of heating. It will convert into 

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

 most olijectionable features of coal are its large amount of 

 troublesome ash, which often interferes with good combus- 

 tion, and its offensi^'e smoke, which is excessive from soft 

 coal. 



Charcoal burns without flame or smoke, and has over 

 twice the heating power of wo(j(l, or as much as the average 

 coal. It is produceil mostly by smothered combustion of 

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

 ered with cartli. 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 jiroducefl at the iiighest temperature yields most 

 heat when l)urned, and is therefore of most use in metallurgy: 

 that produced at as low a temperature as pijssible is tlie 

 most inflammable and thus the nidst suitable foi' mixing 

 with niter and sulphui' to make gunpowdei'. 



Coke bears somewhat tiie sanK> rehition to coal tliat char- 

 coal does to wood. It is similarly obtained l)y smothered 

 combustion in covereil piles, (ir by heating in special ovens or 

 retorts. Like charcoal it is nearly iiun> carbon, and is used 

 extensiveh' in metallurgy and foi- other purpos(>s where a 



