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 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 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 power 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 coy- 
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 
