298 INDUSTRIAL PLANTS 
Anything which burns readily in the air will serve as fuel; 
and, indeed, various sorts of refuse are thus utilized: for ex- 
ample, wheat straw is made to run steam threshing-machines, 
and the crushed stalks of sugar-cane are used in the boiling 
of the juice. But, in general, wood, peat, and coal, and their 
products, charcoal, coke, and illuminating gas, are the fuels 
most extensively used. 
Wood is the most used of all fuels. All woods when per- 
fectly dry consist of nearly 99% of combustible material 
and about 1% of inorganic matter which remains as ash when 
the wood is burned. Air-dry wood contains about 25% of 
water, and in green wood it may be as much as 50%. This 
water reduces the fuel value not only as taking the place 
of combustible substances but also as using up the heat 
necessary for its evaporation. Hence the economy of well- 
seasoned fire-wood. The value of different fuels may be 
conveniently compared when stated in terms of the amount 
of water which a unit weight will evaporate. Thus, green 
wood is found to yield enough heat to convert about twice 
its weight of water at 100 C. into steam; air-dry wood about 
three and a half times; and perfectly dry wood over four times 
its own weight. So far as chemical composition is concerned 
soft woods should yield on burning about the same amount 
of heat as hard woods of equal dryness. In practice, how- 
ever, considerable differences are found, depending in part 
upon the ease with which complete combustion may take 
place, as shown by the amount of smoke, and in part upon 
compactness of structure, and so forth. Wood as being a 
flaming fuel is especially well adapted for heating surfaces 
of large extent, as in the boilers of steam-engines. The small 
amount and the soft crumbly nature of its ash give wood 
a further advantage over peat and coal. 
Peat consists of the more or less carbonized and compacted 
deposits of vegetable substances which accumulate in bogs 
and marshes, and, in the presence of water, slowly decompose. 
Peat-bogs form chiefly in northern countries. Near the sur- 
face they consist largely of moss like that shown in Fig. 227 
with which, however, a number of other plants are found 
growing. In the deeper layers that have been buried for a 
