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 onl}^ 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 Aveight 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 ma}' take 

 place, as shown by tiie 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, slowl}' decompose. 

 Peat-bogs form chiefly in northern countries. Near the sur- 

 face they consist largel}^ 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 



