PLANT PRODUCTS. FERMENTATION AND FUELS 425 



Coal as Fuel. The quality of a fuel coal, and whether it is 

 worth its price, is learned by measuring its calorific (heating) 

 power. A sample (about 1 g.) is burned in a bomb calorimeter. 

 This is a closed, metal vessel, filled with oxygen and submerged 

 in a known weight of water. The coal is set on fire by a wire 

 heated electrically and, after it has burned, the increase in tem- 

 perature of the water is read off. Hence the heat in calories 

 (p. 162) evolved by the burning of 1 g. of coal is obtained. In 

 engineering practice they use the number of British Thermal 

 Units (1 B.T.U. = heat required to raise 1 pound of water 1 F.) 

 developed by 1 pound of coal, and call the result the calorific 

 power. 



Knowing that 100 cal. will raise 1 g. of water from C. to 

 100 C., and 539 cal. more will convert it into steam, it is possible 

 to calculate how much steam should be furnished by 100 kilog. 

 of coal of known heat of combustion. If the quantity falls short, 

 then the furnace, draft, or method of firing may t be defective. 

 Too much draft, for example, merely ifttroduces additional, useless 

 air to be heated. Thus, if the flue gas, upon analysis, is found 

 to contain, not 12 per cent carbon dioxide (normal), but only 3 

 per cent, then for every ton of coal burned, 52 tons of unnecessary 

 air have been raised to the temperature of the furnace. By 

 chemical tests, made in ways like this, the efficiency of every 

 device in the modern factory is (or ought to be) controlled. If 

 the coal is bought without heed to its calorific value, and used 

 without experimental checks, the boiler house alone may easily 

 waste the whole profit earned by the rest of the plant. 



