THE BURNING OF WOOD AND COAL 93 



the fuel suffers decomposition or a breakdown into its ele- 

 ments, hydrogen and carbon, as it burns. These elements 

 then burn separately. If there is a lack of sufficient oxygen, 

 or if the fuel and oxygen are not properly mixed, or if the 

 carbon is cooled below its kindling temperature before it has 

 a chance to meet oxygen, then there will be more or less un- 

 burned carbon and the flame will smoke, due to the escape of 

 this unburned carbon. Now it happens that the volatile matter 

 from wood contains some oxygen in addition to what may mix 

 with it from the air, and consequently wood ordinarily burns 

 without black smoke. It is therefore said to be a cleaner fuel 

 than soft coal which ordinarily burns with a black smoke. 



106. Burning Coal. SOFT or BITUMINOUS coal is the name 

 applied to most of the coal mined in the United States except 

 that which is mined in the eastern half of Pennsylvania. To 

 this latter the name HARD or ANTHRACITE coal is applied. 



Exercise 32. How Soft Coal Burns 



Place two or three thicknesses of wire gauze on a ring attached to a 

 ring stand and lay a piece of soft coal about the size of 

 a marble on the gauze. Heat the lump strongly by 

 means of a flame. Does the coal produce any smoke? 

 Remove the flame (Fig. 71). Can this smoke be 

 ignited? How far above the coal are you able to 

 ignite the smoke? The smoke, or volatile matter, 

 consists of water and various hydrocarbons arising 

 from changes in the coal, due to the heating. After 

 all of the volatile matter of the coal has been driven FIG. 71. 

 off, remove the flame and examine the part which Burning the 



remains. It is called COKE. What is the name of the volatile por- 

 ,. . , , . , , , 10 tion of coal in 



corresponding material obtained by heating wood? air 



See if you can ignite the coke as the charcoal from 

 wood was ignited. Coke is much used as fuel, especially in obtain- 

 ing metals from their ores. 



Exercise 33. How Hard Coal Burns 



Repeat the preceding experiment using a lump of hard coal instead 

 of the soft coal. Can you heat it hot enough to drive off enough 



