FUELS AND CARBON COMPOUNDS 



261 



this. The fuel efficiency of wood depends upon the 

 relative amounts of combustible gas and carbon or 

 charcoal furnished. Hardwood furnishes the most car- 

 bon and is therefore better for fuel than soft wood. 



Coal. If we look at soft coal through a microscope we 

 find that it consists of a black mass of vegetable matter, 

 such as grass, leaves, shrubs, trunks and roots of trees. 

 Because of some change in the earth's surface, this material 

 was covered with water and mud and thus prevented 

 from decaying. In this condition the mass slowly changes 

 from the carbon compounds to pure carbon. The fuel 

 value of the coal depends upon the extent to which this 

 change has taken place. Peat must be dried before it 

 will burn. Bituminous, or soft coal still contains many 

 carbon compounds and burns with a long yellow flame. 

 Hard coal, or anthracite, in which the carbon compounds 

 have been changed to nearly pure carbon, has no flame 

 and can be burned only in a strong draft. 



Experiment 70. Charcoal and Coke. Fill a long test tube one 

 third full of dry sawdust and heat it slowly. Test the gases evolved 

 by bringing a lighted splinter to the mouth of the test tube. 



When wood is 

 heated in a retort 

 from which the air 

 is excluded, several 

 products result : 

 inflammable gases 

 are driven off, tarry 

 liquids appear in 

 the retort, and the wood changes to a porous black solid 

 (Figure 226). In the manufacture of charcoal on a large 

 scale the wood is heated in a large iron cylinder. The 

 valuable liquid products are led away by tubes. Among 



FIG. 226. Wood Arranged for Burning into 

 Charcoal. 



