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When coal is heated in the open air it is burned up and nothing but the
ashes is left. But heat the coal in an enclosed vessel, say a big
fireclay retort, and it cannot burn up because the oxygen of the air
cannot get to it. So it breaks up. All parts of it that can be volatized
at a high heat pass off through the outlet pipe and nothing is left in
the retort but coke, that is carbon with the ash it contains. When the
escaping vapors reach a cool part of the outlet pipe the oily and tarry
matter condenses out. Then the gas is passed up through a tower down
which water spray is falling and thus is washed free from ammonia and
everything else that is soluble in water.
This process is called "destructive distillation." What products come
off depends not only upon the composition of the particular variety of
coal used, but upon the heat, pressure and rapidity of distillation. The
way you run it depends upon what you are most anxious to have. If you
want illuminating gas you will leave in it the benzene.


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