The reason why tar supplies all sorts of useful material is because it is indeed the quintessence of the forest, of the forests of untold millenniums if it is coal tar.  If you are acquainted with a village tinker, one of those all-round mechanics who still survive in this age of specialization and can mend anything from a baby-carriage to an automobile, you will know that he has on the floor of his back shop a heap of broken machinery from which he can get almost anything he wants, a copper wire, a zinc plate, a brass screw or a steel rod.  Now coal tar is the scrap-heap of the vegetable kingdom.  It contains a little of almost everything that makes up trees.  But you must not imagine that all that comes out of coal tar is contained in it.  There are only about a dozen primary products extracted from coal tar, but from these the chemist is able to build up hundreds of thousands of new substances.  This is true creative chemistry, for most of these compounds are not to be found in plants and never existed before they were made in the laboratory.  It used to be thought that organic compounds, the products of vegetable and animal life, could only be produced by organized beings, that they were created out of inorganic matter by the magic touch of some “vital principle.”  But since the chemist has learned how, he finds it easier to make organic than inorganic substances and he is confident that he can reproduce any compound that he can analyze.  He cannot only imitate the manufacturing processes of the plants and animals, but he can often beat them at their own game.

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.  If you are after the greatest yield of tar products, you impoverish the gas by taking out the benzene and get a blue instead of a bright yellow flame.  If all you are after is cheap coke, you do not bother about the by-products, but let them escape and burn as they please.  The tourist passing across the coal region at night could see through his car window the flames of hundreds