nature
and quantity can be made to vary within certain limits by modifying the
temperature of distillation.

Having once realized this principle with respect to coal itself, it is
easy to extend it to the products of its destructive distillation. The
tar, for instance, is a complicated mixture of various substances, among
which hydrocarbons--_i.e._ compounds of carbon with hydrogen--largely
predominate. The different components of coal-tar can be separated by
processes which we shall have to consider subsequently. Of the compounds
thus isolated some few are immediately applicable for industrial purposes,
but the majority only form the raw materials for the manufacture of other
products, such as colouring-matters and medicines. Now these
colouring-matters and other finished products no more exist in the tar
than the latter exists in the coal. They are produced from the
hydrocarbons, &c., present in the tar _by chemical processes_, and bear
much about the same relationship to their parent substances that a
steam-engine bears to the iron ore out of which its metallic parts are
primarily constructed. Just as the mechanical skill of the engineer
enables him to construct an engine out of the raw material iron, which is
extracted from its ore, and converted into steel by chemical processes, so
the skill of the chemist enables him to build up complex
colouring-matters, &c., out of the raw materials furnished by tar, which
is obtained from coal by chemical decomposition.

The illuminating gas which is obtained from coal by destructive
distillation consists chiefly of hydrogen and gaseous hydrocarbons, the
most abundant of the latter being marsh gas. There are also present in
smaller quantities the two oxides of carbon, the monoxide and the dioxide,
which are gaseous at ordinary temperatures, together with other
impurities. Coal-gas is burnt just as it is delivered from the mains--it
is not at present utilized as a source of raw material in the sense that
the tar is thus made use of. In some cases gas is used as fuel, as in
gas-stoves and gas-engines, and in the so-called "gas-producers," in which
the coal, instead of being used as a direct source of heat, is partially
burnt in suitable furnaces, and the combustible gas thus arising,
consisting chiefly of carbon monoxide, is conveyed to the place where it
undergoes complete combustion, and is thus utilized as a source of heat.

Summing up the uses of coal thus far considered, w