ons. The most important of these is methane, with smaller
quantities of many others, some of which are liquids or solids at
ordinary temperatures. The great bulk of the carbon remains behind as
coke and retort carbon.

~The manufacture of coal gas.~ In the manufacture of coal gas it is
necessary to separate from the volatile constituents formed by the
heating of the coal all those substances which are either solid or
liquid at ordinary temperature, since these would clog the gas pipes.
Certain gaseous constituents, such as hydrogen sulphide and ammonia,
must also be removed. The method used to accomplish this is shown in
Fig. 66. The coal is heated in air-tight retorts illustrated by A. The
volatile products escape through the pipe X and bubble into the tarry
liquid in the large pipe B, known as the _hydraulic main_, which runs
at right angles to the retorts. Here is deposited the greater portion of
the solid and liquid products, forming a tarry mass known as _coal tar_.
Much of the ammonia also remains dissolved in this liquid. The partially
purified gas then passes into the pipes C, which serve to cool it and
further remove the solid and liquid matter. The gas then passes into
D, which is filled with coke over which a jet of water is sprayed. The
water still further cools the gas and at the same time partially removes
such gaseous products as hydrogen sulphide and ammonia, which are
soluble in water. In E the gas passes over some material such as lime,
which removes the last portions of the sulphur compounds as well as much
of the carbon dioxide present. From E the gas passes into the large
gas holder F, from which it is distributed through pipes to the places
where it is burned.

[Illustration: Fig. 66]

One ton of good gas coal yields approximately 10,000 cu. ft. of
gas, 1400 lb. of coke, 120 lb. of tar, and 20 gal. of
ammoniacal liquor.

Not only is the ammonia obtained in the manufacture of the gas
of great importance, but the coal tar also serves as the source
of many very useful substances, as will be explained in Chapter
XXXII.

~Water gas.~ Water gas is essentially a mixture of carbon monoxide and
hydrogen. It is made by passing steam over very hot anthracite coal,
when the reaction shown in the following equation takes place:

C + H_{2}O = CO + 2H.

When required merely to produce heat the gas is at once ready for use.
When made for illuminating purposes it must