ureted
hydrogen at least must be removed. One method of accomplishing this is
by washing the gas with water and ammonia, which also removes some of
the carbon dioxide and hydrocyanic acid. Various other undesirable
constituents are removed by chemical means, depending upon the
conditions. The purified gas is now delivered to the gas-holder; but, of
course, all this time the pressure is governed, in order that the
pressure in the mains will be maintained constant.

Much attention has been given to the enrichment of gas for illuminating
purposes; that is, to produce a gas of high illuminating value from
cheap fuel or by inexpensive processes. This has been done by
decomposing the tar obtained during the distillation of coal and adding
these gases to the coal-gas; by mixing carbureted water-gas with
coal-gas; by carbureting inferior coal-gases; and by mixing oil-gas with
inferior coal-gas.

Water-gas is of low illuminating value, but after it is carbureted it
burns with a brilliant flame. The water-gas is made by raising the
temperature of the fuel bed of hard coal or coke by forced air, which is
then cut off, while steam is passed through the incandescent fuel. This
yields hydrogen and carbon monoxide. To make carbureted water-gas,
oil-gas is mixed with it, the latter being made by heating oil in
retorts.

A great many kinds of gas are made which are determined by the
requirements and the raw materials available. The amount of illuminating
gas yielded by a ton of fuel, of course, varies with the method of
manufacture, with the raw material, and with the use to which the fuel
is to be put. The production of coal-gas per ton of coal is of the order
of magnitude of 10,000 cubic feet. A typical yield by weight of a
coal-gas retort is,

10,000 cubic feet of gas 17 per cent.
coke 70 " "
tar 5 " "
ammoniacal liquid 8 " "

The coke is not pure carbon but contains the non-volatile minerals which
will remain as ash when the coke is burned, just as if the original coal
had been burned. On the crown of the retort used in coal-gas production,
pure carbon is deposited. This is used for electric-arc carbons and for
other purposes. From the tar many products are derived such as aniline
dyes, benzene, carbolic acid, picric acid, napthalene, pitch,
anthracene, and saccharin.

A typical analysis of the gas distilled from coal is very a