re system, mantle-burners yield as high as
500 lumens per cubic foot of gas per hour.

The fuels for gas-lighting are natural gas, carbureted water-gas, and
coal-gas obtained by distilling coal, but there are different methods of
producing the artificial gases. Coal-gas is produced analytically by
distilling certain kinds of coal, but water-gas and producer-gas are
made synthetically by the action of several constituents upon one
another. Carbureted water-gas is made from fixed carbon, steam, and oil
and also from steam and oil. Producer-gas is made by the action of steam
or air or both upon fixed carbon. Water-gas made from steam and oil is
usually limited to those places where the raw materials are readily
available. The composition of a gas determines its heating and
illuminating values, and constituents favorable to one are not
necessarily favorable to the other. Coal-gas usually is of lower
illuminating value than carbureted water-gas. It contains more hydrogen,
for example, than water-gas and it is well known that hydrogen gives
little light on burning.

It has been seen in a previous chapter that the distillation of gas from
coal for illuminating purposes began in the latter part of the
eighteenth century. From this beginning the manufacture of coal-gas has
been developed to a great and complex industry. The method is
essentially destructive distillation. The coal is placed in a retort and
when it reaches a temperature of about 700 deg.F. through heating by an
outside fire, the coal begins to fuse and hydrocarbon vapors begin to
emanate. These are generally paraffins and olefins. As the temperature
increases, these hydrocarbons begin to be affected. The chemical
combinations which have long existed are broken up and there are
rearrangements of the atoms of carbon and hydrogen. The actual chemical
reactions become very complex and are somewhat shrouded in uncertainty.
In this last stage the illuminating and heating values of the gas are
determined. Usually about four hours are allowed for the complete
distillation of the gaseous and liquid products from a charge of coal.
Many interesting chemical problems arise in this process and the
influences of temperature and time cannot be discussed within the scope
of this book. Besides the coal-gas, various by-products are obtained
depending upon the raw materials, upon the procedure, and upon the
market.

After the coal-gas is produced it must be purified and the sulph