be enriched, that is,
illuminants must be added, since both carbon monoxide and hydrogen burn
with non-luminous flames. This is accomplished by passing it into
heaters containing highly heated petroleum oils. The gas takes up
hydrocarbon gases formed in the decomposition of the petroleum oils,
which make it burn with a luminous flame.

Water gas is very effective as a fuel, since both carbon monoxide and
hydrogen burn with very hot flames. It has little odor and is very
poisonous. Its use is therefore attended with some risk, since leaks in
pipes are very likely to escape notice.

~Natural gas.~ This substance, so abundant in many localities, varies much
in composition, but is composed principally of methane. When used for
lighting purposes it is usually burned in a burner resembling an open
Bunsen, the illumination being furnished by an incandescent mantle. This
is the case in the familiar Welsbach burner. Contrary to statements
frequently made, natural gas contains no free hydrogen.

TABLE SHOWING COMPOSITION OF GASES

=====================+================+========+========+==========
| PENNSYLVANIA | COAL | WATER | ENRICHED
| NATURAL | GAS | GAS | WATER
| GAS | | | GAS
---------------------+----------------+--------+--------+----------
Hydrogen | | 41.3 | 52.88 | 30.00
Methane | 90.64 | 43.6 | 2.16 | 24.00
Illuminants | | 3.9 | | 12.05
Carbon monoxide | | 6.4 | 36.80 | 29.00
Carbon dioxide | 0.30 | 2.0 | 3.47 | 0.30
Nitrogen | 9.06 | 1.2 | 4.69 | 2.50
Oxygen | | 0.3 | | 1.50
Hydrocarbon vapors | | 1.5 | | 1.50
=====================+================+========+========+==========

These are analyses of actual samples, and may be taken as about
the average for the various kinds of gases. Any one of these
may vary considerably. The nitrogen and oxygen in most cases is
due to a slight admixture of air which is difficult to exclude
entirely in the manufacture and handling of gases.

~Fuels.~ A variety of substances are used as fuels, the most important of
them being wood, coal, and the various gases mentioned above. Wood
consists mainly of compou