pproximately
as follows,

Hydrocarbons 40 per cent.
Hydrogen 50 " "
Carbon monoxide 4 " "
Nitrogen 4 " "
Carbon dioxide 1 " "
Various other gases 1 " "

It is seen that illuminating gas is not a definite compound but a
mixture of a number of gases. The proportion of these is controlled in
so far as possible in order to obtain illuminating value and some of
them are reduced to very small percentages because they are valueless as
illuminants or even harmful. The constituents are seen to consist of
light-giving hydrocarbons, of gases which yield chiefly heat, and of
impurities. The chief hydrocarbons found in illuminating gas are,

ethylene C_{2}H_{4} crotonylene C_{4}H_{6}
propylene C_{3}H_{6} benzene C_{6}H_{6}
butylene C_{4}H_{8} toluene C_{7}H_{8}
amylene C_{5}H_{10} xylene C_{8}H_{10}
acetylene C_{2}H_{2} methane C H_{4}
allylene C_{3}H_{4} ethane C_{2}H_{6}

A gas which has played a prominent part in lighting is acetylene,
produced by the interaction of water and calcium carbide. No other gas
easily produced upon a commercial scale yields as much light, volume for
volume, as acetylene. It has the great advantage of being easily
prepared from raw material whose yield of gas is considerably greater
for a given amount than the raw materials which are used in making other
illuminating gases. The simplicity of the manufacture of acetylene from
calcium carbide and water gives to this gas a great advantage in some
cases. It has served for individual lighting in houses and in other
places where gas or electric service was unavailable. Where space is
limited it also had an advantage and was adopted to some extent on
automobiles, motor-boats, ships, lighthouses, and railway cars before
electric lighting was developed for these purposes.

The color of the acetylene flame is satisfactory and it is extremely
brilliant compared with most flames. An interesting experiment is found
in placing a spark-gap in the flame and sending a series of sparks
across it. If the conditions are proper the flame will became very much
brighter. When the gas issues from a proper jet under sufficient
pressure, the flame is quite steady. Its luminous efficiency gives it an
advantage over other open gas-flames in lighting rooms, because for the
same amount of lig