A typical analysis of the gas distilled from coal is very approximately 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 light it vitiates the air and exhausts the oxygen to a less degree than the others.  Of course, in these respects the gas-mantle is superior.

The reaction which takes place when water and calcium carbide are brought together is a double decomposition and is represented by,

     CaC_{2} + H_{2}O = C_{2}H_{2} + CaO

It will be seen that the products are acetylene gas and calcium oxide or lime.  The lime, being hydroscopic and being in the presence of water or water-vapor in the acetylene generator, really becomes calcium hydroxide Ca(OH)_{2}, commonly called slaked lime.  If there are impurities in the calcium carbide, it is sometimes necessary to purify the gas before it may be safely used for interior lighting.