Several methods of electrical ignition have been evolved which in general employ the electric spark.  Electrical ignition and developments of remote control have added great improvements especially to street-lighting by means of gas.  Gas-valves for remote control are actuated by gas pressure and by electromagnets.  In general, the gas-lighting engineers have kept pace marvelously with electric lighting, when their handicaps are considered.

Various types of burners have appeared which aimed to burn more gas in a given time under a mantle and thereby to increase the output of light.  These led to the development of the pressure system in which the pressure of gas was at first several times greater than usual.  The gas is fed into the mixing tube under this higher pressure in a manner which also draws in an adequate amount of air.  In this way the combustion at the burner is forced beyond the point reached with the usual pressure.  Ordinary gas pressure is equal to that of a few inches of water, but high-pressure systems employ pressures as great as sixty inches of water.  Under this high-pressure 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