The illuminating power of house-gas depends on the presence therein of olefiant gas (ethylene), or, as it is sometimes termed, heavy carburetted hydrogen.  This is the first of the series of hydro-carbons known as the olefines, and is composed of two atoms of carbon to every four atoms of hydrogen (C_{2}H_{4}).  Others of the olefines are present in minute quantities.  These assist in increasing the illuminosity, which is sometimes greatly enhanced, too, by the presence of a small quantity of benzene vapour.  These illuminants, however, constitute but about 6 per cent. of the whole.

Added to these, there are four other usual constituents which in no way increase the value of gas, but which rather detract from it.  They are consequently as far as possible removed as impurities in the process of gas-making.  These are nitrogen, carbonic acid gas, and the destructive sulphur compounds, sulphuretted hydrogen and carbon bisulphide vapour.  It is to the last two to which are to be attributed the injurious effects which the burning of gas has upon pictures, books, and also the tarnishing which metal fittings suffer where gas is burnt, since they give rise to the formation of oil of vitriol (sulphuric acid), which is being incessantly poured into the air.  Of course the amount so given off is little as compared with that which escapes from a coal fire, but, fortunately for the inmates of the room, in this case the greater quantity goes up the chimney; this, however, is but a method of postponing the evil day, until the atmosphere becomes so laden with impurities that what proceeds at first up the chimney will finally again make its way back through the doors and windows.  A recent official report tells us that, in the town, of St Helen’s alone, sufficient sulphur escapes annually into the atmosphere to finally produce 110,580 tons of sulphuric acid, and a computation has been made that every square mile of land in London is deluged annually with 180 tons of the same vegetation-denuding acid.  It is a matter for wonder that any green thing continues to exist in such places at all.

The chief constituents of coal-gas are, therefore, briefly as follows:—­

/ (1)  Hydrogen,
| (2)  Marsh-gas (carburetted hydrogen or fire-damp),
| (3)  Carbon monoxide,
| (4)  Olefiant gas (ethylene, or heavy carburetted hydrogen), with
\      other olefines,
/ (5)  Nitrogen,
| (6)  Carbonic acid gas,
| (7)  Sulphuretted hydrogen,
\ (8) Carbon bisulphide (vapour),

the last four being regarded as impurities, which are removed as far as possible in the manufacture.