ities. 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.

In the process of distillation of the coal, we have seen that various
other important substances are brought into existence. The final residue
of coke, which is impregnated with the sulphur which has not been
volatilised in the form of sulphuro