he _tar-pit_ whilst
the gas proceeds, still further lightened of its impurities. It may be
mentioned that the temperature of the gas in the condenser is reduced to
about 60 deg. F., but below this some of the most valuable of the illuminants
of coal-gas would commence to be deposited in liquid form, and care has
to be taken to prevent a greater lowering of temperature. A mechanical
contrivance known as the _exhauster_ is next used, by which the gas is,
amongst other things, helped forward in its onward movement through the
apparatus. The gas then passes to the _washers_ or _scrubbers_, a series
of tall towers, from which water is allowed to fall as a fine spray, and
by means of which large quantities of ammonia, sulphuretted hydrogen,
carbonic acid and oxide, and cyanogen compounds, are removed. In the
scrubber the water used in keeping the coke, with which it is filled,
damp, absorbs these compounds, and the union of the ammonia with certain
of them takes place, resulting in the formation of carbonate of ammonia
(smelling salts), sulphide and sulphocyanide of ammonia.

[Illustration: FIG. 35.--Filling Retorts by Machinery.]

[Illustration: FIG. 36.--CONDENSERS.]

Hitherto the purification of the gas has been brought about by mechanical
means, but the gas now enters the "_purifier_," in which it undergoes a
further cleansing, but this time by chemical means.

[Illustration: FIG. 37.]

The agent used is either lime or hydrated oxide of iron, and by their
means the gas is robbed of its carbonic acid and the greater part of its
sulphur compounds. The process is then considered complete, and the gas
passes on into the water chamber over which the gas-holder is reared, and
in which it rises through the water, forcing the huge cylinder upward
according to the pressure it exerts.

The gas-holder is poised between a number of upright pillars by a series
of chains and pulleys, which allow of its easy ascent or descent
according as the supply is greater or less than that drawn from it by the
gas mains.

[Illustration: FIG. 38.]

When we see the process which is necessary in order to obtain pure gas,
we begin to appreciate to what an extent the atmosphere is fouled when
many of the products of distillation, which, as far as the production of
gas is concerned, may be called impurities, are allowed to escape free
without let or hindrance. In these days of strict sanitary inspection it
seems strange that the air in the neigh