de at the bottom. Under each is a furnace, the flames of which play on
the bottom, sides, and inner end of the retort. The outer end projecting
beyond the brickwork seating has an iron air-tight door for filling the
retort through, immediately behind which rises an iron exit pipe, A, for
the gases. Tar, which vaporizes at high temperatures, but liquefies at
ordinary atmospheric heat, must first be got rid of. This is effected by
passing the gas through the _hydraulic main_, a tubular vessel half full
of water running the whole length of the retorts. The end of pipe A
dips below the surface of the water, which condenses most of the tar and
steam. The partly-purified gas now passes through pipe B to the
_condensers_, a series of inverted U-pipes standing on an iron chest
with vertical cross divisions between the mouths of each U. These
divisions dip into water, so that the gas has to pass up one leg of a U,
down the other, up the first leg of the second pipe, and so on, till all
traces of the tar and other liquid constituents have condensed on the
inside of the pipe, from which they drop into the tank below.

The next stage is the passage of the _scrubber_, filled with coke over
which water perpetually flows. The ammonia gas is here absorbed. There
still remain the sulphuretted hydrogen and the carbon bisulphide, both
of which are extremely offensive to the nostrils. Slaked lime, laid on
trays in an air-tight compartment called the _lime purifier_, absorbs
most of the sulphurous elements of these; and the coal gas is then fit
for use. On leaving the purifiers it flows into the _gasometer_, or
gasholder, the huge cake-like form of which is a very familiar object in
the environs of towns. The gasometer is a cylindrical box with a domed
top, but no bottom, built of riveted steel plates. It stands in a
circular tank of water, so that it may rise and fall without any escape
of gas. The levity of the gas, in conjunction with weights attached to
the ends of chains working over pulleys on the framework surrounding the
holder, suffices to raise the holder.

[Illustration: FIG. 196.--The largest gasholder in the world: South
Metropolitan Gas Co., Greenwich Gas Works. Capacity, 12,158,600 cubic
feet.]

Some gasometers have an enormous capacity. The record is at present
held by that built for the South Metropolitan Gas Co., London, by
Messrs. Clayton & Son of Leeds. This monster (of which we append an
illustration, Fig. 196) is 300