Gas, for lighting, is made from coal, by subjecting it to heat in
iron tubes and collecting and purifying the gas which is produced;
the following is a summary of the process:--The vessels in which the
coal is put are called “retorts,” they are six or seven feet long,
one-and-a-half broad, and one high, generally fitted up six to a
furnace, in such a way that the flame may entirely surround them (see
fig. 1); from each retort an iron tube passes, and these all dip into
a large horizontal tube, reaching nearly to its bottom (fig. 2), this
is the receiver for all the gas from the retorts; it is connected
with a pit for tar and ammonical liquor, which copiously condenses
from the hot newly-formed gas.
The object of causing the tubes to dip nearly to the bottom of the
receiver is, that the fluid in it may close the end of each tube, and
so prevent the gas returning when any of the retorts are opened for a
fresh supply of coal; from the receiver, the gas passes by tubes bent
up and down inside of a great cistern of water (fig. 3) kept cold by
a constant change of its contents; this is the condenser, it causes
the separation of all the tar and ammonia that remain; but the gas is
still contaminated with sulphur, and this is got rid of by causing it
to pass into a broad iron cylinder perforated with holes and dipping
into a reservoir of lime and water mixed together and stirred about
with a machine contrived for the purpose; the gas enters this lime
mixture in hundreds of bubbles through the holes, and the sulphur
the gas contains is attracted by the lime with which it unites.
The gas now rises to the top of this reservoir, which is, of course,
air tight, and from thence passes for use to the gasometer; this is
the store-house where the gas is kept to supply the service pipes
_b_, _c_, under the streets. The gasometer is made of plates of iron
rivetted together. The lower part of this cylinder (see figs. 4 and
5) is received into a circular deep channel cut in the ground (_b b_,
fig. 5) and filled with water, so that the gasometer _d_, which is
suspended and balanced by means of the chains and weights _e e_, may
be raised or depressed at pleasure.
When the gasometer is quite down, the inside of its top, _a_, rests
on the ground; when the gas enters, it raises it up till the lower
edge is but a few inches below the water, and is prevented from
rising higher by frames which support it; when some of the weights,
_d d_, are removed, the huge cylinder, _a_, slowly sinks down and
forces the gas through the tubes, _c c_, to be burnt by thousands of
consumers. About four cubic feet of gas is produced from every pound
of coal used; the gas is lighter than air in the proportion of 650
to 1000, and it is owing to this lightness that balloons ascend when
filled with it, not from any peculiar power it has of ascending, but
because the air, being heavier, presses downwards and forces it up.
When gas is mixed with air it becomes very explosive, and when any
escape is suspected in a room, the windows of the room should be
opened _at the top_, and the door also opened, before any light is
introduced; a few minutes will then suffice to ensure safety.