scale. It is not the province of the present volume
to deal with the details of manufacture, but a short description of the
process is necessary for the proper understanding of the subsequent
portions of the subject (see Fig. 3). The coal is heated in clay
cylinders, called retorts, provided with upright exit pipes through which
the volatile products escape, and are conducted into water contained in a
horizontal pipe termed the "hydraulic main." In the latter the gas is
partially cooled, and deposits most of the tar and watery liquor which
distil over at the high temperature to which the retorts are heated. The
tar and watery liquor are allowed to flow from the hydraulic main into a
pit called the "tar well," and the gas then passes through a series of
curved pipes exposed to the air, in which it is further cooled, and
deposits more of the tar. From this "atmospheric condenser" the gas passes
into a series of vessels filled with coke, down which a fine spray of
water is constantly being blown. These vessels, known as "scrubbers,"
serve to remove the last traces of tar, and some of the volatile sulphur
compounds which are formed from the small quantity of sulphur present in
most coals. The removal of sulphur compounds is a matter of importance,
because when gas is burnt these compounds give rise to acid vapours, which
are deleterious to health and destructive to property.

[Illustration: FIG. 3.--Sectional diagram of gas plant. The retorts and
furnace are on the right; the gas rises through the upright pipe T into
the hydraulic main B; from there it passes into the atmospheric condensers
D, from the lower cistern of which the condensed tar flows into the
tar-well, H. Passing up through K, the gas is conducted into the scrubber,
O, and from there into the purifier, M. From there it emerges through K'
into the purifier, M, and then into the gas-holder for distribution. (From
Schultz's _Chemie des Steinkohlentheers_.)]

From the scrubbers the gas is sent through another series of vessels
packed with trays of lime or oxide of iron, in order to remove
sulphuretted hydrogen and other sulphur compounds as completely as
possible. A small quantity of carbon dioxide is also removed by these
"purifiers," as the presence of this gas impairs the illuminating power of
coal-gas. From the purifiers the gas passes into the gas-holders, where it
is stored for distribution. It remains only to be stated that the
distillation of coal is