After passing into the scrubbers the gas is cooled, passed into the
scrubbers, and by contact with wooden slat trays, made up like screens;
a large portion of the tar is removed from the gas, the tar passing off
to large receptacles.]
Soft coal was formed in much the same manner, but the process was not
so completely finished. Mixed with the carbon in soft coal we find
quite a good deal of other substances, of which hydrogen forms the
principal part. This is what makes soft coal valuable in the making of
illuminating gas.
When soft coal is heated in a closed receptacle a gas is formed which
will burn. To show this we have only to take an ordinary clay pipe,
put a little piece of coal in the bowl, close the top with wet clay,
and put the bowl part of the pipe in the fire. When it is quite hot, a
gas will be found coming out of the stem of the pipe, which will, when
lighted, burn.
The Story In a Gas Jet.
~HOW ILLUMINATING GAS IS MADE~
Soft coal is heated in large tubes of fire clay called retorts, and the
gas that is formed is then collected in a large tank and sent through
pipes to our homes after being purified. The part of the coal that is
left consists largely of carbon and is what we call coke.
While the gas that comes directly from coal will burn if lighted, it is
not a desirable gas to burn in our homes, because it contains a number
of substances that should be eliminated before it is used for lighting.
How the Gas Is Purified.
From the clay retorts the gas passes through horizontal pipes
containing water. This cools it and takes out of it most of the tar
and water vapor that are driven off with the gas when formed. These
substances settle in the water. The gas then goes through a series
of curved pipes, which are air cooled. These pipes constitute what
is known as an atmospheric condenser. From these the gas goes into
a series of receptacles containing wooden slat trays, made up like
screens. These receptacles are called the scrubbers, and they take out
of the gas the last traces of tar and some of the other compounds found
present. The removal of the sulphur is very important, for burning
sulphur gives off a gas which is not only extremely impure to breathe,
but also injurious to the health.
From the scrubbers the gas goes on through pipes to the purifiers--boxes
which contain wood shavings coated with iron rust upon which the sulphur
is deposited by chemical action. At the same time the lime absorbs a
small quantity of carbonic acid gas, which is formed with the other
gases. From the purifiers the gas passes into the great iron tanks, in
which it is stored until needed.
The gas in the tanks consists chiefly of hydrogen, a number of
compounds of hydrogen and carbon, and a small amount of a compound
of carbon and oxygen containing less oxygen than carbonic acid gas,
known as carbon monoxide. The hydrogen and carbon monoxide burn with
a very pale flame, which gives but little light and much heat. The
light-giving quality of the gas is found in the compounds of carbon and
hydrogen. When these burn, the particles of carbon are heated white hot
and glow very brightly, making a luminous flame.
There are, of course, some impurities in the purified gas. These are
compounds containing sulphur and ammonia. The quantities of these
substances, however, are so small that they are harmless; but the
compounds taken out in the process of purifying the gas are saved, as
considerable use is made of them. The water used for washing the gas is
heavily charged with ammonia and is, in fact, the chief source of the
ammonia sold by druggists.
[Illustration: HOW THE IMPURITIES ARE TAKEN FROM THE GAS