ILLUMINATING GAS.
It has been stated more than once that the compounds of carbon are
very numerous; it might properly be added that their usefulness is
no less striking than their number. This portion of the subject is
so vast, however, that it is often discussed as an entirely separate
branch, called the chemistry of the carbon compounds, and often organic
chemistry. By organic chemistry is meant the chemistry of organic
substances; and by organic substances is meant materials derived from
those existences that possess organs. Now animals and plants, and
they alone, possess organs; whence organic chemistry is described as
the chemistry of animal and vegetable bodies. On some accounts, it is
better defined as the chemistry of the carbon compounds; for while it
was formerly thought that animal and vegetable beings involved in their
processes a chemistry peculiar to themselves, this notion has long
since been dispelled, and it is now clearly perceived that animal and
vegetable compounds are governed by the same chemical laws as others.
But not only is the number of these organic compounds very great;
the variety and importance of animal and vegetable matters give them
a high degree of interest. Thus they include animal and vegetable
juices, extracts, gums, resins, essences, remedial agents, bitter
principles, acids, oils, coloring matters; and of the members of each
one of these classes the name is legion. Moreover, the more any portion
of the subject is studied, the more it seems to reveal a continually
increasing complexity.
Most of the substances of the classes referred to are either compounds
of carbon and hydrogen, or of carbon, hydrogen and oxygen in varied
proportions, or they are compounds containing these elements and yet a
few others combined with them.
It is manifest, from what has been said, that in a book like the
present it is impossible to give any considerable discussion of the
vast field offered by the organic compounds of carbon. It seems better
to choose for description some important manufacturing operation that
involves these compounds and that is on other accounts specially
instructive. Accordingly the manufacture of illuminating gas is
selected for consideration here.
The Manufacture of Illuminating Gas.
The material on which this industry is based is bituminous coal.
This substance is clearly a vegetable product, though it is derived
from a vegetation which lived, flourished and decayed in a period of
prehistoric antiquity.
The manufacture of illuminating gas, although one of the most important
of the chemical industries of to-day, had its beginning but little
before the opening of the present century. A Scotchman named William
Murdoch is generally credited with the first introduction, into
considerable use, of burning gas made from coal. In 1798 he gained
the opportunity to introduce his method of illumination into the
engine works of Boulton & Watt, located at Soho, near Birmingham. From
that date the manufacture and use of illuminating gas from soft coal
has extended and expanded until it has reached its present enormous
development.
General Principles of the Process.
The general principle of the manufacture is exceedingly simple. But its
commercial growth has been assisted by the invention and application of
a multitude of delicate and ingenious appliances.
If any person will take a glass test-tube, place in it a few fragments
of starch, and will then heat the starch strongly over a lamp flame,
he will readily detect three important effects. The first is that a
mass of smoky gas or vapor pours out of the mouth of the test-tube. The
second is that an oily or tarry liquid condenses, on the inside of the
tube, and runs down in streams. The third is that at the close of the
operation a mass of carbon remains in the bottom of the tube where the
starch was. Now the various substances, that have been referred to as
produced by the heating process, are referable to the decomposition of
the molecules of starch.
A Similar Operation on a Large Scale.
In the manufacture of illuminating gas on a large scale there are
developed practically the same series of phenomena as those noted in
the experiment with starch just referred to.
[Illustration: FIG. 92.—Three views of a gas retort.]
In the manufacture of illuminating gas, instead of starch as just
described, soft coal or bituminous coal is used.
In place of a lamp, a large row of furnaces is employed to supply the
heat.
Instead of glass tubes, those of earthenware, ten or twelve feet in
length and between one and two feet in diameter, are used. These tubes,
called retorts, are placed in a horizontal position and so that the
flame of the fire in the furnace may sweep around them and raise them
to a cherry-red heat. At the front end of the retort is attached a
door to prevent the escape of the gases generated, and there is also a
suitable pipe to carry these gases forward to those other portions of
the works which serve to perform upon the crude gas certain necessary
purifications; these are: _First._ The condensation of condensable
vapors. _Second._ The removal of objectionable gases.
[Illustration: FIG. 93.—Section showing five retorts in place.]
The operations spoken of show that the gas must be carried from one
portion of the establishment to another. Now illuminating gas is made
up of material substances and although lighter than air yet they
distinctly possess weight. Gas will not move of itself; to carry it
from place to place the application of force by means of mechanical
appliances is requisite. In fact it is discovered that what is called
an _exhauster_ is necessary for use in gas-works. The exhauster is
simply a kind of rotary pump which pulls the gas from the retorts in
which it is first formed, and pushes it along through the various
purifiers, to the gas holder in which it is stored. If the exhauster
were not used, there would be a constant tendency to the creation of
pressure in the retort, by virtue of which the gas would penetrate the
earthenware into the fire, and so become a source of loss.
From what has been said it will be easily comprehended that the
essential parts of a gas-works are the following:
_First._ The furnace.
_Second._ The retorts, in which the coal is heated.
_Third._ The hydraulic main: a trough of water in which the gas is
cooled, and which also serves as a gate, through which the gas can pass
forward toward the purifiers but not backward toward the retort.
_Fourth._ The out-door condensers, in which the gas is cooled and some
of its vapors condense to tarry liquids.
_Fifth._ The scrubber, in which the gas is cleansed by a spray of water.
[Illustration: FIG. 91.—View in gas-works; drawing coke from the
retorts.]
[Illustration: FIG. 94.—Section to show the processes of the
manufacture of illuminating gas.]
_Sixth._ The purifiers, where sulphuretted hydrogen, and some other
objectionable gases are removed.
_Seventh._ The gas holder, in which the finished gas is collected and
stored prior to delivery to consumers.
The processes by which these various appliances are used in the
manufacture of illuminating gas may be briefly sketched as follows:
A suitable quantity of soft coal is placed in an even layer on the
bottom of the _retort_. Gas at once forms and streams out of the open
door. The door of the retort being now quickly closed by the workmen,
the gas passes out through an exit pipe—called the dip-pipe because it
dips into the water of the _hydraulic main_. The gas bubbles up from
the dip-pipe through the water. Once delivered in the hydraulic main,
the gas cannot go back to the retort.
Next, the gas passes through the _condensers_, a series of connected
up-and-down pipes. As these condensers stand in the open air they cool
the gas so that it deposits tarry liquids that, until this stage, have
been suspended in it in the form of vapor.
Next the gas flows to a large iron box, called the scrubber. In
different works the _scrubber_ varies considerably in outward shape
and internal arrangements. Its essential office however is to wash the
gas, and it does so by the use of water which is applied to the gas
either in sprays or thin films. Ammonia gas is the principal substance
absorbed by the water in the scrubber. Indeed the liquor thus produced
is the main commercial source, at the present day, of ammonia and its
compounds.
The gas next goes to the _purifiers_. These are large iron boxes
supplied with a multitude of shelves upon which, in most works, dry
quicklime is spread. The quicklime absorbs sulphuretted hydrogen and
some other acid gases. From these purifiers the gas is carried on to
the _gas holder_.
The Distillation of Coal, Chemically Considered.
Under the influence of the high temperature of the gas furnace, the
soft coal in the retorts undergoes decomposition. As has before been
intimated, three distinct classes of substances are produced: Solids,
which are left in the retorts; liquids, which are condensed in the
various coolers; gases, which pass on the gas holder.
_First._ _The solids._ These are principally two kinds of carbon. One
is coke,—the principal solid matter found in the retorts as a residue
from the soft coal after the latter has ceased to evolve gas. It is
merely a form of carbon, somewhat spongy in its structure. It is sold
for use as fuel. Beside this the retorts accumulate a sort of scale of
a very different form of carbon called gas carbon. It is extremely hard
and almost non-combustible, being even very difficult to remove from
the retorts. It is at present somewhat used in the manufacture of the
carbon pencils employed in electric lights of the arc variety. Prior to
this use it found scarcely any commercial outlet.
_Second._ _The liquids._ The first condensation of liquids takes
place in the hydraulic main where tarry and oily matters condense and
accumulate, and are drawn off from time to time into the tar well.
Again in the condensers there is a still further deposition of liquids,
also tarry and oily in their nature.
These liquids consist of very complicated mixtures of carbon compounds,
but they are of the most interesting character. In the earlier stages
of the manufacture of coal-gas they were regarded as mere nuisances.
Little by little however chemists have learned to separate the
intermingled products, and have thus been able to obtain a number of
substances of striking interest and usefulness in the arts. Among the
multitudes of substances that go to make up the liquid called coal-tar,
some are as yet hardly classified others are distinctly recognized and
have uses of great commercial importance. Of these latter, two will be
mentioned here. These are anthracene and benzole.
The substance called _anthracene_, a compound of carbon and hydrogen
(C₁₄H₁₀), has within the last ten years sprung into the highest
commercial importance. This is referable to the fact that it has been
found to be a suitable material from which, by chemical processes,
there may be manufactured a substance known as alizarine, besides other
equally valuable and interesting compounds. Alizarine was previously
recognized as the coloring matter of chief value in madder root, a
substance that has been used as a dye-stuff for above a thousand years.
The alizarine, whether of madder or from anthracene, is a coloring
matter of the highest value and usefulness. It affords turkey-red
and other colors that are very important because they are extremely
brilliant and extremely fast. Its _artificial_ manufacture, from the
anthracene of the filthy and offensive coal-tar, is one of the greatest
triumphs of this or any age.
Another substance found in the coal-tar is _benzole_, a compound of
carbon and hydrogen having the formula C₆H₆. This is the principal
material from which, by a variety of well understood though complicated
chemical processes, the well-known aniline colors have been produced.
While these colors may well command the admiration of all, on account
of their unsurpassed beauty and brilliancy, they are of especial
interest to the scientist by reason of the chemical laws they
illustrate. The preparation of these colors, as a group, ranks second
only as a chemical achievement to that of artificial alizarine.
_Third._ _The Gaseous Products._ The gases generated in the process
of the coal-gas manufacture are extremely numerous; some of them are
of high illuminating power, of which that called ethylene (C₂H₄) is
an excellent example. Again there are some that are combustible, but
yet are of slight illuminating power. Substances of this class are
present in the finished product. Hydrogen and carbon monoxide (CO) may
serve as examples. There are always present also gases that are either
injurious to the illuminating power or are otherwise objectionable.
For example, nitrogen is always present, and it is not practicable to
remove it from the gas. It contributes nothing to the value of the
product. Again certain sulphur compounds, like sulphuretted hydrogen,
are usually present. These indeed burn, but they give rise to offensive
and unwholesome oxides of sulphur.
The sketch thus given, while it but imperfectly describes the wonderful
industry in question, with its various well contrived and delicate
appliances, serves however to give some idea of the importance of
the operation, from a chemical point of view, and the mine of rich
materials its carbon compounds offer to chemical students.
CLOSING CHAPTER.