Chapter XXV: 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.

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.

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.

_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.