Bituminous coal, of which there are several varieties, is the best suited for the production of coal gas. The Newcastle coal is principally used in the manufacture of London gas. Scotch parrot coal produces a superior gas, but the coke produced is of inferior quality. Boghead coal is also used for gas making—in fact, every kind of coal, except anthracite, may be used for this purpose. The bituminous shale produces a very good gas, and it is used partly to supply the place of cannel or parrot coal. As carbon and hydrogen, principally with oxygen, are the elements from which gas is formed, most substances containing these elements can be partially converted into gas. And gas has been made from grease or kitchen waste, oil peat, rosin, and wood, besides coal. A ton of Newcastle or caking coal yields about 9,000 cubic feet of gas, Scotch coal about 11,000, English cannel about 10,000, and shale about 7,000, with illuminating powers in the ratio of about 13, 25, 22, and 36 respectively. The coal is put in retorts, r, commonly made of fire clay and often of cast iron. These retorts are from 6 feet to 9 feet long, and from 1 foot to 1 foot 8 inches in breadth. They are made like the letter D, elliptical, cylindrical, or bean shaped. They are built into an arched oven, and heated by furnaces, f, beneath. One, three, five, seven, or more are built in the same oven. The mouthpieces are of cast iron, and project outward from the oven, so as to allow ascension pipes, a p, to be fixed, to convey the gas generated from the coal to the hydraulic main, h m. After the coal has been introduced into the retorts, their mouths are closed with lids luted round the edges with clay, and kept tight by a screw. The retorts are kept at a bright red heat. If the temperature be too low, less gas and more tar are produced, less residue being left; while, should the temperature be too high, the product is more volatile, more residue remaining. And should the gas remain for any length of time in contact with the highly heated retort, it is partially decomposed, carbon being deposited, thereby lessening the illuminating power, and choking up the retort, and more carbon disulphide is produced at a high temperature. The object is to maintain a medium temperature, in order to obtain a better gas having the greatest illuminating power. In about four or five hours the coal in the retort will have given off all its gas. The mouth of the retort is opened, and the coke is raked out into large iron vessels, and extinguished by water. A fresh charge is immediately introduced by means of a long scoop in the cherry-red retort, and the door luted to. The ascension pipes, which convey the gas from the retorts, pass straight up for a few feet, then turn round, forming an arch, then pass downward into the hydraulic main, beneath the level of the liquid contained in it, and bubble up through the liquid into the upper portion of the main. On commencing the main is half filled with water, but after working some time, this water is displaced by the fluid products of distillation. In this way, the opening into each retort is closed, so that a charge can be withdrawn and replaced without interfering with the action of the other retorts and pipes. The liquid tar, ammoniacal water, and gas pass from the end, e, of the hydraulic main, down through the pipe, P, and the liquid falls down into the tar well, T W, while the crude gas goes on into the chest, C, partially filled with the liquid, so that the plates, p p, from the top dip into it to within a few inches of the bottom. These dip plates are placed in the chest, so as to separate the openings into each pair of condensing pipes, c c, so that the gas passing into the chest finds no exit except up c₁, and down c₂; and there being no dip plate between c₂ and c₃ it passes up c₃, and down c₄, and as there is no dip plate to prevent its progress, it passes up c₅, and down c₆, into the lime or iron purifiers, L I. The condensers are kept cool by exposure to the atmosphere, and are often cooled by a stream of water from a tank above. The gas cools quickly, and liquids passing along with the gas in a state of vapor are condensed and fall into the chest, and pass by an overflow pipe into the tar well. The purifier is a cast iron vessel, L I, containing a number of perforated shelves, s₁ s₁ s₁, on which slaked lime, to the depth of about 4 inches, or much greater thickness of iron oxide and sawdust, is placed. The gas passes up through the shelves, s s s, and down through the shelves, s₁ s₁ s₁, through the pipe, G, into the gas holder, and from thence through the pipe, M, to the main pipe. The lime abstracts carbonic anhydride, sulphureted hydrogen, cyanogen, naphthalin, and a portion of the ammonia, but not carbon disulphide, which latter may be absorbed by passing the gas through a solution of sodic hydrate and plumbic oxide, mixed with sawdust. Gas containing CS₂, on burning, produces H₂SO₄, which injures books and furniture in rooms. However, the quantity of CS₂ in gas is generally so minute as to be practically uninjurious. By a proper regulation of the temperature during distillation, the quantity produced is infinitesimal. When the lime is saturated it is removed, and fresh supplied; but the iron, after use, can be reconverted into oxide by exposure to the atmosphere, and used repeatedly. When iron is used a separate lime purifier is necessary to remove carbonic anhydride. The last traces of ammonia are removed before passing to the gas holder, by passing the gas through dilute sulphuric acid, or up through the interior of a tower having perforated shelves covered with coke in small pieces, through which a constant supply of fresh water percolates. This washing removes some of the more condensable hydrocarbons, and lessens the illuminating power of the gas. Before the gas passes from the condensers into the purifiers, it passes through a kind of pump, termed an exhauster, driven by steam power. This action relieves the retorts from the pressure of the gas passing through the hydraulic main, etc. It diminishes the deposit of graphite in the retorts, and lessens leakage in them, should there be any flaws. It also has the beneficial effect of producing a gas of a higher illuminating power, since the relief of pressure in the retorts produces a more favorable condition of combustion.

The following are some of the bodies produced in the manufacture of gas, namely, acetylene, g, the carbonate, s, chloride, s, cyanide, s, sulphide, s, and sulphate, s, of ammonium; aniline, t, anthracene, s, benzine, l, carbonic oxide, g, carbonic anhydride, g, carbonic disulphide, l, chrysene, s, cumene, l, cymene, l, ethylene, g, hydrogen, g, leucoline, l, methyl-hydride, g, naphthaline, s, nitrogen, g, paraffine, s, phenylic alcohol, l, picoline, l, propene, g, quartene, g, sulphureted hydrogen, g, toluene, l, water, l, xylene, l, etc.

The most of the above solid and liquid substances, with the letters s and l written after, are removed by cooling the gas in the condensers, and the gaseous substances marked g, that are injurious in the consumption of the gas, are removed by purification. The impurities in the gas may consist of ammonic carbonate and sulphide, carbonic anhydride and disulphide, nitrogen, oxygen, sulphureted hydrogen, and water in the form of vapor; and acetylene, ethylene, and the vapors of the acetylene, ethylene, and phenylene series of hydrocarbons are the illuminating ingredients diluted with carbonic oxide, hydrogen, and methyl-hydride. The approximate percentage composition of coal gas is: H, 45.6; Me, 34.8; CO, 6.5; C₂H₄, 4; CO₂, 3.6; N, 2.4; C₄H₈, 2.3; SH₂, 0.3, etc.—Hugh Clements in English Mechanic.