Four kinds of illuminating gas are now in practical use, the first being that obtained from the distillation of coal, the second from the retorting of oil, the third from the chemical action of water upon carbide of calcium, and the fourth from the evaporation of light volatile hydrocarbon liquids. Each of these gases has its peculiar advantages and disadvantages, which, with the processes of manufacture, will now be briefly described.

A suitable coal for carbonization can be obtained in small quantities of 10 tons, costing 14s. per ton delivered in London. A ton of this material can be made to produce from 10,000 to 10,500 cubic feet of gas in a small gas-works of capacity sufficient for 50 lights; the cost of this gas, even on such a small scale of working, should not exceed two shillings per 1000 cubic feet.

The buildings and plant for the production of coal-gas consist of a retort-house, coal-store, condensing-apparatus, tar-tank, purifiers, meter, and gasholder.

By J. Murray Somerville, Chief Engineer's Staff.South Metropolitan Gas Company

The retort-house and coal-store should be weather-proof, and well-ventilated, but free from draughts. There is invariably a ventilating roof to retort-houses, as shown in Plate XXIV. The apparatus contained in the building consists of the retort-benches, hydraulic main, the coal, stoking and charging implements, pressure-gauges, valve-keys, and, generally, all tools or apparatus which would deteriorate or perish on exposure to the weather. The retort-bench contains two cast-iron retorts, 7 feet long, of a section about 16 inches wide and 12 inches high, and cast with a closed or "blank" end at one extremity, and an open or "flanged" end at the other. A furnace or fire-grate is placed under each retort, so that they are independent of one another as regards the making of gas. The exit-flue from each furnace runs into a chimney common to both. Retorts and furnaces are contained in a square body of brickwork of fire-clay material, held together by iron straps or buckstaves to prevent cracks, which would otherwise be caused by the expansion and contraction of the brickwork. An arch is turned over the retorts to relieve them of any pressure from the structure, and also to allow the hot gases from the furnace to have full play around the retort.

It is very seldom that a gas-retort is fused, but the continual exposure to atmospheric influences causes an oxidation or " perishing" of the iron. With a badly-constructed furnace, this deteriorating action is rapid; with one having a considerable depth of fuel, the period of life is greatly increased. The minimum life of a retort having a thickness of metal of 2 inches, is about six months; the maximum, with gaseous and careful firing, may exceed eighteen months.

On to the outside flange of the open and front end of the retort, - which is buried about 2 inches into the front wall, - a casting, called the "mouthpiece", attached. This mouthpiece is of a similar section to the retort, and is about 9 inches long. On the upper side is cast a socket, whose inside diameter is an inch to an inch-and-a-half greater than the upright ascension-pipe which it receives. A gas-tight joint is made between the socket and pipe by a "rust-joint" composed of sal-ammoniac and iron filings. At the extremity of the mouthpiece, there is a hinged or slotted-bar arrangement for carrying the door of the retort, which by a mechanical contrivance is pressed upon the smooth surface of the mouthpiece, or upon a "buttering" of moist lime and sand, for the purpose of "sealing" the retort after it has been charged.

The gas given off by the destructive distillation of the coal comes forward to the mouthpiece,and passes upwards and through the "ascension-pipe", by which it is conveyed to the "hydraulic main" placed on the top of the retort-bench. The hydraulic main is generally supported by small brick piers, or cast-iron brackets, at the height of three or four feet above the level of the bench, in order to allow a free circulation of air to protect it from the reflected heat of the retort-bench.

Hydraulic mains are generally of a half-round section, having about 6 inches depth of water at the bottom. The "ascension-pipe" is continued or carried over, either by a half-round pipe, or by a short horizontal length of circular pipe, to

Fig. 600  Longitudinal section of Retort

Fig. 600 -Longitudinal section of Retort.

A. retort. B mouthpiece; c, ascension pipe fitting Into socket of mouthpiece the hydraulic main, and is connected to it in such a manner that the end of the former is covered by one or two inches of the water in the latter. The gas consequently has an upward and downward motion before reaching the hydraulic main. This part of the apparatus is provided with a cover-plate and a gas-outlet, and can be arranged to receive the gas from several retorts. The water covering the ends of the pipes entering the trough, technically known as "dip-pipes", acts as a "seal", preventing any gas returning to the retorts, and consequently rendering these independent of each other, though they are connected with the same hydraulic main.

The piping from the outlet of the hydraulic main to the next portion of the plant, known as the "condenser", is generally taken round the inside of the retort-house, for the pur pose of reducing the temperature of the gas before entering the condensing-apparatus.

The remaining apparatus and plant required in the manufacture are geuerally well coated with paint or tar, and placed in the open; so before proceeding with their description, a short account of the work in the retort-house will be given. Upon the entrance of the stoker to withdraw a charge of coal which has been subjected to a sufficient period of distillation, his first attention should be directed to the pressure-gauges, as the indications of the pressure of every portion of the plant serve to show the condition of the whole works. Any excess exhibited demands instant attention and release before the retorts are re-charged. The fire in the furnace will require disturbance in order to throw down the ash and refuse and brighten the fuel, before re-stoking. Having cleared the fire-grate, the retort-lid or door is taken away, a rake having a "crook " at the end is inserted into the retort, and the solid carbonaceous substance therein withdrawn. A portion of this hot fuel is fed into the furnace, and the remainder quenched into "coke". Any tarry and pitchy particles that may adhere to the orifice of the ascension-pipe are removed, and a short iron-rod pipe-cleaner, or "augur", run up into the pipe. If a "luted" lid or door is used, the luting is now done, and the fresh coal shovelled into the retort; the lid is then placed in its position and screwed home or otherwise fastened, and gas-making again commences.