A longitudinal section of a furnace and two retorts is shown in fig. 7, and also a plan of the different parts of a gas works, so arranged as to give an idea of the process of manufacture, but not showing the parts in their actual position, as this would be impossible in one figure. The two retorts, of a set of five, are shown at a, a, the furnace at b, the chimney at c. The retorts are first raised to a red heat, and then charged about two thirds full with coal by means of a kind of scoop having somewhat the form of the retort, and about the same length, which is introduced by two men, turned over, and withdrawn. The operation is called stoking, and the men who perform it stokers. Machinery is now being introduced in this country and in Europe, by which it will be performed by steam power. From 100 to 200 lbs. are usually introduced at one charge. A lid is then fitted to the mouthpiece of the retort with bolts and a luting of gypsum mixed with iron filings, and a heat of about 2,200° continued for about five hours.

The constituents of the coal are thus converted into the products given in the above table, and those portions, comprising all except the coke, which are volatile at that temperature, pass into the tube d, called the stand pipe, which ascends from the mouth of the retort and is inserted into the hydraulic main, a transverse section of which is shown at e. This is a long tube, usually semi-cylindrical, and from 18 to 24 in. in diameter, running the whole length of a row of furnaces, which may be 100 or 200 ft. When the operation of distillation commences, the main is partly filled with water, into which the stand pipes leading from the retort dip and discharge all the gases and vapors, a considerable portion of which, from the reduction of temperature, are there condensed, forming tar and ammoniactil liquor, which would soon fill the main were it not drawn off from time to time. In this way the fluid contents are kept at about the same level, and there is no necessity of adding water after the first supply. A large pipe, f, carries the still uncondensed gases and vapors to the cooler and condenser g, through which they pass in a series of n-shaped pipes, surrounded with water which is supplied from a cistern, entering at the bottom of the condenser and passing out at the top.

This arrangement is not represented in the diagram, which also only shows a small portion of one condenser, of which there are usually three, through which the gas passes successively, being reduced in temperature in each. At the base of each con-denser are chambers into which the legs of the n-shaped pipes pass, the descending one, or that which carries the gas downward, extending to near the bottom, beneath the surface of the tar and ammoniacal liquor, which then collects and passes off by suitably arranged pipes into the tar well. These tarry matters, together with those collected in the hydraulic main, are the materials from which the beautiful aniline colors used in dyeing are made. (See Alizarine, Aniline, Anthracene, and Coal Products.) From the condensers the gas is conducted into another apparatus for further separation of impurities. This is sometimes made of a box containing lumps of coke or fire brick moistened with water, and is then called a scrubber. At the Manhattan gas works, New York, a box, partially shown at h, called a washer, is used; it consists of several separate vertical chambers, through which the gas is made to pass, under one partition and over the next, and during its passage subjected to the action of jets of water thrown into spray.

In this way nearly all condensible and soluble impurities are abstracted; but there remain several deleterious gases, the principal of which are sulphuretted hydrogen and carbonic acid, which must be removed before the gas is fit to be delivered for consumption. Several methods have been devised for this purpose, such as passing the gas through milk of lime, which is called the wet-lime process, or through layers of moistened slaked lime, and also through layers of mixed protochloride of iron and quicklime, or sulphate of iron and slaked lime, the ferruginous salts being very effectual in removing all traces of sulphuretted hydrogen. These slaked-lime purifiers are placed in large rooms, and require great care in management, as well for safety as for effectiveness, the gas being liable to escape into the room and form an explosive mixture with the air; and serious accidents have resulted from this cause. A single purifier is represented at 1c in the diagram. It consists of a tight double-sided tank from 4 to 6 ft. high and about 20 ft. long by 12 wide. A deep gutter runs around the upper edge, which is nearly filled with water for receiving the edges of the lid, l, by which means the apparatus is effectually sealed.

It is usual to conduct the gas successively through three of these purifiers. In small works, especially those connected with the larger ones for experiment, the gas is forced from the retorts, by the pressure there created, through all the different pieces of apparatus; and formerly this was the only means of urging the gas onward in all of them. The pressure thus created in large works would so retard the flow of the gas from the retorts that it would suffer much decomposition with production of graphite carbon; and if clay retorts were used, much would escape through their walls. The difficulty is avoided by using what are called exhausters to take the gas from the washers and deliver it to the lime purifiers. These machines may be in the form of a rotary fan blower, or of a cylinder and piston blowing machine. The lime purifiers have several lattice-work shelves, placed one above another and covered two or three inches in depth with freshly slaked lime. The gas entering at the bottom ascends through these layers of lime, which absorb the carbonic acid and sulphuretted hydrogen and other impurities by the time it reaches the chamber beneath the lid. In the figure a pipe is seen passing from the bottom of the washer directly to the lime purifier.