Part of the aqueous vapour driven off from the coal is likewise decomposed by the action of carbon at a high temperature forming carbonic acid (CO), carbonic oxide. (CO), and free hydrogen. As the various gases leave the retort, and the diminished temperature permits mutual chemical affinity to have more play, several of the substances previously alluded to re-combine. For example, the ammonia unites with a part of the H2S, CO2, and SO2, and the cyanogen, to form the sulphydrate, carbonate, sulphite, and cyanide of ammonium respectively, and these compounds again mutually react on each other, the ammonium cyanide and sulphide forming sulphocyanide; while another portion of the sulphide, under the influence of SO2, causes the hyposulphite of ammonium to be formed.

A portion of the carbon bisulphide also probably combines with free ammonia to form ammonium sulphocyanide.

The following table shows how the nitrogen present in the original coal is distributed after the latter has been destructively distilled.

The carbonization of the coal is the most important of all the operations which are carried on in a gas-works, the successful working of the concern entirely depending upon the results obtained in the carbonizing department, as the yield of gas per ton of coal, the amount of coke available for sale, and the cost of carbonizing labour are all factors which make a works profitable or not. If good results are

not obtained in the retort-house in the first instance, no amount of after care in the other departments of the works will have much effect.

The first essential for obtaining good carbonizing results, is to be able to heat the retorts in which the carbonization is effected, to the temperature which is necessary for the disengagement of the whole of the gas obtainable from the particular coal employed, with the use of the minimum amount of fuel, and in order to do this, it is necessary to pay especial attention to the retort settings; but, before proceeding to describe the setting of the retorts, it will be advisable to say a few words concerning the retorts themselves, the retorts being the vessels in which the destructive distillation is carried on.

Formerly, retorts made of cast-iron were employed, and they are still used in experimental plants, and in very small works where an exhauster is not in use. At the present time, however, retorts are almost universally made of fireclay, as the latter material possesses the advantage over cast-iron of being cheaper, more durable, and more refractory; that is to say, it is capable of being heated to a higher temperature without injury. One of the most important revolutions in the gas industry, was the introduction of clay retorts in conjunction with the employment of the exhauster.

On the other hand, clay retorts possess the disadvantage that when once they are heated, they do not stand being let down like those made of cast-iron, as they invariably contract, and consequently crack when cooling.

The usual shapes in cross section of retorts employed are known as "the round," "the oval," and "the D-shaped," as shown in fig. 1.

The considerations to be kept in view in selecting a retort best adapted for the economical production of gas are, that the heat shall be capable of easily penetrating the mass of

coal in the interior; and, also, that the gas when generated shall not be liable to be too much exposed to the hot walls of the retort, by there being too much space between the level of the coal in the retort, when the latter is charged, and the crown of the vessel; consequently, of the shapes mentioned above, the one which best fulfils the conditions laid down is the shallow D, or a D with a slightly curved bottom,

known as "the oval D." The "round" retort, owing to its shape, is the strongest, but is the least desirable form to be employed, as the coal cannot be placed in the same in a thin even layer.

Mr. Newbigging, in his "Gas Managers' Handbook," gives the following sizes (inside) of retorts as being useful and convenient :

Round, 15 inches diameter.

Oval, 21 x 15 inches.

D-shaped, 18 x 15 inches.

The D-shaped, 20 × 13 inches, is also a very useful size, while 22 x 16 inches is a convenient size where stoking machinery is employed. Clay retorts are usually made from 2 to 3 inches thick, the flange for attaching to the iron mouthpiece being 4 inches thick and 8 inches broad, the neck tapering gradually down to the thickness of the body of the retort.

Retorts are divided into two classes, "single" retorts, and "through" retorts.

Single retorts have one end closed, the other end having a mouthpiece, and are generally 9 to 10 feet long; a through retort has a mouthpiece at both ends, and is usually from 18 to 22 feet long. The advantages of using through retorts are, that in such retorts the accumulation of carbon is less, owing to the closed ends being absent; and, also, that the current of cold air which is drawn through the retort each time it is charged, helps to loosen any deposited carbon; and further, that more heating surface is available for the same expenditure of fuel, and that in the hottest portion of the setting. It is obvious however that through retorts are not suitable for very small works, as where the stokers are fewer than six in number. Retorts should be hard, well-baked, and glazed on their interior surface; they are generally put together in lengths varying from 4 or 5 feet to 10 feet, but some engineers prefer to construct them with a number of small pieces, or bricks, each separately moulded and burnt. These are put together and jointed with great care, so as to secure the required size and shapes. The average life of a clay retort may be taken as thirty working months, while an iron retort will only be capable of working twelve months.

Retorts are placed in arches made of fire-bricks, the size of the arch depending upon the number of retorts it is

intended to contain, which may vary from two to nine or more. The arch, with its retorts, is technically known as a bed, or setting.

The necessary essentials of a bench of retorts are the furnace for producing the heating gases; the main flue for carrying away the spent products of combustion; and the chimney for creating the necessary draught to draw away the spent products, and to draw in the necessary amount of air for the production of the heating gases.

The width of the arches in which the retorts are set depends upon the size of the retorts and their arrangement in the setting; the form of the arch also depends in some measure on the description of setting employed, as arches of an elliptical and segmental form allow of three retorts being placed in the top tier, where a semicircular arch would only permit of two being so placed. In erecting the retort bench it is necessary to have a good foundation, consisting of a bed of concrete two feet thick laid under the whole surface of the bench, and extending to a distance of from two to three feet beyond the outside walls, arrangements being made for thoroughly draining the ground should the latter happen to be of a damp nature.

It is necessary to make provision in the bed of concrete for the ash-pit, or generator, as the case may be, according as to whether the furnace will be of the ordinary directfired, or generator type. The level of the top of the concrete should be about three or four courses below the intended floor line, and at this level the division walls of the setting should be commenced; the division walls above the footings should be 18 inches thick, and the buttress walls at the two ends of the setting from two to three times that thickness. The floor of oven should be paved with two courses of fire-bricks set on edge and cemented with fire-clay.

The arch should be formed of three rings of 4 inches of fire-bricks, and should be arranged with a 4-inch