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HE apparatus through which the gas is conveyed immediately upon its leaving the retorts is shown in fig. 2, and comprises the iron mouthpiece; the ascension pipe; the arch or saddle pipe; the dip pipe; the hydraulic main; and the gas main.

The mouthpiece is made of cast-iron, and is attached to the clay retort either by bolts and nuts, or by an arrangement described later on; it is provided with a door, which is sometimes made out of a wrought-iron plate cut to the shape of the retort, and provided with lugs, by which it is supported on the mouthpiece.

In order to make a gas-tight joint a luting, composed of one part of spent lime and two parts of sand or clay, is spread on the lid by means of a trowel, the door is placed in position and then tightened up by means of a crossbar and screw fastener, but in modern establishments doors which do not require any luting, and which are known as "self-sealing," are now almost exclusively employed.

There are various descriptions of self-sealing lids, but it will be sufficient to describe the one which is most commonly employed, viz., Morton's, with Holman's eccen

tric fastener.

Morton's lid, fig. 8, is made of cast-iron, cast to the

shape of the mouthpiece, and is faced and planed, the mouthpiece being also faced and planed where the two surfaces are brought into contact. The lid is fixed to the mouthpiece, but is capable of swinging round by the aid of a swivel hinge. When in use it is secured by a catch on the opposite side to the swivel hinge, and is tightened up by the eccentric fastening invented by Mr. Holman.

It is found, in practice, that the best shape of mouthpiece for insuring a tight joint with this description of selfsealing lid is the round, as the pressure in this form of lid is transmitted equally on each and every portion of the same, which is not the case with the other shapes, such as the D, on account of the unequal division above and below the cross-bar; it is consequently advisable, except where mechanical stoking is employed, to make the iron mouthpiece of the same shape as the retort, say a D, but to have the portion which comes into contact with the lid of a round section, and thus

FIG. 8.

secure a tight joint.

As previously remarked, the mouthpiece is usually attached to the retort by bolts and nuts, the bolts being inserted in the head of the retort before the latter is placed in position; but this method of attachment is not to be recommended, as the bolts are liable to be burnt away, with the consequent risk of causing the mouthpiece to come down; and even if the bolts are not burnt through, the weight of the retort mouthpiece, ascension, and saddle pipes, which are connected to the retort by the aid of the bolts, often cause the latter to break; so, in order to avoid this, numerous devices have been adopted in order to get the weight of the various fittings from the retorts themselves. One of the simplest is to brace up the mouth

piece by T-iron girders 4 × 4 × inches, as shown in the figure, the latter in turn being bolted to, and kept in their place, by the wrought-iron buckstays which brace together the setting. Attached to the mouthpieces are the ascension pipes; these are usually made of cast-iron, although at the present time pipes of wrought-iron and steel are being manufactured, their chief recommendation being that of lightness. Ascension pipes are usually made of a diameter of 6 inches at the mouthpiece, and taper to 5 inches at the junction with the arch or saddle pipe, the dip pipe also being 5 inches in diameter. There are various forms of the pipe known as the "arch," "bridge," or "saddle" pipe, the chief essential in designing the same being to make provision for easy access in case of stoppage, as shown in the figure.

The hydraulic main, in conjunction with the dip pipe, acts as an automatic valve, whereby the gas is allowed to go freely forward during the process of carbonizing, but is prevented from returning when the mouthpieces of the retorts are opened during the process of "drawing" and "charging."

The hydraulic main may be described as a large pipe, set perfectly level, and extending the whole length of the retort benches, and receiving the dip pipes from the different benches of retorts in its passage. It is made in various shapes-rectangular, D-shaped, or round; and is usually supported above the retort bench by means of brick piers, or cast-iron standards, placed at suitable intervals. The hydraulic main contains a mixture of tar and ammoniacal liquor which is kept at a predetermined level by an adjustable overflow.

The gas, on leaving the retort, travels by way of the ascension and arch pipes to the dip pipe, down which it passes, and bubbling through the liquid in the hydraulic main, escapes at the top of the liquid, and from thence

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