Some coal deposits tarry substances in the "ascension "-pipe, and thus causes considerable trouble. The ascension-pipe should be provided with caps, which can be opened for the insertion of a clearing-rod. It is sometimes necessary to light a few sticks of wood, previously soaked in naphtha, underneath the pipe, after having opened the cap or joint on the top of the pipe. In most cases, however, the constant use of the clearing-rod will prove an ample means of providing clear ascension-pipes. A quantity - the greater quantity, in fact - of tar is deposited in the hydraulic main, and is allowed to run to the tar-we11 through the same pipe which conveys the gas to the condensers. It is therefore of some importance to keep this residual product in a fluid state during its circulation in the retort-house.

Fig. 661 -Section of Hydraulic Main.

A. rrtoft bench; B, ascension-pipe; c, dip-pipe; D. hydraulic main.

At certain intervals, attention must be given to the quantity of water or "liquor" in the hydraulic main; the tar should never be allowed to rise so as to cover the ends of the dip-pipes, for obvious reasons. The liquor deposited from the gas is frequently insufficient to furnish the requisite supply of "seal", and water most therefore be added as required. A gauge, connected to the top and bottom of the hydraulic main, will serve to indicate the condition of the seal.

It is usual to place a cast-iron trough underneath the furnace for heating the retort-benches, and to run water into it, so that any ash or small pieces of fuel which may escape through the bars of the grate are quenched, and the steam thus produced rises up into the fire, cooling the fire-bars on its way.

In retort-benches of this description furnaces of a very old-fashioned and elementary character are generally placed, requiring attention every two or three hours. As the charge of a retort requires six hours before it is thoroughly carbonized, it would obviously be a saving of time if all the work could be done in the retort-house when the charges must be withdrawn, that is to say, four times per diem; the time so occupied would not exceed one hour for each charge. This method of arranging the manufacture is easily accomplished, with very little extra coat, by having a furnace of suitable capacity only requiring disturbance four times in every twenty-four hours.

In the storage of coal, it is important to "work out" every heap placed in store. Coal allowed to stand deteriorates more or less according to the length of exposure to the atmosphere.

With the exception of charging and discharging the purifiers at various periods according to the quantity of gas made, the foregoing description of the duties attending gas-manufacture will very closely represent the daily routine.

The condenser, winch is the first portion of the outdoor plant to receive the crude gas from the retort-house, aims at the removal of tarry matters and condensable vapours from the gas. In small works, this is accomplished by allowing the gas to slowly ascend and descend a number of somewhat larger pipes, placed in an upright position over a trough or long tank, which is partitioned in such a manner at to ensure the circulation of the gas. The gas leaves a suitable and efficient condensing-apparatus, in a clean and clear condition to the naked eye. The trough which receives the lower ends of the upright condenser-pipes serves to hold the tar, which separates from the gas during its circuit owing to the diminished rate of progress, and the reduction of the temperature of the gas upon exposure to the atmosphere.

The tar is eventually run into a storage-tank, usually placed underground on account of the objectionable odour of the tar. The tar must be allowed to stand in the tank at least three days in order to "settle", i.e. to allow any intermingled liquid to rise and float upon the surface; consequently, in emptying the tank the tar should be withdrawn from the lower level. A gallon of this tar, at a high temperature, - somewhat under boiling-point, - mixed with one-and-a-half to two pints of boiled linseed-oil, makes a very excellent covering-material for iron and wood, not only on account of its preserving qualities, but also from its appearance, the mixture being capable of producing a very lasting lustre. It should undoubtedly be used in gas-works, owing to its resistance to the action of fumes.

A quantity of liquid, known as "ammoniacal liquor", is found on the top of the tar. In works of small size this "liquor", and the lime used in the purification of the gas, are generally known as the two "nuisances". The lime can be used beneficially on the land, but the "liquor" must be disposed of in any convenient and suitable way.

The diminutive size of the works is also a reason for the omission of the apparatus known as the "scrubber", in which gas is subjected to a prolonged contact with moistened surfaces, for the purpose of absorbing the ammonia. That office is therefore performed by the purifiers.

There are two purifiers, or purifying boxes. They are square in shape, and resemble an ordinary box with a cover or lid. The bottom and sides are generally made of cast-iron plates; the box is about 3 feet deep, and has a recess 6 to 8 inches deep and about 6 inches wide running around outside the top of the side plates, technically called " the lute". This recess is filled with water, and the lid or cover is so arranged that, when in position, its sides sink into this water, which thus forms a "seal" or "lute" and confines the gas within the vessel. The cover is usually made of wrought-iron. Within the box, attachments are made for horizontally supporting the "grids", which carry the material used in purification. The grids are made of wooden strips about 4 feet long, 8 inches deep, and three-quarters of an inch in thickness, spaced about half an inch from one another by packing pieces, and each grid carries a layer of lime or oxide of iron 6 inches deep. There are usually three tiers of grids in each box. The gas-inlet to the purifier is attached to the bottom plates, and the gas is forced up through the interstices of the purifying medium, and thus brought into contact with absorbents of its pernicious ingredients. The outlet for the gas is cast at one corner of the box formed by the joint of two side plates.

Fig. 662 -section of purifier.

a. Inlet-pipe: b b b. grids carrying the purifying material. C. lute; D cover. E. outlet-pipe.