vessel 12 to 24 inches wide, 12 to 18 feet high, and of varying length. It is divided internally by mid-feathers, placed at distances equal to the width apart, extending to within a few inches of the top and bottom of the chest alternately, the gas passing from the inlet up and down each division to the outlet. In order to increase its condensing power, small tubes of about 2 inches diameter pass from side to side of the vessel as shown. The air is capable of freely circulating through these tubes, and this helps to cool the gas; they further serve the purpose of breaking up any tarry particles.

The water condenser possesses great advantages over the atmospherical condenser, as it admits of easy regulation, while the cooling agent (water) is much more powerful than that employed in the ordinary or atmospheric condenser, viz., air. By the employment of water we are enabled to neutralize the great variations which occur in the external temperature, and of adequately meeting the variations in the make of gas by the simplest means. Water has a far greater power of absorbing heat-in other words of cooling-than air has.

Peclet gives the following as the results of his experiments on the relative efficiency of water and air as cooling agents.

A water condenser of an excellent kind, the invention of Mr. G. Livesey, has been long employed at the South Metropolitan Gas Company. The condensing pipes are placed in a tank of water divided into separate channels, all on one plane, in which the pipes are led backwards and forwards for several turns, with the water flowing in an opposite direction to the gas in the pipes. In this way the cool entering water comes first in contact with the gas pipes at their outlet from the tank, and becomes gradually warmed in the subsequent winding channels in proportion as it reaches the other end of the tank, where the gas enters at a high temperature from the retorthouse. The temperature of the water, as also that of the gas, can be kept at a steady point by simply increasing or diminishing the admission of water according as it is desired to lower or raise the temperature of the tank.

Morris and Cutler's condenser consists of a rectangular cast-iron tank provided with a series of division plates, which divide it into a series of channels through which the gas is caused to travel alternately from end to end. Through each channel a number of small wrought-iron pipes are fixed, in which water is made to flow in the opposite direction to the flow of gas. The pipes are so arranged that they come into direct contact with the gas,

so that they not only cool the latter, but, in addition, subject it to a certain amount of friction.

The amount of condensing surface required, necessarily varies with the quantity of watery vapour and tar, in the crude gas. In the case of Newcastle coal, which yields a comparatively small quantity of water, say 12 gallons to the ton, an atmospheric cooling surface of 5 superficial feet per 1,000 cubic feet of gas made per day would probably be sufficient. Derbyshire coal yields about 26 gallons of water per ton of coal, so that gas from this coal would require a proportionately larger amount of cooling surface, about 10 superficial feet of cooling surface per 1,000 cubic feet.

By the time the gas has reached the outlet of the condensers, it will have deposited from 10 to 13 gallons of tar, together with from 13 to 20 gallons of water, holding more or less ammonia in solution, per ton of coals carbonized.

Condensers are usually made of cast-iron, although wrought-iron foul mains and annular condensers are now very general.

Pelouze and Audouin's condenser differs in principle from those just described, its object being to break up the suspended tarry particles in the gas. Fig. 16 shows the apparatus, as manufactured by Messrs. R. and J. Dempster, Limited, of Manchester.

The apparatus consists of an outer cylindrical cast-iron chamber, provided with the usual inlet for gas, and outlets for gas and the products of condensation, and contains a cylinder of perforated sheet-iron, which constitutes the condenser. The sides of the condensing chamber are two thin sheets of iron, having a concentric space between The inner sheet is perforated with holes one-twentieth of an inch in diameter, and the outer with slots of a larger size, the outer sheet being so arranged as to offer a

blank surface opposite the small holes in the inner sheet. The passage of the gas through the apparatus is shown by

FIG. 16.

the arrows. On ascending into the cylindrical chamber and passing through the perforations, the gas is formed into jets, which strike against the solid surface sheet placed close to the perforated plates. In passing through the holes the liquid molecules are wire-drawn, and therefore brought into close contact with one another, the action being completed by contact with the solid surface upon which the tarry matter is deposited, from whence it flows down the surface of the plate into a tar pan underneath the apparatus, and from thence to the tar well.

The condensing cylinder, properly balanced, is capable of acting as its own regulator. For this purpose it moves in a hydraulic seal, which allows of the closing of those gas passages which are not required to be in action. When the pressure increases, i.e., when there is an increase of gas, the cylinder rises, and a large number

of openings are uncovered to allow the gas to pass through, thus adjusting itself to night and day, or to winter and summer workings.

CHAPTER VIII.

THE EXHAUSTER.

LACED between the condenser and the scrubbers is the

PLAC

apparatus known as the exhauster, which plays a very important part in the economy of a gas-works. The exhauster is employed for the purpose of withdrawing the gas from the retorts as fast as it is produced, and propelling it onward through the remainder of the apparatus through which the gas has to pass on its way to the gas-holder. On the inlet side of the exhauster, therefore, there is a vacuum sufficient to overcome the resistance of the seal in the hydraulic main, and any resistance due to friction in the condenser, which may be taken at 23 inches, while, on the outlet side, there will be a back pressure, as it is termed, of, roughly speaking, about 24 inches, viz., 2 inches due to the resistance of the scrubber, 14 inches due to the purifiers, and 8 inches to the gas-holder and meter; but these figures will necessarily vary, according to the character of the plant and other circumstances. Without an exhauster the whole of this resistance would be exerted against the gas in the retorts, which would have the effect of causing the latter to leak badly; a large proportion of the gas would in fact find its way into the furnace through the cracks and joints in the retorts.

When the exhauster is at work, the pressure on the retort is only that due to the seal of the dip pipe, which, as a rule, is only a few tenths of an inch; but with the