other (but in the larger sizes open at each end), the ends fit in the covers, and round the circumference of the roller are steel slotted rollers to carry the blades and swivel with them to suit the varying position. Neither rings or segments are used in this exhauster. The blades each have

FIG. 22.

two hinges fitting between each other, making together a continuous bearing on the central pin.

In the working of exhausters it is necessary to guard against both excessive and insufficient exhaustion. Excessive exhaustion means the drawing in of air and furnace gases through the retorts, with a consequent depreciation of the illuminating power of the gas; insufficient exhaustion

means leakage, and loss of illuminating constituents as previously explained.

Owing to the manner in which the retorts are charged, the production of gas varies from hour to hour; the pressure of steam and amount of back pressure also vary; consequently, it is not practicable to work the exhauster satisfactorily so as to avoid the excesses previously referred to, without some special regulating arrangement for controlling the speed so as to adapt it to the varying conditions alluded to. The plan generally adopted is to connect the inlet and outlet pipes of the exhauster with a smaller pipe, known as a bye-pass main, in which is inserted a throttle valve, to the spindle of which is fixed a weighted quadrant. This is connected by a series of rods and chains to the lever arm of a gas governor, which is in connection with a pipe coming direct from the hydraulic main, as shown in fig. 22; the bell of the governor is counterbalanced and weighted so as to maintain a certain exhaust; should this be exceeded, the bell of the governor falls, which causes the throttle valve in the bye-pass main to open, so that a portion of the gas which has passed through the exhauster is churned" over again, which compensates for the extra vacuum resulting from the exhauster running at too high a speed, by giving it more work to do; but this method possesses the defect that, should the exhauster be running too slow, then it is ineffective.

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In order to get over this difficulty, it is customary in many works to attach the governor previously described direct to the steam-valve of the engine, so as to control, the speed of the latter according to the make of gas. Messrs. George Waller and Co. make a governor, specially adapted for this work, which performs its duty admirably.

The exhauster should always be provided with a byepass flap valve in case of a sudden stoppage. The valve,

when the exhauster is at work, is closed, but on the gas having no vent through the exhauster, the excess of pressure forces it open, so that the gas goes forward as usual.

It is advisable to have three gauges fixed in the exhauster house: one leading from a pipe in direct communication with the hydraulic main, one on the main inlet of the exhauster, and another on the outlet of the same. It is necessary to note that the pressure (or vacuum) in the hydraulic main does not tell us the pressure within the retorts, as the latter depends upon the extent to which the dip pipes are sealed in the liquid contained in the hydraulic main. Thus, supposing that the dip pipes are sealed in the liquid contained in the hydraulic main to a depth of two inches, and that the gauge in communication with the hydraulic main shows one and a half inch of vacuum, then the retorts would be under half an inch of pressure. The amount of "pull" exerted by the exhauster on the inlet side, so as to withdraw the gas from the retorts, is known as vacuum," while the resistance on the outlet side is spoken of as "back pressure."

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As remarked at the beginning of this chapter, the exhauster exercises an important effect on the economical working of a gas-works, and it should be so manipulated as to enable the gas to be generated in the retort under the least possible pressure consistent with not drawing in furnace gases.

The following is a brief summary of the advantages which attend the employment of this apparatus:-Increased -production of gas per ton of coals, with an increased illuminating power; longer life of the retorts; decreased consumption of fuel.

THE

CHAPTER IX.

PURIFICATION.

(Abstraction of the Ammonia.)

HE substances present in the crude gas which are regarded as impurities, in addition to the tarry matters and aqueous vapour deposited during the process of condensation, are:

Carbonic acid (CO2),

Ammonia (NH,),

Sulphuretted hydrogen (SH),

Bisulphide of carbon (CS), and other compounds of sulphur.

Before proceeding to discuss the methods by which these impurities are removed from the crude gas, it will be advisable to say a few words concerning their properties, and the reason for their removal.

Carbonic acid (CO2) is a colourless, odourless gas, which will not burn or support ordinary combustion. It has a slight acid taste and reaction. Under ordinary conditions of temperature and pressure water will dissolve its own volume of the gas, i.e., at 60° F. and 30 inches, a pint of water will dissolve a pint of gas. But a change in temperature or pressure will result in a change in the quantity

of

gas dissolved, so that if the water in the illustration just cited were cooled down to 32° F., the pint of liquid would then dissolve nearly a pint and three-quarters of the gas.

Carbonic acid is chiefly objectionable on account of its

exercising a lowering action on the illuminating power of

the gas.

Sulphuretted hydrogen (SH2) is a colourless gas, having a sweet taste and the odour of rotten eggs, and when present in large quantity is poisonous. It is freely soluble in water, which dissolves 4.37 times its bulk of the gas at 23° F. It has a feeble acid action, and burns in air or oxygen, producing SO, or S, according as to whether there is an excess or a deficiency of air.

In addition to its poisonous properties SH2 acts prejudicially by causing all bright metal work, such as picture frames, to become tarnished.

Ammonia is a colourless gas, with a characteristic pungent odour and strong alkaline reaction, and is extremely soluble in water. The gas does not burn, nor does it support combustion in the ordinary sense of the word. If, however, it be ignited in oxygen, it readily burns with a non-luminous flame, forming water and liberating nitrogen.

Being of an alkaline nature it is capable of neutralizing the acid bodies, CO, and SH,, and advantage is taken of this property in the washers and scrubbers.

The presence of ammonia in coal-gas causes destruction of any brass or copper work, such as the index wheels of meters, gas fittings, etc., with which it may come into contact, hence the necessity for its removal.

Carbon disulphide (CS) is prepared by passing the vapour of sulphur over charcoal heated to redness. On making the resulting product flow through a condensingtube, an impure form of carbon disulphide is obtained. Carbon disulphide is a colourless, heavy, and extremely volatile liquid. The commercial product has a very fœtid and repugnant smell; but when pure it has a sweetish, ethereal odour, and an acrid, pungent taste. It is highly refractive, and has a specific gravity of 1.29. Water dissolves about