90 The average composition of the gas supplied to London is, on the authority of the late Dr Letheby, thus stated :- 0r:linm'_v Gus, Canncl Gas, 12 Candles. 20 Candles. Hydrogen ............................... .. -16 '0 27 '7 Light carburetted hydrogen. 39 500 Condcnsible hydrocarbons ........... .. 3'8 13'O Carbonic oxide ........................ .. 7'5 6'8 Carbonic acid .......................... .. 0'6 0'1 Aqueous 2'0 2'0 Oxygen .................................. .. O '1 O '0 Nitrogen . ............................... .. 0'5 0'4 Cannel gas is now, however, supplied only to the Houses of Parliament and to certain of the Government offices. .IANt'F.C'I‘l'RE or C0.-L-GAS. The series of operations connected with the preparation and distribution of coal—gas embrace the processes of dis- tillation, condensation, exhaustion, scrubbing or washing, purification, measuring, storing, and distribution by the governor to the mains, whence the consumers’ supply is drawn. In connexion with consumption, pressure of the gas, measurement of the amount consumed, and the burners and other arrangements for lighting are the most important considerations. GAS Site and Arrangement of ll'or/.-s.—The choice of a site for a gas establishment is necessarily conditioned by local circumstances ; but the facts that a considerable area is re- quired, and that, at best, the works do not improve the amenity of any neighbourhood, are important considerations. A central position with respect to the area to be supplied is certainly desirable, but in tlie circumstances it is seldom to be obtained. Of even greater consequence for a large work is ready access to a iailway or other means of trans- port; and most of the great establishments are now con- nected by. sidings with lines of_ railway, whereby coals, &c., are delivered direct from the waggons to the store or retort- house, and in the same way the coke and residual products are removed. Where the arrangement is practicable, it is also desirable that the works should be erected at the lowest level of the area to be supplied, since coal—gas, being speci- fically lighter than atmospheric air, acquires a certain amount of pressure as it rises in pipes, which pressure facilitates its distribution, and it is much easier to control than to beget pressure. In the planning of works, regard must be given to economy of space and to labonr—saving arrangements, so that the cost of manual labour may be minimized, and operations proceed in an orderly, methodical, and easily—controlled manner. The accompanying ground plan of gas-works (fig. 1) has been kindly furnished by -<3" ” f "E = W KL 5' ‘.1 ‘ @ " ‘ E [E G Mr James Hislop of Glasgow, a gas engineer of known skill la "T ODllI§40$@'UE090 -Ffvr vzm_|lL:ez9_ nine’! "Isa _7tL '9'!) 200
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FIG. 1.——Ground—Plan of Gas—Works. A. a line of rails leading into the works; 13, wageon weighing machine; C, the coal store; D, retort house: E, chimney; F, coke yard: G, condensers: ll, scrubbers; I. tar tank; J, exhnuster house-, K. pump room; L, store room (water tank is placed above K L); .I, purifier house; N. lime store; (L lnut-'1' house: P, gas-holders; Q. governor house; It, photometer room: S, board room; T, office; U. weighing machine; V, superintendent's house; W, jolner's shop; X, smith's shop; Y, cngine boiler. and experience; and while it shows arrangements of the most approved character, it will also enable the reader to recognize the position of the various erections and apparatus as they follow each other, and as they will now be de.-scribecl. Retort .—Retorts for destructive. distillation of coal are formed of cast iron, clay, brick, or wrought iron. Various shapes have been adopted in the construction of these vessels ; nor have their forms been more varied than the modes in which they have been disposed in the furnaces. In many instances they have been constructed of a cylindrical shape varying in length and diameter. These first employed were of iron, with the axis vertical, but experience soon showed that this position was extremely inconvenient, on account of the difliculty which it occasioned in removing the coke. The retorts were therefore next placed in a horizontal position, as being 11ot only more favourable to the most economical distribution of the heat. but better adapted to the introduction of the coal and the subsequent removal of the coke. At first the heat was applied directly to the lower part of the retort ; but it was soon observed that the high temperature to which it was necessary to expose it, for the perfect decomposition of the coal, proved destructive to the lower side, and rendered it useless long before the upper part had sustained much injury. The next improvement was, accordingly, to interpose an arch of brickwork between it and the furnace, and to compensate for the diminished intensity of the heat by a more equally diffused distribution of it over the surface of the retort. This was effected by causing the flue of the furnace to return towards the mouth of the retort, and again conducting it in an opposite direc- tion, till the heated air finally escaped into the chimney. This arrangement was continued so long as iron retorts were in use, but on the general adoption of clay retorts the furnaces were constructed to allow the fire to play freely
around them.
