100 



GAS-LIGHTING 



same thing is done, but the air which meets the 

 furnace gases under the retorts is heated by the 

 waste heat, which would otherwise have been 

 allowed to escape through the flue after the retorts 

 had been heated ; the result being a great economy 

 in fuel and in the wear of the retorts. The retorts, 

 once heated up, are kept continuously at an orange- 

 red heat (2000 F. ); they are charged with coal 

 (2 to 3 cwt. each) ; the charge is raked out after 

 four or six hours, and a fresh charge is put in ; 

 the charging and drawing being now often done by 

 machinery. The duration of clay retorts depends 

 on the treatment they receive ; fifteen to eighteen 

 months where directly exposed to the fire, or, 

 where protected, three or four years, or even longer. 

 In the Yeadon and Adgie revolving retort, small 

 coal is fed in at one end and coke dust withdrawn 

 at the other as the retort revolves ; each granule of 

 coal takes about 15 minutes to traverse the retort. 

 Every retort is provided with a mouthpiece, through 

 which the charge is put in and extracted, and the 

 door of which is pressed home by a screw or lever 

 and may or may not be secured by cement. The 

 gas produced passes from the retort by means 

 of a wide vertical ascending pipe, a very short 

 horizontal bridge -pipe, and a short descending 

 dip-pipe, which dips to a very slight extent 

 below the overflow level of liquid in the hydraulic 

 main. This hydraulic main is a wide tubular 

 closed reservoir of wrought-iron, placed above the 

 retorts ; it has a large descending overflow-pipe ; 

 it is first filled with tar- water as far as it can 

 be filled ; the products of distillation from 

 the retort pass through the hydraulic main ; 

 some tar is deposited, some watery liquid con- 

 densed ; tar accumulates up to the overflow 

 level, so that the gas passing through is washed 

 in hot tar, and the light-giving constituents tend 

 to become dissolved out to a large extent by the 

 tar, unless the tar be kept sufficiently hot or be 

 often enough removed from the hydraulic main. 

 Down the overflow - pipe run the products of 

 distillation, which sink into a tar-well, from 

 which they are pumped out from time to time. 

 This tar-well is also used as a general receptacle 

 for condensation products deposited by the gas in 

 its further course. The gas does not escape by 

 this tar-well, for the overflow-pipe dips to an 

 adequate depth into the liquid in the well ; it 

 passes on by a lateral horizontal tube. This device 

 is repeated as often as is necessary. 



The gas goes on to undergo a gradual process of 

 cooling (to a temperature not below 55 F. ) and 

 farther condensation, partly in pipes led round the 

 retort-house (in which the tar is largely deposited 

 by friction while the gas is still hot), partly in 

 the condenser. There are several types of con- 

 denser: (a) a series of vertical iron tubes in which 

 the gas alternately ascends and descends, the cool- 

 ing oeing due to the exterior air or to the trickling 

 of water down the surface of the tubes ; ( b ) verti- 

 cal iron tubes of large size, concentrically arranged 

 in pairs, so that the gas may slowly descend in the 

 annular space between each two tubes, while the 

 cooling air ascends the inner tube the gas is then 

 led up to the top of another annular space, arid 

 so on (Kirkham's) ; (c) a horizontal spiral; (d) 

 a vertical zig-zag of pipes horizontally laid ; (e ) 

 arrangements for retarding the speed and thus 

 enabling the gas, in comparative repose, more 

 readily to deposit any particles; battery con- 

 denser; Mohr's condenser> in which the gas is 

 guided through hollow cones, so as to run slowly. 

 The cooled gas is then led to the washer, in 

 which it is passed in fine streams through water, 

 which dissolves ammonia, &c. ; but here or farther 

 on, after the scrubber, there is a suction arrange- 

 ment, either a fan, a pump, or a steam-jet 



injector, called the exhauster. The coal being 

 thus distilled in a partial vacuum, gas is more 

 readily given off by it ; and the gas once formed 

 is rapidly removed from the retort and from the 

 decomposing influence of the hot retort-walls, and 

 its percentage in hydrocarbons is thus kept as high 

 as may be ; but there is at the same time a con- 

 trary tendency towards deterioration of quality, 

 along with increase of yield, when the exhaust is 

 at work. After the washer conies the scrubber, 

 in which the gas is made to ascend a lofty 

 column tilled with coke or deal boards, down which 

 water trickles, or is made to ascend a space filled 

 with descending spray. Sometimes the gas is 

 made, as in Pelouze and Audouin's so-called con- 

 denser, to deposit the last traces of tar by impact 

 against solid surfaces ; or may be made to run 

 with or against a stream of hot tar, and thus to 

 pick up hydrocarbons from the tar. Sometimes 

 the functions of washer and scrubber are com- 

 bined in one apparatus ; sometimes a scrubber 

 is used alone. The gas next passes through the 

 purifiers, in which it has to pass slowly up, or 

 better down, through an ample extent of thick 

 layers of porous lime, or of iron oxide somewhat 

 moist and rendered porous by sawdust, chaff, or 

 other vehicle, or aided 'by porous magnesia, or 

 through both, or else through washed Weldon 

 slime. The gas ought, before this stage, to be 

 free from all impurities, except carbonic acid, sul- 

 phuretted hydrogen, and bisulphide of carbon, and 

 these are removed in the purifiers. There are various 

 devices for absorbing these by means of ammonia and 

 hydrocarbons separated in the earlier stages (Young, 

 Glaus, Hills). The British parliamentary standard of 

 purity is that 10 cubic feet of gas shall not stain 

 lead paper (absence of sulphuretted hydrogen); 

 that the ammonia in the gas shall not exceed four 

 grains per 100 cubic feet ; and that the whole 

 sulphur in the gas shall not exceed twenty-two 

 grains per 100 cubic feet. The purifiers are so 

 arranged that while a sufficient large area of puri- 

 fying material shall always be encountered by the 

 gas, one part of the purifiers after another is 

 thrown out of action, and renewal of the material 

 is thus possible, when required, without inter- 

 ruption to the purification. The valves and con- 

 necting pipes are so arranged as to permit this 

 alternation to be readily effected : and throughout 

 a gas-work, the pipes are so arranged as to permit 

 any single piece of apparatus to be cut out of the 

 gas-stream when required. 



The gas goes on from the purifiers to the station- 

 meter-house, in which there are (a) tfie station- 

 meter, a large ' wet ' meter for measuring the 

 whole make of purified gas; (b) the exhaust, 

 previously referred to; (c) pressure gauges, and 

 (d) pressure-recording instruments ; (e) the station- 

 governor, by adjustment of which the pressure of 

 gas as supplied from the gasholder to the mains is 

 to be regulated. From the station-meter the gas 

 goes on to the gasholder, or holders, to be stored 

 and issued as required. The gasholder is an inverted 

 cylindrical vessel of sheet-iron, placed in a tank of 

 stone, brick, concrete, cast or wrought iron, steel, 

 or a combination of these, but generally of brick or 

 stone, lined with Portland cement, or backed with 

 clay puddle, and, where possible, sunk into the 

 ground. The tank contains water, in which the 

 cylindrical vessel floats and rises or sinks. As the 

 floating holder rises and sinks, it is kept vertical 

 by tall columns which surround it, and guide its 

 motion. On the tops of these columns are pulleys, 

 over which run chains which at one end are con- 

 nected to the crown of the gasholder, while at the 

 other they bear suspended balance-weights. These 

 balance- weights are not quite heavy enough to 

 balance the weight of the floating vessel, which 



