LIGHTING. 



GAS ILLUMINATION. 



The existence of an inflammable air, as a natu- 

 ral production, has been known from a period of 

 great antiquity. It was observed to issue spon- 

 taneously from fissures in the earth ; and has 

 been employed in such situations, as a source of 

 light and heat, both in ancient and modern times. 

 This natural gas is also found in abundance in 

 some coal-mines, where it constitutes the 'fire- 

 damp ' so destructive to the miner. 



The artificial production of an inflammable air, 

 by distilling coal in a close vessel, is first men- 

 tioned in a letter by Mr Clayton, rector of Crofton, 

 Yorkshire, addressed to the Royal Society, May 

 12, 1688. Though well known to chemists from 

 this time, it was only esteemed as a philosophical 

 curiosity until the year 1792, when Mr Murdoch, 

 an engineer, then residing at Redruth, in Cornwall, 

 prepared coal-gas on a scale sufficiently large to 

 light up his own house and office. In 1798, he 

 was engaged to put up his apparatus at the manu- 

 factory of Messrs Boulton and Watt, Soho, near 

 Birmingham, where he continued to experiment, 

 with occasional interruptions, until the year 1802. 

 It does not appear, however, that much attention 

 was excited by these first efforts at gas-lighting, 

 except among a very few scientific individuals, 

 until the general illumination at the peace of 

 Amiens afforded an opportunity for a more public 

 display. On this occasion, the front of the manu- 

 factory was brilliantly lighted up by the new 

 method, and it at once attracted the wonder and 

 admiration of every one who saw it. 'All Bir- 

 mingham poured forth to view the spectacle ; and 

 strangers carried to every part of the country an 

 account of what they had seen. It was spread 

 about everywhere by the newspapers ; easy modes 

 of making gas were described ; and coal was dis- 

 tilled in tobacco-pipes at the fireside all over the 

 kingdom.' 



In the course of a few years from this date, gas- 

 lighting, especially for streets and public build- 

 ings, began to spread over the kingdom. The 

 chief contributor to the success of the movement 

 was Mr Clegg, who invented the hydraulic main, 

 the wet-lime purifier, and the wet gas-meter. Its 

 progress in dwelling-houses was at first retarded 

 by the injurious effects of the impurities it con- 

 tains in its crude state. But science has shewn 

 how most of these may be effectually removed, 

 and well-made gas is now almost free from noxious 

 properties. 



The use of gas-lighting in cities and towns is 

 now pretty general over the world. Where coal 

 is scarce, it is prepared from resin, oil, refuse fats, 

 &c. Oil-gas was at one time extensively used in 

 Great Britain, but though it is of very superior 

 quality, the comparative economy of coal-gas 

 made the manufacture be given up. 



The organic substances that form the great 

 bulk of coals are compounds of the elements 

 carbon, hydrogen, and oxygen, with small propor- 

 tions of sulphur and nitrogen. When coals are 

 submitted to a red-heat in close vessels or retorts 

 a process known as destructive distillation the 

 whole of these organic elements, except a portion 

 of the carbon, are volatilised or drawn off in vapour. 

 Coal-gas thus distilled out is not a definite 

 chemical compound, but a mixture of many unlike 



substances consisting chiefly of, first, coal-tar, a 

 very complex mixture of various compounds of 

 carbon and hydrogen ; second, ammonia combined 

 with carbonic acid and sulphuretted hydrogen ; 

 third, light and heavy carburetted hydrogen, in 

 combination with sulphuretted hydrogen, car- 

 bonic acid, carbonic oxide, and certain combina- 

 tions of sulphur and carbon, which it is the busi- 

 ness of the manufacturer to separate as completely 

 as possible from the gas before it is sent out for 

 distribution and consumption. 



Of the uncondensed and permanent gases, the 

 most important are two hydro-carbons. The first 

 namely, olefiant gas is composed of 4 atoms of 

 hydrogen with 2 atoms of carbon. Its specific 

 gravity is -979, common air being considered as 

 unity, or I -ooo. When pure, it burns with a dense 

 white light, combining with three times its bulk of 

 oxygen, and producing carbonic acid and vapour 

 of water. 



The second namely, light carburetted hydro- 

 gen is composed of 4 atoms of hydrogen, com- 

 bined with i atom of carbon. Its specific gravity 

 is -553. It is this gas which is met with in coal- 

 mines, and which forms an explosive mixture 

 with atmospheric air. It burns with a yellowish 

 flame, combining with twice its bulk of oxygen. 



The relative quantities of these gases differ 

 according to the quality of the coals and the mode 

 of manufacture ; there is always much more of 

 the light compound than of the heavy ; but the 

 quality of a gas depends upon having a high pro- 

 portion of the heavy, as it gives a much brighter 

 light ; the proportions of the heavy amounting to 

 10 or 12 per cent, of the volume in the best 

 qualities, usually manufactured from cannel-coal ; 

 and from 4 to 6 per cent, in gas made from New- 

 castle coal. There also remain traces of the other 

 hydro-carbon vapours uncondensed, which, by 

 their large proportion of carbon, add to the white- 

 ness of the light. Of the other gases forming the 

 mixture, the carbonic oxide and the free hydrogen 

 are of no use, but being once formed, are difficult 

 to be got rid of. They burn, adding to the heat of 

 the flame, but not to its light, and yielding watery 

 vapour and carbonic acid. 



Manufacture of Gas. 



The various kinds of coal differ greatly in the 

 extent to which they are volatilised by distillation. 

 In some, as much as 70-80 percent, of coke con- 

 sisting of carbon and earthy matter remains in 

 the retorts ; in general the coke is about 50 to 55 

 per cent, of the weight of coal. The coals best 

 adapted for the manufacture of gas are the com- 

 pact kind, known in England by the name of 

 cannel, and in Scotland by the name of parrot 

 coals. The English caking coals, of which a 

 great part are obtained in the neighbourhood 

 of Newcastle-on-Tyne, are, however, from their 

 cheapness and the superior quality of the coke 

 which remains after distillation, more extensively 

 used than any other. 



English caking-coal yields 8000 to 10,000 feet 

 of gas per ton, of illuminating power equal to 12 

 to 15 candles; cannel-coal yields from 10,000 to 

 14,000 feet per ton, and illuminating power from 

 1 6 to 35 or 40 candles the standard of compari- 

 son being the light from a sperm-candle consum- 

 ing 1 20 grains sperm per hour, and the gas-burner 

 consuming at the rate of five cubic feet per hour 



493 



