CABBON AND THI-: IIVDKOC.MMK.XS 351 



occurs in coal-mines. 3 ' 2 When mixed with air it forms an explosive- 

 mixture, which forms one of the great dangers of coal mining, as 

 subterranean work has always to be carried on by lamp-light. This 

 danger is, however, overcome by the use of Humphry Davy's safety 



generally lighter tliiin marsh gas, as it contains a considerable amount of hydrogen, and is 

 only heavier than marsh gas when it contains much of the heavier hydrocarbons. Thus 

 olefiant gas, CoHj, is fourteen times, and the vapours of benzene thirty-nine times, heavier 

 than hydrogen, and illuminating gas sometimes contains 15 p.c. of its volume of them. 

 The brilliancy of the flame of the gas increases with the quantity of olefiant gas and 

 similar heavy hydrocarbons, as it then contains more carbon for a given volume and a 

 greater number of carbon particles are separated. Gas usually contains from 35 to 60 p.c. 

 of its volume of marsh gas, from 30 to 50 p.c. of hydrogen, from 3 to 5 p.c. of carbonic 

 oxide, from i> to 10 p.c. heavy hydrocarbons, and from 3 to 10 p.c. of nitrogen. Wood 

 gives almost the same sort of gas as coal, and almost the same quantity, but the wood 

 gas contains a great deal of carbonic anhydride, and the vapours of water and tarn- 

 liquids, but on the other hand there is almost complete absence of sulphur compounds. 

 Tar, oils, naphtha, and such like materials furnish a large quantity of good illuminating 

 gas. An ordinary burner of 8 to 12 candle power burns 5 to 6 cubic feet of coal gas per 

 hour, but only 1 cubic foot of naphtha gas. One pood (36 Ibs. Eng.) of naphtha gives 

 ."DO cubic feet of gas that is, one kilogram of naphtha produces about one cubic metre 

 of gas. The formation of combustible gas by heating coal 

 was discovered in the beginning of the last century, but 

 only put into practice towards the end by Le-Bon in France 

 and Murdoch in England. In England, Murdoch, together 

 with the renowned Watt, built the first gas works in 1805. 

 It is worthy of remark that 1 candle-power per hour en- 

 tails a consumption of not less than 7 grams of coal gas, or 1 *> 

 grams of naphtha gas. whilst .lamps consume but 4 grams of 

 kerosene per candle-power, and ordinary-sized candles burn 

 6-9 grams of stearin, wax, or paraffin, or 13 grams of 

 tallow per hour. In practice illuminating gas is not only 

 used for lighting (electricity and kerosene are cheaper in 

 i;u--ia >. but also as the motive-power for gas engines (see 

 p. 172), which consume about half a cubic metre per horse- 

 power per hour ; gas is also used in laboratories for heating 

 purposes. When it is necessary to concentrate the heat, 

 either the ordinary blowpipe (fig. 59) is applied, placing 

 the end in the flame and blowing through the mouthpiece ; 

 or. in other forms, gas is passed through the blowpipe; 

 when a large, hot, smokeless flame is required for heating 

 crucibles or glass-blowing, a foot-blower is used. High 

 temperatures, which are often required for laboratory and 

 manufacturing purposes, are most easily attained by the 

 use of gaseous fuel (illuminating gas, generator gas, and 

 water gas, which will be treated of in the following chap- 

 ter) because complete combustion may be effected without 

 an excess of air. It is evident that for obtaining high tem- 

 peratures means must be taken to diminish the loss of heat 



bv radiation. Fr ; -, 59 -Blowpipe. Air is 



* ... r .,, . . , . Mown in at the trumpet - 



Which is set free in coal mines contains a shaped month pi txv. ami 

 good deal of nitrogen, some carbonic anhydride, and a large escapes in fine stream from 

 quantity of marsh gas. The best means of avoiding tile SrSy of tteltS 

 explosion consists in efficient ventilation. It is best to tuht '- 

 light coal mines with electric lamps. 



