CARBON AND THE HYDROCARBONS 363 



As the decomposition of the organic matter which forms coal, is still 

 going on underground, the evolution of large quantities of marsh gas 



H 2 S, forming FeS and H 2 0, and the gypsum retains the remainder of the ammonia, 

 the excess of lime absorbing carbonic anhydride and sulphuric anhydride. [In English 

 works a native hydrated ferric hydroxide is used for removing hydrogen sulphide.] 

 This purification of the gas takes place in the apparatus L, where the gas passes through 

 perforated trays w, covered with sawdust mixed with lime and sulphate of iron. It is 

 necessary to remark that in the manufacture of gas it is indispensable to draw off the 

 vapours from the retorts, so that they should not remain there long (otherwise the 

 hydrocarbons would in a considerable degree be resolved into charcoal and hydrogen), 

 and also to avoid a great pressure of gas in the apparatus, otherwise a quantity of gas 

 would escape at all cracks such as must inevitably exist in such a complicated arrange- 

 ment. For this purpose there are special pumps (exhausters) so regulated that they 

 only pump off the quantity of gas formed (the pump is not shown in the illustration). 

 The purified gas passes through the pipe n into the gasometer (gasholder) P, a dome 

 made of iron plate. The edges of the dome dip into water poured into a ring-shaped 

 channel g t in which the sides of the dome rise and fall. The gas is collected in this 

 holder, and distributed to its destination by pipes communicating with the pipe o, issuing 

 from the dome. The pressure of the dome on the gas 

 enables it, on issuing from a long pipe, to penetrate- 

 through the small aperture of the burner. A hundred 

 kilograms of coal give. about 20 to 30 cubic metres of gas, 

 having a density from four to nine times greater than that 

 of hydrogen. A cubic metre (1,000 litres) of hydrogen 

 weighs about 87 grams; therefore 100 kilograms of coal 

 give about 18 kilograms of gas, or about one-sixth of its 

 weight. Illuminating gas is generally lighter than 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 defiant gas, C 2 H 4 , is 

 fourteen times, and the vapours of benzene thirty-nine 

 times; heavier than hydrogen, and illuminating gas some- 

 times contains 15 p.c. of its volume of them. The brilliancy 

 of the flame of the gas increases with the quantity of 

 defiant 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 85 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 2 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 con- 

 tains a great deal of carbonic anhydride, although on the 

 other hand there is an almost complete absence of sul- 

 phur compounds. Tar, oils, naphtha, and such materials 

 furnish a large quantity of good illuminating gas. An 

 ordinary burner of 8 to 12 candle-power burns 5 to 6 cubic Pl(j '. 89 ' "T Bto j rp jj| e ' Air 

 feet of coal gas pet hour, but only 1 cubic foot of naphtha sha^l mouthriece^and 

 gas. One pood (36 Ibs. Eng.) of naphtha give's 500 cubic escapes in a fine stream 

 feet of gas-that is, one kilogram of naphtha produces about 

 one cubic metre of gas. The formation of combustible gas tube, 

 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. 



