350 PRINCIPLES OF CHEMISTRY 



decomposition of the organic matter, which forms coal, is still going on 

 underground, the evolution of large quantities of marsh gas frequently 



the furnace. Those products of the dry distillation which most easily pass from the 

 gaseous into the liquid and solid states collect in the hydraulic main. From the hydraulic 

 main the vapours and gases travel along the pipe g and the series of vertical pipes j (which 

 are sometimes cooled by water trickling over the surface), where the vapours and gases 

 cool from the contact of the colder surface, and a fresh quantity of vapour condenses. The 

 condensed liquids pass from the pipes g andy and into the troughs H. These troughs 

 always contain liquid at a constant level (the excess flowing away), so that the gas cannot 

 escape, and thus forms, as it is termed, a hydraulic joint. In the state in which it leaves 

 the condensers, the gas consists principally of the following vapours and gases: (1) vapour 

 of water, (2) ammonium carbonate, (3) liquid hydrocarbons, (4) hydrogen sulphide, H.>S, 

 (5) carbonic anhydride, C(X, (6) carbonic oxide, CO, (7) sulphurous anhydride, SO.,, but a 

 great part of the illuminating gas consists of (8) hydrogen, (9) marsh gas, (10) olefiant 

 gas, C.,H 4 , and other gaseous hydrocarbons. The hydrocarbons (3, 9, and 10), the hydrogen, 

 and carbonic oxide are capable of combustion, and are useful component parts, but the 

 carbonic anhydride, the hydrogen sulphide, and sulphurous anhydride, as well as the 

 vapours of ammonium carbonate, form an injurious admixture, because they do not burn 

 (CO.,, SO.,)) and lower the temperature and brilliancy of the flame, or else, although they 

 are capable of burning, they give out during combustion sulphurous anhydride (for 

 example, H.jS, CS._>, and others), which has a disagreeable smell, is injurious when 

 inhaled, and spoils many surrounding objects. In order to separate the injurious pro- 

 ducts, the gas is washed with water, a cylinder (not shown in the illustration) filled with 

 coke continually moistened with water serving for this purpose. The water coming into 

 contact with the gas dissolves the ammonium carbonate ; hydrogen sulphide, carbonic 

 anhydride, and sulphurous anhydride, being only partly soluble in water, have to be got 

 rid of by a special means. For this purpose the gas is passed through moist lime or 

 another alkaline liquid, as the above-mentioned gases have acid properties and are 

 therefore retained by the alkali. In the case of lime, calcium carbonate, sulphite and 

 sulphide, all solid substances, are formed. It is necessary to renew the purifying 

 material, as its absorbing power ceases. A mixture of lime and sulphate of iron, 

 FeSO 4 , acts still better, because the latter, with lime, Ca(HO)._>, forms ferrous hydroxide, 

 Fe(HO)..,, and gypsum, CaSO t . The suboxide (partly turning into oxide) of iron absorbs 

 H.,S, forming FeS and HoO, 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 

 netting m, 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 arrangement. 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, 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. 100 kilograms of coal give about 20 to 80 cubic 

 metres of gas, having a density from four to nine times greater than that of hydrogen. 

 A cubic metre (1000 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 



