CARBON AND THE HYDROCARBONS 361 



accord. The gas which is evolved consists principally of marsh gas. 30 

 If wood, coal, or many other vegetable or animal substances are decom- 

 posed by the action of heat without access of air that is, are subjected 

 to dry distillation they, in addition to many other gaseous products 

 of decomposition (carbonic anhydride, hydrogen, and various other 

 substances), evolve a great deal of methane. Generally the gas which 

 is used for lighting purposes is obtained by this means and therefore 

 always contains marsh gas, mixed with dry hydrogen and other vapours 

 and gases, although it is subsequently purified from many of them. 31 



30 It is easy to collect the gas which is evolved in marshy places if a glass bottle be 

 inverted in the water and a funnel put into it (both filled with water) ; if the mud of the 

 bottom be now agitated, the bubbles which rise may be easily caught by the inverted 

 funnel. 



51 Illuminating gas is generally prepared by heating gas .coal (see Note 6) in oval 

 cylindrical horizontal cast-iron or clay retorts. Several such retorts BB (fig. 58)^,re 

 disposed in the furnace A t and heated together. When the retorts are heated to a 

 red heat, lumps of coal are thrown into them, and they are then closed with a closely 

 fitting cover. The illustration shows the furnace, with five retorts. Coke (see Note 1, 

 dry distillation) remains in the retorts, and the volatile products in the form of vapours 

 and gases travel along the pipe d, rising from each retort. These pipes branch above 

 the stove, and communicate with the receiver / (hydraulic main) placed above 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 and./ 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 they form, 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 sul- 

 phide, H 2 S, (5) carbonic anhydride, COo, (6) carbonic oxide, CO, (7) sulphurous anhy- 

 dride, SO 2 , but a great part of the illuminating gas consists of (8) hydrogen, (9) marsh 

 gas, (10) olefiant.gas, C 2 H 4 , and other gaseous hydrocarbons. The hydrocarbons (3, 9, and 

 10), the hydrogen, and carbonic oxide are capable of combustion, and are useful com- 

 ponent parts, but the carbonic anhydride, the hydrogen sulphide, a'nd sulphurous anhy- 

 dride, as well as the vapours of ammonium carbonate, form an injurious admixture, 

 because they do not burn (CO 2 , SO 2 ) and lower the temperature and brilliancy of the flame, 

 or else, although capable of burning (for example, H 2 S, CS 2 , and others), they give out 

 during combustion sulphurous anhydride which has a disagreeable smell, is injurious 

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

 products, 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, car- 

 bonic 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 other 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 decreases. A mixture of lime and sulphate of iron, 

 FeSO 4 , acts still better, because the latter, with lime, Ca(HO) 2 , forms ferrous hydroxide, 

 Fe(HO)., and gypsum, CaSO 4 . The suboxide (partly turning into oxide) of iron absorbs 



