THE HYDROCARBONS 145 



presence of hydrocarbons richer in carbon. When the distilla- 

 tion is completed note (i) that coke is left behind in the 

 Florence flask, (2) that coal tar has collected in the wash flask, 

 (3) that the aqueous Hquid in the wash flask is alkaline to 

 litmus, and has the pungent smell of ammonia. Pour off this 

 " ammoniacal liquor " into a distillation flask, add lime, and distil 

 off the ammonia into a little dilute sulphuric acid. Evaporate 

 the solution in a dish to obtain crystals of sulphate of 

 ammonium. To a portion add caustic potash solution. Note 

 the smell of ammonia gas set free. 



Coal Gas. — For the manufacture of coal gas, coal is heated in fire-clay 

 retorts. The volatile products are passed into the "hydraulic main" — 

 a reservoir where tar and an aqueous liquid condense. The gas now 

 traverses " condensers," — upright iron tubes exposed to the air, in which a 

 further quantity of tar and water condense, and "scrubbers," — towers in 

 which the gas is met by a stream of water to dissolve out ammonia. The 

 gas next passes through "purifiers," in which it is first led over 

 hydrated ferric oxide to remove the last traces of sulphuretted hydrogen 

 (a part having previously been removed with the ammonia), the product 

 of combustion of which — sulphur dioxide — is injurious, and then over 

 slaked lime to remove carbonic acid gas, which diminishes the luminosity 

 of burning coal gas. The coal gas is finally stored in gas-holders. 



With respect to its composition, coal gas may contain — 



Heat Givers. 



Hydrogen, H . . .50% 

 Marsh gas, CH4 . . -35% 

 Carbon monoxide, CO . . 8% 



Light Givers. 



Ethylene, C2H4 . . .2% 



Propylene, CgHg . . .1% 



Benzene, CgHg , , .1% 



Impurities. 



Nitrogen 1% 



Carbon dioxide i% 



Sulphuretted hydrogen and oxygen , Traces 



The proportion of illuminants will depend upon (i) the kind of coal used, 

 cannel coal yielding a much larger proportion than other bituminous coal ; 

 and {2) the temperature of distillation, a larger proportion being formed at 

 low than at high temperatures. Gas of low illuminating power may be 

 enriched by vapours of oils of high illuminating power. 



