1 1 8 DESTRUCTIVE DISTILLATION PART i. 



for the gaseous part has a feeble flame, and requires a 

 higher temperature than solid matter, which becomes 

 luminous at about 700 in the dark, and at from 1000 

 to 2000 in bright daylight. Coal gas consists of a 

 combination of illuminants : olefiant gas, which contains 

 86 per cent, of carbon, carburetted hydrogen or marsh 

 gas, which contains 75 per cent., carbonic oxide, carbonic 

 acid gas, hydrogen, sulphuretted hydrogen, and a very 

 small quantity of nitrogen, besides the bisulphide of 

 carbon, and benzol, a pure hydro-carbon, consisting of 

 12 equivalents of carbon and 6 of hydrogen. 



The poisonous quality of coal gas is owing to the 

 carbonic oxide, which is fatal to life, and its explosive 

 quality to carburetted hydrogen, which also is generated 

 by decomposition of vegetable matter in stagnant pools 

 and marshes ; and in the firedamp of mines it still bears 

 testimony to the vegetable origin of coal. That fatal 

 gas increases in explosive force as it mixes with atmo- 

 spheric air, and is at a maximum when it amounts to 1 2 

 per cent. Hydrogen, carburetted hydrogen, and car- 

 bonic oxide do not add much to the light, on account of 

 the feeble flame of hydrogen and the small quantity of 

 carbon they contain, but they force the chief illumi- 

 nating gases out of the iron retorts in which the coal is 

 distilled before the heat has had time to decompose 

 them, and they also enable them to burn without smell 

 or smoke. 



Carbonic acid, bisulphide of carbon, and sulphuretted 

 hydrogen are impurities from which coal gas is freed 

 before it is fit for use. By passing the gas over lime, 

 the lime absorbs both the carbonic acid and the sul- 

 phuretted hydrogen ; one per cent, of carbonic acid 

 diminishes the illuminating power six per cent., and 

 the sulphuretted hydrogen has an abominable smell. 



The bisulphide of carbon, consisting of one equivalent 

 of carbon and two of sulphur, is got rid of by passing 



