SECTION II. FUEL BY-PRODUCTS 



Ammonia. The commercial distillation of coal began when 

 William Murdock invented his process for obtaining illu- 

 mination by the combustion of coal gas. When coal is 

 subjected to dry distillation in a closed retort, a complex 

 series of changes take place, the nature of which depends 

 very largely on the temperature of distillation. When the 

 temperature is only just above that at which decomposition 

 begins, the condensible volatile products of distillation consist 

 chiefly of liquids resembling those contained in petroleum 

 oil, whilst the gases that are formed are small in volume. 

 When the temperature is raised, these products undergo 

 further changes, owing to contact with red-hot coke and with 

 the heated surface of the retort, and also by the action of 

 radiant heat inside the retort; these decompositions pro- 

 duce hydrogen, methane, ethylene, benzene, and complex 

 hydrocarbons. By still further raising the temperature 

 the quantity of hydrogen is increased, but the amounts of 

 the complex substances are decreased. Doubtless, in the 

 limit of ascending temperature, nothing but carbon and 

 hydrogen would be produced. The nitrogen is partly 

 retained by the coke, and partly distilled as pyridine and 

 ammonia. In addition, free nitrogen and traces of hydro- 

 cyanic acid are produced and mixed with the gas. About 

 70 % of the coal remains behind in the retort as coke, 

 which, whilst it consists chiefly of carbon, contains also 

 all the mineral matter of the coal and a fraction of the other 

 elements. 



When the primary object of the distillation of coal is 

 to produce illuminating gas, there are some slight differences 

 of procedure from when the primary object is the production 

 of coke for iron smelting. The horizontal gas retort is 



