542 



GAS, COAL 



HI. SOLID. 



Name 



Chemical formulae 

 NH 4 OCO a 21*H 



In aqueous layer : Carbonate of ammonia 



Hydrosulphato of sulphide of 



ammonium .... NH 4 S + HS 2(VH*)S + H 2 S 



Sulphite of ammonia . . NH*OS0 2 2(WH)SO 3 



Chloride of ammonium . . NH 4 C1 NH'Cl 



In oily layer : Paraffine C <0 H 42 c 20 H 42 



., Naphthaline .... C 20 H 8 C'H 8 



Paranaphthaline (Anthracene) . C 28 H 10 C 14 H 10 



Pyrene . . . . . C 30 H 4 C 15 H 4 



Chrysene C 32 H 12 c' 8 H 12 



In practice, there is not such a perfect separation of the products as is represented 

 in the above table : thus a small proportion of the gases dissolves in the liquid pro- 

 ducts, whilst most of the liqiiids, and even some of the solids, diffuse themselves i n 

 the form of vapour, to a certain extent, into the gases ; and the solids are in most 

 cases almost completely dissolved in the liquids. The relative proportions also in 

 which these products occur greatly depend upon the temperature employed in the 

 destructive distillation, and the length of time during which the volatile products are 

 exposed to it; a low temperature and short exposure favouring the formation of 

 solids and liquids, whilst a higher heat and longer exposure determine the production 

 of a larger proportion of gases at the expense of the solids and liquids. 



The usual process of gas-making consists in exposing coal or cannel to a bright- 

 rod heat, in close vessels of convenient size and shape, until all, or the greater part, 

 of the volatile matter is expelled. Coke is the material left in the retort, and the 

 matters volatilised consist of condonsiblo vapours, and of permanent gases more or less 

 saturated with these vapours. By a simple process of refrigeration nearly the whole 

 of the vapours may be readily condensed, thus separating the gases more or less per- 

 fectly from the liquid and solid products of the distillation. But this preliminary 

 process of purification leaves the gases still in a state totally unfitted for use in the 

 production of artificial light. They still retain constituents, which are either noxious 

 in themselves, or generate noxious compounds when they are burnt, such as sulphu- 

 retted hydrogen, sulphide of ammonium, carbonate of ammonia, and bisulphide of 

 carbon. They also contain carbonic acid, which greatly diminishes the amount of 

 light yielded by the illuminating gases with which it is mixed. 



Besides these injurious ingredients, which may be conveniently included in the term 

 impurities, there are others which do not contribute anything to the illuminating 

 power of the mixture, and which may bo denominated diluents. We can thus classify 

 the constituents of gas-coal as follows : 



As the intelligent manufacture of gas for illuminating purposes requires a know- 

 ledge of the leading properties of the compounds included under the throe heads just 

 mentioned, we will now proceed briefly to describe them. 



L ILLUMINANTS. 



Olefiant gas. This gas has been proved l.y Bcrthelot to exist in coal-gas, and it is 

 probably always a constituent of the illuminating prases from resin, oil, peat, and wood. 

 It is occasionally, though rarely, met with in natmv. ;is a product of the action of 

 volcanic heat upon coal-bearing strata; it never occurs, however, in coal strata under 

 ordinary circumstances, and no trace of it has ever been met with amongst the gases 



