GAS, COAL 543 



issuing from the coal strata of this country, and -which have been investigated by 

 Graham, Playfair, and others. Olefiant gas can be prepared nearly pure by heating 

 in a glass retort a mixture of 1 part by weight of alcohol, and 6 parts oil of vitriol. 

 The gas must bo passed through solution of caustic soda, to remove sulphurous and 

 carbonic acids with which it is generally contaminated. 



Olefiant gas is colourless, and possesses a peculiar and slightly unpleasant odour. 

 Its specific gravity is rather less than that of atmospheric air, being '9784 : 100 

 cubic inches, at 60 F. and 30 inches barometrical pressure, weigh 30-3418 grains. 

 It consists of two volumes of carbon-vapour and four volumes of hydrogen, the six 

 volumes being condensed to two. It contains, in a given bulk, exactly twice as much 

 carbon as is contained in light carburetted hydrogen. Olefiant gas is inflammable, 

 but does not support combustion : when inflamed as it issues from a jet into the atmo- 

 sphere, it burns with a white flame, emitting a very brilliant light without smoke. In 

 burning, it consumes three times its volume of oxygen, and produces twice its volume 

 of carbonic acid. Exposed to a full red heat, as in passing through a red-hot tube, it 

 is rapidly decomposed, carbon being deposited, whilst hydrogen and light carburetted 

 hydrogen are produced ; exposure to a full red heat consequently soon entirely destroys 

 its illuminating power. 



Acetylene. This hydrocarbon may be formed synthetically by the direct union of 

 its elements. Berthelot has shown that when the electric light is produced between 

 carbon poles in an atmosphere of hydrogen, the high temperature thus produced 

 determines the formation of acetylene. Among other modes of generating acetylene 

 may be mentioned the incomplete combustion of gases and vapours containing carbon 

 and hydrogen ; the decomposition of olefiant gas, alcohol-vapour, or ether-vapour, by 

 the action of heat, and the decomposition of marsh-gas by the passage of electric 

 sparks from an induction-coil. 



Acetylene is a colourless transparent gas, of peculiar odour. Its specific gravity is 

 0'92. The gas burns with an intensely luminous flame, much brighter than that of 

 olefiant gas ; for a given volume of acetylene contains the same bulk of carbon with 

 only half as much hydrogen as in an equal volume of olefiant gas. Mixed with 

 chlorine, acetylene explodes, even in diffused daylight, with separation of carbon as a 

 dense black smoke. Acetylene passed through a solution of subchloride of copper 

 (cuprous chloride) yields a red precipitate, which, when dried, forms an amorphous 

 solid substance that readily explodes on being struck with a hammer, or on being 

 suddenly heated to a temperature a little higher than that of boiling water. See 

 ACETYLENE. 



Propylene and Butylene. The first of these highly-illuminating constituents of coal- 

 gas may be obtained by passing the vapour of fusel oil through a red-hot tube, and 

 the second by the electric decomposition of valerate of potash. Both these gases are 

 colourless, possess a light ethereal odour, and burn with a brilliant white flame. Like 

 olefiant gas, they are rapidly decomposed at a bright red heat, depositing much carbon, 

 and being converted into the non-illuminating gases hydrogen and light carburetted 

 hydrogen. Propylene consists of three volumes of carbon-vapour, and six volumes of 

 hydrogen condensed to two volumes. It therefore contains, in a given volume, one- 

 half more carbon than olefiant gas. Its specific gravity is 1-4511. 



Butylene consists of four volumes of carbon-vapour and eight volumes of hydrogen, 

 the twelve volumes being condensed to two ; it consequently contains, in a given 

 volume, twice as much carbon as olefiant gas. Its specific gravity is 1-9348. 



Vapours of hydrocarbons of the formula On H D . A considerable number of compounds 

 having this formula are known to exist in coal-tar, and, as many of them are very 

 volatile, they must be diffused as vapours in coal-gas ; but as they have not yet been 

 successfully disentangled from each other, no account of their individual properties 

 can be given ; they all, however, contain more carbon, in a given volume, than buty- 

 lene, and must therefore contribute, proportionally to their volume, a greater illumi- 

 nating power than any of the gaseous hydrocarbons. They are all readily decomposed 

 at a bright red heat, chiefly into carbon and non-illuminating gases. 



Vapours of hydrocarbons of the formula C n H (n _ 6) . These consist chiefly of benzol, 

 toluol, cumol, and cymol ; compounds which, being components of the more volatile 

 portions of the tar, diffuse themselves into the gaseous products of distillation, con- 

 tributing in no inconsiderable degree to the total illuminating effect of the gas. The 

 composition of these substances has been already given in the Table ; and it is there- 

 fore only necessary here to remark that benzol-vapour contains, in a given volume, 

 three times as much carbon as olefiant gas, whilst the vapours of toluol, cumol, and 

 cymol contain respectively 3, 4^, and 5 times the amount of carbon contained in 

 olefiant gas. For a further account of these and other hydrocarbons, see NAPHTHA ; 

 COAL; DESTRUCTIVE DISTILLATION. 



Vapours of hydrocarbons of the formula C n H (n 12) . The only vapour of this 



