424 COMPOUNDS OF CARBON. 



damp, but remains with its metallic lustre unchanged in that 

 gas. Indeed, in the ordinary fire damp no other combustible 

 gas whatever can be found, besides light carburetted hydrogen. 

 The superiority of coal gas, in illuminating power, depends 

 principally upon the high proportion of olefiant gas and the 

 denser hydro carburets which it contains. The free hydrogen 

 and carbonic oxide present give no light, and are positively inju- 

 rious. As the highly illuminating constituents are dense, and 

 contain much carbon, the value of coal gas is to a certain extent 

 proportional to its density, and to the quantity of oxygen which 

 it requires for complete combustion. In the analysis of coal 

 gas, the different gases may thus be separated : 1st. Olefiant 

 gas, naphtha vapour, and similar hydrocarburets, by mixing the 

 gas, in a dark place, with half its bulk of chlorine, and afterwards 

 washing with caustic potash ; 2ndly, carbonic oxide by potassium 

 gently heated in the gas ; Srdly, the proportion of light carbu- 

 retted hydrogen may be determined by detonating the mixture 

 in a eudiometer (page 2/8), with a measured quantity of oxygen, 

 and ascertaining the quantity of carbonic acid formed, which 

 retains the volume of the carburetted hydrogen ; 4thly, the free 

 hydrogen, by observing the quantity of oxygen remaining, by 

 means of a stick of phosphorus introduced into the gas, and 

 thereby ascertaining the quantity of oxygen consumed in the 

 combustion ; from this quantity deduct twice the measure of 

 the carburetted hydrogen, and half the remaining measure of 

 consumed oxygen represents the hydrogen ; 5thly, the residuary 

 gas after these processes is the nitrogen of the coal gas. 



Structure of flame. The quantity of light obtained from the 

 combustion of coal gas depends entirely upon the manner in 

 which it is burned, which will appear from the consideration of 

 the structure of luminous flames. The flame of a spirit lamp, 

 candle, or gas-jet is hollow, as may be observed by depressing a 

 F 44 snee ^ f w i re trellis upon it, which gives a section of 

 A " * the flame ; the seat of the combustion being the mar- 



k c gin of the flame, where alone the combustible vapour 



is in contact with the air. Of volatile carbonaceous 

 combustibles, the flame consists of three parts, which 

 -A are represented in section, (Fig. 44.) : 



A, cone of vaporized combustible. 



B, sphere of partial combustion. 



C, sphere of complete combustion. 



