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MR. JOHN LEIGH ON THE 



A brief statement of the constituents of coal gas, as de- 

 termined by numerous analyses, without reference to their 

 relative proportions, by affording an idea of the objects of 

 the analytical enquiry, may best precede an enquiry into 

 the nature and value of the methods employed. 



The gases eliminated from coal or cannel consist essen- 

 tially of olefiant gas, volatile arid condensible hydro-carbons, 

 light carburetted hydrogen, hydrogen, carbonic oxide, car- 

 bonic acid, sulphuretted hydrogen, sulphuret of carbon, 

 cyanogen, ammonia, and aqueous vapour. Of these the 

 carbonic acid, sulphuretted hydrogen, and cyanogen, are 

 positively injurious to health. The two latter, and ammo- 

 nia, are injurious to the apparatus employed for the distri- 

 bution of the gas, and these, with the sulphuret of carbon, to 

 the furniture in rooms in which the gas is burnt. The 

 nitrogen and oxygen generally found In gas, proceed 

 from atmospheric air, which gains access by leakage of 

 apparatus, and by opening the retorts when charging with 

 fresh coal. In the process of purification now generally 

 adopted at all well-regulated gas-works, the carbonic acid, 

 sulphuretted hydrogen, cyanogen, and ammonia, are en- 

 tirely removed ; so that there remain olefiant gas, illumina- 

 ting hydro-carbons, light carburetted hydrogen, hydrogen, 

 carbonic oxide, and a minute portion of sulphuret of car- 

 bon, not recognizable by chemical tests in small quantities 

 of the gas. Of these the hydrogen and carbonic oxide, 

 though giving out much heat on combustion, yield scarcely 

 any light, and bum with a very feeble blue flame ; they only 

 dilute the gas, adding nothing to its illuminating power. 

 The light carburetted hydrogen burns with a yellow flame ; 

 the olefiant gas, and condensible hydro-carbons, with a very 

 brilliant white flame, and give to the gas its chief illumina- 

 ting power. The entire illuminating power of the gas, then, 

 depends on the olefiant gas, hydro-carbons, and light 

 carburetted hydrogen; the richness and value of a gas 



