Analysis of the compound inflwrnmalle Gases. 291 

 portion of olefiant gas, which in them amounts to about 

 one-sixth their bulk. 



2. Sulphureted hydrogen gas is, also, most abundantly 

 produced at the early stages of the distillation. Its proportion 

 then varies from 1 to 5 per cent. ; and towards the close of 

 the process it disappears entirely. It increases the illu- 

 minating power of the coal gas ; but is by no means a de- 

 sirable product ; since it yields by combustion, a gas (the 

 sulphurous acid) which is extremely offensive and irritating 

 to the lungs. By the distillation of coal, more sulphureted 

 hydrogen is produced, than is discovered among the aeri- 

 form products ; for a part, uniting with the ammonia which 

 is generated at the same moment, forms sulphuret of am- 

 monia, a compound which I have found among the con- 

 densed products. 



3. Carbonic acid gas, like the two preceding ones, appears 

 only at an early stage of the process, and in small proportion, 

 never amounting to 5 per cent. A portion of this gas, also, 

 unites with ammonia, and hence carbonate of ammonia is 

 found in the condensed fluid. 



4. The gas from coal undergoes a gradual diminution of 

 specific gravity and combustibility, from the commencement 

 to the close of the process. This, is best shown by inspecting 

 the results of the experiments on the Black- mine and Merthyr 

 coal gas in 7 able If. because they were reserved in a greater 

 number of separate portions than usual. The progression 

 would, perhaps, have been more regular, in these as well as 

 in the other instances, if much of the gas had not been al- 

 lowed to escape, in consequence of the immense quantity 

 which was produced. The specific gravity of the coal gas 

 appears to afford a meas-ure of its fitness for illumination, 

 sufficiently accurate for practical uses ; but does not bear an 

 exact correspondence to the chemical properties of the gas, as 

 ascertained by combustion. It may be remarked, also, by 

 comparing the two last columns of the second table, that the 

 carbonic acid produced does-not always bear the same pro- 

 portion to the oxygen expended. Thus the first product of 

 gas from cannel coal combines with 234 measures of oxygen 

 gas ; and gives 139*7 of carbonic acid. But the gas from coal 



T'2 tar. 



