140 Anal t/ses of Books, [Aug. 



proportion of catbonic oxide, that the almost infinite variety of 

 aerifomi products are constituted, which are obtainable by the 

 exposure of moistened charcoal, of alcohol or ether, of oil, tal- 

 low, wax, or coal, to a heat ahttle above ignition. This view of 

 the subject, at least, appears to me much more probable, than 

 that they are so many distinct compounds of carbon and hydro- 

 gen, which, on this theory, would be capable of uniting in all 

 possible proportions with each other. 



" Of these aeriform compounds, the gases from coal and from 

 f)U are of most importance, from their widely extended use in 

 artificial illumination. 



" Coal Gas, — By submitting coal to distillation in an iron 

 Tetort, besides a portion of tar and solution of carbonate of 

 ammonia, which condense in a liquid form, a large quantity of 

 permanent gas is evolved. This gas I have shown (Phil. Trans. 

 T808and 1§20) is extremely variable in composition and proper- 

 ties, not only when prepared from different coals, but from the 

 same kind of coal under different circumstances. AVithin cer- 

 tain limits, the more quickly the heat is applied, the greater is 

 the quantity, and the better the quality, of the gas obtained 

 from coal ; for too slow a heat expels the inflammable matter in 

 the form of tar. The early products of gas are, also, the heaviest 

 and most combustible, and there is a gradual decline in quality 

 towards the close of the distillation, insomuch that the last pro- 

 ducts are inferior, by more than one half, to the first. The ge- 

 neral name of coa/ gfls is, therefore, quite indefinite. It is, in 

 fact, a mixture of the two varieties of carburetted hydrogen, with 

 a third which remains to be more fully investigated, as well as 

 with hydrogen gas, carbonic oxide, carbonic acid, nitrogen, and 

 sulphuretted hydrogen gases, in ever-varying proportions. To 

 -describe the methods of separating these gases from each other, 

 would lead into minute details not suited to an elementary work, 

 and I refer, therefore, to the papers which I have published in 

 the Phil. Trans, for 1808 and 1820, and in the third volume, 

 Second Series, of the Manchester Society's Memoirs, or Annals 

 of Philosophy/, vol. XV. 



" Coal gas, as generally procured, has a very disagreeable 

 odour, arising from sulphuretted hydrogen, and, perhaps, a little 

 sulphuret of carbon ; but both these may be washed out of it by 

 cream of hme, with (as I have shown) very little loss of illumi- 

 nating power, and with an entire removal of all unpleasant smell 

 either before or during burning. The best gas has the specific 

 gravity '650 or upwards ; and each volume consumes about 2^ 

 volumes of oxygen, and gives l^j- volume of carbonic acid ; the 

 last portions have a specific gravity as low as *340, and each 

 volume consumes about 8-lOths of a volume of oxygen, and 

 gives about 3-lOths of a volume of carbonic acid. In the best 

 gas, chlorine, applied as directed, p. 416, detects from 13 to 20 

 per cent, of olefiant gas, and the remainder is almost pure car- 

 buretted hydrogen : but the last products contain little or no 



