Gas-Burners , and on the Illuminating Power of the Gases. 29 
The discrepancy that exists among the opinions of various well known ex- 
perimenters, regarding the relative light of oil and coal gas, is at first sight 
quite incomprehensible. We apprehend, however, that the facts mentioned 
in the previous part of this paper, will account satisfactorily for most of them ; 
and we do not doubt they would have accounted equally well for all, had the 
particulars of the experiments been uniformly detailed. 
In addition to the remarks formerly made concerning the fallacies which 
arise from the use of inaccurate photometers, and likewise from the gas being 
burnt with various lengths of flame, in burners of various constructions, and 
with glasses of various forms, we shall now add a few observations on those, 
of equal importance, which proceed from varieties in the quality of the gases. 
The quality of the gas procured from coal and oil varies greatly with the 
material, with the mode of applying the heat, with the size and form of the 
retorts, with the period of the process at which it is collected, and with other 
circumstances, which it is unnecessary to enumerate. 
The specific gravity of coal-gas, such as is actually made by public compa- 
nies, varies from a little above 400 to 700. That of London is of very infe- 
rior quality. The worst we remember to have read of as employed in the 
streets, is that with which Mr Dewey made his experiments at Whitechapel, 
( Annals of Philosophy, vol. vi. 404. N. $.) Its density was only 407* The gas 
employed by Messrs Phillips and Faraday, was scarcely better, its density being 
429 (lb.). The Westminster coal-gas, as related byMrBrande, in the Bake- 
rian Lecture for 1820, had a specific gravity of 443, and even the best he ever 
examined did not exceed 494. The worst coal-gas we ever examined at Edin- 
burgh was 510, and it very rarely fell short of 580. Dr Henry found, that 
the gas prepared from the Wigan Cannel coal (which is nearly the same as that 
used here), and drawn from the pipes after the retorts had been worked an 
hour, varied from 620 to 650. The coal-gas of Edinburgh rarely exceeds 620 ; 
but we once found it so high as 680. By far the greater number of specimens 
of Edinburgh gas, therefore, will be found to have a specific gravity between 
580 and 620. Its mean specific gravity, deduced from not less than twenty 
trials, made last autumn, was exactly 600. The gas of Glasgow is generally 
said to be at least equal to that of Edinburgh ; but on what good authority we 
have been unable to learn. Mr Anderson of Perth assures us, that the spe- 
cific gravity of the gas of that city is so high as 700. 
There is evidently, therefore, a great difference in the quality of coal-gas 
made in different places. But, if we may judge from our own trials with the 
gas of Edinburgh, and from the data furnished by various experimenters re- 
garding the specific gravity of the London gas, the quality does not differ much 
in the same place, or, at least, in the same establishment. 
This fact must lead to the inference, that the process for the manufacture 
of coal-gas is now so thoroughly understood, that the workmen can follow it 
with perfect regularity. It is exceedingly probable, that the manufacture of 
coal-gas is likewise nearly as perfect as it ever will be made. It is now many 
years since it has been the object of attention among men of science, and since 
numerous public companies have been stimulated by the spirit of commercial 
rivalry to do their utmost for improving its quality. Many experiments have 
therefore been made for that end, both by scientific and by practical men ; 
