(3 ap 
On the Use of Chlorine as an Indication of the Illuminating 
Power of Coal Gas, and on the Comparative Expense of 
Light derived from different sources. By. Anvrew Fyre, 
M.D., F.R.S.E., F.R.S.S.A. Communicated by the Royal 
Scottish Society of Arts.* 
Ix a paper published in the Edinburgh Philosophical Jour- 
nal for 1824, I recommended the condensation of the heavy 
hydro-carbons by chlorine, as an easy and efficacious method 
of ascertaining the comparative illuminating power of coal- 
gas, while, at the same time, it had the advantage of enabling 
us to compare one gas with another, though not brought di- 
rectly into contrast with it, and thus, by fixing on one as a 
standard, to state the illuminating power numerically. 
With regard to the methods now in use, I mean the specific 
gravity, the quantity of oxygen necessary for combustion, and 
the depth of shadow, the last is the only one in which we can 
place any confidence. As to the specific gravity, if the gas be 
pure, that is, free from carbonic acid and sulphuretted hydro- 
gen, then the heavier it is the more likely is it to be of high 
illuminating power; but this is not always the case: thus the 
specifie gravity of olefiant gas and of carbonic oxide is the 
same, but the latter burns with a feeble blue flame, whereas 
the former gives forth a brilliant light. Now, suppose coal- 
gas to contain little of the heavy hydro-carbon, and a large 
proportion of carbonic oxide, then the spécifie gravity may be 
such as to induce us to expect the illuminating power to be 
high, when in fact it is not. 
The same remark is applicable to the mode of testing by the 
quantity of oxygen necessary for complete combustion. A 
gas with much olefiant will no doubt require much oxygen, this 
gas taking no less than thrice its own bulk; but let us sup- 
pose a variety of gases to have the same proportion of olefiant 
* Read before the Royal Scottish Society of Arts, 10th January 1842. 
VOL, XXXII. NO. LXIV.—aPnrip 1842, Q 
