228 Dr Fyfe on the Illuminating Power of Coal-Gas, &c. 
gases may be ascertained as compared with other sources of 
light. 
It is evident from what has been said, that, in finding the 
value of a gas as compared with other sources of light, atten- 
tion must be paid to the quality of the gas; a circumstance 
which by many has been totally disregarded, and hence the 
very discordant results which have been obtained. In com- 
paring the gas by the shadow given by other lights, we must 
in fact not only attend to the different circumstances affecting 
the combustion; we must also at each trial ascertain the 
amount of condensation by chlorine; for the quality of a gas 
manufactured on different days, at the same place, will be 
found to vary considerably. In the trials I am now to state, 
made with the view of finding the comparative expense of 
light as got from candles, oil, &c., I have uniformly kept this 
in view; and by doing so, we are enabled to judge of the 
expense, not only in this town, but also in other places, pro- 
vided, of course, we know the illuminating power of the gas 
by the chlorine test. 
The first series of experiments were those with candles, of 
which ten different kinds were tried. Tallow single wick, 
tallow double wick, cocoa, paim, composite, margerine, dia- 
phane, composition, spermaceti, wax,—all short sixes. 
Tallow.—Very different statements haye been given of the 
illuminating power of coal-gas as compared with that from tal- 
low candles, and which has been accounted for by the difficulty 
of getting the light from the candle to be uniform; the chief 
cause of the discordance is, however, more probably the differ- 
ence in the quality of gases manufactured at different places, 
In conducting my trials, I have paid due attention to the former ; 
trying the candles at different times, so as to have a wick of 
various lengths. The standard gas light, in all the trials, was 
a jet burning under a uniform pressure, with a flame of five 
inches, and consuming exactly one foot per hour. 
From numerous trials, I found that the tallow (single wick, 
short-six), when compared with the gas, and taking the average 
of all the trials, was as 1 to 3.75. A short-six will be found, 
when properly snuffed, to last for six hours, or very nearly so; 
