180 Dr Fyfe on the Comparative Value of Oil and Coal Gas. 
mixture be excluded from light, to prevent any action on the 
carburetted hydrogen ; hence affording an easy method of as- 
certaining its proportion. The mode of performing the experi- 
ment is very simple : A graduated jar, inverted on a water- 
trough, must be filled to the mark 50 with the gas, and 50 mea- 
sures of chlorine are then to be introduced, the tube being co- 
vered with a paper shade to prevent any action on the other 
gases. In the course of from ten to fifteen minutes the conden- 
sation is completed. As chlorine and olefiant gas combine in 
equal proportions the diminution in the mixture will at once in- 
dicate the quantity of the latter in 100 parts of the gas subject- 
ed to trial ; thus, if the water should rise to 40, the gas must 
have contained 40 per cent, of olefiant gas. From the trials I 
have made, I find that it promises to be a very easy, and, as far 
as I have found, an accurate mode of ascertaining the compara- 
tive illuminating power. As tried in this way, it is (with oil- 
gas and coal-gas prepared in Edinburgh), as 17 to 31 ; that is 
very nearly as 1 to 1.8. 
Numerous other trials have been made on oil-gas with this 
test, and in none of them, except in one, did the quantity of gas 
condensed by chlorine come up to double that of coal-gas ,* but 
this gas was prepared by a small apparatus of my own, and is not 
therefore to be brought forward as a fair specimen. In some 
of the others the quantity did not exceed from 25 to 28. 
If this method should ultimately prove correct, it will afford 
a mode, not only of finding the comparative illuminating power 
of gases when tried in the same place ; but it will enable us to 
compare the light given out by one gas with that from any 
other, for we have merely to fix some point to commence with, 
and calling the illuminating power at this as 1, the others will 
bear a ratio to it, according to the quantity of olefiant gas they 
contain above that taken as the standard. 
Though the different methods mentioned may afford a mode 
of ascertaining the comparative illuminating power of one speci- 
men of coal-gas to that of a specimen of oil-gas, yet, I think it 
is impossible, from the experiments of one individual, to come 
to any general conclusion, the quality of the gas depending so 
much on the mode of manufacture. From the different ex- 
periments I have performed on coal-gas it seems to vary more 
