224 Dr Fyfe on the Illuminating Power of Coal-Gas, §¢. 
tus, with the view of having the illuminating power as varied 
as I could possibly obtain. , 
It is well known that the quality of coal-gas, even when 
manufactured from the same kind of coal, depends much on 
the mode of manufacture; when slowly prepared, and when 
the same charge of coal is long subjected to heat, a larger 
quantity of gas is given off, than when the time for the charge 
is shorter ; but then the illuminating power is low, owing to 
the gas which is last evolved having very little of the heavy 
hydro-carbons ; and hence those companies who dispose of 
their coke to advantage, have, besides the quantity of gas 
to be got, another object in view, viz, the freeing of the coke 
from all its gaseous ingredients, otherwise it is not considered 
valuable, indeed will not be purchased by those in the cus- 
tom of using it. It is this which, in addition to the difference 
in the quality of the coal employed, makes such a difference 
between the quality of gas prepared in England and Scotland ; 
for, as the coke from English caking-coal is more prized than 
that from parrot-coal, which is much used in Scotland, the 
English companies may generally be considered, not only as gas 
companies, but also as coke companies, indeed derive a great 
deal of their profit from the coke. Hence, in judging of the 
price of gas, we must take into account its quality ; and hence 
I conceive the importance of having an easy method of ascer- 
taining this, and of comparing different gases with each other. 
In the first series of experiments, the results of which Iam 
now to give, two gases, manufactured under different cireum- 
stances, were compared with the light afforded by a wax 
candle kept burning, as nearly as possible with a uniform 
flame ; the gases being consumed in jet burners with a 5 inch 
flame. Taking the average of several trials, gas A gave alight 
as 2.16, compared to that of the wax-candle as 1; the conden- 
sation by chlorine was 15. Gas B, under similar circum- 
tances, gave a light as 1.98; condensation by chlorine 13, and 
15:13 :: 2.16: 1.86; by the shadow it was 1.98. 
In another trial with other gases the light was compared 
with that afforded hy a ¢allow candle (short six). Gas C, the 
