Dr Fyfe on the Illuminating Power of Coal-Gas, sc. 227 
to any trials, it is therefore necessary to ascertain the amount 
of this, and then to deduct it from the condensation occasioned 
by the action on the coal-gas. In the tube which I have used, 
I found the absorption to be exactly 1 degree for every five 
minutes, and which continues in the same ratio, after the action 
of the chlorine on the hydro-carbon is over. I have, therefore, 
always deducted 1 degree for each five minutes, from the total 
loss, as indicated by the rise of the water in the tube. As, 
however, the action is over in five minutes, I have seldom con- 
tinued the trial beyond that time, of course deducting 1 de- 
gree from the loss sustained. As chlorine and the condensible 
matter act on each other in equal volumes, a condensation 
of 10, when 50 of each are used, indicates ten per cent. of loss 
by the coal-gas. 
Should this method of ascertaining the illuminating power 
of gases be ultimately found to be correct, another important 
result may follow its introduction into practice. If we can, 
by it, fix the illuminating power of one gas compared with 
that of another gas, the quality of which has been previously 
determined, and which is consumed with a burner that is 
known to burn it advantageously, and if the gas which we are 
subjecting to trial by the shadow test does not show such a 
high illuminating power as we are led to expect, from the 
known condensation by chlorine, the probability is, that the 
burners are not adapted for consuming the gas advantageously, 
and hence the necessity of altering the apertures, till the power 
by the shadow is what it ought to be, according to the chlo- 
rine test. 
There is still another advantage attending the introduction 
of the chlorine test in addition to those mentioned; it is the 
facility of comparing different gases with one another, when 
they cannot be brought together, so as to try them by the 
shadow. The illuminating power may be considered just as 
the condensation by chlorine, and thus, then, we may state it 
numerically. Thus taking a coal-gas having only 1 per cent. 
of matter condensible by chlorine, its illuminating power may 
be considered as unity, and all others would be as the per cent-, 
age of condensation. Hence, also, the illuminating power of 
