Dr Fyfe on the Illuminating Power of Coal-Gas, &c. 223 
Drs Christison and Turner, the accuracy of the chlorine test 
was called in question, partly because, when testing the gases 
by the photometrie process, as pointed out by Rumford, due 
attention was not paid to the different circumstances affecting 
the combustion, and partly owing to the opinion expressed in 
the paper by the authors, that other ingredients than olefiant 
exist in coal-gas, which afford light by their combustion, and 
which are also condensible by chlorine. As to the latter ob- 
jection it is of little value, provided we find the results in- 
dicated by the chlorine test, to agree with the photometric 
one. With regard to the latter, it must be admitted, that 
in some of the trials, where two gases were compared with 
each other, due attention was not paid to the height of 
the flame, and to the other circumstances affecting the com- 
bustion, which, at the time that I was engaged in the in- 
quiry, were not known to have an influence on the illumi- 
nating power. The influence of these has now been fully in- 
vestigated, and made known, in the paper by Drs Christison 
and Turner, and also in that which I read to the Society in 
1840. Since then, I have again had my attention drawn to 
the subject, and have had many opportunities of putting the 
chlorine method to the test of experiment ; and I must say that 
Iam more and more inclined to put the most implicit confi- 
dence in it, not only as a very simple, but at the same time 
a correct method of ascertaining the comparative illuminat- 
ing power. I trust the results of the trials will not be devoid 
of interest. 
In fixing the illuminating power of the gases by the shadow, 
two accurately adjusted metres were used, one for the one 
gas, the other for the other. Sometimes the gases were con- 
trasted with each other; in which case, similar burners, con- 
suming the gas under the same circumstances, were employed ; 
and with the view of securing accuracy in the results, the 
burners were sometimes changed from one gas to another; at 
other times, the light given by the gas was contrasted with 
that from candles. The gases subjected to trial were some- 
times those with which Edinburgh is at present supplied, 
sometimes they were prepared by myself, in a small appara- 
