226 Dr Fyfe on the Illuminating Power of Coal-Gas, &c. 
it would have been with a differently constructed burner. 
May not this exception prove the accuracy of the proposed 
test ? 
From what has now been said with regard to the test which 
I have proposed, I think we are warranted in placing implicit 
confidence in it, as a means of indicating the illuminating 
power of coal-gas ; indeed I have no hesitation in stating, that 
when the trial is properly conducted, it leads to results more 
satisfactory than those given by the shadow ; for it has this 
advantage, that while it is much more easily conducted, it 
points out the amount of light that ought to be afforded by 
one gas as compared with another; whereas, unless all the 
different circumstances that affect the combustion of the gases 
are attended to, the results by the shadow will not be correct. 
One of them, in particular, is the kind of burner,—for when 
gas is rich in matter condensible by chlorine, and a common 
coal-gas burner is used, the illuminating power indicated by the 
shadow will, most probably, be below what it really is, owing 
to the burner not being adapted for the combustion of that 
peculiar kind of gas; and hence one of the advantages of the 
chlorine test. 
The process practised in the experiments I have detailed is, 
with a slight modification, the same as that formerly described. 
Two tubes of about half an inch in diameter, and 12 inches 
long, of the same calibre, and graduated to 100 parts, are em~ 
ployed ; into the one 50 degrees of the gas under investiga- 
tion are introduced, and afterwards into the other there are 
put 50 of chlorine; the water of the trough being heated to 
50, or thereabouts. The coal-gas is then transferred into the 
chlorine, and the tube instantly covered with a shade, to pre- 
vent the action of the light. In the course of five minutes, 
the condensation is complete. Should only one graduated 
tube be used, the coal-gas must be measured first, and then 
put into another tube, after which the chlorine is measured, 
and the coal-gas transferred into it; for, if otherwise, a part 
of the chlorine would be absorbed by the water, during its 
passage through it, and thus lead to variable results. As 
chlorine is absorbable by water, a slight absorption takes place 
during the continuance of the experiment. Before proceeding 
