38 Brs Christison and Turner on the Construction of 
It is equally certain, however, that coal-gas, when completely 
free of sulphuretted hydrogen, still contains sulphur. On burn- 
ing a small jet of coal gas, free from sulphuretted hydrogen, so 
as to collect the fluid formed during the combustion, the pre- 
sence of sulphuric acid was uniformly detected, demonstrating 
the existence of some, compound of sulphur. What that com- 
pound is, has not yet been ascertained ; but from its peculiar 
unpleasant odour, and the circumstances under which it is ge- 
nerated, the sulphur is most probably in combination with car- 
bon, either in the form of the volatile liquid, sulphuret of car- 
bon, as Mr Brande conjectures, or, what is perhaps more likely, 
as a gaseous compound, containing a less proportion of sulphur 
than exists in that liquid. 
In whatever state of combination the sulphur may be, it does 
not affect the salts of lead like sulphuretted hydrogen ; nor does 
it act so readily, if at all, on polished silver and gold. Hence 
the gas which contains only this impurity, will be less inju- 
rious, when any of it escapes unburnt, than such as contains 
sulphuretted hydrogen ; but since it uniformly yields acid va- 
pours during its combustion, one part of the objection remains 
in full force. 
These various objections, whatever weight they may have, ap- 
ply to coal-gas only. 
APPENDIX. 
In relating the experiment with fire-light on the photometer, the number 
3° has been inadvertently substituted for 12°, the effect of an Argand oil-lamp 
on the photometer at the distance of 6| inches ; and, therefore, the light of 
the fire, according to the photometer, is only 1 1 times, instead of 40 times, 
that of the lamp. 
We may take this opportunity of mentioning, that we have lately, per- 
formed an experiment which strengthens still more what we have said regard- 
ing the impossibility of applying the thermometric photometer to the mea- 
surement of lights differing in colour. It is well known that coal and oil 
gas burn with a blue flame when mingled with atmospherical air. We have 
found, that a mixture of equal parts of air and gas, about 700 in specific gra- 
vity, will burn in the Glasgow burner, No. 2. with a flame of 2 1 inches, nearly 
all blue, the tips only of the jets being white. At the distance of 64 inches from 
this flame, the photometer of the Astronomical Institution indicated steadily 
5°, while, at the same distance from a tallow candle, it indicated only 2°. Es- 
timating the real illuminating power as in the experiment with the fire-light, 
the relative distances were 15 feet from the candle, and 2| at the utmost for 
the blue flame. Hence the real light of the latter is only a 36th part of that 
