Dr Fyfe on the Comparative Value of Oil and Coal Gas . 179 
with the latter when of equally good quality ; consequently, it 
must be far short of that stated by Mr Dewy. 
Dr Henry, in his paper on the nature of the gases produced 
by the decomposition of coal and oil, proposes to ascertain their 
illuminating power by finding the quantity of oxygen necessary 
for their combustion ; for, according to him, the more a gas will 
consume, the more light it will afford. He has found, that oil 
and coal gas, produced under different circumstances, take dif- 
ferent quantities of oxygen. 
]00 Volumes of Coal-Gas of 
Specific Gravity, 
took of 
Oxygen 
345 
78 
500 
166 
620 
194 
630 
196 
650 
217 
100 Volumes of Oil-Gas of 
464 
116 
590 
178 
758 
220 
906 
260 
From the above tables it would, of course, be inferred, that 
the illuminating power of oil-gas, No. 4., is the greatest, that of 
coal-gas, No. 1., the least, these being to each other as 260 to 78 ; 
that is, as 3J to 1. From this, then, it appears, that the best 
oil-gas is to the worst coal-gas as 3 \ to 1 ; of course a very dif- 
ferent conclusion with respect to their illuminating power would 
be drawn, were we to take an average from the above tables, 
which we should diminish the light given out by the former, 
and increase that from the latter. 
There is still, I conceive, another mode of ascertaining the 
comparative illuminating power of the gases. It has been al- 
ready stated, that the gas from oil and coal, the latter being 
freed from all impurities, is a mixture of variable proportions of 
olefiant-gas, carburet ted hydrogen, hydrogen, carbonic oxide, 
and azote, with an essential oil held in solution ; the first of 
which is probably the principal source of light ; the other com- 
bustible gases affording, by their combustion, very little. If this 
be the case, by finding its quantity we have an easy mode of as- 
certaining the relative illuminating power. The olefiant gas in 
coal and oil gas may be condensed by chlorine, provided the 
m 2 
