178 Dr Fyfe on the Comparative Value of Oil and Coal Gas* 
©ther, the coal-gas from the Imperial Gas Works* the other 
from that at Bow. Being burned so as to afford the same in- 
tensity of light, the quantities were found by accurate metres to> 
be (taking the mean of seven trials) as 4850 to 1368, very 
nearly 3J to 1. Though these experiments seem to have been 
conducted with every degree of care, yet I am inclined to con- 
sider the coal-gas of very inferior quality. It has been supposed 
by some, that the specific gravity of coal-gas is a good test of 
its purity, the lighter it is the greater being its power of illumi- 
nation. The experiments of Dr Henry, and others, however, 
disprove this ; indeed, after the gases are properly purified, I 
believe the heavier they are, so much the more will be the light 
afforded by their combustion. The gas used by Mr Dewy was 
of specific gravity .406, now this is less than pure carburetted ; 
hydrogen, which is -555. In a note to the same paper the edi- 
tor remarks, that the results of Mr Dewy coincide with those ob- 
tained by him and Mr Faraday. The coal-gas they subjected 
to trial was, in one instance, of specific gravity .429, in another,, 
.406. The oil-gas was .965 and .939, and their illuminating 
power to the former was as 3J to 1. As the coal-gas in all of 
these experiments was of inferior specific gravity to carbu retted 
hydrogen, we may reasonably infer, that they contained a con- 
siderable proportion of pure hydrogen, which, it is well known, 
affords a very feeble light. That I am correct in this assertion, 
at least that the coal-gases were of inferior quality, is proved by 
the experiments of Henry and others. Dr Henry has found 
the gas to vary in specific gravity from .345 to .650, its illumi- 
nating power increasing as it approached the maximum. The 
specific gravity of the coal-gas of Edinburgh, which is allowed 
by all to be of very superior quality, I have found to be so high 
as .680. The oil-gas used by Mr Dewy was .939* Dr Henry 
mentions, that in some of his experiments it was .906. I have 
found that, from the small apparatus of Mr Milne (Taylor and 
Martineau's), to be ,940, and which is generally allowed to be 
very fine, so that we take it for granted, that that of Mr Dewy 
was of good quality. If, then, in these trials a good oil-gas, 
pitched against a very inferior coal-gas (which I think I have 
proved it to be), is only as 3| to 1, the illuminating power of 
the former must be much reduced when brought in competition 
