Dr Fyfe on the Illuminating power of Coal and Oil Gas. 369 
the average of ten trials, it was fixed at 1 to 1*5. The gases 
were found, by the chlorine test, to contain coal-gas 15, oil-gas 
25 per cent, of olefiant, making their illuminating as 15 to 25, 
that is, as 1 to 1*6, so that in this also the difference is only 0*1. 
The above experiments, I think, are sufficient to warrant the 
conclusion, that the olefiant gas is the source of light in the gase- 
ous products, derived from the decomposition of coal and oil, 
so that the chlorine test is an accurate indicator of the illumi- 
nating power. Having satisfied myself as to this, I have per- 
formed a number of experiments, with a view of fixing that of 
the gases prepared at the Edinburgh Coal-Gas Works, at the 
Leith Oil-Gas Works, and by Mr Milne’s apparatus. 
The Edinburgh coal-gas is made from parrot-coal, in general 
from a mixture of an inferior with a superior kind ; it is, how- 
ever, occasionally manufactured from that of the best quality. 
The gas from the former I have, by repeated trials, found to 
contain 15, that from the latter 17 per cent, of olefiant. I un- 
derstand it is the intention of the Company to prepare the gas 
from the best coal, when a sufficient supply can be procured, 
and with this view they are forming depots of it ; but at pre- 
sent they are obliged to use the mixed coal, so that we must 
consider the coal-gas now employed by the public as containing 
only 15 of olefiant. 
The quantity of olefiant in oil-gas varies considerably, accord- 
ing to the mode of manufacture. When prepared at Mr Milne’s, 
with all possible care, it contained 32 per cent., but some made 
lately yielded only 25. That prepared at Leith varies from 16 
to 17. The illuminating power of the Leith oil-gas, and of the 
Edinburgh coal-gas, is therefore very nearly the same, while 
that of Mr Milne varies from 1*6 to 2T compared to that of 
the coal-gas as 1, that is, taking the latter as containing 15 per 
cent, of olefiant; but if we consider it as having 17 per cent., 
then Mr Milne’s gas will be as 1*4, and 1*8 to the other as 1. 
As the composition of the gases varies considerably, it is im- 
possible to fix accurately the illuminating power, but, taking the 
average of the above trials, it is as 1 to 1*7, or it maybe stated 
to vary from 1| to 2, compared to that of coal-gas as 1 ; a con- 
clusion very nearly the same as that at which I arrived in my 
former paper. ( Supra , p. 183.) 
vol. xr. no. 22. oct. 1824. 
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