36 Drs Christison and Turner on the Construction of 
ral times found, when the retorts were choked with charcoal, and 
the specific gravity of the gas was only 660, that the quantity fell 
short of 100 cubic feet per gallon, which is said to be about the ave- 
rage produce when the gas is good. When oil-gas has a specific 
gravity of 910, charcoal is still found in the retorts. It may 
therefore be improved by the addition of all this charcoal, and 
-still retain its volume. Besides, it may be possible to improve it 
by the addition of charcoal from other sources. Hence, while 
we at present assign to oil-gas the average specific gravity of 
920, we cannot help anticipating a considerable improvement, 
and positive gain. 
From what has been said of the average quality of coal-gas 
in different quarters of the kingdom, it is clear that the question 
of its economy, compared with oil-gas, can be only answered 
relatively. In Edinburgh and Glasgow, where coal is mode- 
rately cheap, and coal-gas of good quality, oil-gas must be 
somewhat dearer ; in London, where the coal is dear, and the 
gas bad, oil-gas should be positively cheaper; and in other 
places the two will be nearly the same in price. This statement 
is, of course, drawn from our own experiments on their illumi- 
nating power, coupled with the well-known computations of Ac- 
cum, Peckston, Ricardo, and others, regarding their relative cost. 
The second element in the question of their relative advan- 
tages, is their comparative utility. It is certain that, whatever 
difference may exist between them in this respect must be in fa- 
vour of oil-gas. 
In the first place, the quality of the light is superior. It is 
whiter, and has a peculiar sparkling appearance, superior to 
that of coal-gas. It is therefore a more beautiful light, fitter 
for the artificial illumination of colours, and not liable to give 
the human countenance that unpleasant sallow appearance which 
every one has observed to be caused by coal-gas. 
An objection has been urged to the employment of gas in 
general, that it has a disagreeable odour. This objection does 
not apply at all, unless the gas is unconsumed; for neither oil 
nor even coal gas, so far at least as our observation goes, emits 
any odour, if properly burnt. But if they escape, and mix with 
the air, their presence is then readily detected by the smell. 
The odour of oil-gas is purely empyreumatic, but quite dis- 
tinct ; we have possessed occasional specimens, which had a 
