386 Mr Anderson on tfie Illuminating Power of Coal-Gas ^ 
union has already occurred to me, which promises to lead to the 
most favourable results ; but, at present, it would be premature 
to do more than simply advert to the fact. 
I shall conclude these general observations with a few cursory 
remarks on the comparative cost of oil and coal gas ; taking, as 
a standard for the former, the gas produced under the patent of 
Taylor and Martineau ; and for the latter, the gas of the Perth 
Gas- work. From the statements which have already been given, 
it appears, that the illuminating quality of oil-gas, is to that of 
the coakgas in question, as 8 to 5. Now, if it be admitted, that, 
with due management, a gallon of oil , will yield 100 cubic feet 
of gas (and this is certainly rather above than below the truth), 
and that 40 lb. of coal are capable of yielding 160 cubic feet 
of gas (which is rather below than above the truth), we should 
have from a gallon of oil and 40 lb. of coal, the equivalent 
quantities of oil and coal gas, or the quantities of the two gases 
that. afford equal degrees of light. But reckoning the oil at 
L. 9^0 per ton, and the coal at 15s., the cost of the oil-gas would 
be Is. 7d., and that of the coal gas something less than 3J4‘ 
It is no doubt true, that, for the due preparation of coal-gas, 
certain substances are requisite, which are unnecessary in the 
case of oil-gas but the expence of these is very trifling, and, 
at any rate, the value of the coke, tar, and other products aris- 
ing from the coal, more than mee^s the extra outlay which they 
occasion. 
To counterbalance these advantages, however, in favour of 
coal-gas, it is admitted, that the gas-holders, conducting pipes, 
&c. do not, in the case of oil-gas, require to be quite so large, 
for an equal extent of illumination, as in the case of coal-gas. 
But the difference, though it has often been pressed upon the 
public attention by the advocates for oil-gas, is much less than 
it might, at first sight, appear ; for, admitting that even two 
measures of coal-gas were requisite to yield the same degree of 
* At Lord Gray’s oil-gas establishment, erected by Mr Milne of Edinburgh, 
under the patent of Taylor and Martineau, a gallon of oil yields at an average 
only 80 cubic feet of gas. On the other hand, some kinds of cannel coal have been 
found, at the Perth Gas-work, to yield upwards of 5 cubic feet per lb. ; and that, 
too, when the distillation was carried on only for three hours. 
