Dr Fyfe on the Comparative Value of Oil and Coal Gas. 177 
A gentleman connected with the Liverpool Gas Company, in. 
the answers to the queries put to him by the Committee of the 
Dundee Company, replies, that the relative quantity of gas re- 
quisite to supply the same light, is as 14 oil- gas to 51 coal-gas, 
making their power of affording light rather more than 3~ to 1. 
Though the above statements place the illuminating power of 
oil-gas so high, a very different account is given by others. Ac- 
cording to Mr Neilson, Glasgow, it is not to be rated at above 
2, or at all events beyond 2J, to the other as 1 ; and the same 
conclusion is drawn from a series of experiments made at Bris- 
tol, by Messrs Herapath and Rootsey, in whose results, Mr 
Peckstone has remarked, that every reliance may be placed, as 
they could not be actuated by party-feeling, but solely by a de- 
sire to ascertain the truth. These statements, so very discord- 
ant, must arise either from the defective mode of ascertaining 
the intensity of’ the light, or from the variable quality of the gas, 
both of which, I believe, but particularly the latter, have had 
their effect. 
The mode usually followed for ascertaining the illuminating 
power, viz. of producing the same intensity of shadow, and 
marking the quantity of the gas consumed in a given time, is 
liable to many objections. It is extremely difficult, for instance, 
to judge with precision of the depth of shadow ; besides, unless 
each gas is burned under circumstances favourable for produ- 
cing the greatest light, the conclusion with respect to their power 
of illumination is not correct. Some of the experiments in which 
the oil-gas is stated as 3~ to I, it has been said, were conducted 
by using burners of equal dimensions for both : now, it is well 
known that the former requires a smaller one than the latter, 
otherwise the intensity of the light is not in proportion to the 
gas consumed, part of it probably escaping combustion. The 
remark, with respect to the variable quality of the gas, is also of 
equal force. In a paper published by Mr Dewey, in the An- 
nals of Philosophy for December, some experiments on the il- 
luminating power of the gases are stated, with a view of “ setting 
the matter at rest.” This I conceive they have done, as far as 
can be expected ; but I suspect the conclusion to be drawn from 
them, is very different from that at which Mr Dewey arrives . 
The gases were taken from main-pipes running parallel to each 
VOL. XI. no. 21. JULY 1824, M 
