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Art. XXIII . — On the Illuminating Power of Coal-Gas. By 
Adam A-nderson, Esq. A. M. F. R. S. E Rector of the 
Academy, Perth. 
T HERE is no subject, perhaps, capable of being investigated 
in a rigid and scientific manner, with respect to which there 
seems to be so great a diversity of opinion, as that relating to the 
comparative illuminating power of oil and coal gas. Though the 
gaseous products concerning which we have so many discordant 
statements, are publicly and daily exhibited in almost every 
quarter of the empire, and though the properties of both have 
attracted from men of science more than a due share of atten- 
tion, it is not a little singular, that, amidst the multiplicity of 
facts which have been laid before the public, none has yet been 
adduced of a nature sufficiently decisive to fix, beyond dispute, 
the relative values of the rival gases. In such circumstances, I 
cannot imagine that any thing which I may communicate on 
the subject will set the matter at rest ; but as I have some prac- 
tical knowledge, not only of the manipulations by which the 
gases are produced and purified, but also of their chemical con- 
stitution and mechanical properties, it would be an affectation 
of modesty were I not to claim some degree of authority for the 
facts which I am about to state. 
The gas which was the subject of the experiments I have to 
describe, was manufactured, in the ordinary way, at the Perth 
Coal-Gas Work, — an establishment that was planned and exe- 
cuted under my directions, and respecting which, it may be pro- 
per to mention, that the whole of the arrangements for the pro- 
duction of the gas, (the separation of the tar, and the other pu- 
rifying processes), are essentially different from those of any other 
gas-work in the kingdom. In this brief notice, it is not my de- 
sign to enter upon details, but merely to request your permis- 
sion to lay before the public, through the medium of the Edin- 
hurgh Philosophical Journal^ a few general results, that may 
contribute something, at least, to the stock of information which 
is already in their possession. For this purpose, it will be suf- 
ficient to state, that the whole number of retorts at present in 
constant operation at the Perth Gas Work is only three ; Avhile 
