Gas-Burners, and on the Illuminating Power of the Gases. 7 
ting power. We shall afterwards state an experiment, which confirms that 
opinion. At present we may observe, that it does not coincide at all with 
the results obtained by Mr Leslie, (Coal-Gas Company's Report , 24th July 
1824.) 
All the experiments hitherto related were performed with the photo- 
meter of the Astronomical Institution, which has the reputation of being 
made by Mr Leslie’s own hands, and which we procured, because Mr 
Leslie had alleged that our own, not being made by himself, could not 
be accurate. Our own instrument measures correctly the intensity of the 
sun’s light ; at least it agrees in its indications with that of the Astronomical 
Institution. We must admit, however, that it is of faulty construction in re- 
gard to the measurement of artificial light. But the fault, we conceive, is 
one which exists, more or less, in every photometer made by Professor Leslie. 
For, according to the information received from the instrument-maker who 
made our own, and blows all those which are given out as made by Mr 
Leslie, the black ball is always blown somewhat thicker than the other, in order 
to secure its perfect opacity. This construction will not cause any appreciable 
error, so far as concerns the original purpose of his photometer, if, as many be- 
lieve, the sun’s rays are not accompanied with non-luminous heat. But it will 
occasion a material error in the measurement of artificial light, which is always 
accompanied by non-luminous heat. For the instrument, with one ball thicker 
than the other, is not a strict differential thermometer, as it ought to be, and 
as Mr Leslie intended it to be. Non-luminous rays of heat falling equally on 
both balls from one side of them, must make the liquor move in a direction 
away from the thinner of the two. 
The first instrument with which we operated was more faulty in this re- 
spect than that of the Astronomical Institution ; a circumstance which will ac- 
count sufficiently for several discrepancies between the present experiments 
and those formerly published at the request of the Oil-Gas Company. We 
may add, that it is exceedingly difficult to avoid the fault in question. In 
fact, on this account, none of the differential thermometers we have exa- 
mined are really and strictly differential. It will at once be seen, however, 
that the difficulty may be obviated by substituting for the glass-balls cylin- 
ders or balls of metal, which can be easily made quite uniform in thickness. 
A plan of such an instrument was suggested to us last autumn by Professor 
Wallace, and a photometer of the kind has been actually devised and con- 
structed by Mr Ritchie of Tain, as announced in a paper by that gentleman 
lately read to this Society. (See Edin. Journ. of Science , vol. ii.) 
The foregoing observations will explain sufficiently why the photometer 
of Mr Leslie appeared to us inapplicable to our experiments on the proper con- 
struction of gas-burners. We shall next mention our reasons for preferring 
the method for measuring light, which was adopted and perfected by Count 
Rumford. 
The principle of construction of Count Rumford’s photometer, namely, the 
comparison of the intensity of shadows, is the basis of all the attempts (except 
that of Mr Leslie) which have been lately made to determine the relative 
light of the gases. Very few Experimenters, however, have mentioned in 
what way they applied the principle ; none, so far as is stated, nave used the 
