ON COMBUSTION IN RAREFIED AIR. 3sl 
emitted. The lower and blue portion of the flame, which, under 
ordinary circumstances, scarcely rises to within a quarter of an inch 
of the apex of the wick, now extended to the height of 4th of an 
inch above the cotton, thus greatly reducing the size of the luminous 
portion of the flame. 
“On returning to England, I repeated the experiments under cir- 
cumstances which enabled me to ascertain, by photometrical measure- 
ments, the extent of this loss of illuminating effect in rarefied air. 
The results prove that a great reduction in the illuminating power of 
a candle ensues when the candle is transferred from air at the ordi- 
nary atmospheric pressure to rarefied air. It was, however, found 
that, owing to the circumstances mentioned above, no satisfactory 
quantitative experiments could be made with candles in artificially 
rarefied air, and recourse was therefore had to coal-gas, which, 
although also liable to certain disturbing influences, yet yielded 
results, during an extensive series of experiments, exhibiting sufficient 
uniformity to render them worthy of confidence. The gas was in all 
cases passed through a governor to secure uniformity of pressure in 
the delivery tubes. A single jet of gas was employed as the standard 
of comparison, and this was fixed at one end of a Bunsen’s pho- 
tometer, whilst the flame to be submitted to various pressures, and 
which I will call the experimental flame, was placed at the other. 
The experimental flame was made to burn a uniform amount of gas, 
viz. 0°65 cubic foot per hour in all the experiments. 
“The products of combustion were completely removed, so that 
the experimental flame, which burnt with perfect steadiness, was 
always surrounded with pure air, the supply of which was, however, 
so regulated as to secure a maximum of illuminating effect in each 
observation, 
‘Tn all the following series of experiments, the illuminating power 
given under each pressure is the average of twenty observations, 
which accord with each other very closely. In each series, the 
maximum illuminating effect, that is the light given by the experi- 
mental flame when burning under the full atmospheric pressure, is 
assumed to be 100. The following is a summary of the results :— 
