652 DE. E. EEANKLAJfD ON THE INFLUENCE OF ATMOSPHEEIC PEESSUEE 
and even utilized in the wire-gauze and Bunsen’s burners, where heat and not light is 
the object of the combustion of gas. But it may be asked, what conditions are there in 
the combustion of flame in rarefied air, that favour the admission of a larger proportion 
than usual of air to the interior of the flame 1 In reply, it may be stated that there are 
two conditions in such combustion, both of which directly tend to produce this result. 
The first of these conditions, and the one to which I conceive nearly the whole of 
the effect to be due, is the greater mobility of rarefied gaseous bodies, which must 
produce a more rapid admixture of the flame, gases, and external air than would 
otherwise take place. The second condition is the gradual, though slow, increase in 
the volume of the flame as the atmospheric pressure decreases, thus causing the flame to 
present a gradually increasing surface of contact with the exterior air. This alteration 
in the volume of flame by diminished pressure is more strikingly seen with a sperm 
candle than with gas. When such a candle burns under a pressure of two atmospheres, 
its flame presents the appearance of a sharp spike scarcely one-fourth of an inch in 
diameter at its lower and broadest part, the apex being lost in the dense smoke which 
issues from the upper portion of the flame. If the pressure be now diminished, the 
diameter of the spike markedly increases, especially about its centre, until at one-atmo- 
sphere pressure the flame assumes its ordinary appearance. On now rarefying the air, 
the transverse diameter of the flame goes on increasing until, when the pressure is 
reduced to about 6 mercurial inches, the flame becomes nearly globular with a diameter 
of about three-fourths of an inch. 
Now, as the amount of combustible matter in the flame was maintained constant in 
the photometrical experiments detailed above, it follows that the increased external flame 
surface must so alter the conditions of combustion as to bring the constant amount of 
combustible matter into contact with a gradually increasing quantity of oxygen. That 
a large amount of air does, even under ordinary circumstances, gain access to the interior 
of gas- and candle-flames, has been proved by the interesting researches upon the gases 
of these flames recently made by Hilgaed*, who found 64 per cent, of nitrogen in the 
interior of a candle-flame, and by LANDOLTf , who detected 66 per cent, of nitrogen in 
the interior of a gas-flame ; on no occasion, however, did these experimenters find oxygen 
in the luminous portion of the flume, although it was found in the blue or non-luminous 
section. I conceive therefore that these consequences of diminished pressure, viz. 
increased gaseous mobility and augmented volume of flame, are quite competent to explain 
the variations in luminosity resulting from alterations in the pressure of the supporting 
medium, and that these variations in illuminating power depend cliiefiy^ if not entirely, 
upon the ready access or comparative exclusion of atmospheric oxygen as regards the interior 
of the fia'ine. 
. In conclusion, the influence of atmospheric pressure upon the phenomena of com- 
bustion may be thus summed up. 
* Ann. der Chem. und Pharm. toI. xcii. p. 129. f PoGGEXDOEFr’s ‘ Annalen,’ vol. xcix. p. 389. 
