UPON SOME OP THE PHENOMENA OE COMBUSTION. 
635 
fact, an examination into the conditions of combustion in the two cases scarcely leaves 
room for the expectation of any other result. 
In the combustion of a candle, the radiant heat from the flame first melts the com- 
bustible matter in the capsule at the base of the exposed portion of the wick ; the 
capillary action of the latter then elevates the liquefied wax, tallow, or ‘spermaceti into 
the upper part of the wick, where it is exposed to a temperature which effects its volati- 
Hzation and decomposition. It is thus evident that the rate of combustion, or at all events 
the rate of consumption of the combustible matter of the candle, is entirely dependent upon 
the capillarity of the wick, provided that the radiant heat from the flame is sufficient tn 
keep up a supply of liquefied combustible matter at the base of the wick, and that the 
temperature of the flame is high enough to volatilize this matter on its arrival near the 
apex of the cotton. Now, as capillary attraction is not altered by variations of atmo- 
spheric pressure, and as the temperature of the flame is, as will be shown below, almost 
entirely independent of the same influence, a diminution in the consumption of combus- 
tible matter could only arise from the amount of radiant heat striking the capsule at the 
base of the T\dck being insufficient to keep up a supply of melted combustible matter equal 
to the capillary demands of the \vick. There can scarcely be a doubt that the amount 
of heat radiated from a given area of the lower surface of the flame is diminished by 
rarefaction, owing to the decreasing luminosity of the flame : nevertheless this diminution 
is compensated by the increased flame surface, the radiant heat from which strikes the 
capsule when the flame becomes enlarged by rarefaction. "Wliether this compensation 
be complete or not is of little importance, since observation shows that, even at the 
highest degrees of rarefaction, a sufficient amount of heat reaches the capsule to keep 
up an abundant supply of liquefied combustible matter. 
We have therefore no ground for any d priori assumption that the combustion of 
candles ought to go on at a decreased rate in rarefied air ; in fact there is one considera- 
tion which might lead to the opposite opinion ; it is this, — the rapidity of the consump- 
tion of a candle obviously depends upon the amount of liquefied wax, &c. which passes up 
its wick in a given time : this amount is determined, up to a certain maximum limit, by 
the rapidity with which it is got rid of at the upper portion of the capillary tubes where it 
is volatilized by the heat of the flame. Now, as the rapidity of volatilization is known to 
be increased by a reduction of pressure, it follow^s that a larger amount of the combustible 
would be thus removed from the upper portion of the wick in rarefied than in com- 
pressed air. Nevertheless this influence of reduced pressure must be very small in the 
case of bodies possessing such high boiling-points as tallow, wax, &c., and hence its 
influence upon the rate of combustion in rarefied air cannot be perceived. 
Opposed to the above facts and considerations stands the observation of M. Teiger, 
that candles bum much more rapidly in air compressed three times than in air at the 
ordinary pressure. This discrepancy, the cause of which can now only be conjectured, 
may perhaps reside in some of the circumstances, described above, under which the 
experiments were made. The consta,nt supply of compressed air to a chamber such as that 
MDCCCLXI. 4 E 
