UPON SOME OP THE PHENOMENA OF COMBUSTION. 
651 
atmospheres, but also that no reduction of temperature in the flame can occur from 
the same cause. 
A second cause of the diminution of the light of combustion in rarefied atmospheres, 
and its increase in compressed ones, might be sought for in a possible difference between 
the temperatures of the flame in the two cases. It is well known that if air be allowed 
to escape from a vessel into a vacuum, a considerable diminution of temperature ensues 
in the vessel from which the air escapes ; and inasmuch as the gaseous products of com- 
bustion assume a larger volume in rarefied atmospheres than in compressed ones, it can 
scarcely be doubted that the pyrometric thermal effect of a flame must be diminished to 
some extent by rarefying the medium in which it burns ; nevertheless this effect may 
be nearly or quite neutralized by the smaller amount of refrigeration caused by the 
rarefied atmosphere. In order to elucidate this point, a spiral of platinum wire was 
ignited to visible redness in a fiame of hydrogen ; on then rarefying the air around the 
flame and ware, no appreciable alteration in the temperature of the platinum spiral 
could be noticed. A similar experiment was tried with an alcohol flame, and with the 
same result. A spiral of platinum wire placed under the receiver of an air-pump, 
was ignited to \'isible redness by a voltaic current ; on exhausting the receiver, the glow 
of the platinum gradually increased nearly to whiteness. On readmitting the air, it 
again diminished to dull redness, showing that the refrigerating effect of rarefied air is 
much less than that of air at the ordinary pressure. Thus, whilst the temperature pro- 
duced within a given flame is lowered by rarefaction, the escape of heat from its 
exterior is hindered by the same process, — the result apparently being that the actual 
temperature of the flame undergoes but little alteration. This confirms Davy’s con- 
clusion, that rarefaction and compression, within certain limits at least, do not exert any 
cmsideraUe influence upon the heat of flame. 
Although an inquiry into two of the possible causes of the diminution of the light of 
combustion in rarefied atmospheres has thus failed to afford any explanation of the 
phenomenon, yet one of them indirectly points to what I believe to be the conditions 
determining the variation in illuminating power. If it be true that combustion is more 
complete in rare atmospheres than in dense ones, it follows that the light of a smokeless 
flame must decrease with a diminution of pressure, since, with more perfect combustion, 
that is, with freer access of oxygen to eveiy part of the flame, there must be a diminution 
of unconsumed carbon separated within the flame, and consequently a diminished amount 
of light evolved. In fact, the appearance of the experimental flame during the progress 
of rarefaction on the one hand, and of compression on the other, can scarcely leave a doubt 
on the mind of an observer, that the variation of luminosity depends essentially upon 
the admission of oxygen to that portion of the shell of flame where particles of carbon 
are usually precipitated, and where consequently the region of luminosity is situated. 
That an admission of oxygen or air to the interior of a luminous flame has the effect of 
greatly diminishing or even practically annihilating its luminosity, has been long known 
MDCCCLXI. 4 T 
