UPON SOJilE OP THE PHENOMENA OF COMBUSTION. 
649 
of ignited matter closely surrounded on its outside by atmospheric air. The uninterrupted 
supply of gas or Tapour to the core forces the contents of the latter constantly through 
the ignited shell, at the inner wall of which, those hydrocarbons that cannot exist at a 
bright red heat, either undergo decomposition into light carburetted hydrogen and free 
carbon, or imperfect combustionin to water, carbonic oxide, and free carbon, or finally 
perfect combustion into water and carbonic oxide, or even carbonic acid, without any 
separation of free carbon. The nature of the decomposition or combustion which these 
hydrocarbons undergo on coming in contact with the ignited shell, thus evidently depends 
upon the amount of oxygen which gains access to the interior of the shell ; if that 
quantity be insufficient to convert the whole of the carbon of the hydrocarbons into car- 
bonic oxide, the residue will be precipitated, and the fiame Avill be a more or less luminous 
one ; whilst if the amount of oxygen present be sufficient, after burning the hydrogen, 
to consume the whole of the carbon to carbonic acid or even to carbonic oxide, no light 
will be produced from the incandescence of carbon particles. 
Now it is well known that the light of any flame may be increased by increasing the 
number of carbon particles simultaneously floating in it, provided those particles are 
consumed before they leave the flame, and are not evolved as smoke. I have also else- 
where shown* that the light of gas-flames, and doubtless that of candles and oil also, 
greatly depends upon the heat of the flame, the rise in temperature caused by merely 
heating the air supplied to a gas-lamp, by the waste heat of the flame itself, being suffi- 
cient to increase the light to the extent of 67 per cent, without any increased con- 
sumption of gas. Such being the conditions necessary for the increase of light, it is 
scarcely necessary to add that the reversal of these conditions, viz. the decrease in the 
number of particles of carbon existing in the flame at a given time, imperfect combus- 
tion allowing the escape of unconsumed carbon, and decrease of temperature in the 
flame, determine a diminished luminosity. 
One of the first causes which naturally suggests itself to account for the diminution 
of light by decreased atmospheric pressure, is the diminished amount of oxygen in a 
given bulk of the supporting medium rendering combustion imperfect, and thus either 
causing particles of carbon to escape unconsumed, or determining their conversion into 
carbonic oxide instead of carbonic acid. The effect of the first would be to diminish the 
luminosity, whilst the second would have the effect of decreasing the light indirectly by 
diminishing the temperature of the flame. A careful inspection of a gas- or candle-flame, 
burning in an atmosphere undergoing gradual rarefaction, does not afford the slightest 
ewdence of smoke, or even of an increased tendency to throw off fuliginous matter ; on 
the contrar}', the tendency to smoke obviously diminishes as the rarefaction progresses ; 
whilst, on the other hand, the fact that an increase of pressure beyond that of the atmo- 
sphere causes the most smokeless flames to become smoky, renders utterly untenable 
the assumption that the escape of unconsumed carbon is one of the causes of diminished 
luminosity in rarefied atmospheres. Whether or not there is imperfect combustion in 
* Uee’s Dictionary, 1860, article “ Coal-gas.” 
