How oxygen supports combustion no Elementary Treatise explains; but
leaves it to be imagined, that oxygen is somewhat of a burnable nature,
or that it generates heat when blown into a fire. The fact is, it
supports combustion only mechanically. The centrifugal pressure, by
the medium of space, decomposes the fuel; electric matter, entering
the ignited fuel, displaces medium of space, and the fire goes out;
oxygen prevents the entrance of electric matter, and permits the medium
of space to enter the fuel freely, the pressure from without gives
centrifugal force. In this twofold manner of service oxygen promotes
the continuance of the kind of decomposition known as combustion. A
live coal is greatly _deficient_ of electric matter; when just fallen
from the fire it is said to be red and hot, after a few minutes black
and cold; all of which are but mental effects. On the hearth the coal
acquires electric matter from the air, which displaces medium of space,
and becomes extinguished; so would the fire were there no oxygen in the
surrounding air. Hence it would seem, that the interstices of oxygen
are too diminutive for electric matter to enter, but are sufficiently
large for those of the medium of space to pass through, thence into
the fuel. Should the utility of the nitrogen of oxygen in combustion
be questioned, because nitrogen alone puts an end to the combustion
of a candle; it may be answered, that, as the imponderable element of
oxygen air, from being highly evanescent, is not obtainable without
the nitrogen, and as by the service of both together combustion is
increased, so may both be considered supporters of combustion; the
grosser element serving as a carrier to the minor, and, as it were,
giving it momentum sufficient to penetrate beyond the surface of the
half-decomposed, or previously ignited fuel.