However, although it is not possible in practice to supply
a flame with too little air, lest some of its carbon should escape
consumption and prove a nuisance, it is very easy without conspicuous
inconvenience to supply it with too much; and if the flame is supplied
with too much, there is an unnecessary volume of air passing through it
to dilute the true combustion products, which air absorbs its own proper
proportion of heat. It is only the oxygen of the air which a flame needs,
and this oxygen is mixed with approximately four times its volume of
nitrogen; if, then, only a small excess of oxygen (too little to be
noticeable of itself) is admitted to a flame, it is yet harmful, because
it brings with it four times its volume of nitrogen, which has to be
raised to the same temperature as the oxygen. Moreover, the nitrogen and
the excess of oxygen occupy much space in the flame, making it larger,
and distributing that fixed quantity of heat which it is capable of
generating over an unnecessarily large area. It is for this reason that
any gas gives so much brighter a light when burnt in pure oxygen than in
air, (1) because the flame is smaller and its heat more concentrated, and
(2) because part of its heat is not being wasted in raising the
temperature of a large mass of inert nitrogen.
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