But inasmuch as it is
necessary for the evolution of light by the combustion of carbon that
that carbon shall be in the free state, only those carbonaceous fuels
yield light without the mantle in which the carbonaceous ingredient is
dissociated into its elements before it is consumed. For instance,
alcohol and carbon monoxide are both combustible, and both contain
carbon; but they yield non-luminous flames, for the carbon burns to
carbon dioxide in ordinary conditions without assuming the solid form;
ether, petroleum, acetylene, and some of the hydrocarbons of coal-gas do
emit light on combustion, for part of their carbon is so liberated. The
quantity of light emitted by the glowing substance increases as the
temperature of that substance rises: the gain in light being equal to the
fifth or higher power of the gain in heat; [Footnote: Calculated from
absolute zero.] therefore unnecessary dissipation of heat from a flame is
one of the most important matters to be guarded against if that flame is
to be an economical illuminant. But the amount of heat liberated when a
certain weight (or volume) of a particular fuel combines with a
sufficient quantity of oxygen to oxidise it wholly is absolutely fixed,
and is exactly the same whether that fuel is made to give a luminous or a
non-luminous flame.
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