If a substance absorbs heat
during its formation, it cannot be produced unless that heat is supplied
to it; and since heat, being a form of motion, is equally a form of
energy, energy must be supplied, or work must be done, before that
substance can be obtained. Conversely, if a substance evolves heat during
its formation, its component parts evolve energy when the said substance
is being produced; and therefore the mere act of combination is
accompanied by a facility for doing work, which work may be applied in
assisting some other reaction that requires heat, or may be usefully
employed in any other fashion, or wasted if necessary. Seeing that there
is a tendency in nature for the steady dissipation of energy, it follows
that an exothermic substance is stable, for it tends to remain as it is
unless heat is supplied to it, or work is done upon it; whereas,
according to its degree of endothermicity, an endothermic substance is
more or less unstable, for it is always ready to emit heat, or to do
work, as soon as an opportunity is given to it to decompose. The
theoretical and practical results of this circumstance will be elaborated
in Chapter VI., when the endothermic nature of acetylene is more fully
discussed.
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