If the increased temperature to which the vegetable is exposed does not
exceed the heat of boiling water, one part of the hydrogen combines with
the oxygen, and forms water, the rest of the hydrogen combines with a
part of the charcoal, and forms volatile oil, whilst the remainder of
the charcoal, being set free from its combination with the other
elements, remains fixed in the bottom of the distilling vessel.
When, on the contrary, we employ a red heat, no water is formed, or, at
least, any that may have been produced by the first application of the
heat is decomposed, the oxygen having a greater affinity with the
charcoal at this degree of heat, combines with it to form carbonic acid,
and the hydrogen being left free from combination with the other
elements, unites with caloric, and escapes in the state of hydrogen gas.
In this high temperature, either no oil is formed, or, if any was
produced during the lower temperature at the beginning of the
experiment, it is decomposed by the action of the red heat. Thus the
decomposition of vegetable matter, under a high temperature, is produced
by the action of double and triple affinities; while the charcoal
attracts the oxygen, on purpose to form carbonic acid, the caloric
attracts the hydrogen, and converts it into hydrogen gas.
The distillation of every species of vegetable substance confirms the
truth of this theory, if we can give that name to a simple relation of
facts. When sugar is submitted to distillation, so long as we only
employ a heat but a little below that of boiling water, it only loses
its water of cristallization, it still remains sugar, and retains all
its properties; but, immediately upon raising the heat only a little
above that degree, it becomes blackened, a part of the charcoal
separates from the combination, water slightly acidulated passes over
accompanied by a little oil, and the charcoal which remains in the
retort is nearly a third part of the original weight of the sugar.
The operation of affinities which take place during the decomposition,
by fire, of vegetables which contain azote, such as the cruciferous
plants, and of those containing phosphorus, is more complicated; but, as
these substances only enter into the composition of vegetables in very
small quantities, they only, apparently, produce slight changes upon the
products of distillation; the phosphorus seems to combine with the
charcoal, and, acquiring fixity from that union, remains behind in the
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