CHAP. XII.
_Of the Decomposition of Vegetable and Animal Substances by the Action
of Fire._
Before we can thoroughly comprehend what takes place during the
decomposition of vegetable substances by fire, we must take into
consideration the nature of the elements which enter into their
composition, and the different affinities which the particles of these
elements exert upon each other, and the affinity which caloric possesses
with them. The true constituent elements of vegetables are hydrogen,
oxygen, and charcoal: These are common to all vegetables, and no
vegetable can exist without them: Such other substances as exist in
particular vegetables are only essential to the composition of those in
which they are found, and do not belong to vegetables in general.
Of these elements, hydrogen and oxygen have a strong tendency to unite
with caloric, and be converted into gas, whilst charcoal is a fixed
element, having but little affinity with caloric. On the other hand,
oxygen, which, in the usual temperature, tends nearly equally to unite
with hydrogen and with charcoal, has a much stronger affinity with
charcoal when at the red heat[24], and then unites with it to form
carbonic acid.
Although we are far from being able to appreciate all these powers of
affinity, or to express their proportional energy by numbers, we are
certain, that, however variable they may be when considered in relation
to the quantity of caloric with which they are combined, they are all
nearly in equilibrium in the usual temperature of the atmosphere; hence
vegetables neither contain oil[25], water, nor carbonic acid, tho' they
contain all the elements of these substances. The hydrogen is neither
combined with the oxygen nor with the charcoal, and reciprocally; the
particles of these three substances form a triple combination, which
remains in equilibrium whilst undisturbed by caloric but a very slight
increase of temperature is sufficient to overturn this structure of
combination.
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