We have already shown, that, when a vegetable substance is submitted to
the action of fire in distilling vessels, its component elements,
oxygen, hydrogen, and charcoal, which formed a threefold combination in
a state of equilibrium, unite, two and two, in obedience to affinities
which act conformable to the degree of heat employed. Thus, at the
first application of the fire, whenever the heat produced exceeds the
temperature of boiling water, part of the oxygen and hydrogen unite to
form water; soon after the rest of the hydrogen, and part of the
charcoal, combine into oil; and, lastly, when the fire is pushed to the
red heat, the oil and water, which had been formed in the early part of
the process, become again decomposed, the oxygen and charcoal unite to
form carbonic acid, a large quantity of hydrogen gas is set free, and
nothing but charcoal remains in the retort.

A great part of these phenomena occur during the combustion of
vegetables in the open air; but, in this case, the presence of the air
introduces three new substances, the oxygen and azote of the air and
caloric, of which two at least produce considerable changes in the
results of the operation. In proportion as the hydrogen of the
vegetable, or that which results from the decomposition of the water, is
forced out in the form of hydrogen gas by the progress of the fire, it
is set on fire immediately upon getting in contact with the air, water
is again formed, and the greater part of the caloric of the two gasses
becoming free produces flame. When all the hydrogen gas is driven out,
burnt, and again reduced to water, the remaining charcoal continues to
burn, but without flame; it is formed into carbonic acid, which carries
off a portion of caloric sufficient to give it the gasseous form; the
rest of the caloric, from the oxygen of the air, being set free,
produces the heat and light observed during the combustion of charcoal.
The whole vegetable is thus reduced into water and carbonic acid, and
nothing remains but a small portion of gray earthy matter called ashes,
being the only really fixed principles which enter into the constitution
of vegetables.

The earth, or rather ashes, which seldom exceeds a twentieth part of the
weight of the vegetable, contains a substance of a particular nature,
known under the name of fixed vegetable alkali, or potash. To obtain it,
water is poured upon the ashes, which dissolves the potash, and leaves
the ashes which are