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 insoluble; by afterwards evaporating the water, we
obtain the potash in a white concrete form: It is very fixed even in a
very high degree of heat. I do not mean here to describe the art of
preparing potash, or the method of procuring it in a state of purity,
but have entered upon the above detail that I might not use any word not
previously explained.
The potash obtained by this process is always less or more saturated
with carbonic acid, which is easily accounted for: As the potash does
not form, or at least is not set free, but in proportion as the
charcoal of the vegetable is converted into carbonic acid by the
addition of oxygen, either from the air or the water, it follows, that
each particle of potash, at the instant of its formation, or at least of
its liberation, is in contact with a particle of carbonic acid, and, as
there is a considerable affinity between these two substances, they
naturally combine together. Although the carbonic acid has less affinity
with potash than any other acid, yet it is difficult to separate the
last portions from it. The most usual method of accomplishing this is to
dissolve the potash in water; to this solution add two or three times
its weight of quick-lime, then filtrate the liquor and evaporate it in
close vessels; the saline substance left by the evaporation is potash
almost entirely deprived of carbonic acid. In this state it is soluble
in an equal weight of water, and even attracts the moisture of the air
with great avidity; by this property it furnishes us with an excellent
means of rendering air or gas dry by exposing them to its action. In
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