this state it is soluble in alkohol, though not when combined with
carbonic acid; and Mr Berthollet employs this property as a method of
procuring potash in the state of perfect purity.
All vegetables yield less or more of potash in consequence of
combustion, but it is furnished in various degrees of purity by
different vegetables; usually, indeed, from all of them it is mixed
with different salts from which it is easily separable. We can hardly
entertain a doubt that the ashes, or earth which is left by vegetables
in combustion, pre-existed in them before they were burnt, forming what
may be called the skeleton, or osseous part of the vegetable. But it is
quite otherwise with potash; this substance has never yet been procured
from vegetables but by means of processes or intermedia capable of
furnishing oxygen and azote, such as combustion, or by means of nitric
acid; so that it is not yet demonstrated that potash may not be a
produce from these operations. I have begun a series of experiments upon
this object, and hope soon to be able to give an account of their
results.
2. _Of Soda._
Soda, like potash, is an alkali procured by lixiviation from the ashes
of burnt plants, but only from those which grow upon the sea-side, and
especially from the herb _kali_, whence is derived the name _alkali_,
given to this substance by the Arabians. It has some properties in
common with potash, and others which are entirely different: In general,
these two substances have peculiar characters in their saline
combinations which are proper to each, and consequently distinguish them
from each other; thus soda, which, as obtained from marine plants, is
usually entirely saturated with carbonic acid, does not attract the
humidity of the atmosphere like potash, but, on the contrary,
desiccates, its cristals effloresce, and are converted into a white
powder having all the properties of soda, which it really is, having
only lost its water of cristallization.
Hitherto we are not better acquainted with the constituent elements of
soda than with those of potash, being equally uncertain whether it
previously existed ready formed in the vegetable or is a combination of
elements effected by combustion. Analogy leads us to suspect that azote
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