_Of Alkalis._
The class of substances that seems particularly formed by nature to
unite with acids, and thereby form _neutral salts_, are the _alkalis_.
They have all a peculiar acrid taste, not easily defined. They change
the blue juices of vegetables green, or purple, and in common with acids
have an affinity with water, so as to be capable of being exhibited in a
liquid form; though when this water is expelled by heat, some of them
will assume a solid form.
Alkalis are of two kinds; the _fixed_, which have no smell, and the
_volatile_ which have a pungent one.
The fixed alkalis are of _vegetable_ or _mineral_ origin. When in a
solid form, they both melt with a moderate heat, and uniting with earthy
substances, make _glass_. With an intense heat they are volatilized.
Vegetable alkali is procured by burning plants, and lixiviating the
ashes; a purer kind by the burning of tartar, hence called _salt of
tartar_; but the purest of all is got by the deflagration of nitre; the
charcoal uniting with the acid as it assumes the form of
dephlogisticated air, and the alkali being left behind.
Mineral alkali is found in ashes of sea-weed. It is likewise the basis
of sea-salt; from which it is separated by several processes, but
especially by the calx of lead, which has a stronger affinity with the
marine acid with which it is found combined.
Alkalis united with fixed air are said to be _mild_, and when deprived
of it _caustic_, from their readiness to unite with, and thereby
_corrode_, vegetable and animal substances. To render them caustic, they
are deprived of their fixed air by quick-lime; and in this state they
unite with oils, and make _soap_.
Alkalis have a stronger affinity with acids than metals have with them;
so that they will precipitate them from their solutions in acid
menstruums.
The vegetable fixed alkali has a strong attraction to water, with which
it will become saturated in the common state of our atmosphere, when it
is said to _deliquesce_; and having the appearance of _oil_, the salt of
tartar is thus said to become _oil of tartar per deliquium_. On the
other hand, the mineral, or fossil alkali, is apt to lose its water in a
dry atmosphere, and then it is said to _effloresce_. In this state it is
often found on old walls.
Volatile alkali is procured by burning animal substances; in Egypt (from
whence, as contained in _sal ammoniac_, we till of late imported it)
from camel's dung; but now from bones, by distillation. To the liquor
thus procured they add vitriolic acid, or substances which contain it.
This acid unites with the alkali, and common salt being put to it, a
double affinity takes place. The vitriolic acid uniting with the mineral
alkali of the salt, makes _Glauber salt_, and the marine acid uniting
with the volatile alkali, makes _sal ammoniac_. Slaked lime added to
this, unites with the marine acid of the ammoniac, and sets loose the
volatile alkali in the form of _alkaline air_, which combining with
water, makes the liquid caustic volatile alkali. If chalk (containing
calcareous earth united with fixed air) be mixed with the sal ammoniac,
heat will make the calcareous earth unite with the marine acid, while
the fixed air of the chalk will unite with the volatile alkali, and
assume a solid form, being the _sal volatile_ of the apothecaries.