These bodies are distinguished by a highly corrosive and peculiar taste;
they change the blue[329] juices of vegetables to a green, and the
yellow to a brown; they are soluble in water, and have the power of
imparting the same property to oils, by combining with them, and thus
forming saponaceous compounds. With the different acids they constitute
peculiar salts. When applied to the flesh of animals they act as
powerful caustics, destroying its texture, and ultimately dissolving it;
they are accordingly arranged with great propriety under the head of
corrosive poisons.
There are three[330] alkalies—_potass_, _soda_, and _ammonia_. To the
two former the epithet _fixed_ has been applied, since they require a
very high temperature for their sublimation; while to the third, that of
_volatile_ has been assigned, because, when uncombined, it exists in a
state of gas. _Potass_, as it was considered the product of vegetation,
has received the name of the _vegetable_ alkali, while _soda_, as the
base of rock salt, has been distinguished by that of _mineral_ alkali.
The distinctions, however, originally established by _Avicenna_, must
now be abandoned, for they have not the slightest foundation in truth;
_potass_, so far from being the exclusive product of vegetation, exists
as a constituent part of the _Granite_, which forms the foundation of
our globe; it has also been discovered in the _Pumice stone_; in some
minerals of the _Zeolite_ family; in the _Leucite_; in the aluminous
ores of _La Tolfa_, &c. and, although potass is undoubtedly procured by
lixiviation from the ashes of burnt wood, and other vegetable
substances, yet there is ample grounds for supposing that the living
plant receives it from the soil in which it vegetates.