_Lapis infernalis_. P.L. 1720.
QUALITIES. _Form_, a white brittle substance, extremely caustic and
deliquescent, and possessing in an eminent degree all the properties
denominated _alkaline_. SOLUBILITY. f℥j of water dissolves ʒvij; it is
also soluble in alcohol. CHEMICAL COMPOSITION. This preparation,
independent of its impurities, is the _hydrated protoxide of potassium_,
although in the state in which it is cast into sticks it generally
contains a little _Peroxide_,[608] and therefore evolves oxygen when
dissolved in water. MED. USES. It is a most powerful caustic (_causticum
commune acerrimum_), and is frequently employed to establish an ulcer;
or, instead of incision, to open a tumour. It has the advantage of other
caustics, from the circumstance of our being able to neutralize its
powers by touching it with vinegar, and thus to arrest its progress in
an instant; it is however more liable to produce a large eschar than
nitrate of silver, in consequence of the chemical action of the alkali
upon the skin. Within the last few years, surgeons have greatly
preferred the use of this caustic to that of _nitrate of silver_, in
cases of organic stricture of the urethra, as acting more powerfully,
and yet occasioning less irritation than the latter substance; it is
also said that the part which it destroys is sooner detached and more
easily eliminated. Its disposition, however, to extend its sphere of
action has occasioned such a destruction of the membrane as to allow the
urine to pass into the cellular substance, by which very serious effects
have been produced. For this reason there are cases in which the
_nitrate of silver_ is still to be preferred. As an internal remedy it
is only employed in solution. See _Liquor Potassæ_.[609]