Substances whose application to the animal solids, erodes, or decomposes
them.
The operation of these bodies may, in general, be considered chemical;
for having destroyed the life of the part to which they are applied,
they cause, as if by a species of resulting affinity, the elements of
the animal matter to enter into a new state of combination; this is well
exemplified in the action of caustic potass, where the nascent elements
thus disengaged by the decomposition of the animal substance, reunite in
proportions to generate an oily matter, which may be observed to form a
film over the ulcerated surface, while the excess of nitrogen and
hydrogen constitute ammonia, which is disengaged during the action of
the caustic; and may be rendered sensible by inverting over the surface,
a small jar moistened with muriatic acid, when the fumes of _Muriate of
Ammonia_ become visible.[233]
Their surgical value consists in their power to remove excrescencies, to
establish an ulcer, or to convert an ulcerated surface into a simple
sore.