1. Whenever a remedy requires the combination of all, or most of its
principles, to ensure its full effects, as _Bark_, _Ipecacuan_, _Jalap_,
&c.
2. Where medicinal bodies are insoluble, and indisposed to undergo those
essential changes, _in transitu_, which render them operative; for it
must be remembered that by minute division, every particle is presented
to the stomach in a state of activity, being more immediately exposed to
the solvent or decomposing powers of that organ.
3. Where the mechanical condition of the substance is such as to
occasion irritation[294] of the stomach, as the _Sulphuretum Antimonii_,
or in external applications to produce an improper effect upon the skin,
as _Hydrargyri nitrico-oxydum_.
The degree of fineness to which substances should be reduced by
pulverization, in order to obtain their utmost efficacy, is a very
important question. The impalpable form appears to be extremely
injurious to some bodies, as to _cinchona_, _rhubarb_, _guaiacum_, and
to certain aromatics, in consequence, probably, of an essential part of
their substance being dissipated, or chemically changed by the
operation. Fabbroni, for instance, found by experiment that cinchona
yielded a much larger proportion of soluble extractive, when only
coarsely powdered. I think it may be laid down as a general rule, that
_extreme pulverization assists the operation of all substances whose
active principles are not easily soluble, and of compound powders whose
ingredients require, for their activity, an intermixture; whilst it
certainly injures, if it does not destroy, the virtues of such as
contain as their active constituent, a volatile principle which is
easily dissipated, or extractive matter which is readily oxidized_.