pitayensis_, and _C. Pahudiana_, some
agreeing with certain soils and climates better than others, while the
yield of alkaloids and the relative proportions of the different
alkaloids differ in each species.

The official "bark" of the British Pharmacopoeia is that of _Cinchona
succirubra_ or red bark. It is imported in the form of quills or
recurved pieces, with a rough brown outer surface and a deep red inner
surface, forming a reddish brown odourless powder, which has a bitter,
astringent taste. The British Pharmacopoeia directs that the bark, when
used to make the various medicinal preparations, shall contain not less
than 5 nor more than 6% of total alkaloids, of which at least one-half
is to be constituted by quinine and cinchonidine. The preparations of
this bark are four: a liquid extract, standardized to contain 5% of
total alkaloids; an acid infusion; a tincture standardized to contain 1%
of total alkaloids; and a compound tincture which must possess one-half
the alkaloidal strength of the last. The only purpose for which these
preparations of cinchona bark should be used is as tonics; and even when
this is the desired action there are many reasons why the alkaloid
should be preferred, even though the recent introduction of
standardization removes one of the chief objections to their use.

The pharmacology of red bark, dependent as it is almost entirely upon
the contained quinine, will not here be discussed (see QUININE). But
the composition of cinchona bark is a matter of importance and
interest. The bark contains, in the first place, five alkaloids, of
which all but quinine may here be dealt with. Quinidine, C20H24N2O2,
is isomeric with quinine, from which it differs in crystallizing in
prisms instead of needles, in being dextro- and not laevo-rotatory,
and in being insoluble in ammonia except in much excess. Cinchonine
has the formula C19H22N2O, quinine being methoxy cinchonine, i.e.
C19H21(OCH3)N2O. It occurs in inodorous, bitter, colourless prisms;
unlike the two alkaloids already named, does not yield a green colour
with chlorine water and ammonia; is dextro-rotatory; not fluorescent,
and practically insoluble in ammonia and in ether. A fourth alkaloid,
cinchonidine, is isomeric with cinchonine, which yields it when boiled
with amyl alcoholic potash, but is laevo-rotatory, slightly soluble in
ether, and faintly fluorescent. When red bark is extracted with dilute
hydroc