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278
MANUFACTURE
OF
VINEGAR.
white
precipitate,
if
nitric
acid
be
present
an
im-
probable
impurity.
It
may
be
detected
by
its
producing
a
yellow
color
when
boiled
with
indigo.
The
acrid
substances
usually
introduced
into
vinegar
are
red
pepper,
long
pepper,
Guinea
pepper,
pelli-
tory,
and
mustard.
These
may
be
detected
by
eva-
porating
the
vinegar
to
an
extract,
which
will
have
an
acrid,
biting
taste,
if
any
one
of
these
substances
should
be
present.
By
far
the
most
dangerous
impurities
in
vinegar
are
copper
and
lead.
The
former
may
be
detected
by
a
brownish
precipitate
on
the
addition
of
ferro-
cyauuret
of
potassium
to
the
concentrated
\inegar.
The
latter
by
a
blackish
precipitate
with
sulphuret-
ed
barium,
and
a
yellow
one
with
iodide
of
potas-
sium.
Pure
vinegar
is
not
discolored
by
sulphureted
hydrogen.
The
essential
ingredients
of
pure
vinegar
are
acetic
acid
and
water
;
but,
besides
these,
it
con-
tains
various
other
substances
derived
from
the
particular
vinous
liquor
from
which
it
may
have
been
prepared.
Among
these
may
be
mentioned
coloring
matter,
gum,
starch,
gluten,
sugar,
a
small
portion
of
alcohol,
and
frequently
malic
and
tartaric
acids,
with
a
minute
proportion
of
alkaline
and
earthy
salts.