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190
MANUFACTURE
OF
SULPHURIC
ACID.
and
in
three
hours
from
the
time
of
lighting,
the
con
densation
of
the
gases
having
in
that
interval
been
completed,
the
doors
are
thrown
open
for
from
fif-
teen
to
twenty
minutes,
to
admit
fresh
atmospheric
air,
and
to
allow
time
for
the
residuary
nitrogen
to
escape.
Preparatory
to
the
next
burning,
the
ope-
rations
are
repeated
with
fresh
charges
of
the
mix
ture,
every
four
hours,
both
night
and
day,
until
the
water
has
attained
the
requisite
acid-
impregnation.
When
it is
transferred
to
leaden
boilers,
and
other-
wise
treated,
as just
explained,
the
quantity
of
the
charge
for
each
burning
is
determined
by
the
size
of
the
chamber,
allowing
one
pound
of
the
mixture
for
every
three
hundred
cubic
feet
of
atmospheric
air
which
it
may
contain.
As
in
the
manufacture
of
sulphuric
acid,
the
nitre
is
the
most
expensive
material.
Many
plans
have
been
resorted
to
for
the
purpose
of
obtaining
the
necessary
nitrous
acid
at
a
cheaper
rate.
One
plan
has
been
to
treat
molasses,
or
starch,
with
common
nitric
acid.
In
this
case
the
manufacturer
obtains
oxalic acid
as
a
collateral
product,
which
serves
to
diminish
his
expenses.
In
some
manufactories
of
sulphuric
acid
nitrate
of
soda
is
substituted
for
nitre
;
the
advantages
of
the
former
salt
are
its
greater
cheapness,
and
the
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