ACIDS.
455
the  name  of  fusible  spar,  fluor  spar,  false  amethyst,  &c.
was  confounded  with  spar,  till  the  miners,  in  consequence
of  their  practice,  distinguished  it  by  its  useful  property  of
serving  as  a  flux  to  the  most  refractory  ores.
Marggraf  was  the  first  who  examined  fusible  spar,  and
selenitic  spar.  He  determined  their  different  characters,
and  that  an  earthy  sublimate  may  yield  in  distilling  this  spar
with  oil  of  vitriol.
Priestley  first  observed,  that  an  acid  gas  was  disengaged
in  the  distillation  of  this  spar  with  sulphuric  acid,  which
communicated  to  water,  as  soon  as  it  came  into  contact
with  it,  a  strong  acidity,  and  covered  the  surface  of  the
water  with  a  stony  crust.
Scheele,  in  1771,  assigned  it  the  rank  it  was  entitled  to
among  the  mineral  acids.
When  the  fluor  acid  is  obtained  from  a  mixture  of  fluor
spar  and  sulphuric  acid  in  a  glass  retort,  it  is  rendered
impure,  for  it  is  saturated  with  the  silica  it  has  dissolved
from  the  retort,  and  it  is  mixed  with  sulphuric  and  sulphurous ¬
  acids.  The  presence  of  these  is  immediately  shewn
by  the  acetate  of  barytes.  To  obtain  the  pure  fluor  acid,
the  mixture  must  be  distilled  in  lead  or  tin  vessels,  and  the
inside  of  the  receiver  lined  with  a  coat  of  wax.
The  distillation  of  a  mixture  of  four  ounces  of  fluor  spar,
and  twelve  ounces  of  sulphuric  acid,  in  this  way,  is  sufficient ¬
  to  render  eight  ounces  of  water  very-strongly  acid.
The  acetate  of  barytes  does  not  then  discover  any  mixture
of  sulphuric  acid,  though  the  acid  obtained  by  the  distillation ¬
  is  strong  enough  to  dissolve  calcareous  earth  with
effervescence.
This  acid  must  be  kept  in  flint-glass  bottles,  coated  internally ¬
  with  a  mixture  of  wax  and  oil.
The  acid,  when  obtained  this  way,  is,  however,  not  qu’te
pure.  It  is  mixed  with  a  small  quantity  of  oxide  of  lead
or  of  tin,  according  to  the  retort  made  use  of.
Two  ounces  of  vitriolic  acid,  and  half  an  ounce  of  fluor
spar,  were  distilled  in  a  small  retort  of  lead,  in  a  water  bath.
The  retort  weighed  eleven  ounces  eight  drams.  In  the
first  distillation,  it  lost  one  dram  and  a  half  ;  in  the  second,
one  dram  ;  and  in  the  third,  fifty-eight  grains.  The  acid
obtained  was  whitish,  and  had  a  strong  smell  of  liver  of
sulphur.  The  fluor  acid  alone  cannot  dissolve  tin  or  lead  ;
but,  during  the  distillation,  the  superabundant  sulphuric
acid  dissolves  the  metal,  which  is  taken  from  it  by  the
fluor  acid,  and  deprived  of  its  oxygen.  In  this  distillation,