PREPARATION  AND   USE  OF  PERCHLORIDE  OF  IRON.  65 
of  its  retaining  too  much  free  hydrochloric  acid.  If,  instead  of 
stopping  the  evaporation  at  the  point  indicated  by  M.  Burin  du 
Buisson,  it  be  continued  for  a  longer  time,  it  becomes  still  more 
acid.  It  is  known  also  how  difficult  it  is  to  keep  for  any  length 
of  time  a  solution  of  perchloride  of  iron  with  the  deposition  of 
insoluble  oxychloride  and  the  development  at  the  same  time  of 
free  acid  in  the  solution. 
In  order  to  obviate  these  objections,  M.  Burin  du  Buisson  pro- 
posed, in  a  new  memoir  addressed  to  the  Academy  of  Sciences, 
to  substitute  the  following  process  for  that  already  described  : 
Pure  hydrochloric  acid  is  to  be  saturated  as  far  as  possible  with 
hydrated  peroxide  of  iron  ;  the  solution  is  evaporated  over  a 
water-bath,  taking  care  to  remove  the  aqueous  vapors,  as  I  had 
before  indicated,  as  these  cause  the  formation  of  hydrochloric 
acid  and  the  deposition  of  oxychloride  of  iron.  When  the  li- 
quid has  thus  acquired  the  consistence  of  a  thick  syrup  (in  this 
state  it  crystallizes  on  being  cooled,  without,  however,  assuming 
a  solid  mass),  the  evaporation  is  stopped,  and  an  excess  of  ge- 
latinous hydrated  peroxide  of  iron  mixed  with  a  little  water  is 
added ;  they  are  stirred  together  for  a  quarter  of  an  hour,  and 
then  allowed  to  stand  for  several  hours.  Sufficient  distilled 
water  is  added  to  make  the  density  equal  to  30°  Baume,  and  it 
is  then  left  in  contact  with  excess  of  hydrated  oxide  for  eight 
days  when  it  is  filtered,  and  again  allowed  to  stand  for  fifteen 
days.  The  density  of  20°  Baume,  is  that  which  MM.  Valette, 
Desgranges,  and  Petrequin  prefer  for  the  treatment  of  varicose 
veins.  For  aneurisms  these  surgeons  think  it  is  sufficient  to  em- 
ploy a  solution  at  20°  or  even  15°. 
This  new  mode  of  preparing  the  ferric  solution  has  no  mate- 
rial advantage  over  the  former.  The  contact  of  the  liquor  with 
hydrated  oxide  of  iron  does  not  remove  free  hydrochloric  acid 
so  effectually  as  evaporation,  as  M.  Burin  du  Buisson  has  himself 
admitted.  It  is  known  that  solution  of  perchloride  of  iron, 
even  when  made  from  the  sublimed  chloride,  has  always  an  acid 
reaction. 
If  it  be  impossible  to  prepare  a  solution  of  perchloride  of  iron 
without  its  being  acid,  and  if  it  be  important  that  it  should  be 
as  little  acid  as  possible,  would  it  not  be  preferable  to  have  re- 
course to  the  chloride  of  iron  prepared  by  the  process  which  I 
5 
