THE  PREPARATION  OP  HYDROGEN  REDUCED  IRON.  153 
its  purity.  Iron,  prepared  for  industrial  purposes,  is  almost 
always  impure,  for  the  simple  reason  that  in  its  wholesale  manu- 
facture the  purification  of  the  re-agents  and  of  the  products  is 
only  partial ;  there  is  a  limit  at  which  we  must  stop,  but  at 
which  we  do  not  find  that  degree  of  purity  which  should  always 
characterise  substances  introduced  into  the  animal  economy. 
Moreover,  the  reduced  iron  of  commerce  is  often  mixed  with 
fine  iron  filings,  and  sometimes  it  even  consists  entirely  of  ordi- 
nary iron  reduced  to  very  fine  powder  by  a  filing  machine. 
It  is,  however,  easy  to  detect  these  sophistications.  It  is 
sufficient  to  treat  the  suspected  iron  by  a  pure  diluted  acid, 
which,  if  the  iron  is  pure  and  contains  no  ordinary  iron,  will 
dissolve  it,  and  produce  a  limpid  solution  free  from  residue.  This 
process  also  afi"ords  indications  of  the  sulphur  contained  in  almost 
all  reduced  irons  more  or  less  abundantly  ;  it  can  be  detected 
by  paper  saturated  with  a  solution  of  acetate  of  lead,  placed  in 
contact  with  the  hydrogen  which  is  evolved  when  iron  is  treated 
by  a  dilute  acid,  the  paper  becoming  black  if  sulphur  is  contained 
in  the  iron. 
It  is  very  important  to  obtain  iron  exempt  from  sulphur,  but 
it  is  impossible  to  get  it  pure  by  the  ordinary  industrial  process  ; 
it  must,  in  fact,  be  prepared  in  the  laboratory  with  scrupulous 
care.  To  procure  pure  iron,  an  oxide  of  iron  of  almost  absolute 
purity  must  be  first  prepared  ;  but  if  this  oxide  is  obtained  by 
decomposing  the  sulphate  of  iron,  it  is  almost  impossible  to  get 
rid  of  a  portion  of  adherent  sulphate,  which  repeated  washings 
fail  to  remove.  It  is  preferable  to  decompose  an  acid  chloride 
of  iron  by  ammonia,  in  order  to  obtain  the  oxide  pure.  Hydro- 
chloric acid  eliminates  all  the  sulphur  from  the  iron,  under  the 
form  of  sulphuretted  hydrogen,  and  boiling  the  acid  solution  is 
a  sure  means  of  driving  olF  all  trace  of  this  gas  which  may  lurk 
in  the  solution.  Then,  by  precipitating  the  chloride  of  iron  by 
ammonia,  soluble  and  volatile  compounds  are  formed,  easily  eli- 
minated by  heat  and  washing. 
When  we  propose  to  obtain  iron  free  from  sulphur,  it  is  not 
sufficient  to  provide  pure  oxide  of  iron  ;  the  hydrogen  itself 
which  is  employed  in  excess  in  the  operation  of  reduction  must 
contain  no  sulphur.  Those  accustomed  to  the  practical  opera- 
tion of  a  laboratory,  and  familiar  with  chemical  manipulation, 
