70 
NEW  MODE  OE  PREPARING  RED  OXIDE  OP  IRON. 
several  weeks,  in  consequence  of  the  remains  of  coarse  parts 
which  have  not  been  separated,  notwithstanding  the  long  wash- 
ing. Therefore,  a  colcothar  capable  of  being  employed  with 
safety  is  always  in  great  demand  and  of  high  price. 
These  circumstances  have  induced  me  to  seek  a  new  process 
for  procuring  a  colcothar  which  should  be  preferable  to  that  pre- 
pared in  the  ordinary  manner,  however  carefully  washed. 
Numerous  experiments  have  shown  me  that  the  decomposed 
oxalate  of  protoxide  of  iron  presents  the  proper  means  for  ac- 
complishing this  object. 
The  oxalate  of  protoxide  of  iron  heated  in  a  close  vessel  out 
of  contact  with  the  air,  gives  iron  pyrophorus,  that  is  to  say, 
metallic  iron  so  finely  divided  that  it  inflames  on  contact  with 
the  air,  and  red  oxide  of  iron  is  formed*  This  transformation 
takes  place  very  rapidly,  when  we  heat  the  oxalate  of  protoxide 
of  iron  in  the  air  on  a  sheet  of  platinum,  during  which  operation 
the  matter  undergoes  a  considerable  augmentation  of  bulk. 
Owing  to  the  carbonic  acid  and  carbonic  oxide  gases  which  are 
disengaged  at  this  high  temperature,  the  substance  is  thoroughly 
divided,  and  during  this  disengagement  of  gases,  it  absorbs 
oxygen  from  the  air.  We  have  consequently,  in  the  disengage- 
ment of  gases  and  in  the  absorption  of  oxygen,  a  means  of  re- 
ducing the  oxide  of  iron  to  the  state  of  as  fine  a  powder  as 
possible. 
In  order  to  procure  colcothar  according  to  the  mode  indicated, 
we  operate  in  the  following  manner ;  into  a  solution  of  sulphate 
of  iron  made  with  boiling  water  and  filtered,  we  pour  a  concen- 
trated solution  of  oxalic  acid,  until  no  more  yellow  precipitate  of 
oxalate  of  protoxide  of  iron  is  formed.  When  the  liquid  is  quite 
cold  and  deposits  nothing  more,  the  precipitate  is  washed  on  a 
cloth  with  hot  water  until  the  washing  water  no  longer  gives  an 
acid  reaction. 
The  oxalate  is  afterwards  heated  in  the  partially  dry  state  on 
an  iron  plate  or  in  a  boiler  of  the  same  metal,  over  a  small  char- 
coal fire  or  even  a  spirit  lamp.  The  decomposition  commences 
at  the  temperature  of  200°  C.  (392°  F.)  and  on  raising  the  tem- 
perature a  little,  the  red  oxide  of  iron  is  formed,  and  is  found  in 
the  finest  possible  state. 
The  colcothar  formed  by  this  mode  affords  perfect  security  of 
