170 
NOTE  ON  A  PRODUCTION  OF  SULPHURET  OP  IRON, 
NOTE  ON  A  PRODUCTION  OF  SULPHURET  OF  IRON. 
By  M.  Chevreul. 
In  a  memoir  which  I  read  to  the  Academy  in  November, 
1846,  I  examined  the  black  ferruginous  matter  found  under  the 
pavements  of  Paris,  and  I  arrived  at  the  following  conclusions: — 
This  matter  proceeds  from  the  horses'  shoes,  from  the  iron  of 
the  wheels  of  vehicles  and  from  the  debris  of  that  metal  which 
are  scattered  on  the  public  way ;  it  is  carried  into  the  gutters 
and  between  paving  stones,  and  there  it  is  converted  into  mag- 
netic oxide  of  iron  (Fe02-}-Fe)  and  may  afterwards  pass  to  the 
state  of  sulphuret,  likewise  black. 
So  long  as  the  matter  remains  black,  it  prevents  the  oxygen 
of  the  atmosphere,  whether  in  the  gaseous  state,  or  dissolved  in 
water,  from  penetrating  into  the  layer  of  earth  situated  beneath 
it. 
How  is  this  sulphuret  of  iron  produced  ? 
Experiment  has  taught  me  : — 
1.  That  mixtures  of  iron  and  gypsum ;  of  iron,  gypsum  and 
white  of  egg  ;  and  of  iron,  gypsum  and  gum  arabic,  confined  in 
flasks  with  water  and  air,  had  given  only  oxide  of  iron  without 
sulphuret. 
2.  That  protosulphuret  of  iron  was  produced  under  a  pavement 
near  Bie'vre,  and  in  a  place  where  there  was  water  impregnated 
with  organic  matter  and  sulphate  of  lime,  a  portion  of  which 
had  been  converted  into  sulphuret. 
I  concluded  from  this  double  observation,  that  the  sulphuret 
of  iron  which  is  found  in  the  mud  of  Bie'vre,  &c,  results  proba- 
bly from  the  reaction  of  the  oxide,  or  sesquioxide  of  iron,  or 
the  sulphuret  of  calcium,  produced  by  the  reaction  of  the  organic 
matters  on  the  sulphate  of  lime. 
The  object  of  the  note  which  I  now  communicate  to  the 
Academy,  is  to  convert  this  probability  into  certainty.  I  have 
now  profited  by  the  cleansing  of  Bievre  to  examine  a  black  sandy 
mud,  which  was  raised  from  the  bottom  of  the  river.  Ol  placing 
this  mud  on  a  filter,  there  passed  off  a  yellow  liquid,  principally 
formed  of  a  polysulphuret  of  calcium  ;  it  precipitated  acetate  of 
lead  of  a  reddish-brown,  and  disengaged  hydrosulphuric  acid, 
depositing  sulphur ;  it  was  sufficient  to  expose  it  to  the  air  for  it 
to  become  milky  ;  finally,  on  agitating  it  with  hydrated  sesqui- 
