234 
FERROCYANIDE  OF  POTASSIUM. 
water  and  then  through  a  tube  filled  with  asbestos,  moistened 
with  hydrochloric  acid.  After  the  lapse  of  twenty-four  hours 
the  solution  yielded  hydrocyanic  acid  when  treated  with  bichro- 
mate of  potash  (to  destroy  the  smell  of  sulphureted  hydrogen) 
and  sulphuric  acid.  A  faint  ammoniacal  odour  was  likewise  per- 
ceptible at  the  end  of  three,  four,  and  even  five  days,  when  the 
presence  of  cyanide  of  potassium  in  the  solution  could  be  distinct- 
ly recognized. 
8.  The  melt  which  had  been  treated  in  this  manner  for  seven 
days,  was  mixed  with  freshly  precipitated  sulphuret  of  iron,  and 
at  the  end  of  twenty-four  hours  the  solution  no  longer  contained 
cyanide  of  potassium. 
9.  About  twenty  grm.  of  the  melt  were  digested  with  about 
three  grm.  of  fused  potash  and  water  in  the  same  apparatus,  at 
212°F.,  and  after  twenty-four  hours  the  solution  still  contained 
cyanide  of  potassium,  which  did  not  entirely  disappear  even  after 
the  lapse  of  seven  days. 
10.  The  above  mass  was  then  mixed  with  fresh  precipitated 
sulphuret  of  iron,  and  within  twenty-four  hours  all  the  cyanide 
of  potassium  was  converted  into  ferrocyanide. 
The  melts  from  other  factories  gave  corresponding  results, 
Qy article  of  Potassium  (prepared  according  to  Liebig's  method), 
digested  with  fresh  precipitated  sulphuret  of  iron  and  water,  was 
entirely  converted  into  ferrocyanide  within  two  days. 
When  digested  with  fresh  sulphuret,  caustic  potash,  and  water 
the  change  of  the  cyanide  into  ferrocyanide  was  completed  with- 
in twenty-four  hours. 
It  appears  from  the  above  experiments  that  it  is  chiefly  the 
finely  divided  amorphous  sulphuret  of  iron  which  determines  the 
rapid  conversion  of  the  cyanide  into  ferrocyanide,  while  the  crys- 
talline sulphuret  (contained  in  many  melts  produced  at  a  high 
temperature)  acts  much  more  slowly.  Consequently  the  presence 
of  sulphuret  of  potassium  in  the  melts  is  a  favorable  element  in 
the  formation  of  ferrocyanide  of  potassium,  for  then  on  the  ad- 
dition of  sulphate  of  iron  fresh  precipitated  sulphuret  of  iron  is 
produced. 
A  temperature  varying  from  158°  to  176°  F.  appears  to  be 
the  most  favorable  for  the  extraction  of  the  melt,  for  at  that 
temperature  the  formation  of  ammonia  is  but  small,  and  the  sol- 
vent action  of  the  water  is  considerably  augmented. 
