398  PRUSSIAN  BLUE,  OR  HYDROCYANATE  OF  IRON. 
of  the  protoxide  of  iron  and  potassa  (or  soda),  which  at 
once  seize  upon  whatever  oxide  of  iron  is  present  to  form  the 
soluble  combinations  of  ferrocyanide  of  potassium  (or  sodium) 
and  of  iron,  similar  to  the  white  precipitate  brought  on  by  prus- 
siate  of  potassa  in  solutions  of  protosalts  of  iron.  The  manu- 
facture of  prussiate  of  potassa  is  partly  founded  on  this  very 
reaction. 
This  reddish  precipitate  must  be  considered  as  a  combination 
of  the  white  precipitate  mentioned  and  cyanide  of  potassium, 
which,  on  digestion,  gradually  passes  into  yellow  prussiate, 
ferrocyanide  of  potassium.  The  color  can  be  no  objection, 
when  it  is  considered  that  ferrocyanide  of  potassium,  though 
consisting  of  two  supposed-to-be  colorless  salts,  has  a  more  or 
less  rich  yellow  color. 
An  experiment  to  illustrate  this  formation  of  prussiate  of 
potassa  is  easily  made,  even  with  the  common  fused  cyanide  of 
potassium,  (which,  besides  a  varying  proportion  of  cyanide,  con- 
tains hydrate,  carbonate,  cyanate  and  formiate  of  potassa).  By 
adding  a  large  excess  of  this  salt  to  a  solution  of  a  ferrous  salt, 
protosulphate  of  iron  for  instance,  a  reddish  brown  precipitate 
is  formed,  which  turns  rapidly  dark,  but  dissolves  on  heating, 
with  the  exception  of  a  trace  of  peroxide  of  iron,  and  this  solu- 
tion forms  Prussian  Blue  with  perchloride  of  iron. 
On  repeating  the  experiment  with  a  strong  alcoholic  solution 
of  the  cyanide,  pouring  off  most  of  the  supernatant  liquid  and 
replacing  it  by  fresh  spirits,  it  will  be  found  that  as  soon  as  the 
dark  precipitate  has  somewhat  settled,  yellowish  white  flakes 
appear  suspended  in  the  liquid,  which  prove  to  be  ferrocyanide 
of  potassium,  and  in  the  course  of  a  few  days  the  whole  precipi- 
tate left  to  itself  under  alcohol  will  be  found  to  be  converted 
into  that  salt. 
When  the  excess  of  cyanide  of  potassium  has  not  been  so 
large,  the  ferrocyanide  of  iron  and  potassium  can,  of  course,  not 
all  be  converted  into  yellow  prussiate,  and  this  is  especially  the 
case  when  the  common  cyanide  of  potassium  has  been  used, 
which,  in  contact  with  a  salt  of  iron,  forms  also  a  certain 
amount  of  hydrated  or  carbonated  oxide.  Still,  all  cyanide  of 
potassium  forms  a  proportional  quantity  of  ferrocyanide,  which 
partly  forms  a  more  or  less  colored  Prussian  Blue,  partly  re- 
mains, combined  with  the  precipitate,  and  causes  its  solubility. 
