PREPARATION  OF  FERROCYANIDE  OF  POTASSIUM.  113 
of  the  cyanide  of  ammonium  thus  produced  into  cyanide  of  potas- 
sium, representing  ferrocyanide  of  potassium,  in  the  humid  way. 
The  gas  containing  ammonia  is  therefore  passed  through  tubes 
filled  with  charcoal  and  heated  to  redness  ;  the  ammonia  is  thus 
converted  into  cyanide  of  ammonium,  and  this  into  ferrocyanide 
of  potassium  by  contact  with  aqueous  solution  of  potash  and  suit- 
able iron  compounds.  The  particular  advantages  of  this  method 
are  the  following  : — 
1.  The  considerable  loss  of  potash  and  the  expense  of  its  re- 
covery are  got  rid  of. — The  potashes  are  first  dissolved  in  water, 
by  which  they  are  freed  from  the  greater  part  of  the  foreign  salts. 
The  solution  is  then  treated  with  a  quantity  of  the  cyanide  suffi- 
cient to  convert  the  greater  part  of  it  into  ferrocyanide  of  potas- 
sium ;  this  is  allowed  to  crystallize,  and  the  mother-liquor  em- 
ployed without  further  preparation  for  the  same  purpose.  Thus 
we  get  rid  at  once  of  the  contamination  with  silica,  &c,  and  of 
the  necessity  of  bringing  the  dissolved  potashes  again  into  a  solid 
form,  which  is  of  itself  a  cause  of  loss, 
2.  It  becomes  possible  to  replace  the  potashes  by  soda,  zvhich  is 
much  cheaper. — By  the  old  processes  this  was  often  tried  in  vain, 
because  from  the  difficult  reduction  of  sodium,  the  formation  of 
cyanide  of  sodium  in  the  dry  way  does  not  take  place  so  easily, 
and  moreover  ferrocyanide  of  sodium  cannot  easily  be  crystallized 
from  such  impure  solutions  as  the  mother-liquors.  The  'first 
difficulty  is  entirely  got  rid  of  in  its  formation  in  the  humid  way, 
and  the  second  may  also  be  overcome,  as  the  solutions  are  much 
purer.  Ferrocyanide  of  sodium  certainly  does  not  form  such 
beautiful  crystals  as  ferrocyanide  of  potassium  ;  but  if  it  can  be 
prepared  more  cheaply,  it  will  certainly  make  its  way  into  use, 
especially  as  6  parts  of  it  go  as  far  as  7  of  ferrocyanide  of  potas- 
sium, the  equivalent  of  sodium  being  lower. 
3.  Bones  may  be  employed  in  this  process,  their  subsidiary 
product,  bone  charcoal,  covering  the  cost  of  the  bones  and  their 
carbonization,  and  they  would,  therefore,  furnish  the  nitrogenous 
gases  for  nothing.  The  gases  from  the  carbonization  of  bones, 
are  just  as  rich  in  ammonia  in  proportion  as  those  of  most  other 
raw  materials;  in  proportion  to  their  weight,  however,  bones  fur- 
nish less  ammonia  and  much  less  gas. 
4.  It  is  possible  to  bring  back  into  the  manufacture  that  portion 
8 
