PRUSSIAN  BLUB,  OR  HYDROCYANATE  OF  IRON. 
399 
It  is  of  no  avail  to  wash  the  precipitate  with  water,  nor  can 
washing  with  alcohol  produce  a  pure  cyanide  of  iron,  since  the 
prussiate  of  potassa  contained  in  the  precipitate  is  altogether 
insoluble  in  spirits. 
If  the  cyanide  of  potassium  used  was  pure,  the  precipitate 
formed  by  an  excess  of  it  dissolves  completely  in  the  liquid. 
Obtained  from  the  common  cyanide,  it  is  mixed  with  more  or 
less  hydrated  protoxide  of  iron,  which  does  not  at  once  combine 
or  dissolve  with  the  excess  of  cyanide  or  ferrocyanide  already 
formed,  but  does  so  when  it  has  been  converted  into  peroxide 
by  exposure,  in  which  condition  it  combines  with  the  alkaline 
ferrocyanide  to  form  soluble  Blue ;  and  this  is  the  substance  ex- 
amined. 
Neither  the  proto-  nor  the  per-cyanide  of  iron  have  ever  been 
obtained  in  a  pure  form.  The  compound  FeCy,  2HCy,  ferro- 
eyanic  acid,  the  theoretical  acid  of  the  yellow  prussiates,  is  a 
whitish,  generally  slightly  yellow  or  blue,  powder,  which  bears 
heating  to  212°  F.,  a  little  above  that  point  gives  off  hydro- 
cyanic acid  and  water,  leaving  a  grayish  yellow  substance,  of 
what  composition  is  not  exactly  known,  which,  at  a  slightly 
elevated  temperature,  changes  to  a  black  powder,  which  consists 
of  12  equivalents  of  iron,  20  of  carbon,  and  5  of  nitrogen.  A 
solution  of  this  acid  on  boiling  precipitates  a  white  substance, 
which  Berzelius  supposes  to  be  a  protocyanide  of  iron,  though 
it  could  not  be  determined  whether  it  contained  water  or  not,  as 
it  changes  instantly,  in  contact  with  the  air.  The  substance 
obtained  by  heating  ferrocyanide  of  ammonium  is  probably 
purer  than  that  had  from  ferrocyanic  acid,  but,  like  the.  former, 
not  constant  enough  to  allow  of  being  analysed.  The  circum- 
stance that  these  substances,  on  being  heated  with  hydrochloric 
acid,  give  off  hydrocyanic  acid,  speaks  for  the  presence  of 
the  elements  of  water  in  them.  Chemically  pure  Prussian  Blue 
suspended  in  water,  and  thus  treated  with  sulphuretted  hydro- 
gen, furnishes  a  white  product,  which  undoubtedly  contains 
protocyanide  of  iron ;  but  mixed  with  a  quantity  of  sulphur 
equivalent  to  the  peroxide  of  iron  reduced  by  the  hydrogen.  I 
say  peroxide  of  iron,  for  whatever  symbols  may  be  used  for 
convenience'  sake,  all  the  mixtures  comprised  under  the  name  of 
Prussian  Blues  contain  water  chemically  combined,  and  always 
enough  to  constitute  hydrocyanates  of  protoxide  and  of  per- 
