ANTIDOTE  AT  ONCE  FOR  PRUSSIC  ACID,  ETC.  17 
acid  were  now  added,  and  on  now  preparing  to  add  the  alka- 
line solution  to  form  a  cyanide,  before  the  addition  of  the  iron 
solution,  it  occurred  to  us  that  the  excess  of  magnesia  itself 
might  form  the  cyanide  necessary  to  the  formation  of  the 
Prussian  blue.  Kesolving,  therefore,  to  put  the  idea  to  the 
test,  we  at  once  added  the  iron  solution,  and  the  moment  con- 
tact between  the  two  liquids  occurred,  the  blue  color  showed 
that  the  formation  of  Prussian  blue  had,  to  a  certain  extent, 
been  the  result.  After  the  addition  of  a  solution  containing 
11 J  minims  of  solution  of  perchloride  of  iron,  and  8J  grains 
of  green  vitriol,  muriatic  acid  was  added  till  the  excess  of 
magnesia  and  the  excess  of  proto-peroxide  of  iron  had  been 
dissolved.  Prussian  blue  was  left  in  abundance.  On  now  at 
once  filtering,  and  adding  to  the  filtered  liquid  a  few  drops  of 
a  solution  of  persalt  of  iron,  no  Prussian  blue  was  formed,* 
showing  the  absence  of  any  ferro-prussiate.  On  now  adding 
aqua  potassse  to  neutralize  the  excess  of  acid,  and  throw  down 
the  iron  in  solution,  no  tinge  of  blue  was  produced,  not  even 
on  adding  an  excess  of  dilute  muriatic  acid.    The  precipitate 
*  When  freshly-precipitated  Prussian  blue  is  rubbed  up  with  calcined 
magnesia  in  considerable  excess,  after  a  short  time,  the  mixture  having 
been  collected  upon  a  filter  and  washed  with  distilled  water,  it  will  be 
found  that  almost  all  the  Prussian  blue  has  been  converted  into  proto- 
peroxide  of  iron,  along  with  a  simultaneous  corresponding  formation  of 
ferrocyanide  of  magnesium,  as  may  be  proved  by  the  addition  of  a  solu- 
tion of  a  persalt  of  iron  to  the  filtered  liquid,  whereby  almost  all  the 
Prussian  blue  is  reproduced.    This  reaction  does  not  take  place  when  an 
excess  both  of  magnesia  and  of  the  iron  solution  is  used,  as  in  the  case 
of  the  antidote;  for,  if  the  mixture  of  Prussian  blue,  proto-peroxide  of 
iron,  and  magnesia  thus  obtained  be  digested  for  a  long  time  with  water, 
and  that  at  the  temperature  of  the  living  body,  barely  a  trace  of  Prus- 
sian blue  is  produced  upon  the  application  of  a  solution  of  persalt  of  iron 
to  the  filtrate.    In  this  case,  the  excess  of  iron  in  the  form  of  precipita- 
ted oxide,  along  with  the  Prussian  blue,  appears  to  prevent  the  Prussian 
blue  from  being  decomposed  by  the  magnesia,  as  would  occur  with  this 
substance  by  itself.    Two  explanations  of  this  remarkable  result  suggest 
themselves  ;  either  a  compound,  stable  towards  magnesia,  may  be  formed 
between  the  Prussian  blue  and  the  proto-peroxide  of  iron,  or  this  last 
substance  by  enveloping,  and  thus  shielding  the  Prussian  blue  from  the 
action  of  the  magnesia,  may  prevent  the  result  that  would  otherwise  be 
produced.  t 
2 
