416 
ON  NITRO-PRUSSIDE  OP  SODIUM. 
which  the  nitro-prusside  of  sodium  first  crystallized,  by  trying  the 
fluid  on  the  sand-bath  with  the  thermometer  at  all  temperatures 
from  176°  F.  downwards.  By  spontaneous  evaporation  also  he 
always  obtained  a  mixed  crystallization,  both  salts  forming  in  inti- 
mate contact.  The  method  given  by  Kyd  for  the  preparation  of 
the  nitro-prusside  of  copper  has  also  its  particular  difficulty.  With- 
out taking  into  consideration  that  the  smallest  excess  of  caustic 
soda  in  the  decomposition  of  the  nitro-prusside  of  copper  contami- 
nates the  product  with  an  alkaline  ferrocyanide,  a  pure  green  pre- 
cipitate is  not  always  obtained  on  precipitation  with  sulphate  of 
copper;  it  is  frequently  reddish  from  ferrocyanide  of  copper,  the 
formation  of  which  is  not  to  be  avoided  when  there  is  an  excess  of 
carbonate  of  soda,  and  which  is  intimately  connected  with  the 
brown  color  of  the  precipitate  formed  by  boiling  the  neutralized 
fluid. 
According  to  the  experiments  made  by  the  author  with  reference 
to  Playfair's  further  statements,  the  following  method  is  adapted  to 
furnish  the  salt  pure: — 
5|  parts  of  commercial  nitric  acid,  diluted  with  an  equal  weight 
of  water,  are  poured  over  4  parts  of  pounded  prussiate  of  potash  in 
a  retort ;  after  solution  has  taken  place,  which  is  generally  com- 
pleted in  about  ten  minutes  when  the  acid  is  of  the  usual  degree  of 
concentration,  with  strong  evolution  of  the  gases  described  by 
Playfair  and  the  production  of  a  coffee-brown  color,  the  retort  is 
placed  upon  the  water-bath  until  a  drop  of  the  fluid  gives  a  dingy 
green  instead  of  a  blue  precipitate  with  a  solution  of  sulphate  of 
iron.  The  solution  is  allowed  to  cool,  by  which  means  a  large 
quantity  of  nitrate  of  potash  is  separated,  from  which  the  green 
mother-liquor  is  poured  away;  from  this  all  nitrate  of  potash  is 
separated  by  repeated  concentration.  The  fluid  is  then  neutralized 
with  carbonate  of  soda  while  being  heated  on  the  water-bath, 
taking  care  only  to  add  the  carbonate  as  long  as  a  pure  blue  pre- 
cipitate is  produced.  This  is  collected  on  a  filter  and  washed. 
From  the  united  filtrates  the  substance  is  obtained  in  the  most 
perfect  purity  by  spontaneous  evaporation.  The  ruby-red  prismatic 
crystals  are  washed  with  water,  and  dried  between  blotting-paper. 
The  crystals,  as  well  as  the  solution,  are  best  preserved  in  black 
bottles.  At  any  rate,  the  solution  must  be  carefully  kept  from  the 
sunlight. — Chem.  Gaz.,  July  15,  from  Archiv  der  Pharm. 
