ON  THE  PREPARATION  OF  PURE  SILVER. 
247 
cuprous  sulphite,  or  to  a  mixture  of  sulphite  of  ammonium  and 
any  ammoniacal  salt  of  copper. 
At  the  ordinary  temperature  this  reduction  takes  place  slowly 
with  deposition  of  black,  blue,  or  grey  silver,  according  to  the 
dilution  of  the  liquids.  Above  a  temperature  of  60°  C.  the  re- 
duction is  almost  instantaneous,  and  the  silver  is  precipitated  in 
a  state  of  division  corresponding  to  the  dilution  of  the  liquid;  its 
colour  varying  from  grey  to  pure  white. 
This  is  how  I  prepared  silver  by  this  method. 
Silver  coin  is  dissolved  in  dilute  and  boiling  nitric  acid  ;  the 
solution  of  nitrate  of  silver  and  of  copper  is  evaporated  to  dry- 
ness, and  the  saline  mass  fused.  This  fusion  is  necessary  to 
destroy  the  nitrate  of  platinum,  which  is  often  formed  in  dissolving 
silver  coin.* 
After  coolimg,  the  nitrates  are  taken  up  by  an  excess  of  am- 
moniacal water.  The  ammoniacal  solution  is  left  to  rest  for 
fortg-eight  hours.  The  limpid  liquid  is  filtered  through  a  double 
filter  of  paper,  and  then  diluted  with  distilled  water  until  it  con- 
tains no  more  than  two  per  cent,  of  its  weight  of  silver. 
I  procured  neutral  sulphite  of  ammonium  by  mixing  ammonia 
with  sulphurous  acid.  To  ascertain  the  quantity  of  sulphite  re- 
quired for  the  complete  precipitation  of  the  silver  from  the 
ammoniacal  solution  of  nitrate  of  silver  and  copper,  I  heated  to 
the  boiling  point  a  definite  volume  of  solution  of  sulphite  of  am- 
monium, and  I  found  the  volume  of  the  solution  of  silver  and 
copper  which  was  decolorized  by  this  salt.  Experiment  has,  in 
fact,  proved  to  me  that  so  soon  as  the  sulphite  of  ammonium, 
sufficiently  heated,  is  not  colored  blue  by  the  cupric  oxide  dis- 
solved in  the  ammonia,  there  remains  no  trace  of  silver  dissolved 
in  the  liquid,  because  in  this  case  all  the  copper  exists  in  the 
cuprous  state,  the  presence  of  which  is  incompatible  with  that  of 
any  compound  of  silver  dissolved  in  ammonia. 
The  quantity  of  sulphite  of  ammonium  necessary  for  the  pre- 
cipitation of  the  liquid  having  been  determined,  I  added  it  to  the 
argentiferous  solution,  and  after  being  well  mixed,  it  was  left  to 
itself  for  forty-eight  hours  in  a  closed  glass  flask,  to  prevent  the 
*  I  have  found  by  several  trials  that  French  silver  contains  iron,  nickel, 
and  traces  of  cobalt,  platinum  and  gold. 
