248 
ON  THE  PREPARATION  OF  PURE  SILVER. 
contact  of  the  air.  At  the  end  of  this  time  about  a  third  of  the 
silver  was  reduced  at  the  ordinary  temperature,  and  was  pre- 
cipitated in  the  form  of  a  shower  of  crystallized  silver,  of  a  grey- 
ish-white color,  and  very  brilliant. 
I  then  put  the  decanted  blue  liquid,  in  quantities  of  ten  litres 
at  a  time,  in  a  water-bath  at  a  temperature  of  from  60°  to  70°. 
The  time  required  to  cause  the  elevation  of  temperature  was 
quite  sufficient  for  the  complete  reduction  of  the  silver  in  solu- 
tion, and  for  the  reduction  of  the  cupric  sulphite  to  the  state  of 
cuprous  sulphite;*  especially  as  I  was  careful  to  take  a  suffi- 
cient excess  of  solution  of  sulphite  of  ammonium. 
The  silver  being  eliminated,  I  decanted  the  liquid  when  cold, 
and  proceeded  to  wash  separately  the  silver  precipitated  from 
the  cold  and  the  warm  solutions.  This  washing  was  performed 
by  decantation  with  ammoniacal  water  ;  it  was  continued  as  long 
as  the  washing  waters  were  perceptibly  colored  blue  by  exposure 
to  air,  or  precipitated  chloride  of  barium.  I  afterwards  left  the 
silver  for  several  days  in  concentrated  ammonia,  and  then  washed 
it  in  pure  water. 
If  the  solution  from  which  the  silver  is  precipitated  has  been 
diluted  until  it  contains  no  more  than  2  per  cent,  of  silver,  the 
ammonia  left  in  contact  with  this  metal  is  not  colored  even  after 
several  days'  digestion.  There  is  no  longer  any  copper  for  the 
ammonia  to  dissolve ;  it  dissolves  silver  instead,  for  this  metal  is 
feebly  attacked  by  the  alkali  under  the  influence  of  air,  as  it  is 
easily  proved  by  evaporating  liquid  ammonia  which  has  remained 
several  days  in  contact  with  turnings  of  pure  silver.  This  liquid 
always  leaves  a  black  shining  mirror  of  nitride  of  silver  by  its 
spontaneous  evaporation. 
I  prepared  silver  by  this  method  at  four  different  times,  and 
in  the  last  I  operated  upon  2500  grammes  of  silver  at  once.  I 
have  proved  that  by  fulfilling  all  the  conditions  I  have  described, 
and  especially  by  carrying  the  dilution  of  the  ammoniacal  solution 
of  the  nitrates  of  silver  and  copper  to  2  per  cent,  of  silver,  we 
*  The  liquid  in  which  the  reaction  takes  place  becomes  quite  colorless  if 
the  copper  contains  neither  nickel  nor  cobalt.  If  it  contains  nickel,  it  takes 
a  slight  green  tint ;  it  takes,  on  the  other  hand,  a,  reddish  tinge  if  there  is 
cobalt  in  the  dissolved  metal. 
