ON  THE  PREPARATION  OF  PURE  SILVER.  167 
by  decantation,  first  with  water  acidulated  by  hydrochloric  acid, 
and  then  with  pure  water.  This  washing  was  performed  by 
shaking  the  chloride  violently  each  time  in  large  stoppered  bot- 
tles with  the  necessary  quantity  of  liquid. 
It  was  then  spread  upon  a  cloth  that  had  been  washed  with 
hydrochloric  acid,  strongly  compressed,  and  left  to  dry  spon- 
taneously. When  perfectly  dry  it  was  finely  powdered  and  di- 
gested for  several  days  in  aqua  regia.  It  was  then  again  washed 
in  distilled  water. 
As  the  reduction  by  heat  of  chloride  of  silver  with  carbonate  of 
sodium  is  a  most  delicate  operation  when  performed  upon  large 
quantities,  I  made  this  reduction  at  a  low  temperature  under  the 
influence  of  a  solution  of  caustic  potash  and  sugar  of  milk,  as 
first  proposed  by  Levol. 
To  procure  potash  and  sugar  of  milk  free  from  heavy  metals, 
I  added  to  a  concentrated  solution  of  hydrate  of  potassium,  pre- 
viously boiled,  a  slight  excess  of  a  solution  of  sulphydrate  of 
potassium  to  precipitate  traces  of  dissolved  metals.  After  the 
deposit  of  the  metallic  sulphides,  I  decanted  the  alkaline  solution 
and  put  it  in  contact  with  freshly  precipitated  oxide  of  silver  to 
deprive  it  of  the  sulphydrate  of  potassium.  After  a  sufficient 
digestion  and  rest,  I  separated  the  excess  of  oxide  of  silver  and 
the  sulphide  of  silver  that  had  been  produced. 
By  the  same  means  I  eliminated  the  heavy  metals  contained 
in  an  aqueous  solution  of  sugar  of  milk. 
The  chloride  of  silver,  distributed  in  three  large  porcelain  jars, 
was  digested  at  a  temperature  of  from  70°  to  80°  C,  with  the 
mixture  of  solutions  of  hydrate  of  potassium  and  sugar  of  milk, 
until  all  the  chlorine  was  separated  from  the  silver.  The  metal- 
lic silver,  which  was  grey,  was  washed  with  water  until  the  ex- 
cess of  alkali  had  disappeared,  then  digested  with  dilute  pure 
sulphuric  acid,  and  lastly  washed  with  ammoniacal  water.  After 
being  dried,  5  per  cent,  of  its  weight  of  calcined  borax  was  added, 
containing  10  per  cent,  of  nitrate  of  sodium,  and  then,  with  the 
necessary  precautions,  it  was  fused  in  a  u  Paris  crucible." 
The  fused  metal  was  then  poured  into  a  mould~  coated  with  a 
paste  of  mixed  calcined  and  non-calcined  kaolin.  The  bars  of 
silver,  first  cleaned  with  sharp  sand,  were  then  heated  to  red- 
