WET  METHOD  OF  REDUCING  CHLORIDE  OF  SILVER.  269 
tion  of  chloride  of  silver  by  means  of  iron,  zinc,  or  copper  in  an 
acid  liquid  is  well  known  to  be  an  unsatisfactory  method,  as  the 
product  is  always  found  to  be  contaminated  to  a  more  or  less 
degree  with  the  metals  employed  ;  the  same  result  occurs  if  the 
chloride  of  silver  is  reduced  by  means  of  copper  in  an  ammonia 
solution.  At  the  same  time,  no  mention  has  ever  been  made  of 
the  employment  of  zinc  to  reduce  the  chloride  out  of  the  ammo- 
nia solution.  I  find,  however,  that  by  using  this  metal,  silver  of 
a  perfectly  pure  description  may  be  obtained. 
In  order  to  reduce  chloride  of  silver  by  means  of  zinc,  it  is 
first  dissolved  in  ammonia,  and  placed  in  a  bottle,  and  pure  zinc 
in  small  excess,  and  in  fragments  of  notable  size  (so  that  it  may 
be  easily  separated  after  the  process  of  reduction),  is  then  added. 
The  decomposition  commences  at  once,  and  proceeds  so  rapidly, 
especially  if  the  vessel  is  well  agitated,  that  in  the  space  of  three 
hours,  a  quarter  of  a  pound  of  chloride  of  silver  may  be  reduced  ; 
the  duration  of  the  operation  depends,  however,  of  course,  on  the 
greater  or  less  excess  of  zinc  present  in  the  solution.  A  certain 
excess  of  ammonia  will  likewise  have  the  effect  of  accelerating 
the  operation.  At  first,  the  precipitated  silver  assumes  a  light 
grey  or  dirty  white  color,  but  towards  the  end  it  becomes  dark 
grey,  or  even  black.  A  few  drops  of  the  ammoniacal  solution 
should  be  examined  in  a -test-tube  from  time,  to  time,  and-  pre- 
cipitated with  hydrochloric  acid ;  the  operation  is  at  an  end  when 
no  turbidity  is  apparent  on  the  addition  of  the  acid.  When  this 
is  the  case,  the  precipitate  is  allowed  to  settle  down,  and  the 
clear  liquid  then  poured  off  as  completely  as  possible ;  the  silver 
in  the  glass  vessel  is  treated  again  and  again  with  fresh  water 
until  all  smell  of  ammonia  has  disappeared,  and  is  then  trans- 
ferred to  another  bottle  by  means  of  a  funnel,  the  tube  of  which 
has  previously  been  stopped  with  a  few  fragments  of  glass,  to 
prevent  the  pieces  of  zinc  from  passing  through  at  tke  same  time 
as  the  silver  particles.  The  superabundant  water  is  then  decanted 
as  completely  as  possible,  and  the  precipitate  treated  with  con- 
centrated hydrochloric  acid,  in  which  it  is  allowed  to  digest  until 
the  dark  grey  color  has  given  place  to  one  of  a  dirty  white.  If 
•  the  silver  contains  much  water,  it  is  possible  that  it  may  not  be- 
come white  on  a  single  treatment  with  the  hydrochloric  acid ;  in 
