270      WET  METHOD  OF  REDUCING  CHLORIDE  OF  SILVER. 
this  case,  the  liquid  must  be  again  poured  off,  and  a  fresh  quan- 
tity of  acid  added.  By  this  means,  the  silver  may  always  be 
obtained  of  a  white  color,  and  when  the  operation  has  advanced 
thus  far,  the  precipitate  is  repeatedly  washed,  until  the  water 
poured  off  is  only  very  slightly  acid.  The  silver  is  then  collected 
on  a  filter,  and  well  washed  with  distilled  water ;  finally,  a  little 
dilute  ammonia  is  poured  on  it,  and  it  is  again  washed  with 
water.  This  treatment  is  rendered  necessary,  in  order  to  re- 
move any  minute  quantity  of  chloride  of  silver  which  may  have 
been  formed  by  the  action  of  the  concentrated  hydrochloric 
acid,  the  latter  sometimes  containing  small  quantities  of  free 
chlorine. 
The  silver  produced  in  this  manner  is  of  a  very  pure  descrip- 
tion ;  at  any  rate,  I  have  never  been  able  to  detect  the  presence 
in  it  of  any  other  metal.  When  dissolved  in  nitric  acid,  and 
precipitated  with  hydrochloric  acid,  a  fluid  is  obtained,  which, 
when  neutralized  with  carbonate  of  soda,  does  not  become  turbid 
by  treatment  either  with  ferro-cyanide  of  potassium  or  alkaline 
carbonates.  It  assumes  a  brownish  color  on  the  addition  of  sul- 
phide of  ammonium,  owing  to  the  presence  of  dissolved  chloride 
of  silver,  but  no  precipitate  is  formed. 
The  expenditure  of.  ammonia  in  the  operation  is,  of  course, 
not  inconsiderable,  but,  as  the  greater  portion  of  it  may  be  re- 
covered by  distillation,  the  actual  loss  of  the  same  is  hardly  ap- 
preciable. In  working  with  large  quantities  of  chloride  of  silver, 
economy  may  be  exercised  in  the  use  of  ammonia  by  reducing 
the  chloride  in  several  operations,  so  that  the  ammoniacal  solu- 
tion of  chlorine  and  zinc,  when  freed  from  the  silver,  is  again 
used  to  dissolve  the  chloride.  This  appears  somewhat  strange, 
but  is  evidently  due,  in  part,  to  the  presence  of  free  ammonia, 
which  exerts  an  action  upon  the  chloride  of  silver  only  when  the 
silver  has  bejen  separated.  Such  a  mode  of  proceeding  would, 
however,  occur  only  when  manipulating  with  a  considerable  bulk 
of  chloride  of  silver,  for,  in  reducing  small  quantities,  the  neces- 
sary amount  of  water  required  to  dissolve  the  chloride  may  easily 
be  added. 
In  the  same  wTay,  nitrate  of  silver  dissolved  in  ammonia  may 
also  be  reduced  with  zinc,  perfectly  pure  silver  being  obtained ; 
