552  frecipitation  of  Silver  by  Copper.  {Ambll™ijm* 
or  oxygen,  the  tips  of  the  silver  crystals  become  at  once  coated  with 
cuprous  oxide ;  and  when  a  limited  quantity  of  air  is  used,  nitrogen- 
only  remains  in  the  free  condition  (Gladstone  and  Tribe,  Proc.  Roy^ 
Soc,  vol.  xx,  p.  290). 
In  carrying  out  the  above  and  other  experiments,  it  was  frequently 
necessary  to  completely  precipitate  silver  from  the  nitrate  by  copper,, 
and  it  was  observed  that  the  metal  so  obtained,  after  being  washed 
with  water,  invariably  contained  copper,  sometimes  in  considerable 
quantity.  Since  the  above-mentioned  couple  is  formed  the  instant, 
silver  in  solution  is  brought  into  contact  with  copper,  the  idea  sug- 
gested itself  that  the  copper  found  in  silver  precipitated  by  that  metal 
might  be  due  to  dissolved  oxygen  in  the  silver  solutions  ;  or  to  the- 
absorption  of  that  gas,  by  the  liquid,  from  the  air  during  or  subse- 
quent to  the  precipitation  of  the  metal.  The  experiments  made  with, 
the  view  of  ascertaining  the  correctness  of  this  supposition  are  tabu- 
lated below. 
There  was  employed  in  each  experiment  an  excess  of  copper,  and 
in  experiments  C  to  I  about  the  same  volume  of  liquid.  In  A  and 
B,  pieces  of  copper  of  the  same  dimensions  were  placed  in  open  basins, 
and  covered  to  about  a  quarter  of  an  inch  with  ordinary  silver  nitrate, 
L  e.,  impregnated  with  air.  In  C,  D  and  E,  bottles  were  filled  with 
ordinary  silver  solution  and  stoppered  during  the  precipitation.  In, 
F  and  G,  carbonic  anhydride  was  bubbled  through  the  solutions  prior 
to  the  immersion  of  the  copper  plate,  and  the  precipitation  conducted 
as  in  C,  D  and  E.    In  H  and  I  ordinary  solutions  were  employed. 
Per  cent  of     Duration        Copper  in         Copper  per 
Experi-     AgN03  in  in  Precipitated    100  parts  of  Pre- 
ment.       Solution.        Hours.  Metal.       cipitated  Metal. 
A  1-4  24  0-0185  7*45 
B  1-4  48  0-0377  15-23 
C  3-5  24  0-0103  0-32 
D  1-4  24  0-0096  0-77 
E  0-7  24  0-0099  1-61 
F  3-5  24  0  0025  0-08 
G  1-4  24  0-0029  0-23 
H  3*5  i           merest  trace  — 
I  3-5 
It  appears  from  experiments  A,  B  and  D,  that  the  quantity  of 
