78 
ELECTRO- CHEMICAL  COLORING  OF  METALS, 
these  colors— a  disadvantage  I  have  already  mentioned,  and 
which  may  in  great  measure  be  avoided  by  covering  the  colored 
surface  with  alcohol  varnish,  which  acts  but  very  slightly  on 
the  peroxide.  With  a  little  practice  all  the  tints  desired  may 
be  given  to  a  large  object  with  hollows  and  projections,  and 
each  part  painted  with  the  appropriate  color.  These  colors 
are  now  made  fast  by  the  processes  to  be  described  further 
on. 
If  the  solution  of  protoxide  of  lead  in  potash  is  replaced  by 
a  solution  of  protoxide  of  iron  in  ammonia,  and  the  gold,  copper, 
or  platinum  plate  by  one  of  polished  iron,  there  is  deposited  on 
the  latter  a  layer  of  peroxide  of  iron  with  red  or  brown  tints, 
which  deepen  more  and  more  according  to  the  thickness  of  this 
stratum,  which  never  exceeds  a  certain  limit  on  account  of  the 
imperfect  conductibility  of  the  peroxide. 
In  a  paper  on  the  precipitation  of  metals  from  their  solutions 
by  other  more  oxidisable  metals,  I  stated  that  on  plunging  a 
copper  plate  into  a  solution  of  double  chloride  of  potassium  and 
platinum,  heated  to  60°,  the  platinum  is  deposited  and  adheres 
to  the  copper,  producing  a  platinization,  which  is  quickly  acted 
on  by  the  air,  taking  first  a  slightly  brownish  tint,  which  be- 
comes darker  and  darker. 
This  alteration  is  partly  due  to  the  presence  of  protochloride 
of  copper,  which  is  deposited  simultaneously  with  the  platinum 
towards  the  end  of  the  operation.  The  protochloride  may  be  re- 
moved by  washing  the  platinized  copper  with  water  and  acetic 
acid,  or  by  rubbing  its  surface  with  cotton  and  rouge.  The  al- 
teration is  then  avoided,  or  at  least  does  not  appear  till  long 
afterwards,  probably  on  account  of  the  air  traversing  the  inter- 
stices of  the  platinum,  which  with  copper  forms  a  voltaic  pair. 
The  brownish  color  of  the  platinum  surface  is  like  that  which 
protochloride  of  copper  ordinarily  takes  under  exposure  to  air 
and  light. 
If  we  make  use  of  platinized  copper  directly,  it  is  obtained  from 
a  solution  of  double  chloride  as  a  positive  electrode  to  decom- 
pose water  with  a  pile  composed  of  several  elements,  it  produces 
under  the  influence  of  the  oxygen  disengaged  from  the  positive 
pole,  peculiar  coloring  effects,  seeing  that  the  tints  pass  imme- 
diately to  blue  and  deep  crimson,  and  the  protochloride  of  copper 
