ELECTRO-CHEMICAL  COLORING:  OF  METALS. 
79 
is  not  modified  by  light.  While  in  contact  with  platinum  the 
latter  metal  doubtless  contributes  to  the  coloration.  No  similar 
result  is  obtained  by  operating  with  platinized  plates  previously 
treated  with  acidulated  water  or  rouge.  Furthermore,  air  has 
no  effect  on  the  colors  produced — not  an  unimportant  fact,  as 
it  will  make  it  possible  to  obtain  fast  colors  also  with  peroxide 
of  lead. 
Owing  to  the  strata  of  oxide,  the  result  is  the  same,  if  heat  is 
gradually  applied  to  non-preserved  platinized  pieces,  but  the 
colors  are  less  brilliant. 
If  a  copper  plate  with  a  layer  of  peroxide  of  lead  giving  one 
of  the  beautiful  colors  of  the  spectrum  is  employed  as  a  positive 
electrode  to  decompose  the  water,  in  a  few  moments  it  will  be 
found  that  the  coloration  is  preserved — a  result  similar  to  that 
obtained  with  platinized  copper.  By  continuing  the  electro- 
chemical action  for  a  quarter  or  half-an-hour,  according  to  the 
force  of  the  pile,  the  blue-violet  tints  fade,  turning  to  green  and 
yellow,  as  peroxide  of  lead.  The  basis  of  the  coloration  under- 
goes no  change  at  the  positive  pole.  It  is  thought  that  the  acid 
secondary  products  formed  at  the  positive  pole  react  on  the  per- 
oxide and  decompose  it. 
The  colored  plates  thus  preserved  seem  to  be  in  the  same 
condition  as  iron  when  it  has  been  plunged  in  nitric  acid,  or 
when  it  has  been  used  as  a  positive  electrode  to  decompose  this 
same  acid.  It  is  then  in  an  abnormal  state,  being  unattackable 
by  nitric  acid. 
When  a  very  slight  layer  of  platinum  is  electro-chemically  de- 
posited on  a  gold  or  platinum  plate  by  means  of  a  solution  of 
double  chloride  of  potassium  and  platinum  containing  no  copper, 
this  layer  undergoes  no  change  either  by  the  action  of  the  air 
or  when  the  plate  is  employed  as  a  positive  electrode  to  decom- 
pose water  ;  but  it  is  otherwise  when  the  solution  contains  copper. 
The  coloring  effects  before  described  are  then  produced,  when 
the  proportion  of  copper  is  small  and  the  coloration  of  the 
platinum  is  not  destroyed  by  weak  nitric  acid — an  important 
advantage  practically.  In  this  memoir  we  have  indicated  a  great 
improvement  in  the  process  of  coloration,  for  by  its  means  more 
uniform  and  adherent  layers  of  metals  are  deposited. 
Magnificent  coloring  effects  are  produced  by  a  solution  of 
