NEW  PROCESSES  FOR  COVERING  METALS,  ETC.  175 
in  plain  water,  and  finally  in  water  made  alkaline  with  soda ; 
then  cleaned  with  a  scratchbrush,  again  washed,  and  plunged  in- 
to the  copper  bath  in  contact  with  zinc. 
The  articles  are  suspended  in  the  copper  bath  by  means  of 
zinc  wire ;  they  should  be  immersed  from  three  to  seventy-two 
hours,  according  to  the  color,  quality -and  thickness  of  the  copper 
coating  to  be  deposited.  A  tap  is  then  turned  in  the  lower  part 
of  the  apparatus,  the  cuprous  liquor  drawn  off,  and  the  coppered 
articles  washed  in  water.  They  are  then  taken  out,  cleaned 
with  a  scratchbrush,  and  dried,  first  in  sawdust,  then  in  a  stove. 
It  is  much  more  economical  to  use  zinc  wire  than  zinc  plates. 
To  keep  the  bath  of  one  strength  while  operating  on  a  large 
scale,  the  author,  by  the  advice  of  M.  Dumas,  arranges  his  ap- 
paratus in  a  particular  way,  so  that  the  liquid  is  renewed  from 
below,  and  flows  away  drop  by  drop,  or  in  a  fine  stream  from  the 
upper  part,  the  level  being  kept  constant. 
Renewal  of  an  Exhausted  Bath. — After  a  great  number  of 
copperings  the  exhausted  liquids  are  revived  as  follows : — 
Precipitate  the  zinc  in  solution  by  sulphide  of  sodium,  not  in 
excess,  and  re-charge  the  solution  with  sulphate  of  copper.  The 
constant  saturation  of  the  coppering  bath  may  be  effected  by  the 
following  means : — 
Introduce  into  the  reservoir  containing  the  cuprous  liquid  a 
cylinder  furnished  at  the  upper  part  with  an  india  rubber  valve, 
opening  upwards.  The  liquid  should  contain  hydrated  oxide  of 
copper,  and  the  plug  be  controlled  by  a  sort  of  ballcock  floating 
in  the  liquid  of  the  reservoir.  In  proportion  to  the  diminution 
of  the  density  of  the  liquid  from  the  effects  of  the  coppering, 
the  floating  ball  descends,  and  by  so  doing  opens  the  plug.  The 
liquid  enters  the  cylinder  and  dissolves  the  oxide  of  copper ;  it 
should  be  shaken,  and  when  the  bath  is  restored  to  its  normal 
density  the  valve  will  re-close. 
The  raw  materials,  of  a  certain  value,  which  enter  into  the  so- 
lutions of  copper — that  is  to  say,  caustic  soda,  sodio-potassic  tar- 
trate or  glycerin,  serve  thus  for  an  indefinite  time. 
The  advantages  attributed  by  the  author  to  his  process,  when 
all  the  precautions  indicated  are  attended  to,  are : 
