CHEMICAL  AND  PHARMACEUTICAL  MANIPULATIONS.  25 
gold  salt  in  a  little  water,  and  add  it  to  a  solution  of  four  ounces  of  cyanide 
of  potassium  in  two  quarts  of  water  and  filter. 
Silver  Solution. — Take  of  cyanide  of  silver  1  ounce,  cyanide  of  potassium 
10  ounces,  water  6  pints  ;  dissolve  the  cyanide  of  potassium  in  the  water, 
add  the  cyanide  of  silver,  and  filter  the  solution. 
Probably  a  better  way  to  make  the  solutions  of  gold  and  silver  in  cyanide 
of  potassium  is  with  the  battery.  Immerse,  in  a  solution  of  1  part  cyanide 
of  potassium  to  16  parts  of  water,  a  silver  plate,  connected  with  the  positive 
pole  of  a  battery,  complete  the  connection  with  the  negative  pole,  and  keep 
up  the  action  of  the  battery  until  silver  is  freely  deposited  on  the  negative 
pole.  The  same  process  is  followed  for  gold,  care  being  taken  to  substitute 
a  gold  for  a  silver  plate. 
Sulphate  of  Copper  is  the  best  salt  for  the  reduction  of  copper.  A  nearly 
saturated  solution,  acidulated  with  a  few  drops  of  sulphuric  acid,  is  used. 
One  pound  of  the  sulphate  in  six  pounds  of  water  is  a  good  strength. 
Cyanide  of  Copper  is  sometimes  used  for  depositing  copper  or  iron.  It 
is  made  by  dissolving  the  oxide  in  an  excess  of  cyanide  potassium,  or  by 
making  a  sheet  of  copper  the  positive  pole  in  a  solution  of  cyanide  of  potas- 
sium. 
Platinum,  zinc,  and  most  of  the  metals  can  be  reduced  from  their  salts 
by  the  battery  ;  but  for  electrotyping  they  are  seldom  or  never  used. 
To  have  the  metals  adhere  well  in  gilding  and  silvering,  the  articles  to 
be  plated  must  be  well  cleansed.  As  silver  is  generally  precipitated  on 
copper,  the  article  is  boiled  in  caustic  potash  or  soda  well  rinsed  with 
water,  dipped  in  dilute  nitric  acid,  afterwards  immersed  in  a  weak  solution 
of  nitrate  of  mercury,  and  immediately  placed  in  the  silvering  solution. 
Gold  is  usually  deposited  on  silver.  The  silver  object  is  treated  as  before 
with  caustic  lye,  rinsed,  and,  when  dry,  is  thoroughly  scratched  with  a 
scratch-brush,  which  is  a  bunch  of  fine  wire  made  into  a  brush.  It  is  then 
ready  for  the  battery.  In  gilding,  the  solution  should  be  maintained  at 
at  about  150°  F.  by  a  water-bath. 
To  avoid  opposite  currents  of  electricity  in  the  depositing  solution  from 
an  exhaustion  of  the  solution  around  the  negative  pole,  and  a  dense  solution 
forming  around  the  positive  pole,  the  articles  should  be  kept  in  motion 
during  the  deposition ;  for  this  motion  also  prevents  that  crystalline  deposit 
deemed  so  objectionable. 
To  prevent  the  adhesion  of  the  matrix  to  the  deposited  metal,  Mr.  Mathiot, 
of  the  United  States  Coast  Survey,  recommends  that  the  engraved  copper 
plates,  &c,  be  coated  in  a  battery  with  a  thin  film  of  silver,  and  afterwards 
washed  with  a  dilute  solution  of  iodine  in  alcohol,-— about  one  grain  of  the 
former  in  a  quart  of  the  latter. 
Dusting  with  black  lead,  or  spreading  a  little  oil  over  the  surface  of  the 
article,  care  being  taken  not  to  use  an  excess,  will  cause  the  metals  to 
separate  easily.  A  little  wax  dissolved  in  spirits  of  turpentine  also  answers 
well. 
