26        CHEMICAL  AND  PHARMACEUTICAL  MANIPULATIONS. 
Solutions  should  be  kept  covered  from  the  air  and  dust;  and  the  work- 
ing of  the  batteries  is  promoted  by  having  the  surrounding  atmosphere  of  a 
warm  temperature. 
A  few  drops  of  bi-sulphuret  of  carbon  added  to  a  silver  solution  will  pro- 
duce a  bright  deposit. 
In  inserting  the  articles  in  the  solutions  the  air  adhering  to  their  surfaces, 
and  which  prevents  a  contact  of  the  metals,  may  be  dispelled  by  moving  the 
articles  about  in  the  liquid  or  by  heating  the  solution. 
Fig.  475. 
The  plates  attached  to  the  positive  poles  should  be  parallel  to  the  articles 
on  which  the  metal  is  to  be  deposited  and  present  the  same  amount  of 
surface. 
A  battery,  if  in  proper  working  order,  will,  when  the  connections  are 
made,  show  a  disengagement  of  gas  at  its  negative  metal ;  but  no  gas  should 
be  seen  to  escape  at  either  pole. 
Bronzing. — To  give  the  copies  of  medals  and  other  objects  an  antique  or 
bronzed  appearance  like  the  original,  several  means  are  employed.  A  dark 
bronze  is  produced  by  dipping  the  object  in  very  dilute  nitric  acid, — say 
half  an  ounce  of  acid  to  a  pint  of  water, — and,  after  drying,  heating  it 
gradually  and  uniformly.  The  color  is  deepened  in  proportion  to  the  heat 
applied.  Sulphuretted  hydrogen  or  hydrosulphuret  of  ammonia  may  also 
be  used.  Afterwards  polish  with  a  brush.  Green  bronzes  are  formed  by 
immersing  the  articles  in  a  solution  of  chloride  of  ammonium  or  chloride  of 
sodium,  or  by  exposing  them  to  the  fumes  of  chloride  of  lime.  The  depth 
of  the  bronze  is  regulated  by  the  length  of  time  during  which  the  articles 
are  subjected  to  the  galvanic  action.  A  coating  of  black  lead  and  subsequent 
heating  of  the  article,  gives  a  beautiful  bronze.    A  thin  film  of  oil  or  wax, 
