22        CHEMICAL  AND  PHARMACEUTICAL  MANIPULATIONS. 
and  force  the  mercury  up  the  stem  b  ;  and  when  it  is  found,  by  the  use  of  a 
common  thermometer,  that  the  heat  has  risen  to  the  required  degree,  the 
inner  and  smaller  tube  is  to  be  pushed  down  until  its  lower  extremity  reaches 
below  the  surface  of  the  mercury.  This  would,  of  course,  cause  the  flame 
to  be  extinguished ;  but,  as  the  preventive  of  this  occurrence,  a  small  hole 
Is  bored  through  the  inner  tube  above  the  extremity,  to  permit  the  transit 
of  a  small  quantity  of  gas  to  the  burner.  As  the  passage  of  the  gas  is  now 
interrupted,  the  sourc3  of  heat  is  withdrawn,  and  the  cooling  influence  of 
the  surrounding  air  then  causes  the  air  contained  in  a  to  contract,  and  the 
mercury  in  b  to  sink,  and  leave  the  end  of  the  internal  tube  uncovered.  A 
free  channel  for  the  gas  is  thus  opened,  so  that,  as  combustion  proceeds,  the 
temperature  would  again  rise  and  cut  off  the  supply ;  but,  in  a  short  time, 
these  two  opposing  forces  reach  an  equilibrium,  and  scarcely  any  variation 
in  the  size  of  the  flame  occurs.  To  insure  perfect  contact  of  the  end  of  the 
inner  tube  with  the  mercury,  the  former,  to  the  extent  of  a  half  inch,  is 
made  of  platinum,  and  amalgamated  by  dipping  it  into  a  liquid  amalgam 
of  sodium  and  mercury. 
sj<  >jt  >•<  -Jfi.  >;< 
Electro-Metallurgy. —The  deposition  of  metals  by  electric  action  is  one 
of  the  modern  triumphs  of  practical  chemistry.  The  art  dawned  in  1805 
with  the  discoveries  of  Brugnatelli ;  but  no  substantial  benefits  were  derived 
from  it  until  1838,  when  Jacobi,  of  St.  Petersburg,  and  Spencer,  of  England, 
applied  the  principle  to  the  utilitarian  purposes  of  life.  The  subsequent 
invention  of  Danielt,  by  his  well  known  battery,  gave  an  impulse  to  the  art 
which  resulted  in  many  gratifying  and  wonderful  improvements  ;  so  that 
now  it  has  become,  in  its  greatly  advanced  condition,  a  prime  element  of  the 
economy  of  many  branches  of  manufacture.  Plating,  gilding,  stereotyping, 
medal  copying,  engraving,  and  kindred  arts,  are  all  largely  indebted  to 
electro-metallurgy  for  many  of  the  facilities  which  at  present  promote  and 
distinguish  their  progress. 
Those  who  may  wish  to  experiment  in  this  interesting  branch  of  scientific 
art  will  find  ample  instruction  in  the  following  pages. 
Any  of  the  many  forms  of  batteries  previously  described  may  be  used  for 
electrotyping,  but  the  best  is  Smee's.  Care  should  be  taken  to  observe  the 
directions  heretofore  given  for  the  treatment  and  management  of  batteries  ; 
their  good  condition,  proper  arrangement  and  management,  being  necessary 
to  success. 
The  intensity  and  quantity  of  the  galvanic  current  should  be  proportional 
to  the  work  to  be  done. 
Preparation  of  Articles  to  be  Plated  or  Copied.- — In  gilding  and  silvering, 
it  is  merely  necessary  to  have  the  objects  perfectly  clean  and  bright.  This 
is  effected  by  first  boiling  the  articles  in  a  solution  of  caustic  soda  or  potassa, 
and  afterwards  immersing  them  in  dilute  nitric  acid,  and  rinsing  with 
water.  They  are  futher  cleaned  by  rubbing  with  a  hard  brush,  and  some- 
times a  little  fine  sand  or  tripoli. 
