Action  of  Metals  and  Liquids.  69 
wetting  the  sides  of  the  dishes  by  the  liquid  above  the  level  of  their 
immersion. 
To  ascertain  the  influence  of  difference  of  temperature  of  the  air- 
contact  line  I  soldered  two  strips  of  perfectly  similar  sheet  copper, 
each  1 2  inches  long  and  \  inch  wide,  in  the  form  of  circular  hoops 
4  inches  in  diameter  upon  the  bottoms  of  two  tin  cups,  and  ground 
the  edges  of  the  strips  perfectly  lerel,  and  soldered  copper  wires  to 
them  for  connecting  with  the  galvanometer.  Two  glass  triangles 
were  now  put  into  the  apparatus,  fig.  1,  one  in  each  dish,  to  support 
the  cups,  and  a  mixture  of  one  measure  of  nitric  acid  and  12  mea- 
sures of  distilled  water  poured  in  until  it  just  touched  the  edges  all 
round  of  the  perfectly  horizontal  copper  rims  resting  on  the  tri- 
angles. After  the  needles  of  the  galvanometer  had  settled  at  zero, 
about  ten  ounces  of  boiling  water  was  poured  into  one  of  the  cups  ; 
a  temporary  deflection  of  the  value  '0560,  and  a  permanent  one  of 
value  *0759,  were  produced,  the  hot  metal  being  negative.  The  di- 
rection of  the  current  in  this  experiment  agrees  with  that  obtained 
with  the  same  mixture  and  the  copper  dishes  ;  and  the  result  indicates 
that  a  large  proportion  of  the  quantity  of  the  current  obtained  with 
copper  dishes  in  dilute  nitric  acid  was  due  to  the  action  of  the  air- 
contact  line. 
The  influence  of  the  air-line  is  largely  chemical.  "  A  piece  of 
copper  wire  wholly  submerged  in  the  acid  [dilute  sulphuric]  so  as  to 
entirely  exclude  any  portion  of  it  coming  into  contact  with  the  air, 
has  remained  for  many  months  without  imparting  the  slightest  tinge 
to  the^  liquid."  "  But  on  suffering  the  liquid  to  evaporate  so  as  to 
bring  the  upper  end  of  the  metal  near  to  its  surface,  the  instant 
the  slightest  portion  becomes  exposed  chemical  action  immediately 
begins." 
"  Two  equal  portions  of  wire  were  similarly  placed  in  acid,  only  that 
one  was  fully  exposed  to  the  atmosphere  in  an  open  tube,  while  the 
other  was  placed  in  a  phial,  the  acid  occupying  half  its  height,  and 
was  kept  closely  corked  for  several  weeks — after  which  the  fully  ex- 
posed metal  had  lost  in  weight  two-fifths  more  than  the  one  which 
had  been  excluded  from  contact  with  fresh  portions  of  air,  showing 
that  contact  with  the  atmosphere  in  bulk  is  necessary  to  the  fullest 
action"*. 
Experiments  with  Liquids  of  unequal  strength. 
To  throw  some  light  upon  the  questions — 1st,  Is  the  quantity 
of  the  current  simply  a  result  of  the  difference  of  number  of  mole- 
cules of  liquid  which  touch  the  hot  plate  compared  with  those 
which  touch  the  cold  plate  ?  and,  2nd,  What  amount  of  difference 
of  strength  of  a  liquid  is  equal  to  the  amount  of  difference  of  tempe- 
rature employed? — I  brought  the  two  copper  dishes  into  contact  with 
liquids  of  unequal  strength  instead  of  unequal  temperature. 
The  tube  C  (fig.  1 )  was  filled  with  the  stronger  mixture  and  closed 
at  its  end  in  the  dish  A  by  an  india-rubber  bung,  and  the  dish  B  filled 
*  "On  the  Theory  of  the  Voltaic  Pile,"  Bridgman,  Phil.  Mag.  Nov.  186'.). 
