H-  Iron      .     , 
146-18 
Cadmium  . 
9-79 
Zinc 
0-76 
Copper .     . 
0-00 
Silver    . 
1-89 
Gold     .     . 
23*92 
Force  in  the  Contact  of  Metals.  273 
Lead  .  ,  27*27 
Tin  .  .  38*84 
Aluminium  42*15 
Platinum  .  58*41 
Palladium  115*04 
i -Bismuth   .  835*10 
The  metals,  therefore,  keep  the  same  order  in  the  electromotive 
as  in  the  thermoelectric  series. 
§6. 
If  we  compare  the  electromotive  series,  as  determined  in  the 
preceding  pages,  with  the  electric-tension  series,  as  given  by 
Volta,  Pfaff,  Peclet,  and  others,  it  is  impossible  to  find  the  least 
concordance  between  them.  Thus,  for  example,  according  to 
Volta's  series,  zinc  is  positive  to  iron,  while  it  is  the  opposite 
according  to  the  series  above  given ;  according  to  the  tension- 
series,  bismuth  is  positive  to  platinum,  while  in  the  above  series 
bismuth  is  found  far  below  platinum  in  the  negative  direction ; 
according  to  the  tension-series,  lead  is  much  more  positive  than 
copper,  while  it  is  the  contrary  in  the  series  determined  by  me, 
&c.  Yet  the  cause  of  this  want  of  concordance  is  now  not  diffi- 
cult to  discover.  Grove's  gas  pile  and  galvanic  polarization  are 
proofs  that  gases  are  electromotive  in  contact  with  solid  bodies. 
I  have,  I  think,  demonstrated  in  a  former  paper*  that  galvanic 
polarization  produces  a  veritable  electromotive  force  by  the  con- 
tact of  the  gases  precipitated  on  the  polar  surfaces.  The  polari- 
zation-current produced  cannot  be  regarded  as  proceeding  from 
chemical  activity  in  the  polarization-vessel,  but  has  its  true  cause 
in  the  polar  plates  being  covered  by  the  precipitated  gases.  The 
difference,  therefore,  between  the  two  series  results  from  the  fact 
that  gases  are  electromotive  in  contact  with  solid  bodies.  The 
experiments  on  which  the  tension-series  is  founded  were  made 
in  free  air.  If,  then,  with  the  aid  of  the  electroscope  we  inves- 
tigate the  electrical  state  of  a  disk  composed,  for  example,  of 
copper  and  zinc,  we  have  to  do  not  merely  with  the  mutual  con- 
tact of  the  two  metals,  but  also  the  contact  of  both  with  the  sur- 
rounding air.  As  solid  bodies  have  the  property  of  more  or  less 
condensing  gases  and  retaining  them  at  their  surfaces,  the  same 
result  is  in  general  obtained  when  the  experiment  takes  place  in 
a  space  in  which  the  air  is  rarefied,  or  in  a  vacuum,  since  this 
does  not  entirely  remove  the  gas  from  the  surface  of  the  solid. 
In  the  experiment,  therefore,  we  have  to  take  into  consideration 
three  electromotive  contacts.    The  deviation  given  by  the  electro- 
*  Ofversigt  of  K.  Vet.-Akad.  Forh.  1867,  p.  95.  Pogg.  Ann.  vol. 
cxxxi.  p.  381).     Pbil.  Mag.  S.  4.  vol.  xxxv.  p.  103. 
Phil.  Mag.  S.  4.  Vol.  43.  No.  286.  April  1872.  T 
