82  M.  E.  Edlund's  Researches  on  the  Electromotive 
objections  to  the  validity  of  Volta's  fundamental  experiments, 
the  defenders  of  the  electro-chemical  theory  contended  that  it 
was  impossible  to  be  certain  that  the  decomposition  of  the  elec- 
tricity at  the  time  of  the  contact  of  the  two  metals  really  proceeds 
from  that  contact — that,  on  the  contrary,  it  might  have  its  true 
cause  in  the  circumstance  that  the  surface  of  the  metal  is  covered 
with  a  layer  of  moisture  or  of  condensed  gas,  to  which  it  is  im- 
possible to  deny  a  chemical  affinity  with  the  metal  which  they 
cover,  and,  consequently,  that  contact  between  the  metal  and  the 
layer  of  gas  or  moisture  is  the  real  cause  of  the  development  of 
electricity.  In  order  to  refute  this  objection,  Volta's  funda- 
mental experiments  were  repeated  in  various  gases  and  in  vacuo; 
and  it  was  ascertained  that  electricity  is  quite  as  well  developed 
as  when  the  experiments  are  made  under  the  usual  conditions. 
This  attempt  at  refutation  called  forth  the  reply  that,  as  experi- 
ment has  demonstrated  in  other  cases,  condensed  gases  do  not 
entirely  disappear  in  vacuo — that  consequently  the  electromotive 
force  admitted  to  exist  between  the  metals  and  the  gases  or 
moisture  at  their  surface  may  exist  all  the  same  in  the  vacuum 
which  has  been  made.  It  cannot,  then,  be  maintained  that 
Volta's  fundamental  experiments,  as  performed  with  the  aid  of 
the  electroscope,  have  furnished  an  entirely  irrefutable  proof  of 
the  presence  of  an  electromotive  force  in  the  contact  between 
metals.  It  must,  then,  be  confessed  that  this  question  has  not 
yet  been  solved  in  the  way  followed  for  the  purpose  with  the 
aid  of  the  electroscope*. 
The  phenomena,  however,  discovered  by  Peltier,  of  galvanic 
cooling  and  heating,  have  furnished  a  perfectly  distinct  reply  to 
the  question  as  to  the  existence  of  an  electromotive  force  in  the 
contact  between  two  different  metals.  Peltier  found  (as  is  known) 
that,  if  a  galvanic  current  passes  through  the  point  of  contact 
established  between  two  different  metals,  that  point  becomes 
heated  or  cooled,  according  to  the  direction  of  the  current. 
If  the  current  passes  in  a  certain  direction,  there  results  a  true 
production  of  heat ;  if  in  the  opposite  direction,  the  result  is  an 
absorption  of  heat.  It  has  been  found  by  experiment  that  the 
variations  of  temperature  produced  at  the  point  of  contact  are 
proportional  to  the  intensity  of  the  current.  With  the  aid  of  the 
mechanical  theory  of  heat,  and  of  some  known  theses  derived 
from  the  theory  of  electricity,  I  have  proved,  in  a  previous  workf, 
that  Peltier's  phenomena  are  explained  with  the  greatest  facility 
*  Wiedemann,  Die  Lehre  vom  Galvanismus  und  Erdmagnetismus,  T.  2, 
§  849. 
t  Ofversigt  af  Vet.  Akademiens  Fbrhandl.  for  1869,  p.  457-  Pogg. 
Ann.  vol.  cxxxvii.  p.  474.  Arch,  des  Sciences  Phys.  et  Nat.  vol.  xxxvi. 
p.  214.     Phil.  Mag.  S.  4.  vol.  xxxviii.  p.  263. 
