Action  of  Metals  and  Liquids.  67 
viz.  increased  strength  of  solution  and  diminished  conduction-resist- 
ance ;  in  a  very  few  cases,  however,  the  opposite  result  took  place, 
as  with  solutions  of  chloride  and  nitrate  of  strontium. 
Inversions  of  the  direction  of  the  deflection  by  difference  of  strength 
of  the  liquid  occurred  with  solutions  of  chloride  of  copper,  sulphate 
of  zinc,  nitrate,  iodide,  and  sulphate  of  sodium,  bromide  and  iodide 
of  potassium. 
Irregularities  of  the  amount  of  deflection  were  very  apt  to  take 
place  with  liquids  which  gave  strong  deflections,  or  which  acted  much 
upon  the  copper  plates  (for  instance,  nitric  acid),  especially  if  bubbles 
of  air  remained  under  the  plates,  or  the  dishes  were  wetted  on  their 
side  above  the  liquid  by  the  solution. 
In  certain  acid  liquids,  viz.  nitric,  chloric,  hydrobromic,  hydrofluo- 
silicic,  and  sulphuric  acids,  the  hot  copper  was  strongly  negative  (not- 
withstanding the  chemical  action  upon  it  was  distinct,  and  in  some 
cases  even  strong)  ;  this  is  similar  to  the  electrical  behaviour  of  pla- 
tinum in  such  liquids,  and  may  be  attributed  either  to  the  more 
direct  influence  of  the  heat  alone  (such  as  occurs  with  platinum 
plates),  or  to  a  different  influence  of  the  chemical  action  produced 
by  the  heat.     Both  these  causes  probably  operate  in  such  cases. 
It  is  probable  that  in  all  cases  where  the  hot  copper  was  positive 
in  liquids  of  strongly  acid  reaction,  the  positive  condition  was  due  to 
chemical  action  alone. 
AVith  some  liquids,  especially  with  solutions  of  hydrocyanic,  bo- 
racic,  acetic,  tartaric,  and  citric  acids,  the  deflections  were  very  feeble, 
and  the  chemical  action  on  the  plates  not  perceptible ;  whilst  with 
others,  such  as  nitric  and  chloric  acids,  solutions  of  the  chlorides  of 
strontium,  sodium,  potassium,  and  ammonium,  and  of  carbonate,  acid 
carbonate,  and  cyanide  of  potassium,  the  deflections  were  consider- 
able, and  the  chemical  action  distinct,  and  in  some  cases  strong.  In 
none  of  the  liquids  (except  hydrobromie  and  chromic  acids)  did  the 
hot  plate  appear  to  be  less  stained  or  corroded  than  the  cold  one; 
probably  in  all  cases  it  was  the  most  corroded,  although  in  some  cases 
the  corrosion  was  not  perceptible. 
The  amount  of  deflection  was  not  always  proportionate  to  the 
amount  of  chemical  action  ;  for  instance,  with  solutions  of  chloride 
of  copper  and  iodate  of  potassium  there  was  considerable  corrosion, 
but  only  feeble  currents,  probably  because  the  plates  became  covered 
with  a  badly  conducting  film,  whilst  with  hydrochloric  acid,  chloride 
of  cobalt,  chloride  of  manganese,  and  nitrate  of  potassium  the  reverse 
occurred. 
I  consider  the  currents  in  all  these  experiments  of  difference  of  tem- 
perature to  be  due  either,  1st,  to  the  direct  influence  of  heat,  the  effect 
of  which  is  to  make  the  hot  copper  negative  in  acid  liquids  and  po- 
sitive in  alkaline  ones  (see  Phil.  Mag,  1857,  vol.  xiii.  p.  1);  2nd,  to 
chemical  action,  which  sometimes  overpowers  the  direct  influence  of 
heat  and  reverses  the  effect;  or,  3rd,  to  both  these  influences  com- 
bined. The  more  ultimate  cause,  however,  of  the  phenomena  in 
these  cases  must  be  sought  for  in  the  molecular  movements  produced 
by  heat  in  the  metals  and  liquids. 
F2 
