Action  of  Metals  and  Liquids. 
63 
The  negative  condition  excited  in  the  hot  platinum  cup  in  the  so- 
lutions of  citric  and  tartaric  acid  agrees  with  the  results  obtained 
with  copper  in  those  liquids. 
Fig.  3. 
I  have  already  shown  (Phil.  Mag.  1857,  vol.  xiii.  p.  1)  that  the 
currents  obtained  with  platinum  electrodes  are  not  due  to  the  in- 
fluence of  atmospheric  air  upon  the  liquid  and  metal  at  their  line  of 
mutual  contact ;  for,  in  the  experiments  there  recorded,  atmospheric 
air  was  entirely  excluded,  and  the  liquids  were  previously  well  boiled. 
To  test  the  influence  of  size  of  the  cold  electrode,  I  took  a  platinum 
dish,  A  (see  fig.  4),  5  inches  wide  and  1|  inch  deep,  in  a  glass 
vessel  of  the  annexed  form,  B,  closed  at  its  lower  end  by  a  cork, 
and  containing  in  its  neck  two  platinum  electrodes,  one  consisting  of 
a  wire,  C,  and  the  other  of  a  sheet  2  inches  long  and  2  inches  wide 
in  the  form  of  a  cylinder,  D. 
With  a  cold  mixture  composed  of  3j  ounces  of  water  and  J  of  an 
ounce  by  measure  of  strong  sulphuric  acid,  and  the  sheet  of  pla- 
tinum as  the  lower  electrode,  on  pouring  boiling  water  into  the  dish 
a  deflection  of  the  value  of  '0064  was  obtained,  the  cold  electrode 
being  positive ;  but  with  the  wire  as  the  lower  electrode  no  percep- 
tible deflection  occurred.  These  results  were  obtained  repeatedly. 
The  electric  currents  are  therefore  largely  dependent  upon  the  size 
of  the  cold  electrode. 
General  Results. 
The  chief  fact  brought  out  conspicuously  by  these  experiments 
with  copper  dishes  is,  that  in  many  cases  an  increase  of  chemical 
action  produced  by  heat,  instead  of  making  the  hot  metal  electro- 
positive, makes  it  considerably  negative. 
The  results  show  that  hot  copper  was  positive  to  cold  copper  in 
the  following  liquids  : — hydrochloric,  hydrocyanic,  boracic,  and  tri- 
basic  or  orthophosphoric  acids ;  chloride  of  copper  (weak  solution) ; 
chloride  of  cobalt ;  chloride  of  manganese  ;  chromic  acid  ;  chloride 
of  chromium;  sulphate  of  zinc  (weak  solution);  sulphate  of  mag- 
nesia ;  chloride  of  calcium  ;  nitrate  and  chloride  of  strontium  ;  chlo- 
ride of  barium  ;  nitrate  of  sodium  (strong  solution)  ;  chloride,  iodide, 
carbonate,  and  biborate  of  sodium  ;  sulphate  of  sodium  (strong  so- 
lution) ;  tribasic  phosphate  of  sodium  ;  nitrate,  chloride,  and  chlorate 
Phil.  Mag.  S.  4.  Vol.  43.  No.  283.  Jan.  1872.  F 
