Alternating  Current  on  a  Polarlzable  Electrode.       341 
has  been  lost  by  diffusion.  More  mercury  ions  are  brought 
into  solution  during  the  one  phase  o£  the  current  than  were 
brought  out  of  it  in  the  other.  Hence  a  real  increase  of  the 
ionic  concentration  in  the  neighbourhood  of  the  small  electrode 
takes  place. 
It  is  to  be  remarked  that  this  effect  goes  in  the  opposite 
direction  to  the  first.  Its  existence  is  shown  by  the  presence 
of  an  after-effect.  Though  it  is  not  possible  to  separate  the 
two  effects,  it  is  interesting  to  note  which  predominates. 
I  found,  with  all  the  electrolytes  that  I  tried,  a  certain 
cathodic  direct-current  polarization,  at  which  the  two  effects 
exactly  neutralized  each  other.  For  lower  cathodic  polarization 
(greater  Hg-ion  concentration)  the  first  effect  predominates, 
and  a  current  flows  from  small  electrode  to  large  in  the 
electrolyte  ;  and  for  cathodic  polarization  higher  than  this 
"Nullpoint"  {i.  e.  with  smaller  Hg-ion  concentration)  the 
second  effect  predominates,  and  the  current  from  the  large 
electrode  to  the  small  electrode  in  the  electrolyte  is  increased 
by  the  alternating  current.  It  is  clear  that  both  these  effects 
increase  as  the  Hg-ion  concentration  is  diminished,  but  the 
second  increases  more  rapidly  than  the  first. 
I  found  the  Nullpoint  with  H2S04  saturated  with  Hg2S04 
at  about  0*14  volt  ;  with  KCN",  on  the  other  hand,  at 
0*36  volt;  with  K2S  at  about  0'62  volt  cathodic  direct- 
current  polarization. 
In  order  to  test  the  results  obtained  theoretically  for  the 
first  and  more  definite  effect,  it  was  necessary  to  use  an 
electrolyte,  for  which  this  second  effect  was  inappreciable. 
Such  a  case  I  found  to  be  KCX  solution  when  no  direct- 
current  polarization  was  applied,  as  was  shown  by  the  very 
small  after-effect  on  shunting  out  the  alternating  current. 
That  the  second  effect  is  unimportant  in  this  case  is  to  be 
attributed  to  the  fact  that  here  the  mercury  salt  in  solution 
is  a  complex  one,  and  the  excess  of  Hg  ions  brought  into 
solution  mostly  disappear  in  the  form  of  complex  molecules, 
and  therefore  do  not  to  any  great  extent  effect  an  increase  in 
the  Hg-ion  concentration.  It  may  also  in  part  be  accounted 
for  by  the  fact  that  the  solution  is  alkaline  owing  to  hydrolysis, 
and  hence  hydrogen  ions  are  present  only  to  a  very  small 
extent. 
Experiments  made  with  other  electrolytes  are  described 
fully  in  another  place*.  I  will  here  only  bring  forward  the 
results  with  this  electrolyte,  to  show  to  what  extent  the  results 
previously  obtained  hold. 
*  Zeitschr.f.phys.  Chemie,  liii.  p.  177  (LOOo). 
