Alternating  Current  on  a  Polarizable  Electrode.      349 
Ihe  Effect  with  a  Gas-charged  Platinum  Electrode 
in  Sulphuric  Acid. 
I  was  not  able  to  realize  with  complex  salts  of  mercury 
the  case  where  the  effect,  arising  from  the  finiteness  of  the 
velocity  of  reaction,  is  independent  of  the  frequency.  It 
appears  with  KCN-solutions,  from  the  approach  to  constancy 
NI 
of  -^2-,  that  the  velocity  of  reaction  is  sufficiently  great  for 
lo 
the  effect  due  to  diffusion  to  come  first  into  play,  even  where 
the  solution  is  comparatively  strongly  concentrated. 
In  a  case  where  an  asymmetrical  effect  is  large  at  a  very 
high  frequency — at  which  diffusion  must  cease  to  play  an 
important  part — one  is  led,  inversely,  to  consider  that  the 
effect,  if  it  arises  at  all  in  the  way  above  described,  can  only 
do  so  through  the  finiteness  of  some  velocity  of  reaction. 
Such  a  case  is  that  of  the  ordinary  electrolytic  coherer — a 
small  gas-charged  Pt  electrode  in  dilute  sulphuric  acid. 
In  the  first  place,  the  effect  of  the  above  described  asym- 
metry would  be  a  virtual  diminution  of  the  gas  charge,  i.  e.  a 
depolarization  such  as  is  observed. 
As  for  evidence  of  the  reaction  which  takes  place,  the 
following  work  on  the  solution  of  hydrogen  in  palladium  and 
platinum  bears  on  the  subject. 
Hoitsema  *  found  from  the  pressure-concentration  curve  of 
hydrogen  in  palladium,  that  the  hydrogen  is  dissolved  at 
ordinary  temperatures  in  the  atomic  form  and  not  in  the 
molecular. 
A.  Winkelmann  f  found  that  hydrogen  diffuses  through 
glowing  palladium  as  H,  and  not  as  H2. 
The  same  author  J  found  an  exactly  analogous  result  for 
hydrogen  diffusing  through  hot  platinum. 
More  recently,  0.  W.  Richardson,  Nicol,  and  Parnell  § 
have  shown  that  the  dissociation  of  the  hydrogen  takes  place 
in  the  platinum,  and  not  outside  it. 
With  regard  to  polarization  experiments,  it  is  interesting 
to  note  that  M.  Wien  ||  found  for  palladium  electrodes, 
covered  with  Pd-black  and  strongly  charged  with  hydrogen, 
the    result  : — Polarization    capacity    <x  .p, —  -  ,    which 
r  requency 
*  Hoitsema,  Zeitsch.f.  phys.  Chemie,  xvii.  p.  1  (1895). 
t  A.  Winkelmann,  Drude  Annalcn,  vi.  p.  104  (1901). 
X  A.  Winkelmann,  Drude  Annah-n,  viii.  p.  888  (1902). 
§  O.  W.  Richardson,  Nicol,  and  Parnell,  Phil.  Mag.  vii.  (1904). 
||  M.  Wien,  Drude  Annalen,  viii.  p.  o72  (I90i>). 
