Conductivity  of  Metallic  Oxides.  525 
o£  the  conductivity  are  plotted  against  the  corresponding 
temperatures. 
When  the  experiments  already  described  had  been  per- 
formed, it  was  thought  to  be  desirable  to  investigate  further 
the  electrolysis  which  was  found  to  accompany  the  conduction 
in  the  case  of  some  of  the  oxides,  and  to  apply  a  more  delicate 
test  for  it  in  those  cases  in  which  no  trace  of  a  polarization 
current  could  be  detected.  The  oxides  chosen  for  these 
experiments  were  lime,  baryta,  and  cupric  oxide  ;  lime  and 
baryta  because  they  wrere  the  oxides  with  which  electrolysis 
had  been  most  easily  detected,  and  the  cupric  oxide  because 
it  behaved  as  if  its  conductivity  were  entirely  metallic. 
Wehnelt  has  shown  *  that  if  glowing  lime  (or  certain  other 
oxides)  be  used  as  a  cathode  in  a  discharge-tube,  a  strong 
current  can  be  made  to  pass.  If  the  cathode  consists  of  a 
strip  of  platinum  covered  with  lime,  the  whole  of  the  current 
must  pass  through  the  oxide.  If  electrolysis  takes  place, 
oxygen  will  be  liberated  at  the  free  surface  of  the  oxide  and 
calcium  next  to  the  platinum  strip.  The  gas  pressure  in  the 
discharge-tube  will  thus  increase  with  the  passage  of  the 
current,  and  the  amount  of  gas  liberated  in  a  given  time  will 
be  proportional  to  the  current  passing.  Wehnelt  found  in 
his  experiments  that  there  was  u  no  notable  increase  of 
pressure,"  which  seems  to  indicate  that  the  passage  of  the 
current  through  the  oxide  was  unaccompanied  by  electrolysis. 
In  the  following  experiments  much  larger  cathodes  and 
stronger  currents  were  employed,  so  that  if  the  conduction 
of  electricity  is  electrolytic,  the  amount  of  oxygen  evolved 
would  be  quite  easily  detected. 
A  strip  of  platinum  foil  2  cms.  long  and  '3  cm.  wide, 
having  a  thermocouple  of  fine  platinum  and  platinum-rhodium 
wires  fixed  to  its  middle  point,  was  covered  with  lime  and 
used  as  the  cathode  in  a  discharge-tube,  the  anode  being  a 
disk  of  platinum  2  cms.  in  diameter  placed  parallel  to  the  strip 
and  about  2  cms.  away  from  it.  The  discbarge-tube  was 
connected  to  a  mercury-pump  and  McLeod  gauge,  and  the 
whole  apparatus  was  pumped  down  to  a  low  pressure.  The 
platinum  strip  and  lime  were  heated,  by  means  of  the  alternat- 
ing current  from  a  small  transformer,  to  a  white  heat  for 
several  hours.  The  heating  was  accompanied  by  a  considerable 
evolution  of  gas  from  the  oxide.  This  was  pumped  out  of 
the  apparatus  at  intervals,  and  the  heating  was  continued 
until  the  pressure  remained  fairly  constant.     The  transformer 
*  Ann.  der  Phmik,  xiv.  pp.  425-468  (1904) ;  Phil.  Mag-,  yi.  pp.  80  90 
(1905). 
