Conductivity  of  Metallic  Oxides.  507 
a*  non-conductors,  become  conducting  when  their  tempera- 
ture is  raised.  The  conductivity  of  such  substances  was 
studied  by  Faraday  *,  who  found  that  silver  sulphide,  lead 
fluoride,  corrosive  sublimate,  and  periodide  of  mercury,  all 
increase  very  rapidly  in  conductivity  with  rise  of  tempera- 
ture. Since  the  time  of  Faraday  this  subject  has  been 
investigated  by  many  experimenters.  It  has  recently 
received  special  attention  from  Nernstf,  who  has  given  it 
practical  application  in  the  Nernst  filaments  for  incandescent 
lamps.  The  conduction  of  electricity  through  these 
filaments  is  considered  by  Nernst  to  be  electrolytic,  but  the 
recent  experiments  of  Streintz  J  and  of  Guinchant  §  have 
shown  that  a  large  number  of  substances  which  become 
conducting  at  high  temperatures  give  no  trace  of  electrolysis. 
The  conduction  in  these  cases  must  therefore  be  metallic, 
and  on  Professor  Thomson's  theory  due  to  the  negatively 
charged  corpuscles  liberated  from  the  atoms.  If  this  sup- 
position be  true,  it  follows  that  either  the  number  of  free  cor- 
puscles within  these  substances,  or  the  mean  free  path  of  these 
corpuscles,  increases  very  rapidly  with  rise  of  temperature. 
Now  Wehnelt  ||  has  studied  the  emission  of  corpuscles  from 
the  alkaline  earths  when  they  are  raised  to  high  temperatures, 
and  has  found  that  the  rate  of  emission  increases  very 
rapidly  with  the  temperature,  and  at  1500°  C.  is  quite 
enormous.  At  these  high  temperatures,  therefore,  there 
must  be  a  large  number  of  free  corpuscles  inside  the  substance, 
which  probably  owes  its  conductivity  to  their  presence. 
Such  an  explanation  would  account  for  the  great  increase 
in  conductivity  with  rise  of  temperature  observed  by  Faraday, 
Streintz,  and  others,  and  explain  the  loss  of  insulating  power 
of  quartz  and  other  non-conductors  when  raised  to  high 
temperatures. 
The  object  of  the  present  research  was  to  investigate  the 
variation  of  electrical  conductivity  of  solid  metallic  com- 
pounds with  change  of  temperature,  and  to  ascertain  whether 
the  conduction  is  carried  on  electrolytically  or  metallically. 
For  this  purpose  the  metallic  oxides  seemed  to  be  the  most 
*  Faraday,  Exp.  Res.  vol.  i.  §  432  (1833),  §  1340  (1S38). 
+  Nernst,  Zeitsckr.  Elektrochem.  vi.  p.  41  (1899);  Gessel.  JViss. 
Gottingen,  Nachr.  Math.-Phys.  p.  328  (1900). 
%  Streintz,  Ann.  d,  Physik,  iii.  p.  1  (1900) :  Akad.  Wiss.  Wien.  Sitz- 
Ber.  iii.  2  a,  p.  345  (1902;. 
§  J.  Guinchant,  Compter  Rendus,  cxxxiv.  p.  1224  (1902). 
||  Wehnelt,  Ann.  d.  Physik,  xiv.  p.  425  (1904) ;  Phil.  Mag.  [0]  vol.  x. 
p.  80  (1905). 
2  L  2 
