516 
Dr.  F.  Horton  on  the  Electrical 
temperatures.  The  curves  obtained  are  similar  to  those 
given  by  lime. 
It  will  be  observed,  both  from  the  table  and  diagram,  that 
after  1200°  0.  the  conductivity  suddenly  altered  its  behaviour, 
and  began  to  decrease  with  increasing  temperature.  At  these 
temperatures  the  resistance  was  not  constant,  but  increased 
as  time  went  on  as  well  as  with  increasing  temperature.  If 
is,  however,  well  known  that  the  crystalline  structure  of 
magnesia  alters  when  heated  to  high  temperatures,  the 
alteration  being  accompanied  by  a  change  in  density.  Several 
specimens  of  magnesia  were  experimented  on  and  they  all 
behaved  in  the  same  manner,  giving  a  minimum  resistance  at 
from  1200°  0.  to  1250°  C.  It  therefore  appears  probable  that 
this  sudden  change  in  the  conductivity  is  due  to  an  alteration 
of  the  condition  of  the  oxide  into  a  less  conducting  state,  the 
resistance  continually  increasing  Avhile  the  change  is  going  on. 
In  order  to  see  whether  the  increase  of  conductivity  with 
temperature  follows  the  same  law  as  the  emission  of  corpuscles 
Diagram  IV. 
-7 
\© 
0 
•0006 
•0008  -0010 
Scale  ofG~' 
•0012 
•00/4 
from  the  surface,  the  values  of  logi0c— ^logiO0  were  plotted 
against  those  of  6~l,  as  in  the  case  of  lime.     This  is  done  in 
