[  '  206     ] 
XVIII.  The  Radiation  from  Ordinary  Materials.    By  Norman 
E.  Campbell,  B. A.,' Fellow  of  Trinity  College,  Cambridge*. 
[Plate  Y.] 
Experimental. 
§  1.  rjlHE  following  experiments  are  an  extension  o£  those 
X  described  in  a  paper  bearing  the  same  title  in  the 
Phil.  Mag.  for  April  1905.  'They  have  the  same  object — to 
determine  the  nature  of  the  ionizing  radiation  from  ordinary 
materials,  and  they  follow  the  same  method.  But  experi- 
mental improvements  enabled  greater  accuracy  to  be  attained 
and  the  results  interpreted  with  greater  certainty. 
The  observations  consisted,  as  before,  in  determining  the 
relation  between  the  ionization  and  the  volume  of  a  rectan- 
gular vessel  with  two  parallel  sliding  sides.  It  was  pointed 
out  on  page  538  of  the  former  paper  that  difficulties  arose  in 
Fig.  1. 
■ELECTRODE 
W/RE  GAUZE  CAGE 
the"  subsequent  calculation  of  the  penetration  from  the  in- 
fluence of  the  corners  of  the  box.  This  disturbance  has  been 
obviated  by  a  guard-ring  device. 
§  2.  A   sketch  of  the  vessel  employed  is  given  in  fig.  1. 
*  Communicated  by  the  Author. 
