224  Mr.  N.  R.  Campbell  on  the 
(6)  Value  of  X. — X  cannot  be  determined  very  accurately 
from  the  curves,  since  it  will  always  appear  as  the  difference 
of  two  much  larger  quantities.  Its  magnitude  does  not  seem 
to  afford  an  opportunity  for  a  discussion  of  any  relevant 
interest. 
(7)  The  value  of  the  "window  experiments"  lies  in  con- 
firming the  conclusions  already  reached,  and  in  demonstrating 
in  a  more  direct  manner  the  emission  of  ionizing  rays,  similar 
to  a.  rays,  by  ordinary  metals.  The  proof  of  the  absence  of 
radium  from  lead  renders  impossible  one  of  the  suggested 
explanations  of  the  spontaneous  ionization  in  closed  vessels. 
(8)  It  has  been  suggested  to  me,  that  my  estimate  of  the 
ratio  in  which  the  external  radiation  wras  cut  down  by  the 
lead  screen  might  be  wholly  erroneous,  and  that  what  I  have 
called  "intrinsic  radiation"  might  only  be  the  residue  of 
secondary  radiation  due  to  this  error.  If  this  were  so,  the 
surface  ionization  ought  to  be  cut  down  in  the  same  ratio  as 
the  volume  ionization   when  the    screen    was    applied.     To* 
show  that  this  was  not  so,  the  values  of  — and   — — — 
«i  Pi 
are  tabulated  in  columns  8  and  9  of  Table  II.  In  all  but 
two  cases  the  former  is  the  larger,  as  would  be  expected  if 
there  were  present  intrinsic  surface  radiation. 
Conclusion. 
§  16.  If  the  object  of  this  paper  has  been  attained,  it  has 
been  proved  beyond  doubt  that  the  emission  of  ionizing 
radiation  is  an  inherent  property  of  all  the  metals  investigated ; 
and  I  see  no  reason  why  it  should  not  be  extended  to  all 
substances.  It  is  not,  of  course,  necessary  that  this  ray- 
emission  should  be  identified  at  once  with  radioactivity — it 
that  word  is  taken  to  mean  a  process  of  ray-emission 
accompanied  by  atomic  change.  But  the  constant  intensity 
of  the  rays,  and  the  probability  that  the  larger  portion  of 
them  are  a  rays,  which  is  suggested  by  the  investigation  of 
their  charge  and  their  penetration,  afford  considerable 
support  for  that  hypothesis;  while  I  know  of  no  other 
process  which  affords  any  analogy.  But  before  the  identity 
can  be  established  irrefutably,  further  work  is  required, 
which  I  hope  to  be  able  to  supply  in  the  near  future. 
Once  more  it  is  my  privilege  to  acknowledge  my  in- 
debtedness to  the  invaluable  advice  and  kindly  interest  ot 
Prof.  Thomson. 
Cavendish  Laboratory,  Cambridge, 
August  1905. 
