132  Mr.  R.  K.  McClung  on  the 
£5 
radium  A  disappears  in  a  short  time  after  removal  from  the 
emanation,  and  radium  B  does  not  give  out  any  rays  at  all. 
We  have  therefore  practically  a  single  homogeneous  source 
of  radiation  in  radium  0. 
Another  advantage  in  using  this  source  of  radiation  is  that 
the  layer  of  radiating  material  is  infinitely  thin,  and  conse- 
quently there  is  no  absorption  by  the  radiating  matter  itself. 
In  addition,  the  source  of  radiation  can  be  made  as  small  as 
desired  simply  by  using  a  very  thin  wire  on  which  to  con- 
centrate the  excited  activity. 
This  source  of  radiation  has  proved  very  satisfactory,  and 
the  results  which  have  been  obtained  are  quite  in  accordance 
with  those  obtained  by  Bragg  and  Kleeman. 
Although  there  are  several  advantages  in  using  the  excited 
activity  as  a  source  of  radiation,  there  is  one  obvious  disad- 
vantage in  its  use,  namely,  that  the  activity  dies  away  with 
time.  However,  in  all  the  experiments  described  in  this 
paper  the  rate  at  which  the  excited  activity  decayed  was 
measured  at  the  same  time  as  the  otber  observations  were 
made,  and  the  necessary  corrections  were  easily  made  for  the 
decrease  in  the  activity  of  the  source.  This  decay  of  the 
activity  was  therefore  not  at  all  a  serious  difficulty. 
The  method  of  experiment  and  arrangement  of  apparatus 
employed  was  very  similar  to  that  used  by  Bragg  and 
Kleeman.  A  diagram  showing  the  arrangement  and  details 
of  the  apparatus  is  given  in  fig.  1. 
Description  of  Apparatus. 
A  B  C  D  was  a  metal  box  which  inclosed  the  plates  between 
which  the  ionization  was  to  be  measured  and  also  the  source 
of  the  ionization.  E  was  a  zinc  plate  counected  to  one  pair 
of  quadrants  of  a  Dolezalek  electrometer  in  the  usual  way 
and  was  surrounded  by  a  guard-ring,  F  F,  which  was  con- 
nected to  earth.  H  H  was  a  wire  gauze  which  was  about 
•5  cm.  distant  from  the  plate  E,  and  was  connected  to  one 
pole  of  a  battery  of  accumulators,  the  other  pole  being  to 
earth. 
K  was  a  brass  block  made  in  two  sections  and  contained 
the  source  of  radiation.  The  lower  half  wras  a  solid  piece  of 
brass  in  the  centre  of  the  upper  surface  of  which  was  cut  a 
narrow  groove  about  '5  mm.  in  depth.  The  upper  half  con- 
sisted of  a  similar  solid  brass  block  about  5  mm.  in  thickness, 
having  a  hole  2*1  mm.  in  diameter  passing  vertically  through 
the  centre.  The  faces  of  these  blocks  were  made  true  so  as 
to  fit  tightly  together.     The  wire  on  which  was  deposited  the 
