134  Mr.  R.  K.  McCIuno-  on  the 
& 
During  the  time  taken  to  make  these  measurements, 
the  activity  o£  the  excited  wire  gradually  decreased.  It 
takes  radium  C,  as  has  been  shown  by  Rutherford,  about 
55  minutes  to  fall  to  half  its  maximum  value  after  a  long 
exposure  in  the  emanation.  This  falling  off  in  intensity  of 
the  source  was  corrected  for  as  follows.  The  first  observation 
after  placing  the  excited  wire  in  position  was  made  at  a 
definite  distance  of  the  source  from  the  gauze,  and  the  time 
carefully  noted  at  which  the  observation  was  made.  Then 
three  or  four  observations  were  made  at  other  distances, 
noting  carefully  the  time  at  which  each  one  was  made,  and 
then  th^  source  was  replaced  at  the  original  position  and  the 
rate  of  leak  measured  again,  noting  the  time.  Then,  after 
three  or  four  more  observations  at  various  distances,  the 
source  was  again  brought  back  to  the  original  position ;  and 
this  routine  was  followed  throughout  the  whole  series  of 
observations.  The  time  at  which  each  observation  was  made 
was  carefully  noted. 
By  thus  observing  the  amount  of  ionization  at  intervals 
when  the  radioactive  matter  was  placed  at  the  fixed  position, 
a  decay  curve,  extending  over  the  whole  time  of  the  series  of 
observations,  could  be  plotted.  By  observing  the  time  at 
which  each  measurement  was  made  at  the  other  distances,  the 
necessary  correction  could  easily  be  made  by  referring  to 
the  corresponding  time  on  the  decay  curve.  The  amount 
of  ionization  produced  for  each  position  of  the  source  was 
thus  obtained  for  a  constant  source. 
Absolution  of  a  Rays  by  Air. 
The  absorption  of  the  a  rays  by  air  at  atmospheric  pressure 
was  first  investigated,  and  curves  were  obtained  showing  the 
amount  of  ionization  produced  for  different  distances  of  the 
source  from  the  electrode  H  H.  These  curves  were  found  to 
be  quite  in  agreement  with  the  results  obtained  by  Bragg  and 
Kleeman.  One  of  these  curves  is  shown  in  fig.  2,  where  the 
ordinates  represent  the  distance  in  millimetres  of  the  upper 
surface  of  the  radiating  wire  from  the  lower  surface  of  the 
wire  gauze  H  H.  The  abscissae  represent  the  ionization  as 
measured  by  the  rate  of  leak  in  arbitrary  scale-divisions 
per  second. 
This  curve  shows  the  same  characteristics  as  were  pointed 
out  by  the  previous  experimenters.  It  shows  that,  as  the 
source  recedes  from  the  electrode,  there  is  a  very  slow  increase 
in  the  ionization  until  a  distance  of  about  4  cms.  is  reached. 
Beyond  this  distance  the  ionization  increases  quite  rapidly 
until  a  maximum  is  reached  at  a  distance  of  about  5" 8  cms. 
