Ionization  produced  by  a  Rays.  811 
The  measurements  of  pressure  and  ionization  were  taken 
exactly  as  in  the  previous  experiment.  Four  separate 
experiments  were  performed  with  testing-vessels  of  different 
dimensions.  The  four  diameters  used  for  the  tube  D  were 
9'6  cms.,  6'2  cms.,  2*0  cms.,  and  0*6  cm. ;  and  in  each  case  the 
distance  between  E  and  F  was  equal  to  one-half  the  diameter 
of  the  tube. 
Now  let  us  consider  the  case  where  the  diameter  of  the 
tube  D  is  2  cms.  In  this  case  the  average  path  of  the  « 
particles  from  radium  C  will  be  about  1  cm.  If  the  ionization 
is  entirely  due  to  them,  we  should  expect  it  to  be  proportional 
to  the  pressure  of  the  air.  Now  let  us  suppose  that  radium  B 
also  emits  the  same  number  of  a  particles  as  radium  0,  and 
that  each  one  produces  the  same  number  of  ions  per  cm., 
but  that  their  range  of  ionization  is  only  0*2  cm.  If  now 
the  pressure  is  diminished,  the  range  of  the  a  particles  from 
radium  B  will  increase,  and  the  total  ionization  produced  by 
them  will  remain  the  same,  until  the  pressure  has  fallen  to 
one-fifth  of  an  atmosphere,  when  the  path  of  these  a  particles 
will  extend  to  the  sides  of  the  testing-vessel. 
The  following  table  gives  the  theoretical  relation  between 
pressure  and  ionization  calculated  on  the  above  assumptions. 
Pressure  in 
Ionization 
Ionization 
Atmospheres. 
due  to  Eadium  0. 
due  to  Radium  B. 
Total 
1-0 
10 
2 
12 
0-9 
9 
2 
11 
0-8 
8 
2 
10 
0-7 
7 
2 
9 
0-6 
6 
2 
8 
0-5 
5 
v> 
7 
0-4 
4 
2 
6 
0-3 
3 
2 
5 
0-2 
2 
2 
4 
Thus  after  the  pressure  has  been  reduced  to  0*2  of  an 
atmosphere,  the  ionization  should  be  nearly  twice  as  large  as 
though  radium  B  emitted  no  a  particles. 
Curve  1  (fig.  4,  p.  812)  was  obtained  using  the  smallest  of  the 
four  testing- vessels.  Curve  2  was  calculated  on  the  assump- 
tion that  the  ionizing  path  of  the  «  particles  from  radium  B 
is  0*03  cm.  There  is  no  evidence  from  the  experimental 
curve  that  the  ionization  is  not  exactly  proportional  to  the 
pressure.  Exactly  similar  results  were  obtained  with  the 
other  testing- vessels,  and  the  largest  one  would  have  detected 
a  rays  from  radium  B  having  a  range  of  ionization  as  long  as 
2  cms.     It  therefore  seems  safe  to  conclude  that  radium  B 
