Ionization  produced  by  a,  Rays. 
809 
particle  towards  the  end  o£  its  path.  In  the  first  place,  the 
cone  of  rays  had  a  sensible  angle,  but  calculations  showed 
that  this  could  not  account  for  more  than  one-tenth  of  the 
observed   effect.      In   the   second  place,  inequalities  in  the 
Fio-.  2. 
-800 
700 
600 
so: 
|  300 
200 
ICG 
^ 
\ 
\ 
\ 
\ 
X 
X 
2-6         3-0         3-4        3-8         4-2         4-6  5-0         54         5-8         6-2 
D/ STANCE  FROM  SOURCE  TO  TESTING  VESSEL  /N-CMS. 
6-6 
7-0 
thickness  of  the  mica  and  aluminium  used  might  produce 
the  same  effect.  In  order  to  test  this  point,  two  samples  of 
mica  and  four  of  aluminium  were  selected  with  care,  and 
used  successively  with  almost  identical  results.  It  therefore 
seems  certain  that  the  ionization  per  cm.,  produced  by  the 
a.  particle  in  air,  decreases  very  rapidly  between  6*4  cms.  and 
7*1  cms.  from  its  source. 
In  order  to  see  whether  there  was  any  evidence  that  the 
a  particle  continued  to  produce  ions  beyond  about  7*5  cms., 
a  very  strongly  excited  wire  was  used,  and  the  magnitude  of 
the  ionization  due  to  the  different  causes  was  measured.  The 
maximum  ionization  per  cm.  due  to  the  a,  rays  corresponded 
to  about  600  divisions  deflexion  of  the  electrometer,  and 
that  due  to  the  /3  rays  corresponded  to  about  8  divisions. 
When  the  ft  rays  were  bent  away  by  a  magnetic  field  and 
the  effect  due  to  the  y  rays  and  natural  leak  subtracted,  the 
remaining  ionization  per  cm.  did  not  correspond  to  more 
than  04  division.  It  therefore  seems  safe  to  conclude  that 
the  ionizing  power  of  the  a  particle  has  certainly  fallen  below 
Phil.  Mao.  S.  6.  Vol.  11.  No.  66.  June  1906.         3  (J 
74 
