136  Mr.  R.  K.  MoClnng  on  the 
Rutherford,  in  a  recent  paper  in  the  Phil.  Mag.*,  has  also 
investigated  the  question  of  the  distance  at  which  the  photo- 
graphic and  phosphorescent  actions  of  the  a  rays  cease;  and 
he  has  found  almost  the  same  distances  in  those  cases  as  the 
author  found  for  the  ionizing  action. 
The  results  shown  by  this  curve  indicate  that  as  the  velocity 
of  the  a,  particle  from  radium  C  decreases,  the  ionizing  power 
of  the  particle  very  slowly  increases  over  a  certain  range; 
then  for  a  further  decrease  in  the  velocity  the  efficiency  of 
the  «  particle  as  an  ionizer  increases  quite  rapidly  until  a 
maximum  ionization  is  reached.  When  the  velocity  decreases 
beyond  this  point,  the  ionizing  power  of  the  a  particle  falls  off 
extremely  rapidly  and  very  soon  ceases  altogether. 
Rutherford  (loc.  cit.),  in  investigating  this  question  of  the 
velocity  of  the  a,  particle  from  a  photographic  and  phospho- 
rescent point  of  view,  has  shown  that  when  both  the  photo- 
graphic and  phosphorescent  actions  of  the  a  rays  cease,  the 
particles  still  possess  a  considerable  proportion  of  their  original 
velocity  of  projection,  and  consequently  quite  a  large  amount 
of  their  kinetic  energy.  He  also  showed  that  the  distance 
from  the  source  at  which  these  actions  ceased  corresponded 
very  closely  with  the  distances  obtained  by  Bragg  and  by  the 
writer  for  the  cessation  of  the  ionizing  effect.  These  results 
therefore  all  appear  to  indicate  that  the  a  particle  is  not 
capable  of  producing  either  ionization,  nor  photographic  nor 
phosphorescent  effects  when  its  velocity  falls  below  a  certain 
definite  amount.  This  limiting  velocity  appears  to  be  very 
well  defined  for  the  rays  produced  by  a  definite  type  of 
radioactive  product. 
We  should  expect  from  this  point  of  view  that  when  the 
point,  at  which  the  maximum  ionization  takes  place,  is  reached 
a  further  increase  of  the  distance  between  the  source  and  the 
electrode  should  cause  the  ionization  to  cease  abruptly.  The 
curve,  however,  beyond  the  maximum  point  shows  a  decided 
upward  slope  instead  of  being  quite  horizontal.  This  upward 
slope  of  the  curve  is  at  least  partly  due  no  doubt  to  the  fact 
that  the  angle  of  the  cone  of  rays  is  of  considerable  size. 
When  the  distance  between  the  source  of  the  rays  and  the 
electrode  is  gradually  increased,  the  air  between  the  electrodes 
gets  beyond  the  range  of  the  extreme  rays  of  the  cone  before 
it  gets  beyond  the  range  of  the  central  rays,  as  the  extreme 
rays  have  to  travel  slightly  farther,  before  reaching  the 
electrodes,  than  the  central  rays.  Therefore,  as  soon  as  this 
volume  of  air  gets  beyond  the  range  of  the  extreme  outside 
rays  of  the  cone,  the  ionization  will  fall  off,  due  to  the  fact  that 
the  width  of  the  ionizing  cone  is  decreasing  and  the  volume 
of  air  acted  upon  is  decreased. 
*  Phil.  Ma«r.  July  190^ 
