Properties  of  the  a  Rays  from  Radium.  175 
curvature  of  the  path  of  the  rays  in  air  is  in  contradiction  to 
the  hypothesis  of  M.  Becquerel  that  the  value  of  e/m  decreases 
with  distance  from  the  source. 
We  have  seen,  however,  that  this  conclusion  of  M.  Becquerel 
is  based  on  the  incorrect  assumption  that  the  rays  are  homo- 
geneous. Taking  into  account  the  complexity  of  the  a  rays 
from  radium,  the  results  observed  by  M.  Becquerel  receive 
a  simple  explanation  without  the  necessity  of  assuming  that 
the  value  of  e/m  either  increases  or  decreases  with  distance. 
An  explanation  along  these  lines  was  given  last  year  by 
Bragg  and  Kleeman.  Without  entering  into  intricate  calcu- 
lations, the  general  reason  of  the  increase  of  the  radius  of 
curvature  of  the  path  of  the  rays  with  distance  of  air  traversed 
can  easily  be  seen. 
At  a  distance  of  2  cms.  from  the  source,  for  example,  the 
outside  edge  of  the  trace  is  due  to  the  most  slowly  moving 
particles,  which  are  just  able  to  produce  a  photogTaphic 
impression  after  traversing  2  cms.  of  air.  The  inside  edge  is 
due  to  the  most  penetrating  rays,  viz.  those  from  radium  0, 
whose  velocity  has  been  reduced  somewhat  by  passing- 
through  the  2  cms.  of  air. 
Suppose  we  consider  the  trace  of  the  rays  distant  3  cms. 
from  the  source.  As  before,  the  outside  edge  of  the  trace  is 
due  to  particles  which  are  just  able  to  produce  a  photographic 
effect  after  traversing  3  cms.  of  air.  These  particles  are 
thus  projected  initially  with  a  greater  velocity  than  those 
which  were  only  able  to  affect  a  plate  for  a  distance  of  2  cms. 
The  average  velocity  of  the  a  particle  along  its  path  is 
consequently  greater  than  for  the  corresponding  case  for  a 
distance  of  2  cms.  The  outside  edge  of  the  trace  will 
therefore  be  deflected  through  a  smaller  distance  than  would 
be  expected  if  the  average  velocity  were  the  same  in  the  two 
cases. 
The  radius  of  curvature  of  the  outside  edge  of  the  path 
will  consequently  increase  with  increasing  distance. 
Quite  a  contrary  effect  should  be  observed  on  the  inside 
edge  of  the  trace,  which  is  produced  by  the  a  particles  of 
greatest  velocity  escaping  from  the  radium.  This  velocity  is, 
as  we  have  seen,  reduced  by  the  passage  through  air,  and 
consequently  the  average  velocity  of  the  a.  particles,  which 
traverse  the  path  of  3  cms.,  is  less  than  for  the  path  of  2  cms. 
The  inside  trace  should  theoretically  show  evidence  of  de- 
creasing radius  of  curvature.  This  would  have  the  effect  of 
contracting  the  natural  width  of  the  trace  unless  the  scattering 
of  the  rays  in  air  masks  it  experimentally.     Quite  apart  from 
