Properties  of  the  a,  Rays  from  Radium.  167 
were  all  diminished  by  a  constant  amount  after  passing 
through  a  definite  thickness  of  aluminium.  It  was  observed 
that  the  photographic  action  of  the  a  rays  became  relatively 
very  feeble  when  the  velocity  of  the  a  particles  fell  to  about 
60  per  cent,  of  their  maximum  velocity. 
Bragg  and  Kleeman  (Phil.  Mag.  Dec.  1904,  Sept.  1905) 
had  previously  shown  by  a  most  convincing  series  of  ex- 
periments that  the  a  rays  emitted  from  radium  consisted  of 
four  distinct  sets  expelled  from  the  four  a  ray  products 
present  in  radium  in  equilibrium,  viz.  radium  itself,  the 
emanation,  radium  A  and  (J.  Each  product  contained  in  a 
very  thin  film  of  radium  in  equilibrium  emitted  rays  which 
ceased  to  ionize  the  gas  after  passing  through  a  definite  dis- 
tance in  air.  This  "  range "  of  the  a  particles  was,  for 
example,  about  3*5  cms.  for  the  rays  emitted  from  radium 
itself,  and  about  7  cms.  for  the  rays  emitted  from  radium  C. 
In  a  thick  layer  of  radium,  where  the  a  particles  emerge  into 
the  gas  from  different  depths,  they  concluded  that  each  pro- 
duct gave  out  rays  which  had  all  ranges  of  ionization  in  air 
varying  between  zero  and  the  maximum  range. 
In  order  to  explain  their  results  they  supposed  that  the 
a  particles  diminished  in  velocity  and  consequently  in  their 
range  of  ionization  by  passing  through  matter.  The  results 
of  my  experiments  indicated  that  the  velocity  of  the  particles 
was  not  reduced  much  below  60  per  cent,  of  the  velocity  of 
the  particles  from  radium  C  when  they  tailed  to  ionize  the 
gas  or  to  affect  appreciably  a  photographic  plate. 
According  to  the  views  of  Bragg  and  myself,  the  rays 
emitted  from  a  thick  layer  of  radium  in  equilibrium  were 
complex  in  character,  and  consisted  of  four  sets  of  rays  each 
of  which  consisted  of  particles  projected  over  a  considerable 
range  of  velocity. 
As  the  simplest  hypothesis,  it  was  implicitly  assumed  that 
the  a.  particles  from  the  various  radium  products  had  the 
same  mass  and  carried  the  same  charge,  and  differed  from 
each  other  only  in  the  initial  velocity  of  their  projection 
from  the  radium  and  its  products. 
In  a  recent  number  of  the  Comptes  Rendus  (No.  11,  Sept.  11, 
1905),  M.  Henri  Becquerel  has  taken  exception  to  my  ex- 
perimental results  and  deductions.  He  reiterates  his  original 
view  that  the  a  rays  from  radium  are  homogeneous,  and  con- 
cludes that  the  retardation  of  velocity  of  the  a.  particles  in 
their  passage  through  aluminium,  observed  by  me  for  the 
a  particles  from  radium  C,  does  not  exist  in  the  a  particles 
expelled  from  radium,  or  at  any  rate  from  his  radium.  For 
this  reason  he  concludes  that  the  theory  proposed  by  Bragg 
