Various  Gases  by  the  a  Particles  of  Radium.  627 
Sir  William  and  Lady  Hug-gins  made  of  the  phosphorescent 
glow  of  radium  showed  the  bands  of  the  gas  in  which  the 
salt  was  imbedded.  Rutherford  also  has  shown  that  the  a 
particle  can  no  longer  cause  phosphorescence  when  it  has 
lost  its  power  of  ionization. 
The  Magnetic  Deflexion  of  the  a  Rays. 
In  the  Rhysikalische  Zeitschrift  for  Oct.  15th  is  a  paper 
by  M.  Becquerel,  "  Ueber  einige  Eigenschaften  der  a  Strahlen 
des  Radiums."  The  author  discusses  the  theory  that  the 
a  rays  gradually  lose  their  velocity  as  they  spend  their 
energy  on  the  ionization  of  the  media  through  which  they 
pass,  a  theory  which  I  put  forward  about  two  years  ago  *, 
and  which  has  the  support  of  much  experimental  evidence 
accumulated  by  Prof.  Rutherford  f,  and  by  Mr.  Kleeman 
and  myself  %. 
He  maintains  that  the  theory  is  unsuccessful  in  explaining 
the  experiments  which  he  has  himself  performed,  and  in 
particular  he  describes  one  experiment  which  he  has  devised 
as  a  crucial  test  and  which  he  considers  to  show  that  the 
theory  is  incorrect. 
It  is  as  follows  (he.  cit.  p.  688)  :  — 
The  rays  from  a  small  quantity  of  radium  salt  are  allowed 
to  stream  upwards  through  a  narrow  slit  and  fall  upon  a 
photographic  plate.  A  powerful  magnetic  field  deflects  them 
slightly  to  one  side.  The  field  is  reversed  when  the  experi- 
ment is  halfway  through,  and  as  a  result  two  images  of  the 
slit  appear,  slightly  separated,  upon  the  plate.  Now, 
M.  Becquerel  covers  half  the  slit  with  a  thin  sheet  of 
aluminium  ;  and  according  to  the  theory  which  I  have 
advanced,  the  a  rays  which  pass  through  the  sheet  are 
thereby  retarded.  Consequently,  M.  Becquerel  argues, 
these  ol  rays  should  be  more  bent  to  one  side  than  those 
which  have  not  passed  through  the  aluminium,  and  the 
images  on  the  plate  should  show  a  break,  the  lines  being- 
more  widely  separated  in  one  half  of  the  picture  than  in  the 
other. 
But  M.  Becquerel  is  under  a  misapprehension  on  this 
point.     Paradoxical  as  it  may  appear  at  first  sight,  no  such 
*  Australasian  Association  for  the  Advancement  of  Science  Report, 
Dunedin,  January  1904. 
t  Phil.  Mag.  July  1905. 
X  Phil.  Mag-.  Dec.  1904  and  Sept.  190o. 
