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LXI V.  On  some  Properties  of  the  a  Rays  emitted  by  Radium 
and  by  Bodies  rendered  Active  by  the  Radium  Emanation. 
By  H.  Becquerel  *. 
SOME  months  ago  t,  in  consequence  of  an  investigation  oi 
Rutherford's  relating  to  the  retardation  suffered  by  the 
«  rays  in  passing  through  thin  aluminium  sheets,  I  had 
resumed  some  old  experiments  on  the  rays  of  radium.  I 
had  arranged,  in  particular,  a  differential  experiment  which 
enabled  me  to  receive,  on  the  same  photographic  plate,  the 
two  portions  of  a  pencil  of  a  rays  proceeding  from  a  linear 
source,  passing  through  a  slit  parallel  to  the  source  and  plate, 
and  deviated  by  a  magnetic  field — a  pencil  one  half  of  which 
had  traversed  nothing  but  air,  while  the  other  had,  in  addition, 
passed  through  one  or  more  thin  sheets  of  aluminium.  Under 
these  conditions,  the  two  parallel  bands  which  are  the  traces 
of  the  deflected  pencil  corresponding  to  the  two  directions  of 
the  magnetic  field  should  be  more  widely  separated  in  the 
portion  of  the  pencil  which  has  traversed  the  aluminium  than 
in  the  other.  The  first  experiments  made  with  the  a  rays  of 
radium  did  not  exhibit  the  deflexion  expected. 
Immediately  on  my  return  to  Paris  last  October,  I  resumed 
these  experiments,  using  as  sources  either  salts  of  radium  or 
wires  excited  by  the  radium  emanation,  which  M.  Curie  was 
kind  enough  to  render  active  in  his  laboratory.  The  results  ob- 
tained were  in  accordance  with  Rutherford's  statement  (fig.  1). 
Fijr.  1. 
-Aluminium  Sheet. 
-Air  only. 
D.  19/10/1905. 
The  following  are,  for  instance,  the  means  of  the  results  of 
several  experiments : — 
Double  deflexion  of  pencil 
No.  of 
experiment. 
Nature  of 
source. 
Field 
Intensity. 
in  air. 
after  passing  through 
a  sheet  of  aluminium 
e=0-015  mm. 
Eatio 
D.  19/10/1905  ... 
Excited  wire. 
9659 
2-360  mm. 
2*658  mm. 
1122 
A.  21/10/1905  ... 
Radium  salt. 
9659 
2-412  „ 
2671    „ 
1-107 
A.  24/10/1905  ... 
,,         ,, 
9384 
2-222  „ 
2-423    „ 
1090 
The   above    tests    were   made    with    the    same    apparatus 
(distance  a  from  source  to  slit,  2*145  cm.;  distance  b  from 
*  Communicated    by   the   Author:    from    the    Comptes   Rendus    of 
February  12,  1906. 
t  Comptes  Rendus,  t.  cxli.  11th  September,  1905,  p.  485. 
