798  Dr.  0.  Hahn  on  some  Properties  of 
it,  for  instance  Act.  B.  The  latter  would  then,  of  course, 
come  from  actinium  in  the  radiothorium  preparation,  and 
would  show  that  it  was  not  pure  in  the  radioactive  sense  of 
the  word.  There  was,  however,  but  little  probability  of  such 
an  occurrence,  since  no  evidence  of  the  presence  of  actinium 
had  been  obtained  from  an  examination  of  the  decay  curves, 
which  had  been  obtained  for  the  emanation  and  the  active 
deposit.  In  order  then  to  test  the  point  more  completely,  I 
made  use  of  the  method  developed  by  Bragg  and  Kleeman  *, 
who  made  with  so  much  success  a  complete  analysis  of  the  a 
rays  emitted  by  the  various  radium  products.  The  apparatus 
employed  by  the  writer  was  similar  to  that  of  Bragg  and 
Kleeman  and  of  McClung.  A  fairly  large  metal  box 
enclosed  two  parallel  plates,  between  which  the  ionization 
was  to  be  measured.  The  upper  plate  consisted  of  a  solid 
plate  of  zinc,  connected  to  one  pair  of  quadrants  of  a 
Dolezalek  electrometer,  the  other  pair  being  connected  to 
earth.  The  lower  plate  was  of  wire  gauze,  about  0'5  cm. 
away  from  the  upper  plate,  and  was  charged  by  means  of  a 
battery  of  accumulators  to  a  sufficient  potential  to  produce  a 
saturation  current.  To  avoid  any  leak  from  the  lower  to  the 
upper  plate,  the  upper  plate  was  surrounded  by  a  guard-ring 
connected  to  earth.  The  radioactive  source,  in  this  case  the 
active  wire,  was  placed  on  a  small  platform  in  the  metal  box 
directly  beneath  the  two  plates.  In  order  to  get  a  well- 
defined  cone  of  rays,  the  active  wire  was  cut  into  several 
small  pieces  and  laid  in  the  groove  of  a  solid  zinc  block.  On 
this  was  placed  another  zinc  block  about  0'5  cm.  thick, 
having  a  hole  about  0'3  cm.  in  diameter,  just  over  the  groove 
containing  the  active  wire.  The  small  platform  could  be 
moved  vertically  to  any  distance  from  the  two  plates,  and 
this  distance  could  be  measured  on  a  fixed  scale.  The  parallel 
plates  were  sufficiently  large  to  include  the  whole  cross- 
section  of  the  cone  of  rays  over  the  range  required. 
Measurements  of  the  ionization  currents  in  the  testing- 
chamber  were  made  for  different  distances  between  the 
ionization-chamber  and  the  radiant  source.  The  velocity  of 
the  electrometer  needle  was  a  measure  of  the  intensity  of  the 
ionization.  In  the  following  figures  the  ordinates  represent 
the  distances  of  the  radioactive  product  from  the  wire  gauze, 
the  abscissae  the  intensities  of  the  ionization  measured  in 
arbitrary  units,  which  are,  however,  the  same  in  both  figures 
and  for  all  the  curves.  Fig.  1  shows  the  curve  obtained 
from  the  active  wire,  the  latter  being  in  radioactive  equili- 
*  Phil.  Map-.  Dec.  1904 ;  &  Sept.  1905. 
