Absorption  of  a.  Rays.  133 
excited  activity  was  laid  in  the  groove  in  the  lower  section  of 
the  block,  and  the  upper  half  was  then  placed  over  it  so  that 
the  radiations  from  the  wire  could  only  emerge  through  the 
hole  in  the  upper  block.     A  well-defined  cone  of  rays  was 
Fig.  1. 
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thus  obtained.  This  brass  block  was  placed  in  a  definite 
position  on  an  upright  support,  P,  which  could  be  adjusted 
vertically  and  the  amount  of  adjustment  measured  on  a  fixed 
scale. 
The  plate  E  was  made  sufficiently  large  to  include  the  entire 
cross-section  of  the  cone  of  rays  throughout  the  whole  range 
of  the  adjustment  of  K. 
Method  of  Observation. 
The  wire  on  which  the  radioactive  matter  was  deposited 
was  a  thin  copper  one  about  0*5  mm.  in  diameter.  About  an 
inch  of  this  wire  was  attached  to  the  negative  pole  of  a  battery 
of  400  volts,  and  suspended  in  the  emanation  from  about 
20  milligrams  of  radium  bromide  and  allowed  to  remain 
there  for  a  couple  of  hours,  or  longer,  so  as  to  get  the 
maximum  amount  of  excited  activity  upon  it.  It  was  then 
removed  from  the  emanation  and  cut  into  short  pieces  and 
placed  in  the  groove  K.  The  rate  of  leak  produced  between 
the  plates  E  and  H  was  then  measured  for  different  distance.- 
of  the  source  from  the  gauze  H  II . 
