a  Particles  of  Uranium  and  Thorium. 
Uranium. 
761 
The  uranium  was  used  in  the  form  of  the  green  oxide,  U308, 
and  was  freed  for  the  time  from  UrX.  This  was  not  ne- 
cessary, but  convenient,  as  it  diminished  the  /?  ray  correction. 
Fi°-  2. 
To  Q.£ 
To  Battery 
The  uranium  was  ground  to  a  fine  powder  and  placed  in  a 
shallow  depression  turned  in  a  metal  plate,  the  diameter  of 
the  recess  being  3'17  and  the  depth  ■£%  inch,  which  was  far 
more  than  enough  to  make  sure  that  the  a.  rays  from  the 
lowest  stratum  could  not  get  out.  The  surface  of  the  material 
was  carefully  smoothed  by  the  aid  of  a  polished  metal  plate. 
A  potential  of  300  volts  was  used,  which  was  nearly  sufficient 
to  saturate  :  more  was  not  necessary,  as  only  relative  ioni- 
zations were  in  question.  Aluminium-foil  was  used  as  the 
absorbing  layer  :  the  weight  and  area  of  each  piece  being 
measured  so  as  to  obtain  the  product  of  the  density  (p)  and 
the  thickness  (d).  In  the  following  table  (p.  762)  the  first 
column  gives  the  value  of  pd  of  the  foil  used,  and  the  second 
the  corresponding  current,  being  the  mean  of  five  readings 
of  the  leak  for  ten  seconds. 
The  last  line  shows  that  when  two  layers  of  tinfoil  were 
added  to  the  aluminium-foil  already  covering  the  uranium, 
the  leak  was  reduced  to  34.  Each  layer  of  foil  was  equi- 
valent to  about  17  mm.  of  air,  and  the  aluminium  to  about  21; 
so  that  the  whole  cut  off  the  a  rays  completely,  for  their 
range  was  known  to  be  not  more  than  3*5  cm.  This  leak 
of  34  was  therefore  due  to  ft  rays  and  the  normal  leak  of  the 
apparatus.  The  third  column  shows  the  result  of  subtracting 
34  from  all  the  figures  of  the  second  column  and  reducing  to 
a  decimal  of  I.     The  numbers  so  obtained   have  then  been 
Phil.  Mag.  S.  6.  Vol  11.  No.  6Q.  June  1906.        3  D 
