142 
The  Absorption  of  a  Rays. 
Table  III. 
Absorbing 
Medium. 
Distance  at 
which  maximum 
Ionization  occurs. 
Thickness  of  Air 
corresponding  to 
given  thickness  of 
Aluminium. 
Thickness  of  Air 
corresponding  to 
one  layer  of 
Aluminium. 
58  cms. 
4-8    „ 
3-6    „ 
27    „ 
1-5    „ 
10  cm. 
2-2  cms. 
3-1    „ 
43    „ 
'50  cm. 
•55    „ 
•51    „ 
•53    „ 
2  layers  of  Al.    ... 
4      „          „      ... 
6      „          „      ... 
8       „          „      ... 
From  these  curves  we  see  that  we  can  readily  compare 
the  absorptive  power  of  aluminium  with  that  of  air,  and 
obtain  the  thickness  of  air  which  is  equivalent  in  absorbing- 
power  to  any  given  thickness  of  aluminium. 
These  results  are  in  very  fair  agreement  with  those  obtained 
by  Rutherford,  using  the  photographic  and  the  phospho- 
rescent-screen methods.  He  used  specimens  of  aluminium- 
foil  similar  to  those  used  by  the  writer,  and  therefore  the 
results  of  the  two  investigations  may  be  compared.  He 
found  by  the  photographic  and  the  phosphorescent-screen 
methods  that  one  layer  of  aluminium-foil  was  equal  in 
absorbing-power  to  '54  cm.  of  air.  He  obtained  this  result 
by  measuring  the  distance  from  the  source  at  which  the 
a  particle  ceased  to  affect  a  photographic  plate  or  a  phos- 
phorescent screen  after  passing  through  various  thicknesses 
of  aluminium. 
The  close  agreement  of  the  results  obtained  in  the  two 
separate  investigations  is  an  indirect  confirmation  of  the 
hypothesis  put  forward  by  Rutherford  in  his  paper  (loc.  cit.) 
in  regard  to  the  nature  of  the  action  produced  by  the  a  rays 
from  radium  on  a  photographic  plate  or  a  phosphorescent 
screen.  These  two  sets  of  results  taken  together  appear  to 
point  strongly  to  the  conclusion  that  the  ionizing,  photographic, 
and  phosphorescent  effects  of  the  a,  rays  from  radium  are  all 
due  to  the  same  cause,  and  it  is  very  probable  that  in  each 
case  the  effect  produced  is  primarily  one  of  ionization. 
In  conclusion  I  wish  to  express  my  indebtedness  to  Prof. 
Rutherford,  at  whose  suggestion  the  subject  of  this  research 
was  undertaken,  for  his  valuable  suggestions  during  the 
progress  of  the  investigation. 
McDonald  Physics  Building, 
McGill  University, 
Montreal,  Julv  21st,  1905. 
