Absorption  of  a  Rl 
W 
141 
considering  either  the  point  of  maximum  ionization,  or 
the  point  at  which  the  ionization  ceases.  Consider,  for 
instance,  the  point  at  which  the  ionization  ceases  in  the  case 
of  air  and  in  the  case  when  the  rays  pass  through  two  layers 
of  aluminium.  The  distance  between  the  source  and  the 
gauze  at  which  the  ionization  ceases  is  decreased  in  the  second 
instance.  This,  of  course,  is  due  to  the  simple  fact  that  when 
the  rays  emerge  from  the  aluminium  their  velocity  has  already 
been  reduced  to  such  an  extent  by  absorption  that  they  do  not 
now  require  to  travel  so  far  in  air  before  their  velocity  is 
reduced  to  the  point  at  which  they  cease  to  ionize.  The 
differences  in  the  air-spaces  in  the  two  cases  must  be  equi- 
valent to  the  two  layers  of  aluminium  in  absorbing-power. 
Referring  to  the  curves  in  figs.  2  and  3,  the  maximum  ranges 
in  the  two  eases  are  6*8  cms.  and  5' 8  cms.  respectively.  A 
thickness  of  air  of  l'O  cm.  must  therefore  absorb  as  much  of 
the  a  rays  as  two  layers  of  aluminium.  Therefore,  assuming 
that  each  layer  produces  the  same  amount  of  absorption,  one 
layer  of  aluminium  '00031  cm.  in  thickness  absorbs  as  much 
a  radiation  as  '5  cm.  of  air. 
The  same  result  ought  to  be  obtained  if  we  consider  the 
point  of  maximum  ionization  in  the  two  cases.  This  point  in 
air  is  at  a  distance  of  about  5'8  cms.,  while  when  two  layers 
of  aluminium  are  interposed  the  maximum  point  is  at  a 
distance  of  4*8  cms.  The  difference  in  the  two  cases  divided 
by  2  is  "5,  which  agrees  with  the  result  obtained  from  con- 
sidering the  point  at  which  the  ionization  ceases.  Similar 
results  are  obtained  when  the  other  curves  for  the  various 
thicknesses  of  aluminium  are  considered.  These  results  are 
given  in  Tables  II.  and  III. 
Table  II. 
Absorbing 
Medium. 
Distance  at  which 
Ionization  ceases. 
Thickness  of  Air 
corresponding  to 
given  thickness  of 
Aluminium. 
Thickness  of  Air 
corresponding  to 
one  layer  of 
Aluminium. 
Air 
68  cms. 
5-8     „ 
4-8     „ 
3  9    „ 
2-6    „ 
1-0  cm. 
2-0  cms. 
2-9     „ 
42    „ 
*50  cm. 
•50  „ 
•48    „ 
•52    „ 
2  layers  of  Al.    ... 
4       „           „      ... 
6      ,.          „      ... 
8      ,,          „      ... 
