[    131    ] 
X.  The  Absorption  of  a  Rays.  By  R.  K.  McClung, 
M.A.,  Senior  Demonstrator  in  Physics,  McGill  University, 
Montreal  *. 
THE  question  of  the  absorption  of  the  a  rays  from  radium 
by  matter  has  been  discussed  theoretically  in  a  most 
admirable  manner  by  Prof.  Bragg  in  a  paper  which  appeared 
in  a  recent  number  of  the  Philosophical  Magazine  f.  The 
theoretical  considerations  put  forward  in  that  paper  were  also 
verified  experimentally  by  Bragg  and  Kleeman  J.  From  the 
results  set  forth  in  these  papers  it  was  deduced  that  the 
absorption  of  the  a  rays  in  air  is  mainly  due  to  the  fact  that 
their  energy  is  used  up  in  producing  ionization,  and  it  was 
shown  that  these  rays  possess  the  power  of  ionizing  the  gas 
only  within  a  limited  distance  from  the  ionizing  source. 
This  limiting  distance  was  shown  to  be  fairly  sharply  defined, 
and  was  found  to  depend  upon  the  nature  of  the  source  from 
which  the  rays  came. 
The  source  from  which  Bragg  and  Kleeman  obtained  their 
a  rays  was  a  very  thin  layer  of  radium  obtained  by  crystalli- 
zation from  solution.  This  crystalline  layer  was,  as  they 
clearly  pointed  out,  not  a  single  homogeneous  source  of 
a  rays,  but  contained  several  transformation  products  of 
radium,  each  of  which  gave  out  a.  particles  and  some  of 
which  gave  out  ft  and  y  rays  as  well.  In  addition  to  this, 
although  the  layer  of  radiating  matter  was  very  thin,  still 
the  velocity  of  the  rays  proceeding  from  the  lower  parts  of 
the  layer  would  be  appreciably  decreased  by  absorption  before 
emerging  into  the  air,  and  consequently  the  source  would  be 
giving  out  rays  of  different  velocities. 
As  the  source  was  not  homogeneous  the  curves  which  they 
obtained,  as  they  clearly  show,  were  somewhat  more  com- 
plicated than  if  a  single  homogeneous  source  had  been  used. 
Prof.  Bragg  suggested  in  his  paper  that  polonium,  which 
gives  out  only  a  rays,  might  have  been  used  if  it  had  been 
available. 
It  was  therefore  suggested  by  Prof.  Rutherford  that  it 
would  be  of  interest  to  investigate  this  question,  using  as  the 
source  of  a  radiation  the  radioactive  matter  deposited  on  a 
wire  which  had  been  suspended  in  a  vessel  containing  the 
emanation  from  radium.  Although  the  deposit  on  the  wire 
contains  the  three  products  radium  A,  B,  and  C,  still  the 
*  Communicated  bv  Prof.  E.  Rutherford,  F.R.S. 
t  Phil.  Mag.  p.  719,  Dee.  1904. 
\  Phil.  Mau-.  p.  726.  Dec.  1904 
K  2 
