the  a  Rays  from  Radiothorium.  797 
over  the  active  wire,  and  the  maximum  range  again  deter- 
mined. Column  I.  of  the  table  gives  the  number  o£  sheets  of 
aluminium-foil  used,  column  II.  gives  the  distance  at  which 
scintillations  were  still  distinct,  column  III.  gives  the  thickness 
of  air  which  corresponds  always  to  one  layer  of  foil. 
I. 
II. 
III. 
0 
8-0 
1 
7-5 
0-50 
3 
6-75 
0515 
7 
41 
0-55 
9 
3-05 
0-55 
11 
215 
0-53 
13 
no 
0-53 
14 
07 
0-53 
Average  0\r>3 
The  values  given  in  column  III.  are  approximately  constant, 
and  the  mean  0'53is  exactly  that  which  Rutherford  found  for 
the  rays  of  radium  C.  Another  series  of  measurements  was 
made  with  very  thin  foils  of  Dutch  metal,  the  result  of  which 
was  in  even  better  agreement.  For  some  purposes,  it  is  very 
convenient  to  have  a  means  of  cutting  down  the  range  of  the 
ol  particles  by  any  required  amount,  and  it  was  for  this 
reason  that  the  above-mentioned  measurements  were  under- 
taken. 
We  shall  state  later  in  this  paper  the  amount  of  air  equal 
to  a  very  thin  mica  plate — a  screen  which  has  the  great 
advantage  of  cutting  off  radioactive  emanations  completely, 
while  the  sheets  of  metal  are  always  porous  and  almost  useless 
for  that  purpose. 
The  scintillation  method  above  described  for  investigating 
the  maximum  range  of  the  a  particles  in  air  can  be  used  with 
advantage  when  dealing  with  a  single  product  like  radium  0, 
or  when  determining  only  the  range  of  the  most  penetrating 
of  a  number  of  different  rays.  The  method  does  not  throw 
any  light  on  the  range  of  the  a.  rays  which  have  less  than  tin4 
maximum  range,  nor  does  it  indicate  whether  we  are  dealing 
with  a  homogeneous  or  complex  pencil  of  a  rays.  It  was  of 
special  importance  in  the  present  case  to  know  whether  there 
was  present  ouhj  Th.  B  or  some  other  active  product  besides 
