the  a,  Rays  from  Radiotlwrium.  803 
product  must  be  transformed  very  rapidly  compared  with 
thorium  B,  and,  consequently,  the  activity  due  to  its  decay 
pari  passu  with  that  of  the  parent  product  thorium  B. 
It  has  not,  so  far,  been  found  possible  to  chemically 
separate  this  active  product  from  thorium  B.  Some  experi- 
ments of  Pegram  *are  of  importance  in  this  connexion.  By 
electrolysing  thorium  solutions,  he  states  that  he  obtained 
under  certain  conditions  a  very  rapidly  decaying  activity. 
For  example,  he  electrolysed  a  thorium  nitrate  solution  to 
which  a  little  copper  sulphate  had  been  added,  and  found  that 
the  cathode  lost  its  activity  very  rapidly.  A  similar  result 
was  noted  by  precipitating  some  silver  chloride  in  the  solution 
and  electrolysing  immediately  afterwards.  The  activities 
decayed  to  half  value  from  about  40  seconds  to  a  few  minutes. 
I  have  made  a  few  preliminary  experiments  in  this  direction, 
but  so  far  without  success.  The  experiments  will  be 
continued  to  see  if  it  is  possible  to  devise  a  method  of 
separating  the  new  product  and  to  determine  its  period. 
According  to  the  nomenclature  adopted  by  Rutherford  the 
new  a  ray  product  of  thorium  will  be  called  im  thorium  C." 
It  is  not  possible  to  decide  whether  the  a  rays  of  range  8*6  or 
4*7  cms.  must  be  ascribed  to  the  new  product.  From  analogy 
of  the  radium  products,  it  is  probable  that  the  a  particles  of 
greatest  range,  and,  consequently,  greatest  velocity,  are 
emitted  from  the  last  product  thorium  C.  For  a  similar 
reason,  it  is  probable  that  the  /3  and  y  rays  do  not  come  from 
thorium  B  but  from  thorium  C,  but  it  will  be  necessary  to 
separate  these  two  products  before  this  point  can  be  definitely 
settled. 
Decrease  of  range  of  the  a  particles  in  passing 
through  matter. 
It  was  stated  in  the  beginning  of  this  paper  that  the  range 
in  air  of  the  ionization  of  the  a  particles  is  decreased  by  adding 
thin  screens  over  the  active  wire.  The  decrease  of  range 
depends  upon  the  thickness  of  the  screen  and  its  density. 
The  ionization  curve  was  obtained  after  adding  a  mica  screen 
of  thickness  about  0'018  mm.  over  the  active  wire.  The 
absorption  of  the  rays  by  the  screen  was  equivalent  to  about 
2  cms.  of  air.  The  ordinates  of  the  new  curve  are  all  reduced 
by  a  definite  amount  corresponding  to  a  distance  of  2  cms.  of 
air.  A  similar  effect  has  been  observed  by  Bragg  and 
Kleeman  for  radium  rays. 
*  Phys.  Review,  Dee.  190a 
