the  ol  Rays  from  Radiothorium. 
'99 
brium  alter  a  three  days'  exposure  to  the  emanation.     The 
result  obtained  was  most  unexpected. 
Fig.  1. 
The  curve  is  seen  to  consist  of  two  distinct  parts.  I£  the 
portion  BCof  the  curve  is  produced  backwards  to  meet  the 
axis  of  abscissae  at  D,  the  curve  ABGD  represents  the  curve 
to  be  expected  for  the  ol  rays  from  a  thin  film  of  radioactive 
matter  of  one  kind.  Curves  very  similar  in  shape  to  this 
have  been  obtained  by  Bragg  and  Kleeman  for  the  rays  from 
a  thin  film  of  radium  at  its  minimum  activity,  and  by  McClung 
for  the  ol  rays  from  radium  C.  The  abrupt  change  in  the 
curve  at  C  shows  that  at  that  distance  another  set  of  ol  rays 
enters  the  ionization  chamber.  The  complex  curve  clearly 
indicates  that  the  active  deposit  gives  out  two  distinct  types 
of  rays  of  different  ranges  in  air,  and  must  consequently 
consist  of  two  distinct  ol  ray  products.  The  fact  that  the  ioni- 
zation represented  by  G  D  is  nearly  equal  to  that  represented 
by  D  F  shows  that  an  equal  number  of  «  particles  are  emitted 
per  second  by  these  two  products — a  result  to  be  expected 
