800  Dr.  0.  Hahn  on  some  Properties  of 
if  the  two  ol  ray  products  are  successive  aud  in  radioactive 
equilibrium  with  each  other. 
It  may  be  mentioned  that  for  accurate  measurements,  it  is 
necessary  to  make  a  correction  for  the  decay  of  the  activity 
during  the  time  of  the  experiment.  Since  thorium  B  decays 
to  half  value  in  10*6  hours,  after  one  hour  there  is  a  decrease 
of  about  6*5  per  cent.  This  correction  for  change  of  the 
intensity  of  the  rays  during  an  experiment  is  generally  small 
and  is  very  easily  made. 
The  curve  shows  that  the  ionization  of  the  a.  rays  begins  at 
a  distance  of  S'6  cms.  from  the  source — a  result  in  good 
agreement  with  that  obtained  by  the  scintillation  method. 
Beyond  that  distance,  the  curve  is  almost  a  vertical  line,  and 
the  ionization  is  due  to  the  ft  and  7  rays  and  the  natural  leak 
of  the  vessel  and  the  air.  Below  8*6  cms.  the  current  in  the 
ionization  chamber  increases  very  rapidly,  reaches  a  maximum 
at  6'8  cms.,  and  then  decreases  until  at  4*7  cms.  the  rays 
from  the  second  product  add  their  effect. 
If  the  a,  particles  all  ceased  to  ionize  after  traversing  a 
definite  distance  of  air,  the  slope  of  the  curve  A  B,  obtained 
with  a  shallow  ionization  chamber  and  a  narrow  cone  of  ray?, 
should  be  nearly  horizontal.  In  the  experiments,  however, 
a  fairly  wide  cone  of  rays  and  an  ionization  chamber  of  depth 
5  mms.  were  used.  With  a  narrower  cone  of  rays  and  a 
shallower  ionization  chamber,  the  electrical  effect  would  have 
been  smaller,  and  the  errors  of  measurements  consequently 
greater. 
Under  the  experimental  conditions  it  is,  therefore,  not 
unexpected  that  the  curve  A  B  should  have  an  appreciable 
slope  towards  the  axis  of  abscissse.  Using  a  zinc  block  0*8 
cm.  high  instead  of  0*5  cm. — as  used  to  determine  the  curve 
given  in  the  figure — the  slope  of  the  curve  was  less  marked, 
and  the  point  of  maximum  ionization  was  3  mms.  higher. 
The  decrease  of  the  current  below  the  distance  of  6'S  cms. 
shows  that  the  a  particle  is  a  more  efficient  ionizer  near  the 
end  of  its  path,  when  its  velocity  has  been  diminished  by 
passing  through  the  air.  A  similar  effect  for  radium  rays 
has  been  observed  by  Bragg  and  Kleeman  *  and  by  McClungf . 
The  ionization  curve  C  E  F,  due  to  the  rays  from  the 
second  product,  is  very  similar  in  shape  to  the  curve  ABC. 
Below  a  distance  of  4"  7  cms.  the  ionization  rapidly  increases, 
passes  through  a  maximum  at  4"4  cms.,  and  then  diminishes. 
As  we  have  already  pointed  out,  the  equal  ionization 
produced  by  the  two  a  ray  products  suggests  that  the  products 
are  successive.  It  wras,  however,  necessary  to  make  certain 
that  one  of  the  two  a.  ray  products  did  not  arise  from  some 
*  Loc.  cit.  t  Loc.  cit. 
