V]  ACTINIUM  AND   ITS  PRODUCTS  526

be  rate  of  decay  of  this  active  deposit  which  emits  a,  13  and  7
ys  is  independent  of  whether  the  activity  is  collected  on  the
ate  by  diffusion  or  by  the  intermediary  of  an  electric  field.  The
itial  part  of  the  curve  of  decay  measured  by  the  a  rays  for  a  long
:posure  is  shown  in  Fig.  117  (marked  21  hours).  The  activity
lis  slowly  the  first  10  minutes,  but  afterwards  more  rapidly,
jcreasing  finally  according  to  an  exponential  law  with  the  time
ith  a  period  of  36  minutes.  No  doubt,  if  observations  were  made
Lthin  1/500  of  a  second  after  removal,  the  decay  of  activity  due

actinium  A  could  be  observed.  The  half-value  period  was
itially  determined  by  Debierne*,  who  found  a  value  of  40
inutes.  Later  observations  by  Bronsonf,  Elster  and  GeitelJ,
eyer  and  v.  Schweidler§,  Hahn  and  Sackur||,  gave  values  of  about
)  minutes.
Miss  Brooksl,  found  that  the  activity  curves  depended  greatly
I  the  time  of  exposure  to  the  emanation.  Examples  of  the  o  ray
irves  obtained  by  her  for  various  times  of  exposure  are  shown  in
ig.  117.  The  activity  at  first  increases,  passes  through  a  maximum
id  ultimately  decreases  according  to  an  exponential  law  with  a
Bfiod  of  36  minutes.  Bronson  determined  accurately  the  activity
irve  for  a  short  exposure.  This  is  shown  in  Fig.  118,  the
laximum  activity  being  taken  as  100.  The  shape  of  this  curve
idicatod  the  presence  of  two  products,  one  of  which  emits  a  rays
id  the  other  does  not**.  The  activity  /  at  any  time  is  proportional
>  g  A,^_g-Aa^     Tjjjg  jg  ii^Q  relation  (equation  5,  Section  159)  to

3  expected  for  a  short  exposure  where  two  products  are  involved,
le  of  which  emits  a  rays  and  the  other  does  not.
