the  a  Rays  from  Radiothorium.  801 
impurity  in  the  radiothorium,  for  example  from  the  presence 
o£  actinium.  Remembering  that  the  active  deposit  of 
actinium  (half  value  in  36  minutes)  is  transformed  very 
rapidly  compared  with  the  active  deposit  of  thorium  (half 
value  in  10' 6  hours),  there  were  two  simple  methods  of 
definitely  settling  whether  the  observed  shape  of  the 
ionization  curve  resulted  from  the  presence  of  actinium  B. 
Experiment  I. — A  wire  was  exposed  to  the  radiothorium 
preparation  for  about  3  hours.  After  that  interval  the  active 
deposit  due  to  actinium  has  reached  its  maximum  value,  while 
that  of  thorium  is  only  a  small  portion  of  its  equilibrium 
amount.  Under  such  conditions,  the  ionization  due  to  the 
actinium  B  should  predominate. 
Experiment  II. — A  wire  was  exposed  to  the  radiothorium 
preparation  for  three  days  in  order  to  obtain  the  equilibrium 
amount  of  Th.  B,  and,  of  course,  the  equilibrium  amount  of 
actinium  B.  The  wire  was  then  removed  from  the  emanation 
and  tested  four  hours  later.  During  this  interval  the  activity 
due  to  actinium  B  has  disappeared,  and  the  ionization  curve 
should  be  due  to  thorium  B  alone. 
On  the  assumption  that  one  of  the  products  was  actinium 
B,  the  ionization  curve  in  one  case  should  be  mainly  due  to 
rays  of  a  range  S'6  cms.,  and  in  the  other  to  rays  of  range 
4*7  cms.  No  such  effect,  however,  is  observed.  The 
experimental  curves  are  shown  in  fig.  2  (p.  802),  curves  I.  &II. 
Curve  I.  was  obtained  from  experiment  I.,  and  curve  II.  from 
experiment  II. 
The  curves  are  similar  in  shape  to  that  given  in  fig.  1,  and 
show  the  same  equality  in  the  ionization  produced  by  the  two 
products.  These  experiments  thus  prove  that  no  measurable 
amount  of  actinium  B  was  present  on  the  wire.  For  a 
similar  reason,  it  is  seen  that  the  rapidly  changing  active 
deposit  of  radium  is  also  absent. 
We  must,  therefore,  conclude  that  two  a.  ray  products  are 
present  in  the  active  deposit  of  radiothorium  and  not  one  as 
was  previously  supposed.  Neither  of  these  two  products  can 
be  ascribed  to  a  slowly  changing  product  of  thorium  for  the 
reasons   discussed  above.     Such  a  slowly  changing  product 
11  1  1  o         O      I 
would  not  always  be  present  in  the  same  proportion  as 
thorium  B  for  different  times  of  exposure  to  the  emanation  *, 
*  It  may  be  mentioned  that  some  months  ago  the  writer  made  some 
investigations  to  see  whether  there  was  any  slowly  changing-  product  in 
the  active  deposit  of  thorium  obtained  on  a  copper  wire.  A  negatively 
charged  wire  was  exposed  for  three  weeks  to  the  emanation  of  the 
strongest  radiothorium  preparation  in  my  possession.  The  activity 
decayed  at  the  characteristic  rate,  but  no  evidence  of  any  appreciable 
residual  activity  was  obtained.  Of  course,  very  weak  activities  would 
have  been  overlooked,  as  the  air  and  the  instruments  of  the  laboratory 
showed  always  a  slight  activity. 
