Rate  of  Decay  of  Active  Deposit  from  Radium.       151 
of  curve  2,  the  temperature  was  about  800°  C.  2A  was  taken 
immediately  after  heating,  2B  after  about  one  hour,  and  20 
after  about  two  and  a  half  hours.     The  values  obtained  for 
Fi°-.  5. 
f.20fl- 
.905 
5 
q$.602 
^.30! 
V, 
\.    X*         x^ 
xxV 
^^nSx^ 
Xx^TV 
N 
X^Xo 
x-7     x 
X 
the  period  were  respectively  20*1,  21'3,  and  22*8  minutes. 
Thus  it  is  seen  that  the  curves  obtained  after  heating  the 
deposit  were  not  exponential  but  had  a  continually  increasing 
period. 
Decay  Curve  of  the  Excited  Activity  from  Radi 
urn 
Although  19-2  and  19'3  minutes,  calculated  from  F  and  G 
(fig.  4),  are  the  lowest  values  obtained  for  the  period  of 
radium  C,  yet  _  even  in  these  cases  it  would  seem  probable 
that  all  of  radium  B  had  not  been  removed,  and  therefore 
that  the  true  period  of  C  was  not  over  19  minutes.  It  would 
seem  of  interest  to  check  the  above  value  of  the  period  of 
radium  C  by  comparing  the  experimental  decay  curve  with 
the  theoretical  curve,  calculated  on  the  assumption  that  the 
period  of  radium  C  is  nineteen  minutes.  The  period  of  the 
final  part  of  the  decay  curve  of  the  active  deposit  from  radium 
is  usually  taken  as  about  twenty- eight  minutes.  This  is  about 
the  correct  value  for  the  period  two  and  a  half  hours  after 
the  removal  of  the  active  deposit  from  the  presence  of  the 
emanation.  If,  however,  the  period  is  measured  after  five  or 
six  hours,  it  will  be  found  to  be  considerably  less. 
Fig.  6  shows  the  final  part  of  four  decay  curves.  The 
first  point  on  each  curve  is  more  than  four  and  a  half  hours 
after  the  removal  of  the  active  deposit  from  the  emanation  : 
yet  even  in  these  cases  the  curves  are  not  exactly  exponential. 
These  curves  give  26*1  minutes  as  the  mean"  value  of  the 
period  of  the  active  deposit  between  five  and  seven  hours 
after  its  removal  from  the  emanation.     From  this  it  can  be 
