Rate  of  Decay  of  Active  Deposit  from  Radium.        153 
deposit,  and  that  the  periods   of  these   are  twenty-six  and 
nineteen  minutes  respectively. 
Time. 
Long  Exposure. 
■ 
Short  Exposure. 
Observed 
Calculated 
Observed 
Calculated 
Value. 
Value. 
Value. 
Value. 
50 
52-9 
531 
357 
361 
m 
434 
43-7 
349 
317 
70 
35-7 
357 
275 
2-73 
80 
28-9 
28-8 
231 
2-29 
100 
18-5 
18-5 
1-56 
1-54 
120 
11-6 
11:6 
1-01 
1-00 
140 
72 
72 
•64 
•63 
160 
44 
43 
•395 
•393 
180 
263 
263 
•241 
•242 
200 
1-59 
1-58 
•148 
•147 
220 
•95 
•95 
•089 
•089 
240 
•56 
•57 
•054 
•054 
260 
•33 
•33 
•032 
•032 
280 
•19 
•20 
•0186 
•0191 
The  early  part  of  the  curve,  which  is  especially  affected 
by  the  first  rapid  change  in  the  active  deposit,  is  still  under 
investigation.  When  this  is  completed  it  may  be  found 
necessary  to  take  a  value  for  the  period  of  radium  C  slightly 
smaller  than  nineteen  minutes,  in  order  to  make  the  theoretical 
agree  as  well  as  possible  with  the  experimental  curve.  In 
any  case,  the  above  close  agreement  between  the  experimental 
and  theoretical  curves,  added  to  the  separate  measurement  of 
both  periods,  would  seem  to  furnish  conclusive  evidence  that 
twenty-six  and  nineteen  minutes  are  approximately  the 
correct  values  for  the  periods  of  radium  B  and  0  respectively. 
Conclusions. 
The  above  experiments  would  seem  to  prove  : — 
(1)  That  temperatures  between  700°  and  1100°  0.  do  not 
permanently  affect  the  rate  of  decay  of  the  active  deposit 
from  radium. 
(2)  That  radium  B,  and  not  radium  C,  has  the  longer  decay 
period. 
(3)  That  the  previous  values  of  twenty-eight  and  twenty- 
one  minutes  are  both  too  large  for  the  decay  periods  of  radium 
B  and  C  respectively,  and  that  twenty-six  and  nineteen 
minutes  are  much  closer  to  the  true  values. 
In  conclusion,  I  take  pleasure  in  expressing  my  indebted- 
ness to  Professor  Rutherford  for  his  many  valuable  suggestions 
and  kind  supervision  of  this  work. 
Macdonald  Physics  Building, 
McGill  University,  Montreal,  Sept,  25,  1 905. 
