Rate  of  Decay  of  Active  Deposit  from  Radium.        147 
Is   that   temperatures    between   700°  and   1100°   C.    do  not 
permanently  alter  the  rate  of  decay  of  the  active  deposit. 
Miss  Gates  *  showed  that  when  a  platinum  wire  covered 
with  the  active  deposit  from  radium  is  raised  to  a  white  heat, 
the  active  matter  is  removed  from  the  platinum  wire  and 
deposited  on  cooler  bodies  in  the  neighbourhood.  Curie  and 
Danne  still  further  extended  our  knowledge  by  showing  that 
the  constituents  of  the  active  deposit  were  not  equally  volatile. 
In  addition,  they  also  found  that  the  rate  of  decay  had 
apparently  been  permanently  altered  by  the  high  temperature. 
The  following  table  gives  some  of  their  results  :— 
t. 
630° 
830 
1000 
1100 
1250 
1300 
o. 
29-3 
24-6 
21-0 
20-3 
24-1 
25-4 
Here  t  is  the  temperature  in  degrees  centigrade  to  which 
the  active  deposit  was  raised,  and  6  is  the  "  period,'''  that  is 
the  time  in  minutes  required  for  the  activity  to  fall  to  half 
value.  Curie  and  Danne  also  state  that  the  curves  were  all 
exponential.  They  conclude  from  this  that  the  rate  of  decay 
has  been  permanently  altered,  and  that,  as  the  temperature 
is  raised,  the  period  reaches  a  minimum,  at  about  1100°  C, 
and  then  begins  to  increase  again.  It  was  not  unnatural  to 
expect  that  high  temperatures  should  decrease  the  period  of 
the  active  deposit,  but  it  seemed  very  remarkable  that  the 
period  should  reach  a  minimum  and  then  increase  again 
when  still  higher  temperatures  were  used. 
Results  of  the  present  Research. 
It  seemed  possible  that  all  the  results  of  Curie  and  Danne 
might  be  accounted  for  by  the  difference  in  volatility  of  the 
constituents  of  the  active  deposit.  In  order  to  see  whether  this 
were  the  case,  a  copper  wire  on  which  the  active  matter  had 
been  deposited  was  sealed  in  a  piece  of  glass  combustion- 
tubing,  which  entirely  prevented  the  escape  of  any  volatile 
products.  By  exhausting  the  tube  before  sealing,  it  was 
found  that  it  would  stand  temperatures  high  enough  to  melt 
the  copper  wire  inside,  that  is,  temperatures  of  at  least 
1100°  C. 
In  all  cases  where  high  temperatures  were  employed  the 
*  Physical  Review.  May  1903. 
L2 
