a  Particles  of  Uranium  and  Tlio 
num. 
763 
A  very  small  quantity  of  radium  bromide  was  evaporated 
on  a  platinum  plate,  which  was  then  raised  to  a  bright  red 
heat  for  some  minutes.  This  freed  the  radium  from  ema- 
nation, Ra  A,  and  some  Ra  C.  The  ionization  current  due  to 
this  plate  fell  off  quickly;  and  in  three  hours  was  down  to 
half  its  first  value.  The  remaining  RaC  had  then  disappeared. 
It  was  reheated,  to  get  rid  of  any  fresh  emanation  ;  and  it  was 
then  assumed  that  the  great  majority  of  the  a  particles  emitted 
from  the  plate  were  due  to  radium  itself  (Bragg  &  Kleeman, 
Phil.  Mag.  Sept.  1905,  p.  324). 
The  following  table  shows  the  result  of  experiment  with 
the  plate  so  prepared.  The  aluminium-foils  were  the  same 
as  those  used  in  the  uranium  experiments.  The  curve  em- 
ployed was  of  course  not  the  same,  since  the  layer  was  thin. 
According  to  the  experiments  of  Kleeman  and  myself  such  a 
layer  may  be  considered  as  equivalent  to  5  mm.  of  air.  The 
results  were  therefore  calculated  from  curve  C. 
I. 
p^XlC6. 
II. 
i. 
III. 
i/l. 
IV". 
From 
Curve  C. 
V. 
pd  of  fall 
range  X 105. 
0 
1454 
1-000 
317 
1126 
•772 
•075 
423 
633 
927 
•634 
•143 
443 
949 
765 
•522 
•204 
473 
1265 
645 
•426 
•265 
477 
1617 
488 
•331 
•343 
472 
1933 
388 
•261 
•407 
476 
2613 
216 
•150 
•532 
473 
3289 
104 
•072 
•675 
488 
Tinfoil 
11 
0 
These  figures  show  an  agreement  between  calculation  and 
observation  which  is  nearly  as  good  as  in  the  case  of  uranium. 
There  is  indeed  a  gradual  increase  in  the  figures  of  the  last 
column;  but  such  an  effect  should  be  expected,  as  there  were 
present  some  a  particles  of  longer  range  than  those  of  radium, 
but  none  of  shorter  range.  Neglecting  this  effect,  the  mean 
of  the  results  of  the  last  column  is  466 ;  and  we  may  take 
this  to  express  the  product  of  the  thickness  and  density  of 
that  aluminium  sheet  which  the  a  particle  of  radium  can  just 
penetrate,  with  the  reservation  that  it  is  probably  too  high  by 
a  small  but  uncertain  amount,  the  error  being  caused  by  a 
slight  want  of  purity  in  the  material  used.     This  statement 
3D  2 
