762 
Prof.  W.  H.  Bragg  on  the 
I. 
pdxlO6. 
II. 
i. 
III. 
i/L 
IV. 
From 
Curve  A. 
V. 
pd  of  full 
range  XlO5. 
0 
1044 
1-000 
317 
811 
•768 
•0705 
448 
633 
635 
•595 
•139 
462 
949 
494 
•456 
•205 
463 
1265 
376 
•339 
•275 
460 
1620 
275 
•239 
•353 
458 
1930 
201 
•165 
•425 
455 
2610 
97 
•062 
•580 
449 
3290 
+2  layers 
of  tinfoil. 
55 
34 
•021 
0 
•718 
458 
considered  as  so  many  ordinates  of  the  thick-layer  curve  A ; 
and  the  corresponding  abscissae  found  and  placed  in  the 
fourth  column.  It  was  then  possible  to  obtain  from  each 
reading  a  determination  of  the  pd  of  that  aluminium  sheet 
which  the  a  particle  of  Ur  could  just  penetrate.  For 
example,  the  table  shows  that  the  sheet  for  which  pd= '000949 
is  20*5  per  cent,  of  the  necessary  thickness,  and  so  tbe  pd  of 
the  fully  intercepting  sheet  is  '000949/'205  = '00463.  All 
these  separate  determinations  are  shown  in  the  fifth  column. 
Their  close  agreement  with  each  other  is  good  evidence  of 
the  truth  of  the  formula  used  in  obtaining  them. 
Taking  '00456  as  the  mean  of  the  figures  in  the  last  column, 
the  final  result  may  be  thus  stated: — 
The  a  particle  of  uranium  can  just  penetrate  a  sheet  of 
aluminium  for  which  pd  =  '00456. 
Although  this  may  at  once  be  interpreted  so  as  to  give  the 
range  of  the  a  particle  in  air,  yet  it  is  better  to  measure  the 
ranges  of  radium  and  thorium  also  in  terms  of  aluminium-foil. 
Radium. 
The  application  of  this  method  to  thorium  and  radium  is 
somewhat  more  difficult  than  in  the  case  of  uranium,  since  it 
is  necessary  for  success  that  the  radioactive  material  should 
issue  a  particles  of  one  range  only.  It  might  no  doubt  be 
possible  to  allow  for  tbe  existence  of  other  a  particles  if  their 
range  and  relative  number  were  known.  Some  method  of 
this  kind  must  doubtless  be  used  for  ThX,  and  Th  emanation. 
But  it  is  obviously  more  direct,  in  the  case  of  radium  and 
thorium,  to  deal  with  the  pure  materials,  if  possible. 
