592 
Mr.  A.  3.  Eve  on  the  Absorption  of 
When  a  thickness  of  2 '85  mm.  o£  lead  was  reached  a  fairly 
well-marked  change  occurred,  and  A,  became  equal  to  2'7, 
and  this  value  was  maintained  to  a  thickness  of  4*35  mm., 
or  even  to  5*7  mm.  Beyond  that  the  effects  were  small  and 
very  difficult  to  measure,  but  the  mean  of  repeated  observa- 
tions indicated  that  X  was  equal  to  about  2  between  5*7  and 
8*7  mm. 
The  results  are  shown  in  fig.  2*,  but  it  may  be  convenient 
to  state  them  in  tabular  form  also  : — 
Thickness  in  mm.              |          <  ,         .    . 
Zinc.                 Lead.                    X  (cms.)-l. 
•45         +              0 
10*5 
+           -45 
4-1 
4-        2-85 
2-7 
20? 
+        5-7 
+         8-7 
It  appears,  then,  that  actinium  has  two  types  of  ft  rays 
and  one  or  two  of  y  rays.  The  two  types  of  ft  rays  are  both 
homogeneous  and  easily,  but  not  equally,  absorbed.  It  is,  of 
course,  possible  that  there  is  one  type  of  ft  rays,  and  that  all 
the  rest  are  7  rays.  The  question  can  only  be  settled  by 
photographs  of  actinium  rays  deflected  in  a  magnetic  field. 
This  work  is  under  investigation  by  Dr.  Godlewski. 
It  is  noteworthy  that  both  uranium  and  actinium  have 
homogeneous  ft  rays,  and  that  these  substances  give  rise  to 
easily  absorbed  y  rays.  But  thorium  and  radium  have  hetero- 
geneous ft  rays,  and  the  resulting  y  rajs  are  much  more 
penetrating.  We  may  fairly  conclude  that  the  high  velocity 
ft  particles  give  rise  to  the  more  penetrating  y  rays^  and  that 
low  velocity  ft  rays  generate  easily  absorbed  y  rays.  And 
we  have  here  a  striking  similarity  with  the  generation  of 
Bontgen  rays  by  the  cathode  rays.  For  in  a  "  hard  "  bulb, 
with  a  good  vacuum,  the  cathode  rays  have  a  high  velocity, 
and  when  these  are  abruptly  stopped  a  penetrating  type  of 
Bontgen  rays  arises.  But  in  a  "  soft  "  bulb,  the  low  velocity 
cathode  rays  generate  easily  absorbed  Bontgen  rays.  So 
also  the  abrupt   expulsion   of   high  velocity  ft  rays  causes 
*  It  was  not  possible  to  exhibit  all  the  results  of  this  paper  in  a  single 
diagram.  In  fig.  1  radium  and  thorium  are  compared  with  uranium, 
and  in  fig.  2  actinium  and  uranium  are  contrasted.  The  horizontal 
scales  in  the  two  diagrams  are  not  the  same. 
