590 
Mr.  A.  S.  Eve  on  the  Absorption  of 
It  will  be  convenient  to  state  here  the  actual  figures 
obtained  for  the  substances  which  have  been  considered  up 
to  the  present  point.  The  differences  in  the  observed 
intensities  are  probably  within  the  limits  of  experimental 
error,  as  it  is  difficult  to  place  such  different  substances  under 
precisely  similar  conditions  with  respect  to  the  electroscope. 
7  rays. 
Total  thickness 
of  lead  in  cm. 
Intensities. 
Radium. 
Uraninite. 
Thorium 
nitrate. 
Eadio- 
thorium. 
•64 
1-21 
1-79 
2-36 
300 
100 
72 
52 
40 
29 
100 
73 
53 
41 
30 
100 
72 
52 
40 
31 
100 
75 
55 
42 
31 
Uranium. — The  7  ray  effect  of  uranium  was  found  to  be 
surprisingly  small.  A  kilogram  of  thorium  nitrate  gave 
ten  times  the  effect  of  a  kilogram  of  uranium  nitrate, 
measured  through  *64  cm.  of  lead.  The  7  rays  of  uranium 
were  so  readily  absorbed,  that  it  was  not  possible  to  measure 
X  over  a  wide  range,  or  with  great  accuracy.  The  mean  of 
repeated  observations  between  '28  and  "57  cm.,  and  also 
between  '64  and  *92  cm.,  gave  X  =  1'4  ;  so  that  the  7  rays, 
like  the  /3  rays  of  this  substance,  appear  to  be  homogeneous. 
A  specimen  of  pure  uranium  gave  \=146  approximately. 
In  the  case  of  uranium  nitrate  the  loss  of  7  activity  by 
self-absorption  must  be  large. 
The  ratio  of  the  activities  of  uranium  nitrate  and  thorium 
nitrate  was  determined  by  measurement  of  the  «  and  /3  ray 
effect  from  12  grams  of  each  substance. 
a  ray  0'6, 
0  ray  6*0, 
whilst  for  a  kiloo-ram  of  each, 
7  ray  0*1. 
It  will  thus  be  seen  that  these  methods  offer  no  certain 
test  for  comparing  the  total  activities  of  uranium  and  thorium 
or  of  uranium  and  radium,  and  the  same  difficulty  will  be 
found  in  the  case  of  actinium.      On    the    other   hand,  the 
