218 
Mr.  1ST.  Li.  Campbell  on  tit 
In  Table  III.  the  same  quantities  are  given  as  calculated 
from  the  earliest  series  of  experiments.  The  arbitrary  units 
in  which  these  results  were  expressed  previously  have  been 
converted  into  absolute  units  by  the  measurement  o£  the  unit 
o£  capacity  employed  there.  As  this  was  represented  by  the 
capacity  of  a  lead  cylinder  with  a  central  wire,  which  had 
been  lying  about  for  a  year  after  it  was  used  and  before  it 
was  measured,  it  would  not  be  surprising  if  a  considerable 
error  should  attach  to  the  absolute  values  in  the  table. 
Table  III. 
Material. 
„ 
Y. 
V. 
Lead 
Aluminium    
Tin  (tinfoil)  
247 
64 
13S 
0 
30 
0               20-7 
0               205 
74                 5-9 
59               215 
21               177 
1 
12-3 
149 
3-1 
7-4 
9-2 
9 
Zinc    
§  12.  An  attempt  was  also  made  to  measure  more  directly 
the  penetration  of  the  rays  in  a  manner  similar  to  that 
described  on  p.  539  of  the  former  paper.  It  was  found  that 
no  great  accuracy  could  be  attained  by  this  method,  even 
with  the  improved  apparatus  ;  all  the  quantities  examined 
can  be  determined  with  greater  certainty  from  the  curves  *. 
A  box  was  constructed  42  x  42  x  10  cms,,  with  a  hinged  lid 
42  x  42  x  8  cms.  Covering  the  top  of  the  box  was  a  grid  of 
thick  brass  bars  leaving  nine  apertures  12  cms.  square  ;  these 
were  covered  with  aluminium-foil  '000445  cm.  thick.  Thus, 
when  the  lid  of  the  box  was  closed,  the  volume  inside  was 
divided  by  the  aluminium-foil  into  two  compartments.  The 
wralls  of  the  box  and  the  lid  were  of  wood  covered  with 
aluminium  ;  through  the  wall  of  the  box  passed  a  rod  bearing 
an  electrode  insulated  from  the  walls  by  a  guard-ring  in  the 
usual  manner.  By  keeping  a  large  difference  of  potential 
between  the  walls  and  the  electrode,  the  ionization  in  the 
volume  below  the  aluminium  window  could  be  measured. 
The  lid  prevented  the  deposition  of  excited  radioactivity  from 
the  outside  air  upon  the  window. 
*  The  good  agreement  shown  in  Table  III.  (p.  543)  of  the  former 
paper  seems  to  have  been  quite  accidental :  the  values  given  there  are 
not  in  accordance  with  later  results. 
