816  Dr.  0.  G.  Barkla  on 
made  of  iron  wire  in  the  form  of  a  bolometer-grating,  and 
was  exposed  to  primary  X-rays  just  as  sheets  of  metal  had 
been.  The  ionization  produced  in  the  electroscope  was  con- 
siderable on  account  of  the  great  absorbability  of  the  secondary 
rays.  A  current  was  then  passed  through  the  wire  grating 
till  it  became  almost  "  white  hot."  It  was  seen  that  though 
very  flexible,  it  was  no  longer  deflected  under  the  action  of 
a  strong  magnet  placed  near.  With  this  great  rise  of  tem- 
perature, consequent  increase  of  resistance  and  disappearance 
of  magnetic  property,  no  change  was  observed  in  the  ioni- 
zation produced  by  the  secondary  beam,  though  a  change  in 
intensity  or  absorbability  of  the  radiation  by  2  or  3  per  cent, 
would  have  been  detected. 
Thus  there  was  no  appreciable  direct  connexion  between  the 
character  of  the  radiation  and  the  temperature,  conductivity, 
or  magnetic  permeability  of  the  radiating  substance. 
The  observations  wrere  taken  in  the  order  shown  below,  the 
experiments  being  made  with  the  wire  alternately  hot  and 
cold. 
Reading-s  and  Deflexion 
of  Electroscope 
State  of  Radiator.  per  minute. 
HOt  {SI}*" 
CoJd         41-. 
68-7  '  *' 
•5j2S 
4:0'2    , 
Hot  41-5 
40-2"| 
70-1 J 
Selective  Absorption. 
It  has  been  stated  by  experimenters  that  the  radiation 
proceeding  from  a  heavy  metal  is  more  easily  absorbed  by 
that  metal  than  would  be  expected  by  comparing  the  absorb- 
abilities of  other  rays  in  different  substances;  that  is,  that  the 
rays  are  specially  absorbed  by  the  metal  from  which  they  are 
emitted.  If  this  were  so,  it  would  indicate  a  more  or  less 
definite  period  of  vibration  of  corpuscles  or  electrons  in  a 
given  substance  producing  a  radiation  having  some  of  the 
properties  of  Rontgen  radiation  ;  also  that  this  vibration  is 
set  up  by  the  passage  of  X-rays  through  the  substance. 
Experiments  were  made  to  verify  this  important  conclusion 
if  possible,  and  to  learn  the  extent  of  this  special  absorption. 
A  comparison  of  the  absorbability  of  the  secondary  rays 
from  lead  in  aluminium  and  in  tin  was  made.  (The  secondary 
rays   studied  passed  through  about  11  cm.  of  air  at  atmo- 
