Secondary  Rontgen  Radiation.  825 
0  .         .    _  .  ,  Percentage '  Absorption '  * 
Substance.  Atomic  W  eight.         by  .Q1   *m<  Alun)Hliuul. 
Aluminium    27"  1  35-5 
Sulphur     32-06  39 
Calcium     40-1  69 
Chromium     52'1  93-5 
Iron 55-9  89 
Nickel  55*7  81-5 
Cobalt 59-0  80 
Copper 63-6  73 
Zinc 65-4  (54-5 
Arsenic     75-0  43-5 
Selenium 794  39 
Silver    107'9  20 
Cadmium  ., 112-4  21-5 
Tin    118-5  36-8 
Antimony 120-0  68 
Iodine   . 127-0  50 
Tungsten 184-0  56 
Platinum 194*8  50 
Lead 206-9  40 
Bismuth     208-5  38'5 
Percentage  '  absorption  '  of  the  primary  was  about  33. 
*  As  measured  by  diminution  of  ionization  in  an  electroscope. 
secondary  beam,  is  not  that  which  gives  rise  to  the  most 
effective  constituent  in  another  metal;  so  that  the  ionizations 
produced  in  the  electroscope  receiving  the  secondary  beam 
are  not  strictly  due  to  the  same  primary  beam. 
The  absorption  o£  a  given  radiation,  however,  is  not  a 
periodic  function  of  the  atomic  weight*,  so  that  the  general 
features  of  the  curve  (see  fig.  1,  p.  820)  showing  the  relation 
between  percentage  diminution  of  the  ionization  produced  by 
the  secondary  beam  when  a  plate  of  aluminium  =01  cm.  thick 
^as  placed  in  its  path  to  the  electroscope,  and  the  atomic 
weight  of  the  substance  emitting  that  radiation,  are  no  less 
significant.  The  diminution  of  the  ionization  by  this  plate 
has  been  spoken  of  as  the  "  absorption,"  but  this  must  not 
be  taken  as  signifying  the  percentage  diminution  of  energy 
in  the  beam  traversing  the  plate. 
It  will  be  seen  that  as  far  as  these  experiments  have  gone, 
curves   showing  a   rise   and   fall    in   the   absorption   of    the 
*  What  has  been  proved  for  primary  radiation  from  an  X-ray  tube  is 
here  assumed  for  secondary  radiations,  viz.,  that  there  is  not  selective 
absorption,  the  absorption  by  a  given  mass  being  a  steadily  increasing 
junction  of  the  atomic  weight  of  the  absorbing  substance.  This  will  be 
thoroughly  tested  by  obtaining  curves  similar  to  that  shown  in  fig,  1, 
when  the  aluminium  absorbing  plate  is  replaced  by  other  metals. 
Phil.  Mag.  S.  6.  Vol.  11.  No.  66.  June  1906.         3  H 
