814  Dr.  C.  G.  Barkla  on 
These  results  furnished  data  and  suggested  methods  for 
investigating  the  complex  secondary  radiations  proceeding 
from  metals  subject  to  X-rays  more  completely  than  had 
previously  been  attempted. 
The  experiments  described  below  were  made  on  the  more 
penetrating  radiations,  that  is  the  radiations  which  had  passed 
through  several  centimetres  of  air  under  ordinary  atmospheric 
conditions  and  very  thin  aluminium  leaf. 
Density ,  Molecular  Weight,  Atomic  Weight  of  Radiator. 
Preliminary  experiments  on  the  absorbability  of  the 
secondary  radiation  proceeding  from  different  substances 
when  subject  to  X-rays  were  made  in  the  manner  described 
in  previous  papers.  It  was  seen  that  in  general  light 
substances  emitted  radiations  differing  very  little  in  this 
respect  from  the  primary  producing  them,  while  from  heavy 
substances  the  radiation  was  more  easily  absorbed.  Whether 
the  character  depended  on  density,  molecular  weight,  or 
atomic  weight  had  to  be  investigated. 
The  gases  hydrogen,  air,  sulphuretted  hydrogen,  carbon 
dioxide,  and  sulphur  dioxide  had  all  been  found  to  emit 
radiations  closely  resembling  the  primary.  Carbon,  paper, 
wood,  and  even  aluminium  and  sulphur,  were  found  to  emit 
radiations  differing  comparatively  little  from  the  radiation 
producing  them;  while  calcium,  iron,  copper,  zinc,  tin, 
platinum,  and  lead  emitted  radiations  of  considerably  less 
penetrating  power  than  the  primary  producing  them. 
It  should  be  observed  that  this  is  only  a  general  way  of 
classifying  the  radiations  emitted  by  different  substances;  for 
upon  close  examination  it  was  concluded  that  in  all  cases 
there  was  a  difference  between  the  primary  and  secondary 
rays.  The  difference  was,  however,  so  small  in  many  cases — 
including  those  in  which  aluminium  and  sulphur  were  the 
radiators — in  comparison  with  that  when  one  of  the  second 
class  of  substances  was  experimented  upon,  that  these  sub- 
stances may  be  conveniently  spoken  of  as  scattering  and 
transforming  the  radiations  *. 
The  compounds  ammonium  carbonate,  lime,  calcium 
carbonate,  and  copper  sulphate  were  afterwards  tested. 
Ammonium  carbonate  was  found  to  emit  a  radiation  closely 
resembling  the  primary,  and  the  others  radiations  much  more 
absorbable  than  the  primary.  Thus  ammonium  carbonate,  a 
substance  of  greater  molecular  weight  than  calcium  or  lime, 
belonged  to  what  may  be  called  the  scattering  class,  while 
*  Further  reason  for  this  classification  is  given  later. 
