Bary*  has  made  a  very  complete  examination  of  the  class  of
substances  which  become  luminous  under  radium  rays.  He  found
that  the  great  majority  of  substances  belongs  to  the  alkali  metals
and  alkaline  earths.  All  these  substances  were  also  phosphorescent
under  the  action  of  X  rays.
Crystalline  zinc  sulphide  (Sidot's  blende)  phosphoresces  very
brightly  under  the  influence  of  the  rays  from  radium  and  other
very  active  substances.  This  property  of  zinc  sulphide  was
initially  observed  by  Giesel  and  has  proved  invaluable  as  a  delicate
optical  means  of  comparing  quantitatively  the  activity  of  substances
emitting  a  rays.  It  is  an  especially  sensitive  means  of  detecting
the  presence  of  a  rays,  since  it  exhibits  the  "scintillating"  property
already  discussed  in  Section  60.  In  order  to  show  the  luminosity
due  to  the  a  rays,  the  screen  should  be  held  close  to  the  active
substance,  as  the  rays  are  absorbed  in  their  passage  through  a  few
centimetres  of  air.  Zinc  sulphide  is  also  luminous  under  the  action
of  the  /8  rays,  but  the  phosphorescence  is  far  more  persistent  than
vhen  produced  by  the  a  rays  and  does  not  show  obvious  scintil-
lations.
Very  brilliant  luminous  eflfects  can  be  produced  by  admitting
^  large  amount  of  radium  emanation  into  a  glass  tube  coated

•  Bary,  C.  R,  150,  p.  776,  1900.
298  PROPERTIES  OF  THE  RADIATIONS  [CH

internally  with  zinc  sulphide.  In  a  similar  way,  if  the  radium
emanation  is  concentrated  in  a  very  thin- walled  glass  tube  which
allows  the  a  rays  to  escape  with  little  absorption,  intense  lumin-
osity is  produced  on  a  zinc  sulphide  screen  brought  near  it.
