Marsden  {loc.  cit.)  made  a  detailed  examination  of  the  duration
of  the  luminosity  in  zinc  sulphide  and  willemite  after  exposure
to  ff  and  7  rays.  On  exposing  zinc  sulphide  to  a  constant  source
of  fi  rays,  the  luminosity  increased  for  about  20  minutes.  On
removing  the  source  of  y9  rays,  the  luminosity  sank  more  gradually
than  it  had  risen.  Willemite  showed  different  effects.  The
uminosity  due  to  fi  rays  rose  rapidly  to  a  maximum  in  a  few
ninutes,  while,  on  removal  of  the  yS  rays,  the  luminosity  sjink  at
»nce  to  a  small  fraction  of  its  value.  There  was  then  a  slow  decay
0  zero.
*  Stefan  Meyer,  Phyt.  ZeiL  10,  p.  iS3,  1909.
302  PROPERTIES  OF  THE  RADIATIONS  [CH.
107.  laomiiioflity  of  radium  compounds.  All  radiom
impounds  are  spontaneously  luminous.  This  luminosity  is  es-
pecially brilliant  in  the  dry  haloid  salts,  and  persists  for  long
intervals  of  time.  In  damp  air  the  salts  lose  a  large  amount  of
their  luminosity,  but  they  recover  it  on  drying.  With  very  active
radium  chloride,  the  Curies  have  observed  that  the  light  changes
in  colour  and  intensity  with  time.  The  original  luminosity  is
recovered  if  the  salt  is  recrystallized.  The  luminosity  of  radium
persists  over  a  wide  range  of  temperature  and  is  as  bright
at  the  temperature  of  liquid  air  as  at  ordinary  temperatures^
A  slight  luminosity  is  observed  in  a  solution  of  radium,  and  if
crystals  are  being  formed  in  the  solution,  they  can  be  clearly
distinguished  in  the  liquid  by  their  greater  brightness.
