Certain  elements,  such  as  gold  and  radium,  iodine,  sulphur,
and  phosphorus,  chlorine  and  nitrogen,  have  not  been
detected  in  the  Sun.  But  this  does  not  prove  that  they  are
certainly  absent,  as  their  level  in  the  solar  atmosphere,  or
the  low  degree  of  their  absorptive  effects  might  prevent
them  from  being  represented.  On  the  other  hand,  various
substances,  not  yet  found  on  the  Earth,  are  shown  by  many
unidentified  lines  of  the  solar  spectrum  to  be  present  in  the
Sun.  Some,  if  not  all,  of  these,  will  probably  be  discovered
by  chemists,  just  as  helium  was  found  by  Ramsay  in  cleveite
(p.  78).  Rowland  remarked  that  if  the  Earth  were  heated
to  a  sufficiently  high  temperature,  it  would  give  a  spectrum
closely  resembling  that  of  the  Sun.
