They  therefore  suggested  the  following  explanation  of  the  solar  spec-
trum. The  sun  is  probably  an  intensely  glowing  solid  or  liquid  mass
(chromosphere).  This  would  give  a  continuous  spectnun  without  either
bright  or  dark  lines.  But  the  glowing  mass  is  surrounded  by  an  at-
mosphere ( photosphere )  containing  cooler,  brilliant  metallic  vapors.
These  vapors  would  absorb  light  of  the  same  kind  as  they  emit,  and
hence  dark  lines  show  in  the  spectrum  in  the  same  position  as  those
occupied  by  the  ordinary  bright  lines  of  these  metallic  vapors.  This
theory  has  been  rendered  very  probable  by  experiments  in  which  the
bright  lines  of  elements  like  sodium  have  been  reversed  and  caused  to
appear  as  dark  lines  by  passing  the  light  from  a  very  hot  soditim  flame
through  a  cooler  flame  of  the  same  vapor.  We  can  also  increase  the
natural  absorption  by  causing  the  sun's  light  to  pass  through  more
sodium  vapor  placed  in  front  of  the  spectroscope  slit.
The  study  of  the  spectra  of  various  elements,  and  the  acciu^ate  mapping
of  the  dark  lines  of  the  solar  spectrum,  show  that  a  large  niunber  of  the
metals  known  to  us  on  the  earth  occur  as  vapors  in  the  sun's  atmosphere.
With  the  aid  of  this  method  of  spectrum  analysis  chemists  have  dis-
covered a  number  of  new  and  relatively  rare  metals,  such  as  caesium,
rubidium,  thallium,  indium,  gallium,  and  scandium.  It  has  also  furnished
an  invaluable  means  of  detecting  impurities  in  chemical  preparations
which  would  otherwise  escape  notice.
