AmAp0rnr;i?7h9am'}  The  Chemical  Elements.  205 
cnt  to  impurities.  This  is  most  conveniently  done  by  photographing  the  spectra 
on  the  same  plate  one  above  the  other,  so  that  common  lines  are  continuous. 
The  retention  or  rejection  of  lines  coincident  in  two  or  more  spectra  is  deter- 
mined by  observing  in  which  spectrum  the  line  is  thickest;  where  several  elements 
are  mapped  at  once,  all  their  spectra  are  confronted  on  the  same  plate,  as  by  this 
means  the  presence  of  one  of  the  substances  as  an  impurity  in  the  others  can  be  at 
once  detected. 
Lines  due  to  impurities,  if  any  are  thus  traced,  are  marked  for  omission  from  the 
map  and  their  true  sources  recorded,  while  any  line  that  is  observed  to  vary  in  length 
and  thickness  in  the  various  photographs  is  at  once  suspected  to  be  an  impurity  line,, 
and  if  traced  to  such  is  likewise  marked  for  omission.    I  give  a  case. 
The  two  lines  H  and  K  (3933  and  3968),  assigned  both  to  iron  and  calcium  by 
Angstrom,  are  proved  to  belong  to  calcium  in  the  following  way : 
a.  The  lines  are  well  represented  in  the  spectrum  of  commercial  wrought  iron, 
but  are  absolutely  coincident  with  two  thick  lines  in  the  spectrum  of  calcium  chlor- 
ide with  which  the  iron  spectrum  was  confronted. 
b.  The  lines  are  represented  by  mere  traces  in  the  spectrum  of  a  specimen  of  pure 
iron  prepared  by  the  late  Dr.  Matthiessen.  Both  poles  of  the  lamp  were  of  iron,  the 
lower  pole  consisting  of  an  ingot  of  the  metal  which  had  been  cast  in  a  lime-mold. 
c  The  lines  are  altogether  absent  in  a  photograph  of  pure  iron,  where  both  poles 
of  the  lamp  were  of  pure  metal  not  cast  in  lime,  and  they  are  likewise  absent  in  a 
photograph  of  the  spectrum  of  the  Lenarto  meteorite. 
By  eliminating  lines  due  to  impurities  in  the  manner  just  described,  a  spectrum  is 
at  length  obtained,  of  which  every  line  is  assignable  to  the  particular  element  photo  - 
graphed, the  same  temperature  being  employed  in  the  case  of  all  the  elements 
observed. 
With  regard  to  the  second  line  of  work,  I  should  commence  by  stating  that  from 
a  beautiful  series  of  researches  carried  on  by  several  methods,  Mitscherlich  concluded 
in  1864  that  every  compound  oj  the  first  order,  heated  to  a  temperature  adequate  for  the 
production  of  light,  is  not  decomposed,  but  exhibits  a  spectrum  peculiar  to  this  compound. 
In  some  experiments  of  my  own,  communicated  to  the  Royal  Society  in  1873,  I 
observed : 
First.  That  whether  the  spectra  of  iodides,  bromides,  etc.,  be  observed  in  the  flame 
or  a  weak  spark,  only  the  longest  lines  of  the  metals  are  visible,  showing  that  only  a 
small  quantity  of  the  simple  metal  is  present  as  a  result  of  partial  dissociation,  and 
that  by  increasing  the  temperature,  and  consequently  the  amount  of  dissociation,  the 
other  lines  of  the  metal  appear  in  the  order  of  their  length  vjith  each  rise  of  temperature. 
Secondly.  I  convinced  myself  that  while  in  air,  after  the  first  application  of  heat,  the 
spectra  and  metallic  lines  are  in  the  main  the  same,  in  hydrogen  the  spectra  are  different 
for  each  compound,  and  true  metallic  lines  are  represented  according  to  the  volatibility  of 
the  compound,  only  the  very  longest  lines  being  visible  in  the  spectrum  of  the  least  vol- 
atile  compound. 
Thirdly.  I  found  that  with  a  considerable  elevation  of  temperature  the  spectrum 
of  the  compound  faded  almost  into  invisibility. 
These  results  enable  us  to  make  the  following  statement  : 
