It  must  be  remembered  that  under  ordinary  circumstances
the  air  spectrum  lines  will  be  obtained  mixed  with  the  lines  of
the  metallic  spectrum  when  the  spark  passes  between  metal
poles  in  air.  The  relative  brilliancy  of  the  air  and  metallic
lines  obtained  in  this  way  depends  entirely  upon  the  volatility
of  the  metal ;  with  platinum  poles  the  air  lines  are  very  much
stronger  in  relation  to  the  metallic  lines  than  they  are  in  the
case  of  zinc  and  such  metals.  It  is  necessary,  of  course,  to
eliminate  these  lines  in  working  with  the  spark  spectra,  and  this
may  be  done  very  readily  by  taking  a  photograph  of  the
platinum  or  silver  spark  spectrum  (when  the  air  lines  are  strong)
and  comparing  it  with  the  photographs  taken  of  the  metal  in
question ;  the  lines  common  to  the  two  will  be  the  air  lines,
and  these   can   thus  be  eliminated.     In  the  measurement  of

'  Comptes  Rendtis,  65.  1097  (1867);  66.  121  (1868).
■>  Ibid.,  81.  766  (1875).
»  Phil.  Trans.,  175.  49  (1S84).
T.  P.  C.  2D


402


SPECTROSCOPY


wave-lengths  the  air  lines  often  prove  of  considerable  advan-
tage ;  their  wave-lengths  have  been  accurately  determined  by
several  observers,  and  thus  they  may  be  used  as  test  lines  in


Fig.  148.
Fig.  149.
the  comparison  of  the  standard  spectrum,  and  the  spectrum
to  be  measured ;  the  air  lines  are  picked  out  and  their  wave-
lengths determined  by  measurements  from  the  standard
spectrum,  and  if  they  are  found  to  be  correct,  then  one  can


METHODS   OF  ILLUMINATION  403

assume  that  the  two  spectra  are  correctly  placed  in  relation  to
one  another  l^ide  p.  148).
Two  simple  pieces  of  apparatus  have  been  designed  by
Krulla^  for  obtaining  the  spark  spectra  of  salts  in  solution.
