Am.  Jour.  Pharir . ) 
April,  1879.  / 
The  Chemical  Elements. 
203 
There  is  a  simple  experiment  which  shows  clearly  the  different  results  obtained. 
If  we  observe  the  light  of  a  candle  with  the  spectroscope  in  the  ordinary  manner, 
that  is,  by  placing  the  candle  in  front  of  the  slit  at  some  little  distanee  from  it,  we 
see  a  band  of  color — a  continuous  spectrum — and  in  one  particular  part  of  the  band 
we  see  a  yellow  line,  and  occasionally  in  the  green  and  in  the  blue  parts  of  the  band 
other  lines  are  observable.  Now,  if  we  throw  an  image  of  the  candle  on  to  the 
slit — the  slit  being  hoiizontal  and  the  image  of  the  candle  vertical — we  then  get 
three  perfectly  distinct  spectra.  We  find  that  the  interior  of  the  candle,  that  is  the 
blue  part  (best  observed  at  the  bottom  of  the  candle),  gives  us  one  spectrum,  the 
white  part  gives  us  another,  while  on  rhe  outside  of  the  candle,  so  faint  as  to  be 
almost  invisible  to  the  eye,  there  is  a  region  which  gives  as  a  perfectly  distinct  spec- 
trum with  a  line  in  the  yellow.  In  this  way  there  is  no  difficulty  whatever  in  deter- 
mining the  co-existence  of  three  light-sources,  each  with  its  proper  spectrum,  in  the 
light  of  a  common  candle. 
We  see  tin  a  moment  that  much  tl  e  same  condition  of  affairs  will  be  brought 
ahout  if,  instead  of  using  a  candle,  we  use  an  electric  arc,  in  which  the  pure  vapor 
of  the  substance  which  is  being  rendered  incandescent  fills  the  whole  interval 
between  the  poles,  the  number  of  particles  and  degree  of  incandescence  being 
smaller  at  the  sides  of  the  arc.  We  can  throw  an  image  of  such  an  horizontal  arc 
on  a  vertical  slit,-  the  slit  will  give  then  the  spectrum  of  a  section  of  the  arc  at 
right  angles  to  its  length.  The  vapor  which  exists  farthest  from  the  core  of  the  arc 
has  a  much  more  simple  spectium  than  that  of  the  core  of  the  arc  itself.  The 
spectrum  of  the  core  consists  of  a  large  number  of  lines,  all  of  which  die  out  untii 
the  part  of  it  farthest  from  the  centre  gives  but  one  line. 
In  this  way  the  spectrum  of  each  substance  furnishes  us  with  long  and  short  lines^ 
the  long  lines  being  common  to  the  more  and  less  intensely  heated  parts  of  the  arc,, 
and  the  short  lines  more  special  to  the  more  heated  one.    This  is  the  first  step. 
It  has  been  necessary  to  enter  thus  at  length  into  the  origin  of  the  terms  long  and 
short  lines,  because  almost  all  the  subsequent  work  which  need  be  referred  to  now 
has  had  for  its  object  the  investigation  of  the  phenomena  presented  by  them  under 
different  conditions.    The  first  results  obtained  were  as  follows  : 
1.  When  a  metallic  vapor  was  subjected  to  admixture  with  another  gas  or  vapor,, 
or  to  reduced  pressure,  I  found  that  its  spectrum  became  simplified  by  the  abstraction 
of  the  shortest  lines  and  by  the  thinning  of  many  of  the  remaining  ones.  To  obtain 
reduction  of  pressure,  the  metals  were  inclosed  in  tubes  in  which  a  partial  vacuum 
was  produced.  In  all  these  experiments  it  -vas  found  that  the  longest  lines  invariably 
remained  -visible  longest} 
1  In  the  case  of  zinc  the  effect  of  these  circumstances  was  very  marked,  and  they  may  be  given  as  at 
sample  of  the  phenomena  generally  observed.  When  the  .pressure-gauge  connected  with  a  Sprergel 
pump  stood  at  from  35  to  40  millimeters,  the  spectrum  at  the  part  observed  was  normal,  except  that  the  two 
lines  4924  and  4911  (both  of  which,  when  the  spectrum  is  observed  under  the  normal  pressure,  are  lines- 
with  thick  wings)  were  considerably  reduced  in  width  On  the  pump  being  started  these  lines  rapidly 
decreased  in  length,  as  did  the  line  at  4679 — 4810  and  4721  being  almost  unaffected  ;  at  last  the  two  314924. 
and  491 1  vanished,  as  did  4679  and  appeared  only  at  intervals  as  spots  on  the  poles,  the  two  4810  and  4721 
remaining  little  changed  in  length,  though  much  in  brilliancy.  This  experiment  was  repeated  four  times , 
and  on  each  occasion  the  gauge  was  found  to  be  almost  at  the  same  point,  viz  : 
1st  observation,  when  the  lines  4924  and  491 1  were  gone,  the  gauge  stood  at  30  millimeters. 
it 
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