no6  The  Chemical  Elements.  {^m'^;S^rm' 
A  compound  body,  such  as  a  salt  of  calcium,  has  as  definite  a  spectrum  as  that 
given  by  the  so-called  elements;  but  while  the  spectrum  of  the  metallic  element 
Itself  consists  of  lines,  the  number  and  thickness  of  some  of  which  increase  with 
the  increased  quantity,  the  spectrum  of  the  compound  consists  in  the  main  of  chan- 
neled spaces  and  bands,  which  increase  in  like  manner. 
In  short,  the  molecules  of  a  simple  body  and  a  compound  one  are  affected  in  the 
same  manner  by  quantity  in  so  far  as  their  spectra  are  concerned  ;  in  other  words, 
both  spectra  have  their  long  and  short  lines,  the  lines  in  the  spectrum  of  the  element 
being  represented  by  bands  or  fluted  lines  in  the  spectrum  of  the  compound;  and 
in  each  case  the  greatest  simplicity  of  the  spectrum  depends  upon  the  smallest  quan- 
tity, and  the  greatest  complexity  upon  the  greatest. 
The  heat  required  to  act  upon  such  a  compound  as  a  salt  of  calcium,  so  as  to 
render  its  spectrum  visible,  dissociates  the  compound  according  to  its  volatility;  the 
number  of  true  metallic  lines  which  thus  appear  is  a  measure  of  the  quantity  of  the 
metal  resulting  from  the  dissociation,  and  as  the  metal  lines  increase  in  number,  the 
compound  bands  thin  out. 
These  results  bring  us  face  to  face  with  the  subject  matter  of  the  recent  work. 
First  with  regard  to  impurity  elimination.  I  find  that,  although  the  method  is 
good  for  detecting  and  eliminating  impurities,  there  are  still  short-line  coincidences 
between  metals  which  are  pure. 
This  "higher  law"  has  come  out  in  the  following  manner: 
For  the  last  four  years  I  have  been  engaged  upon  the  preparation  of  a  map  of  the 
solar  spectrum  on  a  large  scale,  the  work  including  a  comparison  of  the  Fraunhofer 
lines  with  those  visible  in  the  spectrum  of  the  vapor  of  each  of  the  metallic  elements 
in  the  electric  arc. 
To  give  an  idea  of  the  thoroughness  of  the  work,  at  all  events  in  intention,  I  may 
state  that  the  complete  spectrum  of  the  sun,  on  the  scale  of  the  working  map,  will 
be  half  a  furlong  long;  that  to  map  the  metallic  lines  and  purify  the  spectra  in  the 
manner  described,  more  than  100,000  observations  have  been  made  and  about  2,000 
photographs  taken. 
In  some  of  these  photographs  we  have  vapors  compared  with  the  sun ;  in  others 
vapors  compared  with  each  other  ;  and  others  again  have  been  taken  to  show  which 
lines  are  long  and  which  short  in  the  spectra. 
A  rigorous  application  of  the  system  of  impurity  elimination  formed,  of  course, 
a  large  part  of  the  work. 
The  final  reduction  of  the  photographs  of  all  the  metallic  elements  in  the  region 
39 — 40 — a  reduction  I  began  in  the  early  part  of  last  year — summarized  all  the 
observations  of  metallic  spectra  compared  with  the  Fraunhofer  lines  accumulated 
during  the  whole  period  of  observation,  and  all  the  results  of  the  impurity  elimi- 
nation. 
Now  this  reduction  has  shown  me  that  the  hypothesis  that  identical  lines  in  differ- 
ent spectra  are  due  to  impurities  is  not  sufficient.  I  show  in  detail  in  the  paper  the 
hopeless  confusion  in  which  I  have  been  landed. 
I  find  short-line  coincidences  between  many  metals  the  impurities  of  which  have  been 
t 
