'204  The  Chemical  Elements.  {^t^S^*- 
2.  When  we  use  metals  chemically  combined  with  a  metalloid — in  other  words, 
when  we  pass  from  a  metal  to  one  of  its  salts  (I  used  chlorine) — only  the  longest 
lines  of  the  metal  remain.  The  number  is  large  in  the  case  of  elements  of  low 
atomic  weight,  and  small  in  the  case  of  elements  of  high  atomic  weight,  and  of  twice 
the  atom-fixing  power  of  hydrogen. 
3.  When  we  use  metals  mechanically  mixed,  only  the  longest  lines  of  the  smallest 
constituent  remain.  On  this  point  I  must  enlarge  somewhat  by  referring  to  a  series 
of  experiments  recorded  in  the  "  Philosophical  Transactions,"  1873. 
A  quantity  of  the  larger  constituent,  generally  from  five  to  ten  grams,  was  weighed 
out,  the  weighing  being  accurate  to  the  fraction  of  a  milligram  ;  and  the  requisite 
quantity  of  the  smaller  constituent  was  calculated  to  give,  when  combined,  a  mix- 
ture of  a  definite  percentage  composition  by  weight  (this  being  more  easily  obtain- 
able than  a  percentage  composition  by  volume). 
The  quantities  generally  chosen  were  10,  5,  1  and  cri  per  cent. 
In  a  few  cases,  with  metals  known  to  have  very  delicate  spectral  reactions,  a  mix- 
lure  of  0  01  per  cent  was  prepared. 
Observations  were  then  made  of  the  spectrum  of  each  specimen,  and  the  result 
was  recorded  in  maps  in  the  following  manner:  First,  the  pure  spectrum  of  the 
smallest  constituent  was  observed,  and  the  lines  laid  down  from  Thalen's  map. 
The  series  thus  mapped  was  as  follows  : 
Tin    -f"  Cadmium,    percentages  of  Cd   10,  5,  1,  015 
Lead  -j-  Zinc  "  Zn   10,  5,  i,  o'i 
Lead  -j-  Magnesium,  "  Mg  :o,  1,  o'i,  0.01 
The  observations  showed  that  the  lines  of  the  smallest  constituent  disappeared  as 
the  quantity  got  less.  Although  we  had  here  the  germs  of  a  quantitative  spectium 
analysis,  the  germs  only  were  present,  because  from  the  existence  of  several  u  criti- 
cal points,"  and  great  variations  due  to  other  causes,  the  results  obtained  were  not 
constant. 
In  a  subsequent  research  on  the  gold-copper  alloys  used  in  the  coinage,  Mr. 
Roberts,  the  Chemist  of  the  Mint,  and  myself  were  able  to  show  that  the  shortening 
in  the  length  of  the  lines  by  reduced  quantity  was  such  a  definite  physical  effect 
following  upon  reduced  quantity,  that  a  difference  of  jq^qq  part  of  copper  in  gold 
could  be  detected. 
We  are  now  in  possession  of  the  facts  utilized  in  the  work  which  has  led  up  to 
the  subject  discussed  in  the  present  paper. 
They  have  been  utilized  along  two  perfectly  distinct  lines  of  thought : 
(1  )  They  have  been  used  in  an  attempt  to  enable  us  to  produce  a  spectrum  of  a 
substance  free  from  lines  due  to  the  impurities  which  are  almost  always  present. 
(2)  They  have  been  used  to  indicate  the  existence  and  amount  of  dissociation 
when  acknowledged  compounds  have  been  submitted  to -the  action  of  different  and 
increasing  temperatures. 
I  will  deal  with  (1)  first. 
The  elimination  of  impurity  lines  is  conducted  as  follows:  The  spectrum  of  the 
element  is  first  confronted  with  the  spectra  of  the  substances  most  likely  to  be  pres- 
