Whether  the  expelled  particles  are  original  constituents  of  the  atom,  or
incident  particles  absorbed  by  the  atom  and  subsequently  expelled,  does  not
really  amount  to  any  essential  difference.  It  is  true  that  from  the  latter
point  of  view  we  might,  in  a  sense,  regard  the  secondary  particles  as  scattered
primary  particles,  but  scattering  in  this  special  sense  would,  as  just  stated,  be
practically  identical  with  the  explanation  advanced  in  this  and  the  previous
papers.
516


On  Scmidium*

By  Sir  William  Cuookes,  D,Sc.,  F.B.S.
(Revived  Marct  4,— Read  April  30,  1908.)

(Abstract)

Scandia  is  one  of  the  rarest  aixd  least  known  of  the  recognised  t^xe  earths.
It  was  discovered  in  1879  by  Nilson,  who  separated  it^  together  with
yttei'bia,  from  erbia  extracted  from  euxenite  and  gadolinite.  Later  in  the
aame  year  Clave  extracted  scandia  from  gadolinite,  yttrotitanite,  and  keilhauite,
and  deacribed  the  scandium  sulphate,  double  sulphates,  nitrate,  oxalate,  double
oxalates,  selenate,  acetate,  formate,  oxide,  and  hydrate,  and  gave  some  of  the
chief  reactiona  of  the  new  body.
In  the  courae  of  my  20  years*  work  on  the  fractionation  of  the  rare  earths
I  have  repeatedly  tested  my  products  by  examining  their  photographed
spectra,  using  the  dominant  lines  of  the  various  elements  as  tests  for  their
presence,  Scandium  has  an  extremely  characteristic  group  of  lines  in  ita
spectrmn,  situated  between  wave-lengths  3535'864  and  365fl'^83,  the
strongest  being  at  86I3'984,  midway  between  two  strong  iron  lines*  By  using
.a  part  of  the  spectrum  in  which  this  occupies  the  centre  of  the  photograph
it  is  easy  to  see  if  scandium  is  present.  Detecting  the  dominant  line,  the
presence  of  scandium  can  be  verified  by  reference  to  the  other  lines  of  the  group.
Scandium  I  found  in  some  of  my  fractions,  but  only  in  small  quantities.
