AmApriir'i9ih5arm" }    Views  on  Constitution  of  the  Atom.  193 
ing  to  Sir  Joseph  Thomson,  when  an  atom  has,  suppose,  three  rings 
of  revolving  electrons,  the  outer  ring  is  that  influenced  by  ionization 
and  is  investigated  by  spectrum  analysis,  the  inner  ring  is  connected 
with  the  penetrating,  or  hard,  or  K  series  of  characteristic  Rontgen 
radiations,  and  the  middle  ring  with  the  less  penetrating,  or  soft, 
or  L  series.  To  the  nucleus  we  must  look  for  the  main  effects  of 
mass  and  inertia,  and  gravitational  attraction,  and,  when  they  exist, 
for  the  radio-active  transformations  and  expulsions. 
Bohr's  Theories. — It  has  been  pointed  out  that  when  a  single 
electron  revolves  round  a  unit  charge  nucleus,  as  in  the  case  of 
hydrogen,  that  by  electromagnetic  theory  there  should  be  a  continued 
stream  of  energy  radiated  away  into  space,  necessarily  demanding 
that  the  revolving  electron  would  lose  speed  and  fall  into  the  nucleus. 
To  overcome  this  and  similar  difficulties,  Bohr  has  suggested  a 
theory  of  stable  states,  non-radiating,  with  the  angular  momentum 
an  exact  multiple  of  h/2ir,  where  h  is  Planck's  constant.  When  the 
multiple  is,  for  reasons  unknown,  diminished  by  unity,  the  electron 
abruptly  changes  from  the  old  stable  state  to  the  next  stable  position, 
and  in  doing  so  radiates  away  an  exact  quantum  such  as  Planck  first 
conceived.  It  must  be  admitted  that  these  ideas  appear  at  first  highly 
imaginative,  and  they  might  well  be  ignored  but  for  the  remarkable 
degree  of  success  which  they  have  already  achieved.  Notably  he 
foretold  the  existence  of  certain  lines  in  the  ultra-violet  spectrum 
of  hydrogen,  and  these  were  subsequently  discovered  by  Lyman.  He 
identified  lines  found  by  Paschen  in  the  infra-red,  and  also  obtained 
an  excellent  numerical  verification  between  his  theory  and  the  value 
of  the  constant  of  Rydberg  which  links  up  the  spectral  series  of 
various  elements.  Perhaps  the  most  interesting  suggestion  is  that 
lines  seen  in  the  spectrum  of  certain  stars  may  be  assigned  to 
hydrogen  in  a  state  of  extreme  tenuity,  so  that  the  electron  revolves 
in  a  greatly-enlarged  orbit  about  the  nucleus,  a  state  which  it  is  im- 
possible to  imitate  in  the  small  vessels  of  our  laboratories. 
Valency  Lazv  of  Radiants. — A  band  of  chemists  who  have 
made  a  careful  study  of  the  properties  of  the  radio-elements  have 
succeeded  in  the  discovery  of  an  interesting  law  which  removes  the 
difficulty  of  finding  places  in  the  periodic  table  for  all  the  radiants, 
about  thirty-seven  in  number.  A  full  account  of  this  work  is  given 
by  Soddy  in  "  The  Chemistry  of  the  Radio-elements  "  (Longmans, 
Green  &  Co.,  1914). 
Briefly,  it  has  been  found  that  when  a  radiant  expels  an  alpha 
