﻿246 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  192 
  9 
  

  

  and 
  the 
  difficulty 
  of 
  satisfactorily 
  accommodating 
  the 
  rare 
  earth 
  group 
  

   of 
  metals. 
  Some 
  16 
  years 
  ago 
  the 
  science 
  of 
  chemistry, 
  primarily 
  

   concerned 
  as 
  it 
  is 
  with 
  the 
  elements 
  which 
  form 
  the 
  basis 
  of 
  its 
  studies 
  

   was, 
  in 
  one 
  important 
  respect, 
  somewhat 
  in 
  the 
  dark, 
  in 
  spite 
  of 
  the 
  

   periodic 
  table, 
  as 
  regards 
  a 
  matter 
  of 
  primary 
  importance, 
  the 
  total 
  

   number 
  of 
  elements 
  possible 
  in 
  the 
  scheme 
  of 
  nature. 
  As 
  its 
  name 
  

   implies 
  the 
  classification 
  was 
  largely 
  guided 
  by 
  the 
  natural 
  recur- 
  

   rence 
  of 
  periodic 
  chemical 
  and 
  physical 
  properties 
  of 
  the 
  elements. 
  

   Full 
  credit 
  is 
  due 
  to 
  Mendeleef 
  for 
  his 
  remarkable 
  prediction 
  of 
  the 
  

   "eka" 
  elements, 
  though 
  this 
  depended 
  on 
  analogies 
  suggested 
  by 
  

   the 
  table, 
  and 
  lacked 
  definite 
  mathematical 
  support 
  such 
  as 
  was 
  

   to 
  be 
  supplied 
  by 
  the 
  discovery 
  about 
  to 
  be 
  reviewed. 
  

  

  In 
  1913 
  a 
  young 
  English 
  scientist, 
  H. 
  G. 
  Moseley, 
  developed 
  a 
  

   study 
  of 
  the 
  X-ray 
  spectra 
  of 
  the 
  elements 
  in 
  the 
  light 
  of 
  the 
  nuclear 
  

   theory 
  of 
  the 
  atom 
  advanced 
  in 
  1911. 
  According 
  to 
  this 
  theory, 
  

   which 
  is 
  now 
  firmly 
  established, 
  the 
  elements 
  are 
  regarded 
  as 
  consist- 
  

   ing 
  of 
  a 
  central 
  positively 
  charged 
  nucleus 
  around 
  which 
  rotate 
  elec- 
  

   trons 
  or 
  negative 
  electrical 
  particles, 
  in 
  definite 
  orbits. 
  The 
  suc- 
  

   cessive 
  elements 
  are 
  built 
  up 
  b}^ 
  the 
  addition 
  of 
  an 
  increasing 
  number 
  

   of 
  orbital 
  electrons 
  accompanied 
  by 
  a 
  more 
  complex 
  development 
  of 
  

   the 
  nucleus. 
  Starting 
  from 
  hydrogen 
  as 
  unity, 
  having 
  one 
  orbital 
  

   electron, 
  the 
  elements 
  can 
  be 
  arranged 
  in 
  order 
  of 
  increasing 
  number 
  of 
  

   orbital 
  electrons, 
  and 
  to 
  each 
  successive 
  element 
  an 
  ordinal 
  number 
  

   can 
  be 
  assigned. 
  These 
  numbers 
  are 
  termed 
  atomic 
  numbers, 
  and 
  

   Moseley 
  found 
  a 
  simple 
  numerical 
  relation 
  between 
  these 
  numbers 
  

   and 
  the 
  frequency 
  of 
  the 
  characteristic 
  line 
  in 
  the 
  X-ray 
  spectrum 
  of 
  

   any 
  particular 
  element. 
  This 
  epoch-making 
  discovery 
  threw 
  a 
  flood 
  

   of 
  light 
  on 
  the 
  problem 
  of 
  classifying 
  the 
  elements; 
  atomic 
  weights 
  

   were 
  now 
  displaced 
  from 
  their 
  position 
  of 
  importance 
  by 
  the 
  still 
  

   more 
  fundamental 
  atomic 
  numbers, 
  to 
  which 
  the 
  former 
  were 
  found 
  

   to 
  be 
  approximately 
  proportional. 
  He 
  thus 
  corrected 
  those 
  instances 
  

   in 
  the 
  periodic 
  table 
  in 
  which 
  it 
  was 
  known 
  that 
  the 
  properties 
  of 
  

   certain 
  pairs 
  of 
  elements 
  demanded 
  that 
  their 
  positions 
  should 
  be 
  

   interchanged, 
  a 
  course 
  which 
  could 
  not 
  be 
  reconciled 
  with 
  their 
  

   arrangement 
  in 
  order 
  of 
  increasing 
  atomic 
  weight. 
  Laborious 
  re- 
  

   searches, 
  e. 
  g. 
  in 
  the 
  case 
  of 
  iodine 
  and 
  tellurium, 
  had 
  in 
  the 
  past 
  been 
  

   conducted 
  under 
  the 
  impression 
  that 
  the 
  accepted 
  atomic 
  weights 
  

   were 
  inaccurate, 
  and 
  the 
  application 
  of 
  Moseley's 
  discovery 
  cleared 
  

   away 
  the 
  difficulty. 
  

  

  The 
  attendant 
  difficulty 
  of 
  fractional 
  atomic 
  weights 
  was 
  cleared 
  

   up 
  at 
  about 
  the 
  same 
  time 
  by 
  the 
  work 
  of 
  Soddy 
  and 
  later 
  Aston 
  on 
  

   isotopes, 
  which 
  were 
  shown 
  to 
  be 
  elements 
  existing 
  in 
  two 
  or 
  more 
  

   chemically 
  inseparable 
  forms 
  having 
  the 
  same 
  atomic 
  number, 
  but 
  

   slightly 
  different 
  atomic 
  weights, 
  the 
  varying 
  proportions 
  of 
  these 
  

   forms 
  accounting 
  for 
  the 
  fractional 
  atomic 
  weights 
  as 
  determined 
  

  

  