﻿162 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  192 
  9 
  

  

  fragments 
  were 
  identical, 
  no 
  matter 
  what 
  type 
  of 
  atom 
  they 
  came 
  

   from; 
  they 
  were 
  of 
  equal 
  mass, 
  and 
  they 
  carried 
  equal 
  negative 
  charges 
  

   of 
  electricity, 
  and 
  so 
  were 
  called 
  electrons. 
  Two 
  years 
  later, 
  Lorentz's 
  

   explanation 
  of 
  the 
  newly 
  discovered 
  Zeeman 
  effect 
  provided 
  evidence 
  

   that 
  the 
  moving 
  parts 
  in 
  atomic 
  interiors 
  were 
  precisely 
  similar 
  

   electrons. 
  

  

  The 
  series 
  of 
  researches 
  so 
  initiated 
  were, 
  after 
  a 
  few 
  years, 
  coordi- 
  

   nated 
  in 
  the 
  Rutherford 
  view 
  of 
  atomic 
  structure, 
  which 
  supposed 
  the 
  

   chemical 
  properties 
  and 
  nature 
  of 
  the 
  atom 
  to 
  reside 
  in 
  an 
  excessively 
  

   minute 
  central 
  nucleus 
  carrying 
  a 
  positive 
  charge 
  of 
  electricity, 
  

   about 
  which 
  the 
  negatively 
  charged 
  electrons 
  described 
  wide 
  orbits. 
  

   By 
  clearing 
  a 
  space 
  around 
  the 
  central 
  nucleus, 
  and 
  so 
  preventing 
  other 
  

   atoms 
  from 
  coming 
  too 
  near, 
  these 
  electronic 
  orbits 
  gave 
  size 
  to 
  the 
  

   atom. 
  The 
  volume 
  of 
  space 
  kept 
  clear 
  by 
  the 
  electrons 
  is 
  enormously 
  

   greater 
  than 
  the 
  total 
  volume 
  of 
  the 
  electrons; 
  roughly, 
  the 
  ratio 
  of 
  

   volumes 
  is 
  that 
  of 
  the 
  battlefield 
  to 
  the 
  bullets. 
  The 
  atom, 
  with 
  a 
  

   radius 
  of 
  about 
  2X 
  10~^ 
  cm., 
  has 
  about 
  100,000 
  times 
  the 
  dimensions, 
  

   and 
  so 
  about 
  10^^ 
  times 
  the 
  volume, 
  of 
  a 
  single 
  electron, 
  of 
  which 
  the 
  

   radius 
  is 
  about 
  2X10~'^ 
  cm. 
  In 
  all 
  probability 
  the 
  nucleus 
  is 
  even 
  

   smaller 
  than 
  the 
  electrons. 
  The 
  number 
  of 
  orbital 
  electrons 
  in 
  an 
  

   atom 
  is 
  called 
  the 
  atomic 
  number 
  of 
  the 
  atom; 
  it 
  ranges 
  from 
  unity 
  in 
  

   hydrogen, 
  the 
  lightest 
  and 
  simplest 
  of 
  atoms, 
  to 
  92 
  in 
  uranium, 
  which 
  

   is 
  the 
  most 
  massive 
  and 
  complex 
  atom 
  known. 
  

  

  Simultaneously 
  with 
  this, 
  physical 
  science 
  was 
  discovering 
  that 
  

   the 
  nuclei 
  themselves 
  were 
  neither 
  permanent 
  nor 
  indestructible. 
  

   In 
  1896, 
  Becquerel 
  had 
  found 
  that 
  uranium 
  salts 
  had 
  the 
  remark- 
  

   able 
  property, 
  as 
  it 
  then 
  appeared, 
  of 
  spontaneously 
  affecting 
  pho- 
  

   tographic 
  plates 
  in 
  their 
  vicinity. 
  This 
  observation 
  led 
  to 
  the 
  dis- 
  

   covery 
  of 
  a 
  new 
  property 
  of 
  matter, 
  namely, 
  radioactivity, 
  and 
  all 
  

   the 
  results 
  obtained 
  in 
  the 
  next 
  few 
  years 
  were 
  coordinated 
  in 
  the 
  

   hypothesis 
  of 
  spontaneous 
  disintegration 
  advanced 
  by 
  Rutherford 
  

   and 
  Soddy 
  in 
  1903, 
  according 
  to 
  which 
  radioactivity 
  indicates 
  a 
  

   spontaneous 
  break-up 
  of 
  the 
  atomic 
  nuclei. 
  So 
  far 
  from 
  the 
  atoms 
  

   being 
  permanent 
  and 
  indestructible, 
  their 
  very 
  nuclei 
  were 
  now 
  seen 
  

   to 
  crumble 
  away 
  with 
  the 
  mere 
  lapse 
  of 
  time, 
  so 
  that 
  what 
  was 
  once 
  

   the 
  nucleus 
  of 
  a 
  uranium 
  atom 
  was 
  transformed, 
  after 
  sufficient 
  time, 
  

   into 
  the 
  nucleus 
  of 
  a 
  lead 
  atom, 
  and 
  eight 
  a-particles, 
  which 
  are 
  the 
  

   nuclei 
  of 
  helium 
  atoms. 
  Radiation 
  is 
  given 
  off 
  in 
  the 
  process, 
  the 
  

   radiation 
  that 
  affected 
  Becquerel's 
  photographic 
  plates, 
  and 
  so 
  led 
  

   to 
  the 
  detection 
  of 
  the 
  radioactive 
  property 
  of 
  matter. 
  

  

  With 
  the 
  unimportant 
  exceptions 
  of 
  potassium 
  and 
  rubidium, 
  the 
  

   property 
  of 
  radioactivity 
  occurs 
  only 
  in 
  the 
  most 
  complex 
  and 
  mas- 
  

   sive 
  of 
  atoms, 
  being 
  indeed 
  limited 
  to 
  those 
  of 
  atomic 
  numbers 
  above 
  

   S3. 
  Yet, 
  although 
  the 
  lighter 
  atoms 
  are 
  not 
  liable 
  to 
  spontaneous 
  

  

  