﻿TWENTIETH 
  CENTURY 
  PHYSICS 
  MILLIKAN 
  193 
  

  

  mind 
  for 
  the 
  startling 
  changes 
  that 
  were 
  to 
  come. 
  I 
  shall 
  cata- 
  

   logue 
  some 
  of 
  the 
  most 
  significant 
  of 
  these 
  changes 
  under 
  eight 
  

   different 
  heads, 
  taking 
  the 
  discovery 
  of 
  X 
  rays 
  as 
  the 
  first. 
  

  

  Second. 
  Rontgen's 
  discovery 
  furnished 
  an 
  instrument 
  and 
  a 
  tech- 
  

   nique 
  which 
  made 
  possible 
  the 
  rapid 
  development 
  of 
  the 
  electron 
  

   theory 
  of 
  matter 
  — 
  one 
  of 
  the 
  grandest, 
  because 
  the 
  simplest, 
  of 
  all 
  

   physical 
  generalizations. 
  Although 
  this 
  is 
  in 
  a 
  sense 
  the 
  very 
  heart 
  

   and 
  soul 
  of 
  the 
  new 
  physics, 
  I 
  shall 
  pass 
  over 
  it 
  here 
  with 
  only 
  such 
  

   mention 
  as 
  is 
  necessary 
  to 
  give 
  it 
  a 
  place 
  in 
  the 
  catalogue 
  of 
  great 
  

   new 
  developments, 
  because, 
  superficially 
  at 
  least, 
  the 
  electron 
  tlieory 
  

   did 
  not 
  at 
  first 
  set 
  itself 
  in 
  opposition 
  to 
  nineteenth-century 
  points 
  

   of 
  view. 
  It 
  represented 
  the 
  discovery 
  of 
  a 
  wonderful 
  new 
  world, 
  the 
  

   subatomic 
  world 
  of 
  extraordinary 
  simplicity 
  and 
  orderliness, 
  but 
  

   it 
  left 
  the 
  world 
  of 
  large-scale 
  phenomena, 
  the 
  old 
  " 
  macroscopic 
  " 
  

   world 
  which 
  we 
  had 
  known 
  before, 
  functioning 
  pretty 
  much 
  in 
  its 
  

   nineteenth 
  century 
  fashion. 
  

  

  Third. 
  Within 
  a 
  year 
  of 
  Rontgen's 
  discovery, 
  namely, 
  in 
  1896, 
  

   there 
  came 
  the 
  discovery 
  of 
  radioactivity, 
  and 
  with 
  that 
  discovery, 
  

   as 
  soon 
  as 
  its 
  significance 
  began 
  to 
  be 
  seen, 
  man's 
  view 
  of 
  the 
  nature 
  

   of 
  this 
  physical 
  world 
  changed 
  overnight. 
  Matter 
  had 
  theretofore 
  

   been 
  put 
  up 
  in 
  a 
  definite 
  number 
  — 
  we 
  knew 
  not 
  how 
  many— 
  of 
  

   eternal, 
  unchangeable 
  chemical 
  elements. 
  In 
  radioactivity 
  we 
  found 
  

   two 
  of 
  these 
  elements 
  first 
  spontaneously 
  shooting 
  off 
  parts 
  of 
  them- 
  

   selves 
  with 
  speeds 
  comparable 
  with 
  the 
  speed 
  of 
  light 
  — 
  speeds 
  which 
  

   nobody 
  had 
  ever 
  dreamed 
  that 
  matter 
  in 
  any 
  form 
  could 
  under 
  any 
  

   circumstance 
  attain 
  — 
  and 
  second, 
  by 
  virtue 
  of 
  this 
  process, 
  trans- 
  

   forming 
  themselves 
  into 
  new 
  elements; 
  so 
  that 
  now 
  we 
  definitely 
  

   know 
  the 
  life 
  period 
  of 
  a 
  considerable 
  number 
  of 
  the 
  erstwhile 
  

   eternal 
  elements. 
  

  

  The 
  discovery 
  of 
  X 
  rays 
  in 
  1895 
  had 
  revealed 
  a 
  whole 
  domain 
  of 
  

   ether 
  physics, 
  of 
  whose 
  existence 
  prior 
  to 
  1895 
  we 
  had 
  been 
  com- 
  

   pletely 
  unconscious. 
  The 
  discovery 
  of 
  radioactivity 
  in 
  1896 
  had 
  

   revealed 
  an 
  entirely 
  new 
  property 
  of 
  matter 
  and 
  quite 
  as 
  important 
  

   a 
  property, 
  so 
  far 
  as 
  its 
  influence 
  upon 
  our 
  conceptions 
  of 
  our 
  world 
  

   are 
  concerned, 
  as 
  any 
  which 
  had 
  ever 
  been 
  discovered. 
  For 
  it 
  

   forced 
  us, 
  for 
  the 
  first 
  time, 
  to 
  begin 
  to 
  think 
  in 
  terms 
  of 
  a 
  universe 
  

   which 
  is 
  changing, 
  living, 
  growing, 
  even 
  in 
  its 
  elements 
  — 
  a 
  dynamic 
  

   instead 
  of 
  a 
  static 
  universe. 
  It 
  has 
  exerted 
  the 
  most 
  profound 
  

   influence 
  not 
  only 
  upon 
  physics 
  which 
  gave 
  it 
  birth, 
  but 
  also 
  upon 
  

   chemistry, 
  upon 
  geology, 
  upon 
  biology, 
  upon 
  philosophy. 
  Indeed, 
  

   it 
  is 
  at 
  this 
  point 
  that 
  one 
  of 
  the 
  great 
  contributions 
  of 
  science 
  to 
  

   religion 
  is 
  now 
  being 
  made. 
  

  

  To 
  the 
  general 
  public 
  the 
  wonder 
  of 
  radioactivity 
  is 
  now 
  wearing 
  

   off 
  a 
  bit 
  merely 
  because 
  the 
  phenomena 
  have 
  become 
  familiar, 
  but 
  

   to 
  the 
  thoughtful 
  observer 
  the 
  mj^stery 
  is 
  in 
  some 
  particulars 
  as 
  

  

  