﻿STRUCTURE 
  OF 
  MATTER 
  — 
  COMPTON. 
  149 
  

  

  miles 
  per 
  second. 
  Similarly, 
  the 
  a 
  particles 
  are 
  atoms 
  of 
  helium 
  

   which 
  have 
  lost 
  two 
  negative 
  electrons 
  and 
  which 
  consist, 
  therefore, 
  

   of 
  four 
  positive 
  and 
  two 
  negative 
  electrons, 
  forming 
  a 
  very 
  com- 
  

   pact 
  and 
  stable 
  group. 
  These 
  have 
  velocities 
  as 
  large 
  as 
  aboul 
  

   one-tenth 
  that 
  of 
  light. 
  The 
  (3 
  particles 
  set 
  up 
  oscillations 
  of 
  nega- 
  

   tive 
  electrons 
  in 
  neighboring 
  atoms 
  which 
  they 
  strike, 
  and 
  these 
  oscil- 
  

   lations 
  produce 
  radiation 
  called 
  f 
  radiation 
  or 
  wave 
  motion 
  in 
  the 
  

   ether. 
  The 
  atoms 
  of 
  radium 
  do 
  not 
  " 
  explode 
  " 
  in 
  this 
  manner 
  fre- 
  

   quently. 
  In 
  fact, 
  the 
  occurrence 
  is 
  so 
  rare 
  that 
  the 
  chances 
  are 
  even 
  

   that 
  any 
  given 
  atom 
  will 
  or 
  will 
  not 
  explode 
  within 
  a 
  time 
  of 
  2,000 
  

   years. 
  When 
  it 
  does 
  explode 
  there 
  remains 
  not 
  an 
  atom 
  of 
  radium 
  

   (atomic 
  weight 
  226), 
  but 
  an 
  atom 
  of 
  radium 
  emanation 
  (atomic 
  

   weight 
  222) 
  and 
  an 
  a 
  particle 
  (helium, 
  atomic 
  weight 
  4). 
  

  

  In 
  spite 
  of 
  their 
  smaller 
  velocity, 
  the 
  a 
  particles 
  possess 
  much 
  

   greater 
  kinetic 
  energy 
  than 
  do 
  the 
  (3 
  particles, 
  being 
  nearly 
  7,400 
  

   times 
  heavier. 
  It 
  was 
  by 
  means 
  of 
  bombardment 
  of 
  nitrogen 
  and 
  

   other 
  atoms 
  by 
  these 
  a 
  particles 
  that 
  Professor 
  Rutherford 
  has 
  

   effected 
  their 
  atomic 
  disintegration, 
  yielding 
  hydrogen 
  as 
  a 
  product. 
  

  

  When 
  the 
  a 
  particles 
  shoot 
  out 
  through 
  a 
  gas, 
  such 
  as 
  air, 
  their 
  

   paths 
  may 
  be 
  seen 
  and 
  photographed, 
  provided 
  the 
  air 
  is 
  saturated 
  

   with 
  water 
  vapor 
  and 
  suddenly 
  cooled 
  by 
  expansion. 
  The 
  air 
  mole- 
  

   cules 
  in 
  the 
  path 
  of 
  the 
  a 
  particles 
  have 
  negative 
  electrons 
  forced 
  

   out 
  of 
  them 
  by 
  the 
  action 
  of 
  the 
  positively 
  charged 
  a 
  particle 
  as 
  it 
  

   comes 
  very 
  close. 
  These 
  positively 
  and 
  negatively 
  charged 
  residues 
  

   of 
  the 
  air 
  molecules 
  serve 
  as 
  nuclei 
  for 
  the 
  condensation 
  of 
  water 
  

   vapor. 
  Thus 
  the 
  path 
  of 
  the 
  a 
  particle 
  is 
  visible 
  as 
  a 
  thin 
  line 
  of 
  

   water 
  droplets. 
  In 
  air 
  at 
  atmospheric 
  pressure 
  these 
  paths 
  may 
  be 
  

   as 
  long 
  as 
  11 
  centimeters. 
  

  

  Now, 
  the 
  diameters 
  of 
  air 
  molecules 
  are 
  known 
  to 
  be 
  about 
  3(10) 
  -8 
  

   cm., 
  and 
  there 
  are 
  about 
  2.7(10) 
  19 
  of 
  them 
  in 
  each 
  cubic 
  centimeter. 
  

   An 
  a 
  particle, 
  in 
  traversing 
  11 
  cm. 
  of 
  air, 
  would 
  pass 
  through 
  about 
  

   200,000 
  molecules. 
  Yet 
  many 
  a 
  particles 
  go 
  this 
  entire 
  distance 
  

   without 
  changing 
  the 
  direction 
  of 
  their 
  motion, 
  and 
  most 
  of 
  them 
  

   go 
  at 
  least 
  several 
  centimeters 
  without 
  swerving 
  from 
  their 
  course. 
  

   This 
  can 
  only 
  mean 
  that 
  an 
  a 
  particle 
  may 
  pass 
  right 
  through 
  thou- 
  

   sands 
  of 
  atoms 
  without 
  colliding 
  with 
  that 
  part 
  of 
  an 
  atom 
  in 
  which 
  

   practically 
  all 
  of 
  its 
  mass 
  is 
  situated. 
  We 
  must, 
  therefore, 
  think 
  of 
  

   all 
  of 
  the 
  positive 
  electrons 
  (and 
  possibly 
  some 
  of 
  the 
  negative 
  elec- 
  

   trons) 
  of 
  an 
  atom 
  as 
  grouped 
  within 
  a 
  region 
  which 
  is 
  excessively 
  

   small 
  as 
  compared 
  with 
  the 
  size 
  of 
  the 
  atom. 
  Around 
  this 
  compact 
  

   group, 
  or 
  " 
  nucleus," 
  the 
  remaining 
  negative 
  electrons 
  are 
  situated 
  

   at 
  relatively 
  large 
  distances 
  — 
  distances 
  comparable 
  with 
  the 
  atomic 
  

   radius. 
  

  

  With 
  all 
  the 
  heavy 
  positive 
  electrons 
  and 
  only 
  some 
  of 
  the 
  nega- 
  

   tive 
  electrons 
  constituting 
  this 
  nucleus, 
  it 
  is 
  evidently 
  positively 
  

  

  55379—24 
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