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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  production 
  of 
  light 
  rays 
  and 
  of 
  all 
  such 
  rays, 
  including 
  the 
  infra-red, 
  

   the 
  visible, 
  the 
  ultra-violet, 
  and 
  the 
  X 
  rays. 
  

  

  The 
  diameter 
  of 
  an 
  atom 
  — 
  that 
  is 
  to 
  say, 
  the 
  diameter 
  of 
  the 
  sphere 
  

   within 
  which 
  all 
  the 
  protons 
  and 
  electrons 
  of 
  a 
  single 
  atom 
  find 
  them- 
  

   selves 
  — 
  does 
  not 
  exceed 
  one 
  ten-millionth 
  part 
  of 
  the 
  diameter 
  of 
  an 
  

   ordinary 
  bird 
  shot. 
  Hence 
  a 
  single 
  atom 
  is 
  quite 
  too 
  small 
  to 
  see 
  

   even 
  with 
  a 
  microscope. 
  Moreover, 
  an 
  atom, 
  as 
  we 
  have 
  remarked, 
  is 
  

   not 
  solid, 
  but 
  itself 
  composed 
  of 
  a 
  number 
  of 
  particles 
  — 
  the 
  protons 
  

   and 
  electrons 
  — 
  well 
  separated. 
  Indeed, 
  these 
  constituents 
  of 
  the 
  

   atom 
  are 
  excessively 
  small 
  compared 
  to 
  what 
  we 
  have 
  just 
  described 
  

   as 
  the 
  diameter 
  of 
  an 
  atom. 
  A 
  single 
  electron 
  is 
  only 
  one 
  fifty- 
  

   thousandth 
  part 
  of 
  the 
  diameter 
  of 
  an 
  atom. 
  Comparing 
  this 
  to 
  one 
  

   of 
  the 
  planets, 
  the 
  diameter 
  of 
  Mars 
  bears 
  roughly 
  the 
  same 
  ratio 
  to 
  

   the 
  diameter 
  of 
  its 
  orbit 
  that 
  holds 
  between 
  an 
  electron 
  and 
  an 
  atom. 
  

   Similarly 
  the 
  nucleus 
  of 
  a 
  heavy 
  atom, 
  like 
  that 
  of 
  lead, 
  containing 
  

   many 
  protons 
  and 
  electrons, 
  bears 
  somewhat 
  the 
  same 
  proportion 
  to 
  

   the 
  whole 
  atomic 
  volume 
  that 
  our 
  sun, 
  865,000 
  miles 
  in 
  diameter, 
  bears 
  

   to 
  a 
  sphere 
  just 
  inclosing 
  the 
  orbit 
  of 
  the 
  planet 
  Jupiter. 
  From 
  these 
  

   figures 
  we 
  see 
  that 
  the 
  inclosure 
  we 
  call 
  an 
  atom 
  is 
  almost 
  wholly 
  

   given 
  up 
  to 
  free 
  space. 
  The 
  occupied 
  parts 
  form 
  hardly 
  any 
  greater 
  

   volume, 
  proportionally, 
  than 
  the 
  sun 
  and 
  planets 
  do 
  to 
  the 
  solar 
  

   system. 
  

  

  Matter 
  exists 
  in 
  the 
  three 
  states 
  — 
  solid, 
  liquid, 
  and 
  gaseous. 
  The 
  

   two 
  former 
  have 
  densities 
  that 
  are 
  nearly 
  equal. 
  For 
  instance, 
  water 
  

   is 
  of 
  about 
  twice 
  the 
  density 
  of 
  ice. 
  But 
  gases 
  are 
  of 
  all 
  densities, 
  

   from 
  that 
  of 
  their 
  liquids 
  to 
  as 
  little 
  as 
  one 
  pleases, 
  depending 
  on 
  

   the 
  pressure 
  one 
  applies 
  to 
  keep 
  them 
  so. 
  What, 
  then, 
  are 
  the 
  dis- 
  

   tances 
  apart 
  of 
  the 
  several 
  atoms 
  of 
  a 
  solid? 
  This 
  is 
  known 
  for 
  

   many 
  crystals 
  by 
  experiments 
  with 
  X 
  rays. 
  In 
  common 
  salt, 
  for 
  

   example, 
  the 
  distances 
  are 
  of 
  only 
  two 
  or 
  three 
  times 
  the 
  magnitude 
  

   as 
  given 
  above 
  for 
  the 
  diameter 
  of 
  an 
  atom. 
  Such, 
  then, 
  are 
  the 
  

   distances 
  between 
  the 
  nuclei 
  of 
  the 
  atoms 
  for 
  solids 
  and 
  liquids, 
  and 
  

   from 
  this 
  up 
  to 
  anything 
  greater 
  for 
  gases, 
  depending 
  on 
  the 
  pres- 
  

   sure. 
  In 
  solids, 
  and 
  to 
  a 
  less 
  extent 
  in 
  liquids, 
  the 
  atoms 
  are 
  so 
  far 
  

   restricted 
  by 
  their 
  mutual 
  proximity 
  and 
  interacting 
  forces 
  of 
  attrac- 
  

   tion 
  and 
  repulsion 
  that 
  there 
  is 
  no 
  wandering, 
  or 
  but 
  little, 
  of 
  the 
  

   atoms 
  through 
  the 
  mass, 
  so 
  long 
  as 
  new 
  forces 
  are 
  not 
  introduced. 
  

   Individual 
  electrons, 
  freed 
  from 
  atoms 
  temporarily, 
  may 
  easily 
  travel 
  

   along 
  under 
  the 
  urging 
  of 
  electric 
  field, 
  difference 
  of 
  temperature, 
  or 
  

   bombardment 
  by 
  radium 
  or 
  X 
  rays. 
  In 
  gases, 
  however, 
  especially 
  

   very 
  rare 
  gases, 
  as 
  in 
  a 
  vacuum, 
  the 
  whole 
  atoms, 
  or 
  the 
  combinations 
  

   of 
  atoms 
  called 
  molecules, 
  fly 
  about 
  over 
  relatively 
  long 
  paths 
  before 
  

   hitting 
  each 
  other, 
  and 
  so 
  are 
  continually 
  remixing 
  within 
  the 
  

   gaseous 
  volume. 
  

  

  