﻿Demonstrating 
  Crystal 
  Structure. 
  261 
  

  

  for 
  the 
  central 
  rod, 
  the 
  model 
  presents 
  the 
  aspect 
  of 
  a 
  

   centered 
  cube, 
  fig. 
  3. 
  

  

  Introducing 
  rods 
  appropriately 
  spaced 
  with 
  beads 
  of 
  

   a 
  different 
  color 
  the 
  atomic 
  grouping 
  of 
  cuprite 
  may 
  be 
  

   produced 
  as 
  in 
  fLg. 
  4 
  where 
  the 
  introduced 
  beads 
  repre- 
  

   sent 
  the 
  copper 
  atoms. 
  This 
  structure 
  may 
  be 
  carried 
  

   further 
  by 
  replacing 
  the 
  white 
  beads 
  of 
  fLg. 
  2 
  and 
  adding 
  

   more 
  copper 
  atoms, 
  as 
  in 
  fig. 
  5. 
  It 
  now 
  becomes 
  appar- 
  

   ent 
  that 
  the 
  spacing 
  of 
  the 
  colored 
  beads 
  representing 
  

   copper 
  atoms 
  are 
  arranged 
  in 
  a 
  face-centered 
  cubic 
  lat- 
  

   tice. 
  This 
  may 
  be 
  demonstrated 
  by 
  removing 
  the 
  white 
  

   beads 
  of 
  the 
  cube 
  centered 
  lattice 
  which 
  represented 
  the 
  

   oxygen 
  atoms. 
  

  

  Retaining 
  the 
  rods 
  carrying 
  the 
  colored 
  beads 
  of 
  the 
  

   face-centered 
  lattice 
  and 
  introducing 
  rods 
  carrying 
  

   another 
  face-centered 
  cubic 
  lattice, 
  the 
  atomic 
  grouping 
  

   of 
  sphalerite 
  may 
  be 
  reproduced 
  as 
  in 
  fig. 
  6 
  in 
  which 
  the 
  

   white 
  beads 
  represent 
  sulphur 
  atoms 
  and 
  the 
  colored 
  

   beads 
  zinc 
  atoms 
  or 
  vice 
  versa. 
  

  

  The 
  point 
  system 
  of 
  calcium 
  fluoride 
  (fluorite) 
  may 
  be 
  

   demonstrated 
  by 
  retaining 
  the 
  white, 
  face-centered 
  cubic 
  

   lattice 
  of 
  fLg. 
  6, 
  and 
  substituting 
  for 
  the 
  colored 
  face- 
  

   centered 
  cubic 
  lattice 
  rods 
  which 
  carry 
  a 
  closer-spaced 
  

   cubic 
  lattice 
  or 
  colored 
  beads 
  which 
  center 
  every 
  tetra- 
  

   hedral 
  group 
  of 
  the 
  white 
  beads 
  as 
  in 
  fig. 
  7. 
  Here 
  the 
  

   white 
  beads 
  represent 
  calcium 
  atoms 
  and 
  the 
  colored 
  ones 
  

   fluorine. 
  

  

  An 
  atomic 
  structure 
  which 
  is 
  somewhat 
  difficult 
  to 
  

   visualize 
  is 
  that 
  of 
  the 
  diamond. 
  It 
  may 
  be 
  demon- 
  

   strated 
  on 
  the 
  model 
  by 
  setting 
  up 
  two 
  face-centered 
  

   cubic 
  lattices 
  of 
  the 
  white 
  beads, 
  one 
  shifted 
  along 
  the 
  

   cubic 
  diagonal 
  of 
  the 
  other 
  one 
  quarter 
  of 
  its 
  length, 
  as 
  

   in 
  fig. 
  8. 
  It 
  will 
  be 
  noted 
  that 
  this 
  network 
  presents 
  the 
  

   same 
  aspect 
  as 
  that 
  of 
  sphalerite 
  fig. 
  6, 
  with 
  the 
  excep- 
  

   tion 
  that 
  the 
  atoms 
  represented 
  are 
  of 
  one 
  kind. 
  

  

  •The 
  atomic 
  structure 
  of 
  calcium 
  chloride 
  (halite) 
  is 
  

   another 
  example 
  of 
  the 
  shifting 
  of 
  the 
  position 
  of 
  one 
  

   face-centered 
  cubic 
  lattice 
  with 
  respect 
  to 
  another 
  simi- 
  

   lar 
  lattice. 
  This 
  may 
  be 
  demonstrated 
  with 
  rods 
  upon 
  

   which 
  two 
  sets 
  of 
  colored 
  beads 
  are 
  spaced 
  so 
  that 
  they 
  

   occupy 
  the 
  relative 
  positions 
  shown 
  in 
  fig. 
  9, 
  in 
  which 
  one 
  

   lattice 
  is 
  shifted 
  along 
  the 
  cubic 
  edge 
  of 
  the 
  other 
  one 
  

   half 
  its 
  length. 
  The 
  beads 
  of 
  two 
  lattices 
  represent 
  

   respectively 
  atoms 
  of 
  calcium 
  and 
  of 
  chloride. 
  

  

  