﻿264 
  //. 
  P. 
  Whitlock 
  — 
  Crystal 
  Structure. 
  

  

  Fig. 
  10 
  shows 
  a 
  centered 
  square 
  prismatic 
  lattice 
  con- 
  

   structed 
  from 
  the 
  same 
  rod 
  and 
  bead 
  elements 
  used 
  in 
  

   fig. 
  2 
  and 
  capable 
  of 
  being 
  " 
  taken 
  down" 
  to 
  show 
  a 
  

   single 
  centered 
  square 
  prism 
  of 
  the 
  tetragonal 
  system. 
  

   Similarly 
  the 
  principal 
  lattices 
  of 
  the 
  orthorhombic 
  sys- 
  

   tem 
  may 
  be 
  demonstrated 
  with 
  the 
  same 
  rod 
  and 
  bead 
  

   elements 
  by 
  varying 
  the 
  spacing 
  of 
  the 
  holes 
  used. 
  By 
  

   also 
  varying 
  the 
  length 
  of 
  the 
  rods 
  lattices 
  of 
  the 
  mono- 
  

   clinic 
  and 
  triclinic 
  systems 
  may 
  be 
  demonstrated. 
  

  

  The 
  triangular 
  spaced 
  stand 
  is 
  shown 
  in 
  fig. 
  11 
  ar- 
  

   ranged 
  to 
  show 
  a 
  hexagonal 
  space 
  lattice 
  which 
  of 
  course 
  

   may 
  be 
  varied 
  both 
  by 
  using 
  a 
  system 
  of 
  holes 
  closer 
  or 
  

   further 
  apart 
  or 
  by 
  varying 
  the 
  bead 
  spacing 
  on 
  the 
  rods. 
  

  

  A 
  rhombohedral 
  space 
  lattice 
  is 
  illustrated 
  in 
  fig. 
  12. 
  

   This 
  again 
  may 
  be 
  varied 
  as 
  in 
  the 
  preceding 
  instance,. 
  

   and 
  by 
  removing 
  appropriate 
  sets 
  of 
  rods 
  the 
  relation 
  of 
  

   this 
  lattice 
  to 
  interpenetrated 
  triagonal 
  lattices 
  may 
  be 
  

   demonstrated. 
  

  

  Fig. 
  13 
  shows 
  the 
  triangular 
  frame 
  arranged 
  to 
  show 
  

   Bragg 
  's 
  suggestion 
  as 
  to 
  the 
  atomic 
  structure 
  of 
  quartz, 
  

   the 
  colored 
  beads 
  representing 
  silicon 
  atoms 
  and 
  the 
  

   white 
  ones 
  oxygen 
  atoms. 
  

  

  All 
  of 
  the 
  foregoing 
  concrete 
  examples 
  are 
  constructed 
  

   from 
  diagrams 
  and 
  data 
  given 
  by 
  W. 
  H. 
  and 
  W. 
  L. 
  

   Bragg. 
  4 
  They 
  have 
  been 
  chosen 
  more 
  or 
  less 
  at 
  random 
  

   and 
  by 
  no 
  means 
  exhaust 
  the 
  possibilities 
  of 
  the 
  device. 
  

   Eod 
  and 
  bead 
  elements 
  may 
  be 
  added 
  to 
  the 
  accessories 
  

   of 
  the 
  model 
  ad 
  libitum, 
  and 
  a 
  little 
  practise 
  will 
  render 
  

   possible 
  the 
  construction 
  and 
  demonstration 
  of 
  a 
  very 
  

   extended 
  series 
  of 
  group 
  movements 
  including 
  all 
  but 
  the 
  

   most 
  complex 
  groups. 
  With 
  a 
  framework 
  constructed 
  

   with 
  a 
  closer 
  system 
  of 
  perforations 
  even 
  these 
  latter 
  

   may 
  be 
  reproduced. 
  

  

  The 
  simplicity 
  of 
  this 
  model, 
  its 
  low 
  cost 
  of 
  construc- 
  

   tion 
  and 
  its 
  ease 
  of 
  manipulation 
  should 
  render 
  it 
  pos- 
  

   sible 
  of 
  adoption 
  as 
  a 
  lecture 
  accessory 
  by 
  many 
  teachers 
  

   of 
  crystallography. 
  

  

  Department 
  of 
  Mineralogy, 
  

  

  American 
  Museum 
  of 
  Natural 
  History, 
  

   New 
  York. 
  

  

  4 
  X-Kays 
  and 
  Crystal 
  Structure, 
  London, 
  1916. 
  

  

  