﻿250 
  JOURNAL 
  OF 
  THE 
  WASHINGTON 
  ACADEMY 
  OF 
  SCIENCES 
  VOL. 
  12, 
  NO. 
  10 
  

  

  however, 
  so 
  difficult 
  to 
  reconcile 
  with 
  any 
  simple 
  structure 
  for 
  this 
  

   salt 
  that 
  further 
  work 
  will 
  be 
  necessary 
  to 
  establish 
  its 
  structure 
  with 
  

   certainty. 
  

  

  CAESIUM 
  FLUORIDE 
  

  

  The 
  density 
  of 
  this 
  salt, 
  as 
  approximately 
  determined 
  from 
  a 
  mea- 
  

   surement 
  of 
  its 
  refractive 
  index"* 
  indicates 
  that 
  four 
  molecules 
  are 
  to 
  

   be 
  associated 
  with 
  the 
  unit 
  cube. 
  The 
  diffraction 
  data 
  are 
  in 
  such 
  

   good 
  agreement 
  with 
  a 
  structure 
  containing 
  this 
  number 
  of 
  molecules 
  

   in 
  the 
  unit 
  that 
  there 
  is 
  little 
  reason 
  for 
  doubting 
  the 
  correctness 
  of 
  the 
  

   structure 
  here 
  assigned 
  (which 
  is 
  different 
  from 
  that 
  of 
  all 
  of 
  the 
  other 
  

   caesium 
  halides). 
  

  

  Calculated 
  density 
  = 
  4.52. 
  

   Spacing: 
  dioo 
  = 
  6.03 
  ± 
  0.02 
  A. 
  U. 
  

   Structure: 
  NaCl 
  grouping. 
  

  

  Discussion 
  of 
  these 
  structures. 
  — 
  The 
  data 
  concerning 
  (1) 
  the 
  struc- 
  

   tures, 
  (2) 
  the 
  dimensions 
  of 
  the 
  unit 
  cells, 
  and 
  (3) 
  the 
  distance 
  of 
  near- 
  

   est 
  approach 
  of 
  unlike 
  atoms 
  in 
  each 
  crystal 
  of 
  this 
  series 
  are 
  collected 
  

   in 
  table 
  1.^ 
  

  

  On 
  the 
  basis 
  of 
  the 
  available 
  crystal 
  structure 
  data, 
  volumes 
  of 
  

   "spheres 
  of 
  influence" 
  have 
  been 
  assigned^ 
  to 
  various 
  atoms 
  and 
  

   crystals 
  imagined 
  as 
  resulting 
  from 
  a 
  close 
  packing 
  of 
  these 
  atomic 
  

   spheres. 
  The 
  extent 
  to 
  which 
  these 
  measurements 
  are 
  in 
  agreement 
  

   with 
  such 
  an 
  hypothesis 
  may 
  be 
  tested 
  by 
  assigning 
  to 
  some 
  one 
  

   atom 
  an 
  indefinite 
  radius 
  a 
  and 
  obtaining 
  the 
  radii 
  of 
  the 
  other 
  atoms 
  

   in 
  terms 
  of 
  a. 
  If 
  this 
  is 
  done 
  the 
  calculated 
  distances 
  R-X 
  of 
  table 
  

  

  * 
  We 
  wish 
  to 
  thank 
  Dr. 
  H. 
  E. 
  Merwin 
  of 
  this 
  Laboratory 
  for 
  determining 
  the 
  refrac- 
  

   tive 
  index 
  of 
  caesium 
  fluoride, 
  and 
  suggesting 
  the 
  estimation 
  of 
  its 
  density 
  therefrom. 
  

   The 
  index 
  is 
  1.478 
  =•= 
  0.005. 
  Applying 
  Gladstone's 
  law 
  and 
  using 
  the 
  specific 
  refractive 
  

   energies 
  given 
  by 
  E. 
  S. 
  Larsen 
  (U. 
  S. 
  Geol. 
  Survey 
  Bull. 
  679: 
  31), 
  the 
  density 
  of 
  caesium 
  

   fluoride 
  is 
  found 
  to 
  be 
  approximately 
  4.38. 
  

  

  ^ 
  After 
  completing 
  our 
  determinations 
  of 
  the 
  structures 
  of 
  the 
  alkali 
  halides 
  we 
  have 
  

   become 
  aware 
  of 
  previous 
  work 
  upon 
  some 
  of 
  them 
  through 
  a 
  paper 
  by 
  A. 
  W. 
  HuLi<. 
  Journ. 
  

   Frankl. 
  Inst., 
  193, 
  217. 
  1922. 
  

  

  « 
  W. 
  L. 
  Bragg. 
  Phil. 
  Mag. 
  VI, 
  40: 
  169. 
  1920. 
  

  

  