﻿Separation 
  of 
  Dextro- 
  and 
  Lcevo-rotating 
  Crystals. 
  

  

  137 
  

  

  crystals 
  undergo 
  the 
  expected 
  change 
  in 
  color 
  and 
  are 
  beauti- 
  

   fully 
  differentiated. 
  During 
  the 
  turning 
  thej 
  may 
  all 
  change 
  

   color, 
  but 
  never 
  so 
  as 
  to 
  leave 
  the 
  result 
  uncertain. 
  For 
  it 
  is 
  

   evident 
  that, 
  if 
  the 
  field 
  crystal 
  is 
  selected 
  so 
  as 
  to 
  be 
  thicker 
  

   than 
  any 
  of 
  the 
  regular 
  crop, 
  the 
  crystal 
  examined 
  can 
  only 
  

   increase 
  or 
  diminish 
  its 
  effect 
  but 
  never 
  reverse 
  the 
  rotation. 
  

   Hence 
  the 
  blue 
  color 
  will 
  only 
  appear 
  at 
  the 
  two 
  characteristic 
  

   points 
  in 
  rotating 
  the 
  analyzer 
  through 
  360°. 
  But 
  if 
  this 
  con- 
  

   dition 
  is 
  not 
  fulfilled, 
  then 
  a 
  thicker 
  crystal 
  of 
  opposite 
  rotation 
  

   would, 
  at 
  some 
  point, 
  turn 
  blue, 
  just 
  as 
  would 
  a 
  crystal 
  of 
  the 
  

   same 
  rotation 
  whose 
  thickness 
  was 
  the 
  difference 
  between 
  the 
  

   field 
  crystal 
  and 
  the 
  one 
  under 
  consideration. 
  Usually 
  the 
  field 
  

   crystal 
  was 
  3 
  or 
  4™°^ 
  thick. 
  The 
  crystals 
  grown 
  rarely 
  exceed 
  

   this 
  thickness, 
  and 
  if 
  they 
  do, 
  their 
  determination 
  affords 
  no 
  

   difficulty 
  by 
  the 
  simple 
  use 
  of 
  the 
  two 
  Nicols 
  as 
  before 
  stated. 
  

  

  If 
  the 
  analyzer 
  be 
  adjusted 
  so 
  that 
  the 
  smallest 
  of 
  the 
  blue 
  

   crystals 
  is 
  distinctly 
  blue, 
  then 
  if 
  the 
  field 
  crystal 
  is 
  3'^'°' 
  thick, 
  

   a 
  crystal 
  of 
  the 
  same 
  kind 
  10"°^ 
  thick 
  would 
  still 
  have 
  a 
  bluish 
  

   tint 
  in 
  this 
  field, 
  while 
  a 
  Isevo 
  crystal 
  of 
  that 
  size 
  would 
  be 
  dis- 
  

   tinguished 
  by 
  a 
  yellowish 
  tint, 
  or 
  at 
  least 
  lack 
  the 
  bluish 
  tint. 
  

   This 
  of 
  course 
  is 
  an 
  extreme 
  test, 
  the 
  crystals 
  never 
  growing 
  to 
  

   that 
  thickness 
  under 
  ordinary 
  conditions 
  : 
  but 
  it 
  gives 
  some 
  

   idea 
  as 
  to 
  the 
  range 
  within 
  which 
  the 
  detection 
  may 
  be 
  made. 
  

  

  Manifestly, 
  the 
  greatest 
  efficiency 
  of 
  this 
  device 
  would 
  only 
  

   be 
  obtained 
  when 
  the 
  crystals 
  under 
  examination 
  are 
  shielded 
  

   from 
  diffused 
  light 
  : 
  which, 
  in 
  my 
  work, 
  was 
  accomplished 
  by 
  

   aid 
  of 
  a 
  piece 
  of 
  dull 
  black 
  paper, 
  or 
  by 
  working 
  in 
  a 
  moder- 
  

   ately 
  dark 
  room. 
  

  

  the 
  Growth 
  of 
  Crystals. 
  — 
  It 
  may 
  be 
  worth 
  while 
  here 
  to 
  

   add 
  some 
  of 
  the 
  results 
  which 
  the 
  writer 
  has 
  obtained 
  in 
  

   a 
  little 
  investigation 
  of 
  the 
  causes 
  which 
  determine 
  the 
  result- 
  

   ant 
  optical 
  activity 
  of 
  a 
  crop 
  of 
  these 
  crystals, 
  to 
  which 
  Pro- 
  

   fessor 
  Japp's 
  inaugural 
  address, 
  and 
  the 
  discussion 
  to 
  which 
  it 
  

   gave 
  rise, 
  adds 
  some 
  interest. 
  

  

  First, 
  I 
  have 
  to 
  add 
  to 
  the 
  evidence 
  adduced 
  by 
  Marbach,* 
  

   Grenezf 
  and 
  Landolt:}: 
  in 
  favor 
  of 
  the 
  optical 
  neutrality 
  of 
  the 
  

   ultimate 
  molecule 
  of 
  sodium 
  chlorate, 
  the 
  results 
  of 
  a 
  number 
  

   of 
  experiments 
  in 
  which, 
  starting 
  with 
  only 
  one 
  kind 
  of 
  crys- 
  

   tal, 
  I 
  have 
  obtained, 
  by 
  repeated 
  recrystallization, 
  successive 
  

   crops 
  of 
  crystals 
  whose 
  resultant 
  rotations, 
  i. 
  e., 
  the 
  excess 
  of 
  

   dextro 
  or 
  Isevo 
  forms, 
  bear 
  no 
  connection 
  whatever 
  with 
  that 
  

   of 
  the 
  original 
  lot. 
  For 
  example, 
  I 
  have 
  selected 
  a 
  number 
  of 
  

   pure 
  dextro 
  crystals 
  and 
  allowed 
  their 
  solution 
  to 
  crystallize 
  by 
  

   spontaneous 
  evaporation. 
  The 
  dextro 
  and 
  Isevo 
  crystals 
  of 
  the 
  

   crop 
  were 
  then 
  separated, 
  counted 
  and 
  weighed 
  ; 
  after 
  which 
  

   they 
  were 
  again 
  mixed 
  and 
  dissolved. 
  These 
  were 
  then 
  allowed 
  

   to 
  recrystallize 
  and 
  the 
  whole 
  process 
  repeated. 
  But 
  the 
  

  

  * 
  Pogg. 
  Ann., 
  xci, 
  487. 
  f 
  Ber., 
  xxix, 
  2, 
  2410. 
  X 
  Compt. 
  Rend., 
  Ixvi, 
  855. 
  

  

  Am, 
  Jouk. 
  Sci.— 
  Fourth 
  Series, 
  Tol. 
  A^III, 
  No. 
  44. 
  — 
  August, 
  1899. 
  

   10 
  

  

  