﻿102 
  

  

  Mr. 
  G. 
  F. 
  Rodwell 
  on 
  the 
  [Dec. 
  10, 
  

  

  to 
  the 
  crystalline 
  condition 
  takes 
  place 
  at 
  116° 
  C, 
  it 
  would 
  appear 
  that 
  

   between 
  the 
  point 
  of 
  fusion, 
  450° 
  C. 
  (p. 
  100), 
  and 
  the 
  temperature 
  at 
  which 
  

   the 
  amorphous 
  iodide 
  becomes 
  crystalline 
  it 
  follows 
  the 
  ordinary 
  law 
  

   and 
  contracts 
  as 
  it 
  cools, 
  while 
  below 
  that 
  temperature 
  (and, 
  as 
  will 
  be 
  

   shown, 
  as 
  low 
  as 
  —18° 
  C, 
  — 
  o, 
  4 
  F.) 
  it 
  expands 
  on 
  getting 
  cooler, 
  and 
  

   possesses 
  a 
  negative 
  coefficient. 
  It 
  thus 
  appears 
  that 
  when 
  the 
  iodide 
  is 
  

   in 
  the 
  amorphous 
  condition 
  at 
  116° 
  C, 
  immediately 
  before 
  the 
  change 
  to 
  

   the 
  crystalline 
  condition, 
  it 
  is 
  at 
  its 
  point 
  of 
  maximum 
  density. 
  

  

  Several 
  unsuccessful 
  attempts 
  were 
  made 
  to 
  burst 
  metal 
  bottles, 
  after 
  

   the 
  manner 
  of 
  the 
  familiar 
  ice-experiment, 
  by 
  the 
  expansion 
  of 
  the 
  iodide 
  

   at 
  the 
  moment 
  when 
  it 
  passes 
  from 
  the 
  amorphous 
  to 
  the 
  crystalline 
  con- 
  

   dition. 
  On 
  one 
  occasion, 
  when 
  a 
  somewhat 
  large 
  cylindrical 
  mass 
  had 
  

   been 
  cast 
  in 
  a 
  tube 
  of 
  thin 
  brass, 
  the 
  latter 
  was 
  burst 
  by 
  the 
  expanding 
  

   iodide, 
  but 
  thick 
  metal 
  bottles, 
  furnished 
  with 
  a 
  screw, 
  which 
  was 
  forced 
  

   down 
  into 
  the 
  molten 
  mass, 
  were 
  not 
  broken. 
  Thick 
  porcelain 
  and 
  

   glass 
  tubes 
  were 
  invariably 
  broken 
  by 
  the 
  expansion 
  ; 
  and 
  a 
  good 
  lecture 
  

   experiment 
  to 
  illustrate 
  the 
  anomalous 
  expansion 
  is 
  furnished 
  by 
  the 
  

   following 
  means. 
  Let 
  20 
  or 
  30 
  grammes 
  of 
  fused 
  iodide 
  be 
  cast 
  into 
  a 
  

   thick 
  cylindrical 
  tube 
  of 
  porcelain 
  a 
  centimetre 
  diameter 
  ; 
  in 
  the 
  course 
  

   of 
  a 
  minute 
  or 
  two 
  the 
  mass 
  has 
  cooled 
  down 
  to 
  the 
  temperature 
  at 
  

   which 
  it 
  changes 
  from 
  the 
  amorphous 
  to 
  the 
  crystalline 
  condition 
  ; 
  it 
  then 
  

   expands, 
  cracks 
  the 
  tube 
  with 
  a 
  loud 
  noise, 
  and 
  sometimes 
  jerks 
  portions 
  

   of 
  the 
  tube 
  to 
  a 
  distance 
  of 
  several 
  feet. 
  

  

  A 
  curious 
  effect 
  was 
  noticed 
  in 
  the 
  case 
  of 
  bars 
  of 
  the 
  iodide 
  during 
  

   cooling. 
  If 
  a 
  bar 
  be 
  cast 
  in 
  a 
  tube 
  and 
  pushed 
  out 
  before 
  it 
  begins 
  to 
  

   expand, 
  it 
  is 
  seen 
  to 
  curve 
  considerably 
  during 
  cooling. 
  In 
  the 
  case 
  of 
  

   a 
  bar 
  15 
  centimetres 
  long 
  by 
  6 
  millimetres 
  diameter, 
  the 
  curvature 
  was 
  

   such 
  as 
  would 
  be 
  produced 
  with 
  a 
  radius 
  of 
  48 
  centimetres, 
  and 
  was 
  

   always 
  the 
  same 
  with 
  bars 
  of 
  the 
  same 
  length 
  and 
  diameter. 
  A 
  very 
  

   slight 
  pressure 
  resisted 
  the 
  tendency 
  of 
  the 
  bar 
  to 
  curve. 
  The 
  effect 
  

   was 
  not 
  due 
  to 
  conduction 
  of 
  heat 
  from 
  one 
  part 
  of 
  the 
  bar 
  while 
  the 
  

   rest 
  remained 
  perfectly 
  hot 
  ; 
  for 
  the 
  effect 
  was 
  the 
  same 
  whether 
  the 
  bar 
  

   was 
  allowed 
  to 
  cool 
  on 
  a 
  flat 
  copper 
  plate, 
  in 
  an 
  air-bath, 
  or 
  even 
  if 
  it 
  

   were 
  suspended 
  by 
  a 
  thread 
  of 
  non-conducting 
  matter. 
  It 
  takes 
  place 
  

   when 
  the 
  iodide 
  passes 
  from 
  the 
  amorphous 
  to 
  the 
  crystalline 
  condition, 
  

   and 
  is 
  no 
  doubt 
  due 
  to 
  the 
  inequality 
  of 
  strain 
  produced 
  between 
  the 
  

   outside 
  portions, 
  which 
  first 
  become 
  crystalline 
  and 
  expand, 
  and 
  the 
  

   internal 
  portions, 
  which 
  assimilate 
  the 
  change 
  less 
  rapidly, 
  for 
  the 
  iodide 
  

   is 
  a 
  very 
  bad 
  conductor 
  of 
  heat. 
  

  

  It 
  has 
  been 
  stated 
  above 
  that 
  at 
  the 
  moment 
  of 
  solidification 
  of 
  a 
  

   mass 
  of 
  iodide 
  of 
  silver 
  a 
  considerable 
  contraction 
  takes 
  place. 
  The 
  

   following 
  experiments 
  were 
  made 
  in 
  order 
  to 
  determine 
  the 
  amount 
  

   of 
  this 
  contraction. 
  A 
  copper 
  tube, 
  which 
  contained 
  105*548 
  grammes 
  

   of 
  mercury, 
  was 
  found 
  to 
  contain 
  42-080 
  grammes 
  of 
  iodide 
  in 
  a 
  

   molten 
  condition 
  (say 
  450° 
  C.) 
  — 
  that 
  is, 
  a 
  little 
  above 
  the 
  fusing-point 
  

  

  