﻿1874.] 
  

  

  Effects 
  of 
  Heat 
  on 
  Iodide 
  of 
  Silver. 
  

  

  101 
  

  

  takes 
  place, 
  often 
  accompanied 
  by 
  a 
  land 
  cracking, 
  and 
  large 
  fissures 
  

   appear 
  in 
  the 
  mass. 
  

  

  Many 
  attempts 
  were 
  made 
  to 
  determine 
  the 
  precise 
  temperature 
  at 
  

   which 
  the 
  change 
  from 
  the 
  amorphous 
  to 
  the 
  crystalline 
  condition 
  takes 
  

   place 
  ; 
  but 
  the 
  results 
  were 
  somewhat 
  discordant, 
  depending 
  apparently 
  

   on 
  the 
  mass 
  of 
  the 
  iodide, 
  and 
  perhaps 
  on 
  the 
  number 
  of 
  times 
  it 
  had 
  

   been 
  previously 
  fused. 
  The 
  iodide 
  was 
  fused 
  in 
  a 
  glass 
  tube 
  or 
  porcelain 
  

   crucible, 
  and 
  when 
  fusion 
  was 
  quite 
  complete 
  was 
  placed 
  in 
  an 
  air-bath 
  

   at 
  150° 
  C. 
  (302° 
  E.) 
  and 
  allowed 
  to 
  cool. 
  The 
  exact 
  temperature 
  at 
  

   which 
  the 
  tube 
  was 
  broken 
  by 
  the 
  expanding 
  mass 
  was 
  noticed. 
  About 
  

   15 
  grammes 
  of 
  iodide, 
  which 
  had 
  been 
  often 
  fused, 
  changed 
  suddenly 
  

   from 
  the 
  amorphous 
  to 
  the 
  crystalline 
  condition 
  at 
  120° 
  C. 
  (248° 
  F.). 
  

   Another 
  specimen 
  cracked 
  the 
  tube 
  at 
  116° 
  C. 
  (240 
  o# 
  8). 
  A 
  porcelain 
  

   crucible 
  containing 
  10 
  grammes 
  of 
  the 
  fused 
  iodide 
  commenced 
  to 
  change 
  

   at 
  118° 
  C. 
  (244°-4 
  F.) 
  ; 
  the 
  crucible 
  was 
  violently 
  riven 
  open 
  at 
  105° 
  C. 
  

   Two 
  small 
  test-tubes, 
  about 
  6 
  millimetres 
  diameter 
  and 
  each 
  contain- 
  

   ing 
  2 
  grammes 
  of 
  iodide, 
  were 
  placed 
  in 
  the 
  hot-air 
  bath 
  ; 
  the 
  two 
  

   masses 
  of 
  iodide 
  simultaneously 
  changed 
  to 
  the 
  crystalline 
  condition 
  at 
  

   109° 
  C. 
  (228°'2 
  F.). 
  On 
  one 
  occasion 
  a 
  small 
  mass 
  weighing 
  3 
  grammes, 
  

   prepared 
  by 
  dissolving 
  silver 
  in 
  hydriodic 
  acid, 
  was 
  fused 
  in 
  a 
  tube 
  and 
  

   slowly 
  cooled. 
  It 
  cooled 
  down 
  to 
  the 
  ordinary 
  temperature 
  of 
  the 
  air 
  

   without 
  breaking 
  the 
  tube 
  ; 
  on 
  moving 
  the 
  tube, 
  however, 
  the 
  mass 
  sud- 
  

   t 
  denly 
  underwent 
  molecular 
  change, 
  and 
  the 
  tube 
  was 
  broken. 
  The 
  same 
  

   iodide 
  fused 
  with 
  some 
  which 
  had 
  been 
  similarly 
  prepared 
  suddenly 
  

   changed 
  to 
  the 
  crystalline 
  variety 
  at 
  121° 
  C. 
  (249°-8 
  F.). 
  From 
  the 
  

   above 
  results 
  we 
  cannot 
  be 
  far 
  wrong 
  in 
  stating 
  that 
  the 
  change 
  from 
  

   the 
  amorphous 
  to 
  the 
  crystalline 
  variety 
  of 
  the 
  iodide 
  takes 
  place 
  at 
  a 
  

   temperature 
  of 
  about 
  116° 
  C. 
  (240°-8 
  F.). 
  

  

  Presumably 
  heat 
  is 
  evolved 
  when 
  the 
  amorphous 
  modification 
  of 
  the 
  

   iodide 
  passes 
  into 
  the 
  crystalline. 
  Several 
  attempts 
  were 
  made 
  to 
  ascer- 
  

   tain 
  this 
  by 
  plunging 
  a 
  mass 
  of 
  hot 
  amorphous 
  iodide 
  into 
  hot 
  mercury, 
  

   inserting 
  a 
  thermometer, 
  and 
  allowing 
  the 
  whole 
  to 
  cool, 
  but 
  no 
  rise 
  of 
  

   temperature 
  was 
  observed 
  at 
  any 
  given 
  point 
  of 
  the 
  cooling. 
  

  

  If 
  the 
  fused 
  iodide 
  be 
  cast 
  into 
  a 
  tube 
  of 
  porcelain 
  or 
  brass 
  the 
  fol- 
  

   lowing 
  effects 
  may 
  be 
  observed 
  : 
  — 
  (a) 
  The 
  mass 
  contracts 
  considerably 
  

   at 
  the 
  moment 
  of 
  solidification, 
  the 
  level 
  liquid 
  surface 
  sinking 
  into 
  a 
  

   deep 
  conical 
  depression 
  when 
  it 
  becomes 
  solid. 
  (/3) 
  For 
  many 
  seconds 
  

   after 
  the 
  solidification 
  the 
  solid 
  cylinder 
  of 
  iodide 
  will 
  freely 
  slip 
  out 
  of 
  

   the 
  tube, 
  and 
  is 
  then 
  seen 
  to 
  be 
  red 
  and 
  transparent, 
  in 
  fact 
  in 
  the 
  

   amorphous 
  condition; 
  but 
  (y) 
  if 
  the 
  mass 
  cools 
  until 
  it 
  assumes 
  the 
  

   crystalline 
  condition 
  it 
  can 
  no 
  longer 
  be 
  got 
  out 
  of 
  the 
  tube 
  ; 
  and 
  if 
  the 
  

   latter 
  be 
  of 
  glass 
  or 
  porcelain, 
  it 
  is 
  infallibly 
  broken 
  by 
  the 
  expansion. 
  

   Hence 
  if 
  a 
  mass 
  of 
  iodide 
  be 
  allowed 
  to 
  cool 
  in 
  a 
  tube 
  which 
  it 
  cannot 
  

   break 
  when 
  it 
  expands, 
  it 
  may 
  be 
  made 
  to 
  contract 
  and 
  slip 
  easily 
  out 
  of 
  

   the 
  tube 
  by 
  heating 
  it. 
  Hence 
  also, 
  as 
  the 
  change 
  from 
  the 
  amorphous 
  

  

  