﻿98 
  

  

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

  

  the 
  expansion 
  of 
  platinum 
  for 
  100° 
  C. 
  He 
  also 
  found 
  that 
  a 
  large 
  

   hexagonal 
  crystal 
  exhibited 
  a 
  very 
  considerable 
  contraction 
  in 
  the 
  direc- 
  

   tion 
  of 
  the 
  axis 
  of 
  symmetry, 
  while 
  a 
  slight 
  expansion 
  was 
  produced 
  in 
  a 
  

   direction 
  normal 
  to 
  the 
  axis 
  of 
  the 
  crystal*. 
  The 
  contraction 
  was 
  

   observed 
  in 
  the 
  case 
  both 
  of 
  the 
  crystal, 
  a 
  confused 
  crystalline 
  mass, 
  and 
  

   an 
  amorphous 
  mass 
  produced 
  by 
  strongly 
  compressing 
  the 
  precipitated 
  

   iodide 
  until 
  it 
  became 
  a 
  hard 
  mass 
  capable 
  of 
  receiving 
  a 
  fine 
  polish, 
  and 
  

   possessing 
  a 
  specific 
  gravity 
  of 
  5*569. 
  Fizeau 
  considers 
  that 
  the 
  iodide 
  

   possesses 
  its 
  maximum 
  of 
  volume 
  or 
  minimum 
  of 
  density 
  at 
  a 
  tempera- 
  

   ture 
  of 
  -60° 
  C. 
  ( 
  -76° 
  F.). 
  

  

  The 
  iodide 
  of 
  silver 
  employed 
  in 
  the 
  following 
  experiments 
  was 
  pre- 
  

   pared 
  : 
  — 
  (1) 
  By 
  precipitation. 
  Pure 
  iodide 
  of 
  potassium 
  was 
  added 
  to 
  

   nitrate 
  of 
  silver, 
  both 
  in 
  dilute 
  solution. 
  The 
  precipitated 
  iodide 
  was 
  

   thoroughly 
  washed 
  in 
  the 
  dark, 
  slowly 
  dried, 
  fused 
  in 
  a 
  porcelain 
  crucible, 
  

   and 
  cast 
  into 
  cylindrical 
  masses, 
  either 
  in 
  a 
  warm 
  porcelain 
  or 
  brass 
  

   mould 
  t. 
  (2) 
  By 
  dissolving 
  pure 
  silver 
  in 
  strong 
  hydriodic 
  acid, 
  eva- 
  

   porating 
  to 
  dryness, 
  fusing. 
  (3) 
  By 
  exposing 
  pure 
  silver 
  leaf 
  for 
  

   several 
  hours 
  to 
  the 
  vapour 
  of 
  iodine 
  produced 
  by 
  spontaneous 
  eva- 
  

   poration. 
  

  

  Before 
  we 
  examine 
  the 
  effects 
  of 
  heat 
  upon 
  the 
  iodide, 
  it 
  may 
  be 
  well 
  

   to 
  say 
  a 
  word 
  or 
  two 
  concerning 
  the 
  action 
  of 
  light 
  upon 
  it. 
  A 
  con- 
  

   siderable 
  amount 
  of 
  misconception 
  appears 
  to 
  exist 
  in 
  regard 
  to 
  this. 
  

   Gmelin 
  says 
  " 
  it 
  turns 
  brown 
  on 
  exposure 
  to 
  light, 
  but 
  less 
  quickly 
  than 
  

   the 
  chloride 
  ; 
  " 
  Miller 
  says 
  "it 
  is 
  but 
  slowly 
  acted 
  upon 
  by 
  light 
  ; 
  " 
  

   Fizeau 
  describes 
  it 
  as 
  " 
  noircissant 
  lentement 
  a 
  la 
  lumiere;" 
  while 
  

   Vogel 
  (' 
  Jahresbericht,' 
  1863) 
  affirm^ 
  that 
  if 
  it 
  be 
  precipitated 
  with 
  

   excess 
  of 
  iodide 
  of 
  potassium 
  it 
  is 
  scarcely 
  affected 
  by 
  light, 
  whereas 
  if 
  

   precipitated 
  with 
  excess 
  of 
  nitrate 
  of 
  silver 
  it 
  changes 
  colour, 
  but 
  under- 
  

   goes 
  no 
  chemical 
  change. 
  The 
  general 
  idea 
  that 
  it 
  is 
  nearly 
  as 
  sensitive 
  

   to 
  light 
  as 
  the 
  chloride, 
  has 
  no 
  doubt 
  arisen 
  from 
  the 
  fact 
  that 
  iodides 
  

   and 
  chlorides 
  are 
  known 
  to 
  have 
  many 
  points 
  of 
  resemblance, 
  and 
  that 
  

   the 
  iodide 
  is 
  largely 
  used 
  in 
  photography 
  ; 
  moreover 
  we 
  remember 
  that 
  

   a 
  thin 
  film 
  of 
  iodide 
  of 
  silver 
  was 
  the 
  sensitive 
  medium 
  in 
  the 
  original 
  

   daguerreotype. 
  But 
  we 
  must 
  bear 
  in 
  mind 
  that 
  the 
  change 
  produced 
  by 
  

   light 
  is 
  not 
  apparent 
  until 
  the 
  so-called 
  " 
  developing 
  solution," 
  which 
  

   contains 
  reducing 
  agents, 
  has 
  been 
  employed. 
  The 
  change 
  is 
  indeed 
  

   most 
  obscure 
  : 
  the 
  author 
  of 
  the 
  article 
  on 
  Photography 
  in 
  Watt's 
  

   'Dictionary 
  of 
  Chemistry' 
  says 
  of 
  it, 
  "The 
  atoms 
  have 
  apparently 
  

   acquired 
  a 
  certain 
  degree 
  of 
  mobility, 
  in 
  consequence 
  of 
  which, 
  wheii 
  

   submitted 
  to 
  the 
  action 
  of 
  reducing 
  agents, 
  such 
  as 
  ferrous 
  sulphate 
  or 
  

   pyrogallic 
  acid, 
  they 
  suffer 
  decomposition, 
  the 
  silver 
  being 
  reduced 
  to 
  the 
  

  

  * 
  " 
  Sur 
  la 
  propriete 
  que 
  possede 
  l'iodure 
  d'argent 
  de 
  se 
  contracter 
  par 
  la 
  chaleur 
  et 
  

   de 
  se 
  dilater 
  par 
  le 
  froid," 
  Comptes 
  Rendus, 
  1867. 
  

  

  t 
  I 
  must 
  express 
  my 
  indebtedness 
  to 
  Mr. 
  Valentin 
  for 
  allowing 
  me 
  to 
  have 
  a 
  

   quantity 
  of 
  iodide 
  prepared 
  at 
  South 
  Kensington. 
  

  

  