﻿100 
  

  

  Mr. 
  G. 
  F. 
  Rodwell 
  on 
  the 
  

  

  [Dec. 
  10, 
  

  

  t. 
  A 
  developing 
  solution 
  composed 
  of 
  ferrous 
  sulphate, 
  alcohol, 
  acetic 
  

   acid, 
  and 
  water, 
  when 
  applied 
  to 
  the 
  exposed 
  sheets 
  of 
  rj 
  and 
  d, 
  which 
  had 
  

   been 
  purple, 
  but 
  on 
  continued 
  exposure 
  nearly 
  regained 
  their 
  original 
  

   colours, 
  produced 
  a 
  reddish-brown 
  colour. 
  

  

  k. 
  A 
  sheet 
  of 
  silver 
  leaf 
  was 
  exposed 
  to 
  the 
  vapour 
  of 
  iodine 
  for 
  many 
  

   hours 
  ; 
  it 
  was 
  found 
  to 
  be 
  converted 
  into 
  a 
  slightly 
  coherent 
  film 
  of 
  

   lemon-yellow 
  iodide. 
  Light 
  had 
  no 
  effect 
  upon 
  it, 
  even 
  after 
  long 
  

   exposure 
  to 
  a 
  July 
  sun 
  ; 
  neither 
  was 
  any 
  colour 
  produced 
  on 
  the 
  addi- 
  

   tion 
  of 
  a 
  developing 
  solution. 
  

  

  The 
  pure 
  iodide 
  of 
  silver 
  would 
  thus 
  appear 
  to 
  be 
  scarcely 
  affected 
  by 
  

   light, 
  except 
  when 
  silver 
  is 
  present, 
  either 
  in 
  the 
  form 
  of 
  nitrate 
  or, 
  as 
  in 
  

   the 
  case 
  of 
  the 
  silver 
  films, 
  as 
  metallic 
  silver. 
  

  

  If 
  the 
  precipitated 
  iodide 
  of 
  silver 
  be 
  fused 
  it 
  is 
  found 
  to 
  cool 
  to 
  a 
  

   greenish-grey 
  mass, 
  which 
  in 
  thin 
  layers 
  is 
  translucent. 
  The 
  surface 
  

   has 
  sometimes 
  a 
  dark 
  steel-grey, 
  semimetallic 
  appearance, 
  but 
  this 
  does 
  

   not 
  affect 
  the 
  composition. 
  Sometimes, 
  without 
  any 
  apparent 
  cause, 
  the 
  

   ordinary 
  greenish 
  surface 
  and 
  the 
  dark 
  steel-grey 
  may 
  exist 
  side 
  by 
  side 
  

   in 
  the 
  same 
  fused 
  mass. 
  A 
  second 
  fusion 
  may 
  produce 
  a 
  uniformly 
  

   greenish 
  surface 
  or 
  a 
  uniformly 
  steel-grey 
  surface. 
  But 
  whatever 
  the 
  

   appearance 
  of 
  the 
  fused 
  mass, 
  it 
  always 
  furnishes 
  when 
  pulverized 
  a 
  

   lemon-yellow 
  powder, 
  which, 
  when 
  heated, 
  remains 
  unaltered 
  in 
  colour 
  

   up 
  to 
  about 
  105° 
  C. 
  (221° 
  F.). 
  At 
  that 
  temperature 
  it 
  begins 
  to 
  darken, 
  

   and 
  between 
  105° 
  C. 
  and 
  180° 
  C. 
  (356° 
  F.) 
  it 
  assumes 
  darker 
  and 
  darker 
  

   shades 
  of 
  yellow, 
  passing 
  into 
  orange 
  and 
  orange-red 
  ; 
  above 
  180° 
  C. 
  it 
  

   becomes 
  decidedly 
  red, 
  and 
  darkens 
  through 
  temperatures 
  which 
  may 
  be 
  

   roughly 
  indicated 
  by 
  the 
  fusing-points 
  of 
  tin, 
  lead, 
  and 
  zinc, 
  until 
  at 
  the 
  

   latter 
  temperature 
  (412° 
  C, 
  773° 
  F.) 
  it 
  possesses 
  a 
  very 
  dark 
  brick-red 
  

   colour. 
  At 
  this 
  temperature 
  the 
  powder 
  becomes 
  coherent, 
  but 
  does 
  not 
  

   commence 
  to 
  fuse. 
  At 
  a 
  somewhat 
  higher 
  temperature, 
  probably 
  about 
  

   450° 
  C. 
  (842° 
  F.), 
  the 
  iodide 
  fuses 
  to 
  a 
  dark-red 
  liquid, 
  the 
  colour 
  of 
  

   bromine, 
  or 
  of 
  melted 
  sulphur 
  shortly 
  before 
  its 
  boiling-point. 
  At 
  a 
  red 
  

   heat 
  the 
  iodide 
  begins 
  to 
  volatilize 
  and 
  to 
  decompose, 
  and 
  at 
  a 
  bright 
  

   red 
  heat 
  this 
  takes 
  place 
  readily. 
  If 
  the 
  iodide 
  .be 
  fused 
  and 
  poured 
  

   into 
  cold 
  water, 
  it 
  becomes 
  a 
  lemon-yellow, 
  amorphous, 
  very 
  brittle 
  

   mass. 
  

  

  If 
  the 
  fused 
  mass 
  of 
  iodide 
  is 
  allowed 
  to 
  cool, 
  it 
  solidifies 
  to 
  a 
  dark- 
  

   red 
  transparent 
  body, 
  which 
  is 
  somewhat 
  plastic. 
  On 
  further 
  cooling 
  

   it 
  becomes 
  much 
  paler 
  in 
  colour, 
  still 
  remaining 
  transparent 
  ; 
  and 
  if 
  

   cooled 
  as 
  a 
  thin 
  film 
  in 
  contact 
  with 
  a 
  hot 
  surface, 
  it 
  passes 
  to 
  a 
  pale 
  

   yellow 
  transparent 
  variety. 
  The 
  transparent 
  varieties, 
  at 
  a 
  temperature 
  

   which 
  varies 
  with 
  the 
  mass 
  of 
  the 
  substance, 
  and 
  which 
  in 
  the 
  case 
  of 
  a 
  

   thin 
  film 
  may 
  be 
  as 
  low 
  as 
  105° 
  C, 
  become 
  crystalline, 
  opaque, 
  and 
  of 
  a 
  

   pale 
  greenish-grey 
  colour, 
  somewhat 
  brittle, 
  and 
  of 
  a 
  granular 
  fracture. 
  

   At 
  the 
  moment 
  of 
  the 
  change 
  from 
  the 
  amorphous, 
  transparent, 
  plastic 
  

   variety 
  to 
  the 
  opaque, 
  brittle, 
  crystalline 
  variety, 
  considerable 
  expansion 
  

  

  