﻿1874.] 
  On 
  the 
  Effects 
  of 
  Heat 
  on 
  Iodide 
  of 
  Silver. 
  

  

  97 
  

  

  III. 
  " 
  On 
  the 
  Effects 
  of 
  Heat 
  on 
  Iodide 
  of 
  Silver." 
  By 
  G. 
  F. 
  

   Rodwell, 
  F.R.A.S., 
  F. 
  C.S. 
  } 
  Science 
  Master 
  in 
  Marlborough 
  

   College. 
  Communicated 
  by 
  Professor 
  Frederick 
  Guthrie, 
  

   F.R.S. 
  Received 
  August 
  14, 
  1874. 
  

  

  Professor 
  Clerk 
  Maxwell, 
  when 
  discussing 
  the 
  expansion 
  of 
  matter 
  by 
  

   heat 
  (' 
  Theory 
  of 
  Heat,' 
  p. 
  8), 
  says, 
  " 
  The 
  body 
  generally 
  expands 
  (the 
  

   only 
  exception 
  among 
  solid 
  bodies, 
  as 
  far 
  as 
  I 
  am 
  aware, 
  is 
  the 
  iodide 
  

   of 
  silver, 
  which 
  has 
  been 
  found 
  to 
  contract 
  as 
  the 
  temperature 
  rises)." 
  

   M. 
  H. 
  Fizeau, 
  speaking 
  of 
  the 
  same 
  substance 
  (' 
  Nouvelles 
  Observations 
  

   relatives 
  a 
  l'iodure 
  d'argent 
  '), 
  writes 
  as 
  follows 
  : 
  — 
  " 
  Ce 
  corps, 
  en 
  effet, 
  

   parait 
  offrir 
  l'exemple 
  d'une 
  inversion 
  complete 
  des 
  phenomenes 
  ordi- 
  

   naires 
  de 
  la 
  dilatation 
  par 
  la 
  chaleur, 
  car 
  son 
  volume 
  diminue 
  tres-cer- 
  

   tainement 
  pendant 
  l'echauffement 
  et 
  augmente 
  pendant 
  le 
  refroidisse- 
  

   ment." 
  

  

  It 
  was 
  thought 
  that 
  a 
  substance 
  possessing 
  so 
  marked 
  a 
  property 
  

   would 
  probably 
  exhibit 
  peculiarities 
  of 
  molecular 
  structure 
  ; 
  and 
  the 
  fol- 
  

   lowing 
  experiments 
  were 
  made 
  in 
  order 
  to 
  determine 
  whether 
  such 
  

   peculiarities 
  exist, 
  to 
  note 
  the 
  effects 
  of 
  higher 
  temperatures 
  upon 
  the 
  

   iodide 
  than 
  those 
  employed 
  by 
  Fizeau 
  (which 
  in 
  no 
  instance 
  exceeded 
  

   100° 
  C), 
  and 
  to 
  determine 
  the 
  point 
  of 
  maximum 
  density 
  of 
  the 
  

   iodide. 
  The 
  phenomena 
  which 
  most 
  closely 
  approximate 
  to 
  those 
  

   assigned 
  to 
  the 
  iodide 
  of 
  silver 
  when 
  heated 
  are 
  to 
  be 
  found 
  in 
  the 
  

   case 
  of 
  the 
  anomalous 
  expansion 
  of 
  ice 
  and 
  bismuth, 
  and 
  a 
  few 
  other 
  

   substances 
  which 
  at 
  the 
  moment 
  of 
  fusion, 
  and 
  for 
  a 
  few 
  degrees 
  above 
  

   their 
  point 
  of 
  solidification, 
  exhibit 
  contraction 
  on 
  being 
  heated 
  ; 
  but 
  in 
  

   these 
  instances 
  we 
  have 
  to 
  bear 
  in 
  mind 
  that 
  a 
  change 
  of 
  state 
  is 
  simul- 
  

   taneously 
  effected, 
  or 
  about 
  to 
  be 
  effected, 
  in 
  the 
  substance. 
  Again, 
  

   certain 
  crystals 
  contract 
  in 
  the 
  direction 
  of 
  one 
  of 
  their 
  axes 
  on 
  the 
  

   application 
  of 
  heat 
  ; 
  but 
  they 
  expand 
  in 
  the 
  direction 
  of 
  another 
  axis, 
  

   and 
  the 
  total 
  expansion 
  is 
  greater 
  than 
  the 
  contraction, 
  so 
  that 
  they 
  

   possess 
  a 
  positive 
  coefficient 
  of 
  expansion. 
  Carnets 
  and 
  a 
  few 
  other 
  

   crystals 
  undergo 
  an 
  increase 
  of 
  specific 
  gravity 
  on 
  being 
  strongly 
  heated, 
  

   and 
  slowly 
  recover 
  their 
  original 
  density. 
  

  

  The 
  iodide 
  of 
  silver, 
  on 
  the 
  other 
  hand, 
  when 
  far 
  removed 
  from 
  the 
  

   point 
  at 
  which 
  it 
  undergoes 
  any 
  change 
  of 
  state, 
  appears 
  to 
  exhibit 
  con- 
  

   traction, 
  to 
  possess 
  what 
  M. 
  Fizeau 
  calls 
  a 
  " 
  negative 
  coefficient 
  of 
  

   expansion 
  and 
  this 
  is 
  the 
  more 
  remarkable 
  when 
  we 
  remember 
  

   that 
  the 
  chlorides, 
  bromides, 
  and 
  iodides 
  of 
  potassium, 
  sodium, 
  and 
  

   ammonium, 
  and 
  the 
  chloride 
  and 
  bromide 
  of 
  silver 
  expand 
  consider- 
  

   ably 
  when 
  heated, 
  more 
  so, 
  indeed, 
  than 
  the 
  most 
  expansible 
  metals 
  5 
  

   such 
  as 
  lead, 
  tin, 
  and 
  zinc. 
  The 
  contraction 
  of 
  the 
  iodide 
  of 
  silver 
  is, 
  

   according 
  to 
  Fizeau, 
  quite 
  regular 
  between 
  —10° 
  C. 
  (14° 
  F.) 
  and 
  

   + 
  70° 
  C. 
  (158° 
  F.) 
  ; 
  and 
  he 
  calculates 
  that 
  the 
  contraction 
  is 
  equal 
  tc 
  

   about 
  of 
  its 
  volume 
  at 
  0° 
  C. 
  for 
  100° 
  C, 
  or, 
  again, 
  equal 
  to 
  one 
  sixtt 
  

  

  