﻿1874.] 
  

  

  Effects 
  of 
  Heat 
  on 
  Iodide 
  of 
  Silver. 
  

  

  105 
  

  

  and 
  more 
  compact 
  than 
  that 
  produced 
  by 
  precipitation. 
  Deville 
  found 
  the 
  

   specific 
  gravity 
  of 
  the 
  unfused 
  precipitated 
  iodide 
  to 
  be 
  5*807, 
  and 
  of 
  the 
  

   fused 
  iodide 
  5-687, 
  while 
  Damour 
  found 
  the 
  native 
  crystals 
  to 
  have 
  a 
  

   specific 
  gravity 
  of 
  5*667 
  ; 
  hence 
  the 
  amorphous 
  precipitate 
  has 
  a 
  higher 
  

   density 
  than 
  either 
  the 
  fused 
  crystalline 
  iodide 
  and 
  their 
  native 
  crystals. 
  

   Thus 
  the 
  density 
  of 
  the 
  amorphous 
  precipitate 
  coincides 
  almost 
  perfectly 
  

   with, 
  the 
  density 
  of 
  the 
  fused 
  iodide 
  at 
  its 
  point 
  of 
  maximum 
  density 
  

   (116° 
  C.) 
  when 
  in 
  the 
  amorphous 
  condition, 
  as 
  deduced 
  from 
  the 
  above 
  

   experiments. 
  

  

  If 
  we 
  place 
  in 
  a 
  specific-gravity 
  flask 
  a 
  quantity 
  of 
  fused 
  iodide 
  of 
  

   silver, 
  fill 
  it 
  up 
  with 
  mercury 
  (taking 
  every 
  precaution 
  to 
  displace 
  the 
  air 
  

   from 
  the 
  intercrystalline 
  spaces), 
  and 
  place 
  in 
  the 
  ground 
  neck 
  of 
  the 
  

   flask 
  a 
  perforated 
  stopper 
  continued 
  as 
  a 
  capillary 
  thermometer-stem, 
  we 
  

   have 
  obviously 
  a 
  thermometer 
  in 
  which 
  we 
  can 
  observe 
  the 
  effect 
  pro- 
  

   duced 
  by 
  the 
  anomalous 
  contraction 
  of 
  the 
  iodide 
  on 
  the 
  regularly 
  

   expanding 
  mercury. 
  On 
  applying 
  heat 
  to 
  such 
  an 
  arrangement 
  we 
  

   observe 
  that 
  for 
  a 
  while 
  the 
  mercury 
  rises 
  in 
  the 
  stem, 
  until 
  on 
  further 
  

   heating 
  the 
  contraction 
  of 
  the 
  iodide 
  exceeds 
  the 
  expansion 
  of 
  the 
  mercury, 
  

   and 
  the 
  column 
  retreats 
  if 
  much 
  iodide 
  is 
  present 
  into 
  the 
  very 
  bulb 
  of 
  

   the 
  instrument. 
  

  

  If 
  the 
  heating 
  be 
  now 
  discontinued 
  the 
  mercury 
  slowly 
  rises 
  as 
  the 
  

   iodide 
  cools, 
  until 
  the 
  contraction 
  of 
  the 
  mercury 
  exceeds 
  the 
  expansion 
  

   of 
  the 
  iodide, 
  beyond 
  which 
  point 
  the 
  mercury 
  continues 
  to 
  sink 
  as 
  the 
  

   bulb 
  cools. 
  Nothing 
  could 
  better 
  illustrate 
  the 
  complete 
  inversion 
  of 
  the 
  

   effects 
  usually 
  produced 
  by 
  heat 
  on 
  bodies 
  in 
  the 
  case 
  of 
  the 
  iodide 
  of 
  

   silver. 
  

  

  Professor 
  Gruthrie 
  suggested 
  to 
  me 
  that 
  a 
  convenient 
  method 
  of 
  deter- 
  

   mining 
  the 
  amount 
  of 
  contraction 
  produced 
  by 
  heat 
  in 
  the 
  iodide 
  would 
  

   be 
  to 
  fill 
  a 
  specific-gravity 
  flask 
  with 
  mercury, 
  and 
  determine 
  the 
  amounts 
  

   of 
  mercury 
  exuded 
  from 
  the 
  flask 
  for 
  every 
  ten 
  or 
  twenty 
  degrees 
  of 
  

   temperature 
  ; 
  then 
  to 
  place 
  in 
  the 
  flask 
  a 
  known 
  weight 
  of 
  fused 
  iodide 
  

   with 
  a 
  known 
  weight 
  of 
  mercury, 
  and 
  repeat 
  the 
  determinations. 
  This 
  

   was 
  accordingly 
  done. 
  About 
  440 
  grammes 
  of 
  mercury 
  were 
  employed, 
  

   and 
  weighings 
  made 
  at 
  intervals 
  of 
  a 
  few 
  degrees. 
  The 
  method 
  was 
  

   found 
  to 
  be 
  satisfactory, 
  and 
  the 
  results 
  concordant. 
  For 
  example 
  (to 
  

   take 
  a 
  few 
  instances 
  from 
  many), 
  the 
  amounts 
  of 
  mercury 
  driven 
  from 
  

   the 
  flask 
  by 
  expansion 
  for 
  10° 
  C. 
  were 
  found 
  to 
  be 
  as 
  follows 
  : 
  — 
  

  

  Between 
  29° 
  C. 
  and 
  53° 
  C 
  -066062 
  

  

  22° 
  C. 
  „ 
  74° 
  C 
  -067250 
  

  

  20° 
  C. 
  „ 
  84° 
  C 
  -067390 
  

  

  29° 
  C. 
  „ 
  86° 
  C 
  -068526 
  

  

  47° 
  C. 
  „ 
  84° 
  C 
  -069297 
  

  

  numbers 
  which, 
  when 
  the 
  necessary 
  corrections 
  have 
  been 
  made 
  for 
  

  

  