﻿108 
  Mr. 
  G. 
  F. 
  Rodwell 
  on 
  the 
  [Dec. 
  10, 
  

  

  I 
  have 
  endeavoured 
  to 
  prove 
  in 
  the 
  foregoing 
  pages 
  the 
  following 
  

   main 
  facts 
  : 
  — 
  

  

  1. 
  That 
  the 
  iodide 
  of 
  silver 
  exists 
  in 
  three 
  allotropic 
  forms, 
  viz. 
  (a), 
  

   at 
  temperatures 
  between 
  116° 
  C. 
  and 
  its 
  fusing-point, 
  as 
  a 
  plastic, 
  

   tenacious, 
  amorphous 
  substance 
  possessing 
  a 
  reddish 
  colour 
  and 
  trans- 
  

   parent 
  to 
  light 
  ; 
  (jg), 
  at 
  temperatures 
  below 
  116° 
  C, 
  as 
  a 
  brittle, 
  opaque, 
  

   greenish-grey, 
  crystalline 
  mass 
  ; 
  and 
  (y), 
  if 
  fused 
  and 
  poured 
  into 
  cold 
  

   water, 
  as 
  an 
  amorphous, 
  very 
  brittle, 
  yellow, 
  opaque 
  substance. 
  

  

  2. 
  That 
  the 
  iodide 
  possesses 
  a 
  point 
  of 
  maximum 
  density 
  at 
  or 
  about 
  

   116° 
  C. 
  at 
  the 
  moment 
  before 
  passing 
  from 
  the 
  amorphous 
  into 
  the 
  crys- 
  

   talline 
  condition. 
  

  

  3. 
  That 
  if 
  we 
  allow 
  a 
  mass 
  of 
  molten 
  iodide 
  to 
  cool, 
  the 
  following 
  

   effects 
  ma,y 
  be 
  observed 
  : 
  — 
  (a) 
  at 
  the 
  moment 
  of 
  solidification 
  a 
  very 
  

   considerable 
  contraction 
  takes 
  place 
  ; 
  (/3) 
  the 
  solid, 
  on 
  further 
  cooling, 
  

   undergoes 
  slight 
  and 
  regular 
  contraction 
  after 
  the 
  manner 
  of 
  solid 
  bodies 
  

   in 
  general, 
  until 
  (y) 
  at 
  or 
  about 
  116° 
  C. 
  it 
  undergoes 
  sudden 
  and 
  violent 
  

   expansion, 
  passing 
  from 
  the 
  amorphous 
  into 
  the 
  crystalline 
  condition 
  ; 
  

   (3) 
  after 
  undergoing 
  this 
  expansion 
  the 
  mass 
  on 
  further 
  cooling 
  undergoes 
  

   slight 
  expansion, 
  and 
  (e) 
  the 
  coefficient 
  of 
  contraction 
  diminishes 
  as 
  the 
  

   temperature 
  decreases 
  (or 
  otherwise 
  expressed, 
  the 
  coefficient 
  of 
  con- 
  

   traction 
  augments 
  with 
  the 
  temperature). 
  

  

  I 
  must, 
  in 
  conclusion, 
  express 
  my 
  great 
  indebtedness 
  to 
  Dr. 
  Guthrie 
  

   for 
  allowing 
  me 
  to 
  carry 
  out 
  many 
  of 
  the 
  foregoing 
  experiments 
  in 
  the 
  

   Physical 
  Laboratory 
  at 
  South 
  Kensington. 
  

  

  IV. 
  " 
  On 
  the 
  Coefficient 
  of 
  Expansion 
  of 
  a 
  Paraffine 
  of 
  high 
  boiling- 
  

   point." 
  By 
  G. 
  F. 
  Rodwell, 
  F.R.A.S., 
  F.C.S., 
  Science 
  Master 
  

   in 
  Marlborough 
  College. 
  Communicated 
  by 
  Prof. 
  Stokes, 
  

   Sec. 
  U.S. 
  Received 
  November 
  17, 
  1874. 
  

  

  In 
  the 
  search 
  for 
  a 
  liquid 
  of 
  high 
  boiling-point 
  which 
  could 
  be 
  used 
  

   for 
  the 
  exact 
  determination 
  of 
  the 
  coefficient 
  of 
  expansion 
  of 
  iodide 
  of 
  

   silver, 
  and 
  which 
  at 
  the 
  same 
  time 
  should 
  be 
  free 
  from 
  certain 
  objections 
  

   which 
  apply 
  to 
  the 
  employment 
  of 
  mercury 
  for 
  this 
  purpose, 
  a 
  paraffine 
  

   of 
  high 
  boiling-point 
  naturally 
  suggested 
  itself. 
  Mr. 
  W. 
  H. 
  Hatcher, 
  to 
  

   whom 
  I 
  express 
  my 
  acknowledgments, 
  procured 
  for 
  me 
  a 
  specimen 
  of 
  

   paraffine 
  which 
  was 
  taken 
  from 
  one 
  of 
  the 
  stills 
  at 
  Messrs. 
  Price's 
  works 
  

   at 
  a 
  high 
  temperature. 
  "With 
  it 
  the 
  following 
  experiments 
  were 
  made. 
  

  

  The 
  paraffine 
  in 
  question 
  is 
  perfectly 
  white 
  and 
  pearly, 
  and 
  in 
  thin 
  

   layers 
  translucent 
  ; 
  it 
  fuses 
  to 
  a 
  colourless 
  liquid, 
  which 
  slightly 
  darkens 
  

   when 
  heated 
  for 
  some 
  time 
  to 
  a 
  temperature 
  exceeding 
  600° 
  F. 
  

   (315°-55 
  C). 
  Its 
  specific 
  gravity 
  at 
  32° 
  F. 
  (0° 
  C.) 
  is 
  -921. 
  It 
  is 
  hard 
  

   and 
  somewhat 
  granular 
  when 
  broken, 
  and 
  at 
  a 
  low 
  temperature 
  may 
  be 
  

   crudely 
  pulverized 
  by 
  a 
  blow. 
  At 
  temperatures 
  exceeding 
  100°F. 
  

  

  