﻿of 
  various 
  Substances 
  on 
  Solidification. 
  

  

  229 
  

  

  of 
  the 
  facts, 
  is, 
  that 
  when 
  a 
  very 
  thick 
  iron 
  mould 
  of 
  this 
  sort 
  is 
  suddenly- 
  

   heated 
  by 
  pouring 
  molten 
  iron 
  into 
  its 
  interior, 
  as 
  the 
  heat 
  abstracted 
  

   from 
  the 
  latter 
  can 
  only 
  pass 
  into 
  the 
  material 
  of 
  the 
  mould 
  at 
  a 
  rate 
  

   determined 
  by 
  its 
  conductivity, 
  so 
  the 
  interior 
  part 
  rapidly 
  becomes 
  

   raised 
  to 
  a 
  temperature 
  enormously 
  higher 
  than 
  the 
  exterior 
  portions, 
  

   which 
  for 
  a 
  time 
  remain 
  almost 
  cold. 
  The 
  expanded 
  interior 
  walls 
  of 
  

   the 
  mould 
  push 
  inwards 
  as 
  towards 
  the 
  points 
  of 
  least 
  resistance, 
  and 
  so 
  

   actually 
  diminish 
  the 
  capacity 
  of 
  the 
  mould 
  for 
  a 
  time, 
  the 
  inner 
  surfaces 
  

   of 
  which 
  press 
  upon 
  the 
  consolidating 
  crust 
  of 
  metal 
  within 
  it, 
  and 
  so 
  

   squeeze 
  out 
  in 
  part 
  its 
  liquid 
  contents, 
  just 
  as 
  water 
  might 
  be 
  squeezed 
  

   from 
  an 
  india-rubber 
  bottle*. 
  

  

  It 
  seemed 
  desirable 
  to 
  obtain 
  some 
  experimental 
  results 
  in 
  reference 
  to 
  

   the 
  objects 
  of 
  this 
  communication 
  with 
  lead. 
  It 
  has 
  never, 
  so 
  far 
  as 
  the 
  

   author 
  is 
  aware, 
  been 
  even 
  suggested 
  that 
  this 
  metal 
  expands 
  in 
  consoli- 
  

   dating. 
  Its 
  coefficient 
  of 
  dilatation 
  by 
  heat 
  is 
  enormously 
  greater 
  than 
  

   that 
  of 
  cast 
  iron, 
  being, 
  according 
  to 
  the 
  determination 
  of 
  Lavoisier 
  and 
  

   Laplace, 
  between 
  32° 
  and 
  212° 
  Fahr. 
  =0-0000474 
  of 
  its 
  volume 
  for 
  one 
  

   degree 
  Fahr.; 
  so 
  that, 
  taking 
  its 
  fusing-point 
  at 
  617° 
  (Eudberg), 
  and 
  

   assuming 
  the 
  coefficient 
  constant 
  for 
  the 
  entire 
  range 
  from 
  60° 
  to 
  617° 
  

   (which 
  is 
  much 
  below 
  the 
  truth), 
  its 
  dilatation 
  when 
  in 
  fusion 
  would 
  

   be=0-0264 
  of 
  its 
  volume, 
  and 
  the 
  specific 
  gravity 
  of 
  lead 
  at 
  60°= 
  11-36 
  ; 
  

   that 
  of 
  liquid 
  lead 
  must 
  be 
  below 
  11-07. 
  Indeed 
  this 
  enormous 
  amount 
  

   of 
  dilatation 
  is 
  impressed 
  upon 
  any 
  observer 
  who 
  sees 
  the 
  rate 
  at 
  which 
  

   the 
  lead 
  in 
  casting 
  a 
  common 
  bullet 
  sinks 
  into 
  the 
  neck 
  of 
  the 
  mould, 
  

   and 
  the 
  comparatively 
  large 
  cavity 
  which 
  always 
  exists 
  in 
  the 
  ball 
  when 
  

   cut 
  in 
  two. 
  From 
  its 
  low 
  temperature 
  of 
  fusion 
  and 
  the 
  suddenness 
  

   with 
  which 
  lead 
  passes 
  from 
  the 
  solid 
  to 
  the 
  liquid 
  state 
  without 
  

   any 
  phase 
  of 
  intermediate 
  viscidity, 
  and 
  only 
  a 
  brief 
  one 
  of 
  crystalline 
  

   brittleness, 
  and 
  the 
  facility 
  with 
  which 
  its 
  surface 
  can 
  be 
  kept 
  free 
  from 
  

   dross 
  or 
  oxide, 
  this 
  metal 
  presents 
  a 
  " 
  crucial 
  " 
  example 
  for 
  experiment 
  

   in 
  reference 
  to 
  our 
  subject. 
  

  

  The 
  following 
  experiments, 
  by 
  the 
  kind 
  permission 
  of 
  Messrs. 
  Pontifex 
  

   and 
  Wood, 
  London, 
  were 
  made 
  at 
  their 
  works 
  : 
  — 
  

  

  1st. 
  Upon 
  the 
  surface 
  of 
  a 
  large 
  pot 
  of 
  melted 
  lead, 
  the 
  temperature 
  

  

  * 
  The 
  enormous 
  disparity 
  in 
  temperature 
  between 
  the 
  interior 
  and 
  the 
  exterior 
  

   couches 
  of 
  a 
  very 
  thick 
  spherical 
  or 
  cylindrical 
  iron 
  mould 
  is 
  only 
  partially 
  shown 
  by 
  

   Biot's 
  expression, 
  

  

  for 
  the 
  distribution 
  of 
  temperature 
  along 
  a 
  bar 
  heated 
  at 
  one 
  extremity 
  ; 
  for 
  in 
  the 
  case 
  

   here 
  before 
  us, 
  as 
  in 
  that 
  of 
  all 
  forces 
  radiating 
  from 
  a 
  centre, 
  the 
  temperature 
  at 
  any 
  

   given 
  point, 
  and 
  quite 
  independent 
  of 
  any 
  question 
  of 
  conductivity, 
  varies 
  inversely 
  as 
  

   the 
  square 
  of 
  its 
  distance 
  from 
  the 
  centre. 
  If 
  a 
  unit 
  in 
  volume 
  situated 
  in 
  any 
  point 
  of 
  

   a 
  radius 
  lose 
  as 
  much 
  heat 
  as 
  will 
  lower 
  its 
  temperature 
  1°, 
  that 
  heat, 
  when 
  trans- 
  

   mitted 
  to 
  double 
  the 
  distance 
  along 
  the 
  radius, 
  can 
  only 
  raise 
  the 
  temperature 
  of 
  a 
  unit 
  

   in 
  volume 
  there 
  by 
  0-25 
  of 
  1° 
  having 
  been 
  diffused 
  amongst 
  four 
  units 
  of 
  volume. 
  (See 
  

   Biot, 
  « 
  Traite 
  de 
  Physique,' 
  and 
  Mallet,' 
  Trans. 
  E. 
  I. 
  A./ 
  1856.) 
  

  

  