﻿212 
  

  

  Mr. 
  R. 
  Mallet 
  on 
  the 
  alleged 
  Expansion. 
  

  

  cast 
  irons 
  is 
  known 
  to 
  be 
  somewhat 
  greater 
  than 
  that 
  of 
  the 
  less 
  dense 
  j 
  

   but 
  as 
  the 
  increase 
  in 
  volume 
  may 
  not 
  be 
  sufficient 
  to 
  equalize 
  the 
  spe- 
  

   cific 
  gravity 
  of 
  a 
  very 
  dense 
  iron 
  when 
  in 
  fusion 
  with 
  that 
  of 
  a 
  very 
  light 
  

   iron 
  when 
  cold, 
  so 
  it 
  is 
  obvious 
  that 
  a 
  piece 
  of 
  cold 
  cast 
  iron 
  might 
  be 
  

   so 
  selected 
  in 
  reference 
  to 
  its 
  specific 
  gravity, 
  as 
  referred 
  to 
  that 
  of 
  

   another 
  sort 
  of 
  cast 
  iron 
  in 
  fusion, 
  that 
  the 
  former 
  should 
  either 
  sink 
  or 
  

   swim 
  upon 
  the 
  latter 
  by 
  mere 
  buoyancy, 
  were 
  that 
  free 
  to 
  act 
  alone. 
  

  

  2nd. 
  Assuming 
  the 
  cold 
  and 
  the 
  molten 
  cast 
  iron 
  originally 
  identical 
  

   in 
  qualities, 
  whether 
  a 
  piece 
  of 
  the 
  former 
  shall 
  float 
  or 
  not 
  float 
  upon 
  

   the 
  latter 
  depends 
  not 
  only 
  upon 
  buoyancy 
  as 
  above, 
  but 
  also 
  upon 
  the 
  

   form 
  of 
  the 
  piece 
  of 
  cold 
  metal 
  — 
  that 
  is 
  to 
  say, 
  on 
  the 
  relation, 
  all 
  other 
  

   things 
  being 
  the 
  same, 
  that 
  subsists 
  between 
  its 
  volume 
  and 
  its 
  surface. 
  

  

  3rd. 
  The 
  force, 
  whatever 
  be 
  its 
  nature, 
  which 
  keeps 
  the 
  piece 
  of 
  cold 
  

   cast 
  iron 
  floating 
  is 
  of 
  sufficient 
  energy 
  to 
  overcome 
  a 
  considerable 
  want 
  of 
  

   buoyancy 
  in 
  the 
  cold 
  iron 
  under 
  certain 
  conditions, 
  so 
  that 
  it 
  may 
  float 
  

   upon 
  molten 
  cast 
  iron 
  whose 
  specific 
  gravity, 
  as 
  such, 
  is 
  much 
  less 
  than 
  

   that 
  of 
  the 
  colder 
  iron 
  which 
  floats 
  upon 
  it. 
  Messrs. 
  Nasmyth 
  and 
  

   Carpenter 
  assume, 
  without 
  any 
  sufficient 
  proof, 
  that 
  solid 
  cast 
  iron 
  floats 
  

   on 
  liquid 
  iron 
  of 
  the 
  same 
  quality 
  in 
  virtue 
  of 
  buoyancy 
  alone, 
  and 
  pro- 
  

   ceed 
  to 
  state 
  that 
  " 
  the 
  inevitable 
  inference 
  from 
  this 
  is 
  that 
  in 
  the 
  case 
  

   of 
  cast 
  iron 
  the 
  solid 
  is 
  specifically 
  lighter 
  than 
  the 
  molten, 
  and 
  there- 
  

   fore 
  that, 
  in 
  passing 
  from 
  the 
  molten 
  to 
  the 
  solid 
  condition, 
  this 
  substance 
  

   undergoes 
  expansion 
  in 
  bulk 
  " 
  (' 
  The 
  Moon,' 
  pp. 
  20, 
  21). 
  

  

  I 
  proceed 
  to 
  prove 
  that 
  this 
  view 
  is 
  altogether 
  contrary 
  to 
  fact. 
  The 
  

   determination 
  of 
  the 
  specific 
  gravity 
  of 
  cast 
  iron 
  in 
  its 
  molten 
  condition 
  

   is 
  a 
  problem 
  of 
  considerable 
  difficulty, 
  and 
  can 
  only 
  be 
  solved 
  by 
  indirect 
  

   means 
  ; 
  we 
  cannot 
  ascertain 
  its 
  specific 
  gravity 
  by 
  any 
  of 
  the 
  methods 
  

   ordinarily 
  employed, 
  nor 
  can 
  any 
  are- 
  

   ometric 
  method 
  be 
  used, 
  as 
  any 
  hydro- 
  

   meter 
  or 
  solid 
  of 
  known 
  specific 
  gra- 
  

   vity 
  at 
  common 
  temperatures, 
  when 
  

   dipped 
  into 
  liquid 
  cast 
  iron, 
  changes 
  

   its 
  volume 
  as 
  well 
  as 
  gets 
  incrusted 
  

   with 
  adherent 
  cast 
  iron 
  or 
  its 
  oxides, 
  

   &c. 
  By 
  an 
  indirect 
  method, 
  and 
  by 
  

   operating 
  upon 
  a 
  sufficiently 
  large 
  

   scale 
  to 
  eliminate 
  certain 
  sources 
  of 
  

   error, 
  the 
  specific 
  gravity 
  of 
  molten 
  

   cast 
  iron 
  may, 
  however, 
  be 
  approxi- 
  

   mately 
  ascertained 
  with 
  considerable 
  

   accuracy. 
  The 
  method 
  adopted 
  by 
  

   the 
  author 
  was 
  as 
  follows 
  : 
  — 
  A 
  

   conical 
  vessel, 
  of 
  the 
  form 
  shown 
  

   in 
  figure 
  1, 
  was 
  formed 
  of 
  wrought- 
  

   iron 
  plate 
  by 
  welding 
  up 
  only, 
  

  

  Fig. 
  1. 
  

  

  