﻿226 
  Mr. 
  R. 
  Mallet 
  on 
  the 
  alleged 
  Expansion 
  

  

  to 
  stand 
  at 
  rest, 
  the 
  thin 
  coat 
  of 
  scoria 
  or 
  molten 
  oxide 
  which 
  forms 
  on 
  

   the 
  surface 
  of 
  the 
  metal 
  is 
  seen, 
  as 
  fast 
  as 
  it 
  forms 
  at 
  the 
  circumference 
  

   of 
  the 
  ladle, 
  to 
  be 
  swept 
  by 
  active 
  convergent 
  currents 
  towards 
  the 
  centre, 
  

   where 
  it 
  accumulates 
  in 
  a 
  patch. 
  As 
  the 
  fluid 
  metal 
  parts 
  with 
  some 
  of 
  

   its 
  heat 
  and 
  the 
  ladle 
  gets 
  hot 
  by 
  absorbing 
  it, 
  this 
  remarkable 
  surface- 
  

   disturbance 
  becomes 
  less 
  energetic." 
  This 
  arises 
  from 
  " 
  the 
  expansion 
  of 
  

   that 
  portion 
  of 
  the 
  molten 
  mass 
  which 
  is 
  in 
  contact 
  with 
  the 
  compara- 
  

   tively 
  cool 
  sides 
  of 
  the 
  ladle, 
  which 
  sides 
  act 
  as 
  the 
  chief 
  agent 
  in 
  dispers- 
  

   ing 
  the 
  heat 
  of 
  the 
  melted 
  metal 
  ; 
  careful 
  observation 
  will 
  show 
  that 
  the 
  

   motion 
  in 
  question 
  is 
  the 
  result 
  of 
  an 
  upward 
  current 
  of 
  the 
  metal 
  around 
  

   the 
  circ 
  umf 
  erence 
  of 
  the 
  ladle, 
  as 
  indicated 
  by 
  the 
  arrows 
  A, 
  B, 
  C 
  in 
  the 
  

   accompanying 
  sectional 
  drawing 
  

   of 
  the 
  ladle 
  " 
  [fig. 
  7 
  ; 
  the 
  figure 
  in 
  Fig. 
  

   the 
  margin 
  is 
  essentially 
  the 
  same 
  

   as 
  that 
  of 
  these 
  authors]. 
  " 
  The 
  

   upward 
  current 
  of 
  the 
  metal 
  can 
  

   be 
  seen 
  at 
  the 
  rim 
  of 
  the 
  ladle, 
  

   where 
  it 
  is 
  deflected 
  into 
  the 
  con- 
  

   vergent 
  horizontal 
  direction, 
  and 
  

   where 
  it 
  presents 
  an 
  elevatory 
  ap- 
  

   pearance, 
  as 
  shown 
  in 
  the 
  figure. 
  

   It 
  is 
  difficult 
  to 
  assign 
  to 
  this 
  any 
  

   cause 
  but 
  that 
  of 
  expansion 
  and 
  

   consequent 
  reduction 
  of 
  specific 
  

   gravity 
  of 
  the 
  fluid 
  metal 
  in 
  con- 
  

   tact 
  with 
  the 
  sides 
  of 
  the 
  pot, 
  as, 
  

   according 
  to 
  the 
  generally 
  enter- 
  

   tained 
  idea, 
  the 
  surface-currents 
  above 
  referred 
  to 
  would 
  be 
  in 
  the 
  con- 
  

   trary 
  direction 
  to 
  that 
  which 
  they 
  invariably 
  take, 
  i. 
  e. 
  they 
  would 
  diverge 
  

   from 
  the 
  centre 
  instead 
  of 
  converging 
  to 
  it." 
  

  

  The 
  facts, 
  so 
  far 
  as 
  they 
  are 
  above 
  described, 
  are 
  generally 
  correct, 
  but 
  

   the 
  explanation 
  given 
  is 
  not 
  the 
  true 
  one. 
  The 
  currents 
  observable 
  for 
  

   some 
  time 
  after 
  a 
  large 
  ladle 
  (say, 
  holding 
  10 
  tons) 
  is 
  first 
  filled 
  with 
  

   molten 
  iron 
  are 
  not 
  produced 
  by 
  difference 
  of 
  temperature 
  in 
  different 
  

   parts 
  of 
  the 
  mass, 
  but 
  in 
  the 
  following 
  way 
  : 
  — 
  Such 
  a 
  ladle 
  is 
  of 
  wrought 
  

   iron, 
  about 
  half 
  an 
  inch 
  in 
  thickness 
  ; 
  and 
  to 
  preserve 
  this 
  tolerably 
  cool, 
  

   even 
  for 
  several 
  hours, 
  it 
  is 
  lined 
  with 
  a 
  coating 
  of 
  earthy 
  material 
  daubed 
  

   upon 
  the 
  interior 
  in 
  a 
  tough 
  and 
  plastic 
  state, 
  from 
  an 
  inch 
  to 
  an 
  

   inch 
  and 
  a 
  half 
  in 
  thickness, 
  and 
  dried 
  within 
  it. 
  The 
  lining 
  mate- 
  

   rial 
  consists 
  of 
  plastic 
  clay, 
  with 
  a 
  proportion 
  of 
  siliceous 
  sand 
  beaten 
  up 
  

   together 
  with 
  horsedung, 
  chaff, 
  plasterer's 
  cow-hair, 
  or 
  other 
  fibrous 
  

   material, 
  conferring 
  toughness 
  upon 
  the 
  mass 
  when 
  soft 
  and 
  porosity 
  

   when 
  dry. 
  This 
  material, 
  after 
  drying 
  at 
  a 
  temperature 
  averaging 
  500° 
  

   to 
  700° 
  Fahr., 
  on 
  being 
  exposed 
  to 
  contact 
  with 
  the 
  molten 
  cast 
  iron, 
  

   exhales 
  torrents 
  of 
  gas 
  and 
  vapour, 
  which 
  pass 
  upwards 
  through 
  the 
  

  

  