﻿of 
  various 
  Substances 
  on 
  Solidification. 
  227 
  

  

  molten 
  mass 
  and 
  determine 
  the 
  direction 
  of 
  its 
  currents 
  ; 
  and 
  it 
  will 
  be 
  

   obvious, 
  on 
  inspecting 
  the 
  figure, 
  that 
  these 
  currents 
  will 
  be 
  most 
  powerful 
  

   round 
  the 
  outer 
  circumference 
  of 
  the 
  mass, 
  where 
  each 
  unit 
  of 
  its 
  top 
  sur- 
  

   face 
  has 
  a 
  larger 
  proportion 
  of 
  lining 
  in 
  proximity 
  to 
  it 
  than 
  at 
  the 
  cen- 
  

   tral 
  parts 
  of 
  the 
  mass, 
  where 
  downward 
  currents 
  are 
  the 
  necessary 
  con- 
  

   sequence 
  of 
  those 
  produced 
  upwards 
  at 
  the 
  circumference. 
  The 
  organic 
  

   matters 
  mixed 
  with 
  the 
  lining 
  are 
  carbonized, 
  and 
  give 
  forth 
  the 
  elements 
  

   of 
  water 
  as 
  well 
  as 
  nitrogen. 
  The 
  clay, 
  which 
  is 
  a 
  hydrous 
  silicate 
  of 
  

   various 
  earthy 
  bases, 
  gives 
  forth 
  its 
  water 
  and 
  some 
  of 
  the 
  oxygen 
  of 
  the 
  

   peroxide 
  of 
  iron 
  which 
  most 
  clays 
  contain. 
  More 
  or 
  less 
  carbonate 
  of 
  

   lime 
  is 
  almost 
  always 
  interspersed, 
  and 
  this 
  gives 
  forth 
  carbonic 
  acid 
  and 
  

   water. 
  The 
  gases 
  thus 
  streamed 
  forth 
  act 
  mechanically 
  by 
  their 
  ascent 
  

   and 
  also 
  chemically 
  upon 
  molten 
  iron, 
  the 
  water 
  being 
  decomposed, 
  oxi- 
  

   dizing 
  portions 
  of 
  the 
  iron 
  and 
  forming 
  scorise, 
  which 
  is 
  again 
  more 
  or 
  

   less 
  reduced 
  by 
  contact 
  of 
  the 
  hydrogen 
  and 
  nitrogen 
  when 
  the 
  latter 
  is 
  

   present. 
  These 
  rapid 
  combinations 
  and 
  decompositions 
  are 
  no 
  doubt 
  the 
  

   main 
  cause 
  of 
  those 
  singular 
  vermicular 
  startings 
  referred 
  to 
  by 
  Messrs. 
  

   Nasmyth 
  and 
  Carpenter, 
  which 
  are 
  familiar 
  to 
  every 
  iron-founder, 
  but 
  

   which 
  are 
  entirely 
  distinct 
  from 
  the 
  ascending 
  and 
  descending 
  currents 
  

   due 
  to 
  the 
  ascent 
  of 
  the 
  evolved 
  gases. 
  That 
  this 
  is 
  the 
  true 
  explanation 
  

   is 
  supported 
  by 
  the 
  following 
  facts 
  : 
  — 
  1. 
  After 
  a 
  large 
  ladle 
  has 
  stood 
  full 
  

   of 
  molten 
  metal 
  for 
  some 
  hours, 
  and 
  time 
  has 
  been 
  given 
  thus 
  for 
  the 
  

   whole 
  of 
  the 
  gaseous 
  contents 
  of 
  the 
  lining 
  to 
  be 
  driven 
  off, 
  the 
  ascending 
  

   and 
  descending 
  currents 
  cease 
  to 
  be 
  perceptible, 
  and 
  if 
  any 
  currents 
  at 
  all 
  

   can 
  be 
  discerned 
  they 
  are 
  in 
  the 
  opposite 
  directions. 
  2. 
  If, 
  after 
  this, 
  

   such 
  a 
  ladle 
  be 
  emptied 
  of 
  its 
  contents, 
  the 
  lining 
  remaining 
  untouched 
  

   and 
  only 
  coated 
  with 
  a 
  thin 
  shell 
  of 
  adherent 
  cast 
  iron 
  [and 
  oxides 
  and 
  

   silicates 
  of 
  iron], 
  and 
  the 
  ladle 
  being 
  again 
  filled 
  with 
  molten 
  iron, 
  no 
  

   such 
  currents 
  as 
  at 
  first 
  are 
  produced 
  in 
  the 
  molten 
  mass, 
  the 
  lining 
  

   having 
  been 
  previously 
  exhausted 
  of 
  its 
  gases 
  and 
  vapours. 
  That 
  the 
  

   currents 
  described 
  by 
  Messrs. 
  Nasmyth 
  and 
  Carpenter 
  are 
  not 
  due 
  to 
  dis- 
  

   sipation 
  of 
  heat 
  from 
  the 
  mass 
  through 
  the 
  sides 
  of 
  the 
  ladle 
  is 
  evident 
  

   from 
  the 
  following 
  considerations 
  : 
  — 
  

  

  A 
  10-ton 
  ladle, 
  which 
  is 
  about 
  4^ 
  feet 
  by 
  3 
  feet 
  in 
  depth, 
  loses 
  heat 
  so 
  

   slowly 
  that 
  after 
  standing 
  for 
  six 
  hours 
  the 
  molten 
  metal 
  is 
  still 
  fluid 
  

   enough 
  to 
  make 
  castings. 
  Let 
  us 
  suppose 
  it 
  filled 
  into 
  the 
  ladle 
  at 
  a 
  

   temperature 
  of 
  2800° 
  to 
  2900° 
  Fahr., 
  and 
  that 
  after 
  six 
  hours 
  it 
  is 
  still 
  

   200° 
  above 
  the 
  temperature 
  of 
  solidification 
  of 
  cast 
  iron, 
  or 
  at 
  2600°. 
  

   The 
  molten 
  mass 
  has 
  thus 
  lost 
  300° 
  of 
  heat 
  in 
  360 
  minutes, 
  or 
  -0138 
  of 
  

   a 
  degree 
  per 
  second. 
  "We 
  may 
  assume 
  this 
  at 
  any 
  instant 
  as 
  represent- 
  

   ing 
  the 
  difference 
  in 
  temperature 
  between 
  two 
  vertical 
  columns, 
  one 
  at 
  

   the 
  centre 
  and 
  the 
  other 
  at 
  the 
  circumference 
  of 
  the 
  molten 
  mass. 
  The 
  

   linear 
  dilatation 
  of 
  cast 
  iron 
  for 
  one 
  degree 
  of 
  Fahrenheit 
  being 
  0*0000111, 
  

   as 
  deduced 
  from 
  its 
  total 
  cubic 
  dilatation 
  between 
  60° 
  Eahr. 
  and 
  the 
  tem- 
  

   perature 
  of 
  fusion 
  at 
  which 
  it 
  was 
  poured 
  into 
  the 
  cone, 
  as 
  given 
  in 
  this 
  

  

  