﻿492 
  

  

  Mr. 
  W. 
  C. 
  Roberts 
  on 
  

  

  the 
  lower 
  portions 
  of 
  the 
  curve 
  similar 
  to 
  those 
  which 
  they 
  hold 
  on 
  

   Matthies 
  sen's 
  curve 
  of 
  electric 
  conductivity. 
  

  

  The 
  range 
  of 
  temperature 
  which 
  these 
  7. 
  

   melting-points 
  exhibit 
  appears 
  to 
  justify 
  the 
  

   conclusion 
  that 
  liquation 
  is 
  in 
  some 
  way 
  the 
  

   result 
  of 
  the 
  unequal 
  cooling 
  of 
  a 
  mass 
  of 
  

   silver 
  and 
  copper, 
  and 
  that 
  if 
  the 
  cooling 
  

   could 
  be 
  greatly 
  protracted 
  the 
  liquation 
  

   would 
  be 
  considerably 
  modified. 
  In 
  order 
  

   to 
  ascertain 
  whether 
  this 
  were 
  the 
  case, 
  I 
  

   used 
  cubical 
  moulds 
  (about 
  45 
  millims. 
  side) 
  

   of 
  firebrick 
  (fig. 
  7), 
  which 
  were 
  easily 
  heated 
  

   to 
  bright 
  redness, 
  and 
  in 
  which 
  the 
  alloys 
  

   could 
  be 
  slowly 
  and 
  uniformly 
  cooled*. 
  

  

  All 
  the 
  cubes 
  represented 
  on 
  pp. 
  490 
  & 
  491 
  were 
  cast 
  in 
  moulds 
  of 
  this 
  

   description. 
  The 
  first 
  of 
  these 
  (fig. 
  1), 
  the 
  composition 
  of 
  which 
  was 
  about 
  

   925 
  parts 
  of 
  silver 
  per 
  1000 
  of 
  the 
  alloy, 
  was 
  cooled 
  rapidly. 
  Its 
  structure 
  

   confirms 
  Levol's 
  general 
  conclusion, 
  as 
  the 
  centre 
  contains 
  12-8 
  parts 
  per 
  

   thousand 
  more 
  silver 
  than 
  the 
  external 
  portions. 
  On 
  the 
  other 
  hand, 
  

   fig. 
  2 
  shows 
  that 
  when 
  the 
  same 
  alloy 
  is 
  slowly 
  cooled 
  the 
  constituents 
  

   hardly 
  undergo 
  any 
  molecular 
  re-arrangement, 
  the 
  maximum 
  difference 
  

   being 
  only 
  1*4 
  per 
  thousand. 
  A 
  cube 
  of 
  the 
  alloy 
  used 
  for 
  the 
  French 
  

   coinage, 
  when 
  rapidly 
  cooled, 
  exhibits 
  a 
  difference 
  of 
  10*1 
  parts 
  per 
  

   thousand 
  between 
  the 
  centre 
  and 
  the 
  corners 
  ; 
  but 
  when 
  the 
  same 
  metal 
  

   is 
  slowly 
  cooled 
  (fig. 
  3), 
  the 
  variation 
  is 
  only 
  1*3 
  parts 
  per 
  thousand. 
  The 
  

   maximum 
  difference 
  found 
  by 
  Levol 
  in 
  his 
  homogeneous 
  alloy 
  was 
  0*44 
  

   part 
  per 
  thousand. 
  I 
  find 
  (fig. 
  4) 
  that 
  if 
  the 
  cooling 
  is 
  slowly 
  effected 
  this 
  

   homogeneity 
  is 
  disturbed, 
  the 
  external 
  portions 
  being 
  slightly 
  richer 
  in 
  

   silver 
  than 
  the 
  centre. 
  Fig. 
  5 
  exhibits 
  the 
  results 
  of 
  an 
  experiment 
  on 
  

   the 
  alloy 
  which 
  has 
  the 
  lowest 
  melting-point 
  and 
  the 
  simple 
  chemical 
  

   formula 
  AgCu. 
  Its 
  structure 
  is 
  interesting, 
  as 
  the 
  action 
  of 
  gravity 
  

   appears 
  to 
  have 
  influenced 
  the 
  arrangement, 
  the 
  lower 
  parts 
  of 
  the 
  cube 
  

   being 
  richer 
  than 
  the 
  upper. 
  The 
  maximum 
  difference 
  is 
  21*1 
  parts 
  per 
  

   thousand. 
  I 
  should 
  observe 
  that 
  Levol 
  found 
  the 
  corners 
  of 
  a 
  cube 
  of 
  

   this 
  alloy 
  to 
  be 
  15 
  parts 
  richer 
  than 
  the 
  centre 
  ; 
  but 
  the 
  only 
  alloy 
  in 
  

   which 
  he 
  detected 
  any 
  effect 
  of 
  gravity 
  was 
  that 
  containing 
  690 
  parts 
  of 
  

  

  * 
  In 
  treating 
  of 
  ingots 
  of 
  low 
  standard, 
  Jars 
  stated 
  in 
  1781, 
  in 
  the 
  work 
  to 
  which 
  I 
  have 
  

   already 
  alluded 
  (note, 
  p. 
  481) 
  : 
  — 
  " 
  Je 
  remarquai 
  par 
  des 
  experiences 
  que 
  pour 
  rendreles 
  

   lingots 
  d'une 
  teneur 
  plus 
  egale 
  dans 
  toutes 
  les 
  parties 
  il 
  falloit 
  que 
  les 
  lingotieres 
  fussent 
  

   aussi 
  chaudes 
  qu'il 
  est 
  possible." 
  I 
  should 
  state 
  that 
  certain 
  unpublished 
  experiments 
  by 
  

   Dr. 
  Boycott, 
  formerly 
  Assay 
  Master 
  in 
  the 
  Calcutta 
  Mint, 
  hare 
  shown 
  that 
  the 
  liqua- 
  

   tion 
  of 
  silver-copper 
  alloys 
  is 
  modified 
  by 
  casting 
  the 
  metal 
  in 
  sand 
  moulds, 
  and 
  that 
  Mr. 
  

   E. 
  Seyd 
  suggested 
  in 
  1871, 
  in 
  a 
  work 
  printed 
  for 
  private 
  circulation, 
  the 
  use 
  of 
  hot 
  

   iron 
  moulds 
  in 
  casting 
  gold 
  and 
  silver, 
  as 
  an 
  improvement 
  in 
  the 
  process 
  of 
  coinage, 
  

   the 
  bars 
  being 
  "more 
  equal 
  in 
  temper 
  and 
  in 
  molecular 
  arrangement" 
  (patented 
  in 
  

   1872, 
  No. 
  368). 
  

  

  