﻿482 
  

  

  Mr. 
  W. 
  C. 
  Roberts 
  on 
  

  

  The 
  alloys 
  in 
  question 
  have, 
  during 
  the 
  present 
  century, 
  been 
  the 
  

   subject 
  of 
  many 
  excellent 
  researches 
  in 
  this 
  country 
  and 
  on 
  the 
  continent, 
  

   the 
  earliest 
  systematic 
  experiments 
  being 
  those 
  of 
  D'Arcet, 
  Inspecteur 
  

   General 
  des 
  Essais 
  at 
  the 
  French 
  Mint, 
  who 
  in 
  1824 
  investigated 
  the 
  

   phenomena 
  which 
  attend 
  the 
  cooling 
  of 
  molten 
  mixtures 
  of 
  silver 
  and 
  

   copper. 
  He 
  does 
  not 
  appear 
  to 
  have 
  published 
  his 
  results 
  ; 
  but 
  in 
  1852 
  

   Levol 
  stated, 
  in 
  a 
  well-known 
  memoir*, 
  that 
  the 
  object 
  D'Arcet 
  had 
  in 
  

   view 
  was 
  the 
  discovery 
  of 
  a 
  method 
  of 
  rendering 
  homogeneous 
  the 
  bars 
  

   of 
  metal 
  used 
  in 
  coinage. 
  He 
  adds 
  that 
  the 
  researches 
  in 
  this 
  direction 
  

   offered 
  little 
  prospect 
  of 
  obtaining 
  such 
  a 
  result. 
  I 
  shall 
  presently 
  show, 
  

   as 
  the 
  result 
  of 
  my 
  experiments, 
  that 
  this 
  conclusion 
  may 
  now 
  be 
  

   modified. 
  

  

  Levol, 
  in 
  his 
  own 
  experiments, 
  cast 
  the 
  alloy 
  to 
  be 
  examined 
  either 
  in 
  

   a 
  cubical 
  iron 
  mould 
  of 
  45 
  millims. 
  side, 
  or 
  in 
  a 
  sphere 
  50 
  millims. 
  in 
  

   diameter. 
  He 
  concluded 
  that 
  the 
  only 
  homogeneous 
  alloy 
  contains 
  

   718-93 
  parts 
  of 
  silver 
  and 
  281*07 
  of 
  copper 
  in 
  1000 
  ; 
  and 
  he 
  considers 
  

   this 
  to 
  be 
  a 
  definite 
  combination 
  of 
  the 
  two 
  metals, 
  having 
  the 
  formula 
  

   Ag 
  3 
  Cu 
  4 
  (or 
  Ag 
  3 
  Cu 
  2 
  if 
  63*34 
  be 
  taken 
  to 
  be 
  the 
  equivalent 
  of 
  copper). 
  

   All 
  other 
  alloys 
  of 
  silver 
  and 
  copper 
  he 
  views 
  as 
  mixtures 
  of 
  this 
  definite 
  

   alloy 
  with 
  excess 
  of 
  either 
  of 
  the 
  metals. 
  

  

  In 
  1860 
  f 
  Matthiessen 
  studied 
  these 
  alloys 
  with 
  the 
  minute 
  accuracy 
  

   which 
  characterized 
  all 
  his 
  work, 
  and 
  he 
  described 
  them 
  as 
  " 
  mechanical 
  

   mixtures 
  of 
  allotropic 
  modifications 
  of 
  the 
  two 
  metals 
  in 
  each 
  other." 
  

   The 
  curve 
  of 
  electric 
  conductivity 
  led 
  him 
  to 
  doubt 
  Levol's 
  conclusion 
  

   that 
  they 
  were 
  mixtures 
  of 
  the 
  definite 
  alloy 
  Ag 
  3 
  Cu 
  2 
  (new 
  notation) 
  with 
  

   silver 
  or 
  copper 
  ; 
  for, 
  as 
  he 
  observes, 
  if 
  this 
  were 
  the 
  case, 
  we 
  should 
  

   expect 
  to 
  find 
  that 
  the 
  curve 
  consisted 
  of 
  two 
  straight 
  lines, 
  connecting 
  

   Levol's 
  alloy 
  with 
  pure 
  silver 
  and 
  copper 
  respectively. 
  But 
  on 
  examin- 
  

   ing 
  the 
  curve, 
  starting 
  from 
  the 
  copper 
  side 
  of 
  it, 
  we 
  see 
  that 
  a 
  rapid 
  

   decrement 
  in 
  conducting-power 
  is 
  caused 
  by 
  copper 
  being 
  alloyed 
  with 
  a 
  

   small 
  quantity 
  of 
  silver. 
  From 
  the 
  alloy 
  containing 
  10 
  per 
  cent, 
  of 
  

   silver 
  to 
  that 
  containing 
  65 
  per 
  cent, 
  we 
  have 
  a 
  straight 
  line 
  ; 
  and 
  we 
  

   may 
  regard 
  the 
  intermediate 
  alloys 
  as 
  mixtures 
  or 
  solutions 
  of 
  these 
  two 
  

   alloys. 
  Again, 
  from 
  the 
  latter 
  of 
  these 
  to 
  that 
  containing 
  72 
  per 
  cent, 
  

   of 
  silver, 
  we 
  may 
  have 
  a 
  mixture 
  or 
  solution 
  of 
  alloys 
  containing 
  65 
  and 
  

   72 
  per 
  cent, 
  respectively. 
  This 
  last 
  point 
  (which 
  corresponds 
  to 
  a 
  con- 
  

   ducting-power 
  of 
  63*7, 
  fine 
  silver 
  being 
  taken 
  as 
  100) 
  is 
  the 
  minimum 
  

   point 
  of 
  the 
  curve, 
  and 
  corresponds 
  almost 
  exactly 
  to 
  Levol's 
  Ag 
  3 
  Cu, 
  alloy. 
  

   The 
  alloys 
  intermediate 
  between 
  this 
  and 
  pure 
  silver 
  may 
  be 
  mixtures, 
  or 
  

   perhaps 
  solutions 
  of 
  it 
  in 
  silver 
  containing 
  a 
  small 
  quantity 
  of 
  copper. 
  

   Now 
  it 
  would 
  appear 
  as 
  if 
  there 
  was 
  an 
  alloy 
  of 
  constant 
  composition 
  

   at 
  about 
  the 
  point 
  which 
  represents 
  the 
  one 
  containing 
  65 
  per 
  cent. 
  

  

  * 
  " 
  Sur 
  les 
  alliages 
  consideres 
  sous 
  le 
  rapport 
  de 
  leur 
  composition 
  chimique," 
  Ann. 
  

   Chira. 
  et, 
  Pkys. 
  (3) 
  t. 
  xxxvi. 
  

   t 
  Phil. 
  Trans. 
  1860, 
  p. 
  173. 
  

  

  