﻿31. 
  C. 
  Lea 
  — 
  Allotropic 
  Silver. 
  313 
  

  

  color. 
  (It 
  is 
  always 
  a 
  little 
  difficult 
  to 
  characterize 
  these 
  sub- 
  

   stances 
  by 
  their 
  colors 
  since 
  the 
  surface 
  color 
  which 
  they 
  show 
  

   when 
  dry 
  — 
  either 
  in 
  mass 
  or 
  in 
  films 
  — 
  is 
  mostly 
  complement- 
  

   ary 
  to 
  their 
  color 
  when 
  wet. 
  As 
  the 
  surface 
  color 
  is 
  much 
  the 
  

   more 
  characteristic, 
  I 
  have 
  adopted 
  the 
  course 
  of 
  naming 
  them 
  

   by 
  that.) 
  

  

  The 
  behavior 
  of 
  the 
  red 
  solution 
  obtained 
  by 
  soda 
  and 
  

   dextrine 
  with 
  dilute 
  sulphuric 
  is 
  very 
  interesting 
  and 
  instruc- 
  

   tive. 
  When 
  100 
  c.c. 
  of 
  solution 
  are 
  poured 
  into 
  100 
  c.c. 
  of 
  

   water 
  to 
  which 
  3 
  c.c. 
  of 
  sulphuric 
  acid 
  have 
  been 
  previously 
  

   added, 
  a 
  dark 
  red 
  precipitate 
  falls, 
  which, 
  when 
  dry, 
  especially 
  

   in 
  films, 
  is 
  blue. 
  The 
  mixed 
  liquid 
  from 
  which 
  the 
  precipi- 
  

   tate 
  is 
  formed 
  is 
  acid. 
  Increasing 
  the 
  proportion 
  of 
  acid 
  to 
  4, 
  

   5 
  and 
  6 
  c.c. 
  successively, 
  the 
  substance 
  obtained 
  has 
  a 
  green 
  

   surface 
  color 
  becoming 
  more 
  yellowish 
  green 
  in 
  proportion 
  as 
  

   the 
  acid 
  is 
  increased 
  in 
  quantity. 
  With 
  7-J- 
  c.c. 
  the 
  substance 
  

   no 
  longer 
  dries 
  green 
  but 
  yellow. 
  Increased 
  proportions 
  of 
  

   acid 
  produce 
  substances 
  drying 
  with 
  a 
  coppery 
  shade. 
  

  

  It 
  will 
  be 
  seen 
  that 
  from 
  a 
  single 
  solution, 
  and 
  using 
  one 
  

   substance 
  only 
  as 
  a 
  precipitant, 
  we 
  can 
  obtain 
  the 
  whole 
  range 
  

   of 
  different 
  forms 
  of 
  allotropic 
  silver, 
  by 
  simply 
  varying 
  the 
  

   proportions 
  of 
  the 
  precipitant. 
  

  

  That 
  these 
  forms 
  of 
  silver 
  should 
  subsist 
  in 
  the 
  presence 
  of 
  

   sulphuric 
  acid 
  in 
  excess 
  is 
  remarkable. 
  For 
  the 
  most 
  part 
  the 
  

   presence 
  of 
  this 
  acid 
  tends 
  to 
  quickly 
  convert 
  allotropic 
  to 
  

   ordinary 
  silver. 
  For 
  example, 
  bright 
  yellow 
  allotropic 
  silver 
  

   obtained 
  with 
  ferrous 
  tartrate 
  was 
  washed 
  on 
  a 
  filter 
  with' 
  

   water 
  containing 
  1/500 
  its 
  volume 
  of 
  sulphuric 
  acid 
  : 
  in 
  two 
  

   or 
  three 
  hours 
  the 
  entire 
  mass 
  was 
  converted 
  into 
  gray 
  ordinary 
  

   silver. 
  

  

  It 
  is 
  observable 
  that 
  the 
  substances 
  precipitated 
  with 
  the 
  

   least 
  acid, 
  have 
  a 
  very 
  splendid 
  luster, 
  and 
  that 
  this 
  luster 
  

   diminishes 
  steadily 
  as 
  the 
  proportion 
  of 
  acid 
  is 
  increased. 
  Up 
  

   to 
  6 
  c.c. 
  to 
  100 
  the 
  effect 
  is 
  hardly 
  noticeable, 
  after 
  that 
  it 
  be- 
  

   comes 
  more 
  marked. 
  

  

  But 
  we 
  can 
  also 
  obtain 
  'the 
  converse 
  of 
  this 
  reaction. 
  Just 
  

   as 
  the 
  solution 
  which 
  naturally 
  would 
  yield 
  the 
  blue 
  product, 
  

   can 
  be 
  made 
  to 
  yield 
  the 
  yellow 
  by 
  the 
  presence 
  of 
  excess 
  of 
  

   strong 
  acid, 
  so 
  the 
  solution 
  which 
  normally 
  yields 
  the 
  yellow 
  

   substance, 
  may 
  be 
  made 
  to 
  produce 
  blue 
  (or 
  rather 
  green) 
  

   silver 
  by 
  adding 
  alkali. 
  Thus 
  a 
  mixture 
  of 
  dilute 
  solutions 
  of 
  

   ferrous 
  sulphate 
  and 
  of 
  Rochelle 
  salt 
  added 
  to 
  mixed 
  solutions 
  

   of 
  silver 
  nitrate 
  and 
  of 
  Rochelle 
  salt, 
  results 
  in 
  the 
  formation 
  

   of 
  gold-colored 
  silver. 
  But 
  if 
  we 
  add 
  a 
  little 
  sodium 
  hydrox- 
  

   ide, 
  either 
  to 
  the 
  iron 
  solution 
  or 
  the 
  silver 
  mixture, 
  we 
  shall 
  

   get 
  a 
  bluish 
  green 
  product, 
  whose 
  properties 
  show 
  that 
  it 
  

   belongs 
  to 
  the 
  blue 
  class 
  and 
  not 
  to 
  the 
  yellow. 
  Even 
  if 
  a 
  

  

  