﻿314 
  M. 
  C. 
  Lea 
  — 
  Allotropic 
  Silver. 
  

  

  solution 
  of 
  the 
  hydroxide 
  is 
  added 
  immediately 
  after 
  the 
  iron 
  

   solution 
  has 
  been 
  poured 
  into 
  the 
  silver, 
  the 
  result 
  is 
  the 
  same. 
  

  

  There 
  is 
  therefore 
  a 
  well 
  marked 
  tendency 
  of 
  acids 
  to 
  give 
  

   rise 
  to 
  the 
  formation 
  of 
  the 
  yellow 
  product 
  and 
  of 
  alkalies 
  to 
  

   the 
  blue. 
  But 
  this 
  is 
  a 
  tendency 
  only. 
  Both 
  substances 
  can 
  

   be 
  produced 
  from 
  neutral 
  solutions, 
  and 
  slight 
  changes 
  are 
  

   sufficient 
  to 
  alter 
  the 
  product 
  formed. 
  Thus, 
  ferrous 
  tartrate, 
  

   in 
  dilute 
  solution 
  acting 
  on 
  silver 
  tartrate 
  gives 
  rise 
  to 
  the 
  for- 
  

   mation 
  of 
  the 
  gold-colored 
  substance, 
  but 
  when 
  citrates 
  are 
  

   substituted, 
  the 
  blue 
  substance 
  is 
  obtained. 
  

  

  Production 
  of 
  Allotropic 
  Silver 
  by 
  Inorganic 
  Substances. 
  — 
  

   For 
  reasons 
  which 
  will 
  be 
  mentioned 
  presently, 
  the 
  reduction 
  

   of 
  silver 
  must 
  take 
  place 
  gradually 
  to 
  produce 
  the 
  allotropic 
  

   form, 
  and 
  for 
  a 
  time 
  it 
  seemed 
  an 
  invariable 
  condition 
  that 
  an 
  

   organic 
  substance 
  of 
  some 
  sort 
  should 
  be 
  present. 
  This, 
  how- 
  

   ever, 
  proves 
  not 
  to 
  be 
  essential. 
  In 
  a 
  paper 
  presented 
  to 
  the 
  

   American 
  Academy 
  and 
  kindly 
  read 
  for 
  me 
  by 
  Professor 
  

   Bemsen 
  at 
  the 
  meeting 
  in 
  April 
  last, 
  I 
  alluded 
  briefly 
  to 
  having 
  

   found 
  a 
  reaction 
  depending 
  upon 
  inorganic 
  agents 
  only. 
  It 
  is 
  

   as 
  follows 
  : 
  Sodium 
  hypophosphite 
  added 
  to 
  silver 
  nitrate 
  does 
  

   not 
  effect 
  reduction, 
  but 
  when 
  hypophosphorous 
  acid 
  is 
  set 
  

   free 
  by 
  the 
  addition 
  of 
  phosphoric 
  acid, 
  a 
  red 
  coloration 
  

   appears, 
  indicating 
  the 
  presence 
  of 
  allotropic 
  silver. 
  The 
  

   coloration 
  is 
  transitory, 
  no 
  doubt 
  because 
  of 
  the 
  strong 
  ten- 
  

   dency 
  of 
  free 
  mineral 
  acids 
  to 
  convert 
  allotropic 
  to 
  normal 
  

   silver, 
  but 
  red 
  and 
  blue 
  stains 
  form 
  on 
  the 
  sides 
  of 
  the 
  vessel. 
  

  

  Phosphorous 
  acid 
  gives 
  similar 
  results, 
  though 
  perhaps 
  less 
  

   well 
  marked. 
  

  

  Action 
  of 
  Light 
  on 
  Blue 
  Silver. 
  — 
  This 
  action 
  differs 
  with 
  

   different 
  varieties 
  : 
  it 
  was 
  more 
  especially 
  examined 
  with 
  the 
  

   form 
  that 
  is 
  obtained 
  from 
  the 
  soda 
  dextrine 
  silver 
  solution 
  

   already 
  described 
  by 
  pouring 
  the 
  solution 
  into 
  an 
  equal 
  bulk 
  of 
  

   water 
  to 
  which 
  sulphuric 
  acid 
  had 
  been 
  added 
  in 
  the 
  propor- 
  

   tion 
  of 
  4 
  c.c. 
  to 
  each 
  100 
  c.c. 
  of 
  water. 
  This 
  form 
  was 
  selected 
  

   because 
  it 
  is 
  easy 
  to 
  obtain 
  with 
  great 
  constancy 
  of 
  result, 
  and 
  

   because 
  it 
  is 
  one 
  of 
  the 
  forms 
  of 
  blue 
  silver 
  most 
  sensitive 
  to 
  

   light. 
  

  

  Exposed 
  to 
  light, 
  this 
  substance 
  first 
  becomes 
  more 
  distinctly 
  

   blue, 
  losing 
  a 
  slight 
  greenish 
  shade. 
  With 
  continued 
  exposure 
  

   it 
  passes 
  to 
  a 
  yellow-brown 
  shade, 
  and 
  finally 
  to 
  a 
  perfectly 
  

   pure 
  golden-yellow 
  of 
  great 
  brilliancy 
  and 
  luster. 
  The 
  last 
  is 
  

   the 
  intermediate 
  or 
  crystalline 
  form. 
  

  

  The 
  action 
  of 
  light 
  on 
  this 
  form 
  of 
  silver 
  is 
  remarkable 
  in 
  

   this 
  respect, 
  that 
  its 
  first 
  effect 
  is 
  to 
  increase 
  the 
  sensitiveness 
  

   to 
  reagents. 
  

  

  This 
  result 
  was 
  so 
  unexpected 
  and 
  a 
  priori 
  so 
  improbable, 
  

   that 
  it 
  was 
  subjected 
  to 
  the 
  most 
  careful 
  verification 
  before 
  

  

  