﻿M. 
  C. 
  Lea 
  — 
  Allotropic 
  Silver. 
  315 
  

  

  being 
  accepted. 
  The 
  action 
  is 
  very 
  easily 
  shown 
  by 
  exposing 
  

   a 
  film 
  of 
  the 
  substance 
  to 
  light, 
  covering 
  part 
  of 
  the 
  surface 
  

   with 
  an 
  opaque 
  screen. 
  After 
  twenty 
  or 
  thirty 
  minutes 
  of 
  

   exposure 
  to 
  strong 
  summer 
  sunshine, 
  the 
  film 
  may 
  be 
  plunged 
  

   into 
  a 
  one 
  per 
  cent 
  solution 
  of 
  potassium 
  ferridcyanide, 
  when 
  

   the 
  part 
  exposed 
  colors 
  much 
  sooner 
  and 
  more 
  strongly 
  than 
  

   that 
  which 
  was 
  covered. 
  The 
  effect 
  is 
  shown 
  still 
  better 
  by 
  

   placing 
  the 
  film 
  in 
  a 
  frame, 
  covering 
  part 
  with 
  paper 
  rendered 
  

   absolutely 
  opaque 
  by 
  coating 
  it 
  with 
  thick 
  tin 
  foil, 
  part 
  with 
  

   translucent 
  paper 
  (thick 
  white 
  writing 
  paper 
  or 
  very 
  thin 
  

   brown 
  paper) 
  and 
  leaving 
  part 
  wholly 
  exposed. 
  After 
  four 
  or 
  

   five 
  hours 
  action 
  of 
  strong 
  summer 
  sunshine, 
  the 
  film 
  is 
  to 
  be 
  

   treated 
  with 
  weak 
  ferridcyanide. 
  The 
  part 
  wholly 
  exposed 
  

   having 
  passed 
  into 
  the 
  gold-colored 
  crystalline 
  condition 
  (if 
  

   the 
  exposure 
  has 
  been 
  sufficient) 
  is 
  wholly 
  unacted 
  upon, 
  the 
  

   part 
  covered 
  by 
  the 
  translucent 
  paper 
  is 
  rapidly 
  attacked, 
  that 
  

   wholly 
  protected 
  is 
  attacked 
  slowly. 
  So 
  that 
  the 
  portion 
  

   moderately 
  acted 
  on 
  by 
  light 
  has 
  very 
  markedly 
  increased 
  in 
  

   sensitiveness 
  thereby. 
  

  

  It 
  follows 
  that 
  upon 
  this 
  form 
  of 
  silver 
  light 
  has 
  a 
  reversing 
  

   action, 
  first 
  exalting 
  its 
  sensitiveness, 
  then 
  completely 
  destroy- 
  

   ing 
  it. 
  

  

  It 
  is 
  impossible 
  to 
  overlook 
  the 
  analogy 
  which 
  exists 
  between 
  

   this 
  action 
  of 
  light, 
  and 
  that 
  which 
  light 
  exerts 
  on 
  silver 
  

   bromide. 
  

  

  The 
  latter 
  substance 
  though 
  incomparably 
  more 
  sensitive 
  to 
  

   light, 
  is 
  subject 
  to 
  the 
  same 
  reversing 
  action, 
  first 
  gaining 
  in 
  

   sensitiveness 
  to 
  reducing 
  agents 
  and 
  then, 
  by 
  continued 
  expos- 
  

   ure, 
  becoming 
  less 
  sensitive 
  than 
  originally, 
  a 
  change 
  com- 
  

   monly 
  known 
  as 
  solarization. 
  

  

  Causes 
  determining 
  vjhether 
  in 
  the 
  reduction 
  of 
  Silver, 
  

   the 
  Allotropic 
  or 
  the 
  formal 
  form 
  shall 
  he 
  produced. 
  — 
  I 
  have 
  

   examined 
  the 
  phenomena 
  connected 
  with 
  the 
  reduction 
  of 
  

   silver 
  under 
  a 
  great 
  variety 
  of 
  conditions. 
  These 
  for 
  the 
  most 
  

   part 
  do 
  not 
  deserve 
  particular 
  mention 
  but 
  seem 
  to 
  lead 
  up 
  to 
  

   this 
  generalization 
  : 
  that 
  the 
  reduction 
  of 
  silver 
  may 
  be 
  direct 
  

   or 
  indirect, 
  direct 
  when 
  it 
  passes 
  from 
  the 
  condition 
  of 
  the 
  

   normal 
  salt 
  or 
  oxide 
  to 
  that 
  of 
  the 
  metal, 
  indirect 
  when 
  the 
  

   change 
  is 
  first 
  to 
  sub-oxide 
  or 
  to 
  a 
  corresponding 
  sub-salt. 
  So 
  

   far 
  as 
  my 
  observation 
  has 
  gone 
  when 
  the 
  reduction 
  is 
  direct 
  

   the 
  reduced 
  silver 
  always 
  appears 
  in 
  its 
  ordinary 
  form. 
  But 
  

   when 
  the 
  reduction 
  is 
  indirect 
  the 
  silver 
  presents 
  itself 
  in 
  one 
  

   of 
  its 
  allotropic 
  states. 
  

  

  The 
  following 
  reactions 
  support 
  this 
  view. 
  

  

  Three 
  of 
  the 
  principal 
  modes 
  of 
  formation 
  of 
  allotropic 
  

   silver 
  are 
  : 
  (1) 
  reduction 
  of 
  silver 
  citrate 
  or 
  tartrate 
  by 
  ferrous 
  

   citrate 
  or 
  tartrate 
  ; 
  (2) 
  acting 
  on 
  silver 
  nitrate 
  or 
  oxide 
  by 
  

  

  