﻿622 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  

  

  the 
  patches 
  sometimes 
  reaches 
  the 
  yellow 
  of 
  the 
  first 
  order, 
  and 
  its 
  crystallisation 
  

   has 
  a 
  very 
  singular 
  appearance. 
  

  

  When 
  the 
  molecules 
  of 
  the 
  same 
  body, 
  or 
  those 
  of 
  different 
  bodies, 
  are 
  com- 
  

   bined 
  under 
  the 
  influence 
  of 
  disturbing 
  causes, 
  we 
  may 
  reasonably 
  expect 
  that 
  

   their 
  union 
  will 
  neither 
  be 
  strong 
  nor 
  permanent. 
  When 
  regular 
  crystals 
  are 
  

   melted 
  by 
  heat, 
  either 
  alone 
  or 
  along 
  with 
  other 
  bodies, 
  their 
  molecules 
  are 
  forced 
  

   into 
  positions 
  of 
  unstable 
  equilibrium, 
  and 
  the 
  natural 
  tendency 
  of 
  similar 
  poles 
  

   to 
  unite 
  is 
  aided 
  by 
  every 
  mechanical 
  vibration, 
  and 
  every 
  variation 
  of 
  tempera- 
  

   ture 
  to 
  which 
  they 
  are 
  exposed. 
  Different 
  kinds 
  of 
  glass, 
  for 
  example, 
  in 
  which 
  

   earths, 
  alkalies, 
  and 
  metals 
  may 
  have 
  been 
  combined 
  by 
  fusion, 
  are 
  thus 
  com- 
  

   pletely 
  decomposed 
  by 
  time, 
  and 
  the 
  elementary 
  particles, 
  liberated 
  from 
  their 
  

   constrained 
  position, 
  resume 
  their 
  place 
  in 
  crystals 
  regularly 
  formed. 
  The 
  spe- 
  

   cimens 
  of 
  ancient 
  glass 
  found 
  at 
  Nineveh, 
  and 
  in 
  various 
  parts 
  of 
  Italy 
  and 
  

   Greece, 
  have 
  undergone 
  the 
  most 
  remarkable 
  decomposition, 
  and 
  some 
  of 
  it 
  

   converted 
  into 
  a 
  sort 
  of 
  indurated 
  mass, 
  which 
  can 
  be 
  broken 
  between 
  the 
  fingers. 
  

   The 
  character 
  of 
  these 
  decompositions, 
  and 
  the 
  process 
  by 
  which 
  they 
  are 
  effected, 
  

   I 
  have 
  had 
  occasion 
  to 
  describe 
  in 
  the 
  Appendix 
  to 
  Mr 
  La 
  yard's 
  new 
  work 
  on 
  

   Nineveh 
  and 
  Babylon.* 
  The 
  same 
  principles 
  operate 
  in 
  the 
  decomposition 
  of 
  cir- 
  

   cular 
  crystals, 
  and 
  the 
  same 
  phenomena 
  are 
  exhibited 
  in 
  their 
  restoration 
  to 
  their 
  

   original 
  state. 
  

  

  In 
  circular 
  crystals 
  the 
  decomposition 
  takes 
  place 
  in 
  different 
  ways. 
  In 
  those 
  

   from 
  borax 
  and 
  phosphoric 
  acid, 
  which 
  I 
  have 
  had 
  occasion 
  to 
  watch 
  month 
  after 
  

   month 
  for 
  several 
  years, 
  the 
  decomposition 
  generally 
  begins 
  at 
  the 
  centre, 
  which 
  

   is 
  dissolved, 
  or 
  occupied 
  by 
  a 
  number 
  of 
  minute 
  prisms, 
  with 
  their 
  axes 
  lying 
  in 
  

   every 
  direction. 
  These 
  prisms 
  sometimes 
  are 
  arranged 
  in 
  a 
  ring 
  round 
  the 
  centre, 
  

   and 
  I 
  have 
  seen 
  them 
  like 
  a 
  St 
  Andrew's 
  cross. 
  In 
  other 
  crystals, 
  the 
  decompo- 
  

   sition 
  goes 
  on 
  in 
  radial 
  lines 
  or 
  streaks, 
  where 
  the 
  optical 
  contact 
  has 
  not 
  been 
  

   complete 
  ; 
  but 
  in 
  the 
  more 
  perfect 
  crystals 
  it 
  takes 
  place 
  in 
  concentric 
  circles, 
  

   sometimes 
  double, 
  the 
  colours 
  between 
  each 
  pair 
  of 
  circles 
  being 
  different. 
  Nu- 
  

   merous 
  cavities 
  are 
  formed, 
  — 
  pieces 
  of 
  the 
  crystal 
  separate, 
  and 
  irregular 
  crystals 
  

   are 
  often 
  formed 
  in 
  the 
  solution. 
  Decomposition 
  sometimes 
  takes 
  place 
  without 
  

   solution 
  : 
  the 
  crystal 
  preserves 
  its 
  form, 
  the 
  black 
  circles 
  are 
  granulated, 
  and 
  the 
  

   colours 
  wholly 
  disappear. 
  In 
  one 
  of 
  the 
  specimens 
  in 
  my 
  possession, 
  every 
  

   crystal 
  has 
  vanished, 
  and 
  their 
  elements 
  converted 
  into 
  beautiful 
  prisms, 
  united 
  

   like 
  a 
  bunch 
  of 
  straw 
  tightened 
  at 
  the 
  middle. 
  Between 
  these 
  groups 
  there 
  are 
  

   numerous 
  flat 
  crystals, 
  of 
  considerable 
  size, 
  and 
  of 
  a 
  perfectly 
  uniform 
  tint. 
  All 
  

   these 
  decompositions 
  have 
  been 
  the 
  work 
  of 
  several 
  years 
  ; 
  and 
  in 
  the 
  course 
  of 
  

   one 
  year 
  more 
  there 
  will 
  not 
  be 
  found 
  a 
  vestige 
  of 
  the 
  original 
  crystals. 
  

  

  In 
  Manna 
  the 
  transformation 
  of 
  the 
  circular 
  into 
  their 
  component 
  crystals 
  

  

  * 
  Discoveries 
  in 
  the 
  Ruins 
  of 
  Nineveh 
  and 
  Babylon. 
  By 
  Austen 
  H. 
  Layard, 
  M.P. 
  Appendix, 
  

   p. 
  674-676. 
  

  

  