﻿604 
  dr 
  davy's 
  observations 
  on 
  fish, 
  

  

  the 
  contrary, 
  a 
  free 
  acid, 
  the 
  phosphoric, 
  analogous 
  to 
  what 
  occurs 
  in 
  the 
  ash 
  of 
  

   the 
  yolk 
  of 
  the 
  domestic 
  fowl, 
  — 
  and 
  in 
  consequence 
  of 
  which, 
  the 
  complete 
  incine- 
  

   ration 
  of 
  the 
  roe 
  of 
  the 
  fish 
  and 
  its 
  milt, 
  like 
  that 
  of 
  the 
  yolk 
  of 
  the 
  egg, 
  is 
  very 
  

   difficult. 
  

  

  The 
  same 
  conclusion, 
  on 
  the 
  same 
  ground, 
  viz., 
  the 
  absence 
  of 
  iodine, 
  is 
  appli- 
  

   cable 
  to 
  fresh-water 
  fish, 
  — 
  a 
  conclusion 
  that 
  can 
  hardly 
  be 
  tested 
  by 
  experience, 
  

   nor 
  is 
  it 
  of 
  practical 
  importance, 
  since 
  fish 
  of 
  this 
  kind 
  enters 
  so 
  sparingly 
  into 
  

   the 
  ordinary 
  diet 
  of 
  the 
  people. 
  

  

  Lesketh 
  How, 
  Ambleside, 
  

   April 
  14, 
  1853. 
  

  

  P.S. 
  — 
  I 
  have 
  mentioned 
  briefly 
  the 
  test 
  employed 
  to 
  detect 
  iodine. 
  To 
  prevent 
  

   obscurity, 
  may 
  I 
  be 
  permitted 
  to 
  add 
  a 
  few 
  particulars 
  relative 
  to 
  the 
  mode 
  of 
  

   proceeding 
  ? 
  On 
  a 
  portion 
  of 
  starch 
  in 
  fine 
  powder, 
  that 
  is, 
  in 
  its 
  granular 
  state, 
  

   aqua 
  regia 
  is 
  poured, 
  or 
  about 
  equal 
  parts 
  of 
  nitric 
  and 
  muriatic 
  acid, 
  in 
  a 
  platina 
  

   capsule, 
  and 
  then 
  well 
  mixed, 
  using 
  a 
  glass 
  rod. 
  The 
  salt 
  to 
  be 
  tested, 
  either 
  in 
  

   solution 
  or 
  solid, 
  is 
  then 
  added. 
  The 
  blue 
  tint 
  due 
  to 
  the 
  presence 
  of 
  iodine 
  is 
  

   immediately 
  produced, 
  if 
  any 
  of 
  this 
  substance, 
  or 
  a 
  sufficiency 
  of 
  it 
  to 
  take 
  effect, 
  

   be 
  present. 
  The 
  delicacy 
  of 
  this 
  test 
  is, 
  I 
  believe, 
  well 
  known. 
  I 
  have 
  by 
  means 
  

   of 
  it 
  detected 
  iodine, 
  when 
  one-tenth 
  of 
  a 
  grain 
  of 
  the 
  iodide 
  of 
  potassium 
  was 
  

   dissolved 
  in 
  1 
  6775 
  grains 
  of 
  water. 
  Relative 
  to 
  this 
  method, 
  I 
  may 
  further 
  remark, 
  

   that 
  by 
  well 
  mixing 
  the 
  acid 
  and 
  starch, 
  not 
  only 
  is 
  the 
  starch 
  reduced 
  to 
  a 
  gela- 
  

   tinous 
  state 
  favourable 
  for 
  being 
  acted 
  on 
  by 
  the 
  iodine 
  as 
  liberated, 
  by 
  the 
  action 
  

   of 
  the 
  chlorine, 
  but 
  also 
  that 
  the 
  excess 
  of 
  chlorine 
  is, 
  to 
  a 
  great 
  extent, 
  got 
  rid 
  of. 
  

   The 
  platina 
  capsule 
  has 
  appeared 
  preferable 
  to 
  one 
  of 
  glass, 
  as 
  shewing 
  the 
  effect 
  

   of 
  colour 
  by 
  reflected 
  light 
  more 
  readily 
  and 
  distinctly 
  ; 
  and 
  also, 
  I 
  am 
  disposed 
  

   to 
  think, 
  from 
  some 
  peculiar 
  influence 
  which 
  the 
  metal 
  exercises, 
  favouring 
  the 
  

   combination 
  of 
  the 
  starch 
  and 
  iodine, 
  similar, 
  it 
  may 
  be, 
  to 
  that 
  of 
  spongy 
  plati- 
  

   num, 
  in 
  effecting 
  the 
  union 
  of 
  oxygen 
  and 
  hydrogen. 
  

  

  In 
  seeking 
  for 
  iodine 
  in 
  animal 
  substances 
  by 
  incineration, 
  it 
  may 
  be 
  well 
  to 
  keep 
  

   in 
  mind, 
  that, 
  experimentally 
  considered, 
  the 
  liability 
  to 
  error 
  lies 
  in 
  underrating, 
  

   rather 
  than 
  in 
  overrating 
  the 
  result 
  by 
  the 
  methods 
  employed, 
  and 
  that 
  mainly 
  in 
  

   consequence 
  of 
  more 
  or 
  less 
  of 
  loss 
  of 
  iodine 
  being 
  sustained 
  in 
  the 
  process 
  of 
  com- 
  

   bustion, 
  incineration, 
  and 
  evaporation 
  used. 
  To 
  illustrate 
  this 
  by 
  a 
  simple 
  experi- 
  

   ment, 
  I 
  may 
  mention 
  that 
  a 
  portion 
  of 
  water, 
  equivalent 
  to 
  about 
  1525 
  grains, 
  in 
  

   which 
  were 
  dissolved 
  10 
  grains 
  of 
  common 
  salt, 
  and 
  -09 
  grain 
  of 
  iodide 
  of 
  potas- 
  

   sium, 
  was 
  quickly 
  evaporated 
  to 
  dryness 
  by 
  boiling. 
  Previously, 
  the 
  iodine 
  could 
  

   be 
  detected 
  in 
  the 
  mixture 
  by 
  the 
  test 
  I 
  have 
  used 
  ; 
  but 
  not 
  afterwards, 
  when 
  the 
  

  

  