﻿174 
  

  

  Mr. 
  W. 
  H. 
  Johnson 
  on 
  the 
  Action 
  of 
  [Jan. 
  14, 
  

  

  cannot 
  be 
  explained 
  by 
  it 
  alone, 
  but 
  which 
  seem 
  to 
  show 
  that 
  the 
  occlu- 
  

   sion 
  of 
  hydrogen 
  is 
  accompanied 
  by 
  the 
  absorption 
  of 
  a 
  minute 
  portion 
  of 
  

   the 
  acid 
  by 
  the 
  pores 
  of 
  the 
  iron. 
  

  

  In 
  proof 
  of 
  this 
  the 
  following 
  well-established 
  facts 
  are 
  adduced 
  : 
  — 
  

   1st. 
  Iron 
  much 
  sooner 
  regains 
  its 
  natural 
  state 
  after 
  immersion 
  in 
  

   hydrochloric 
  than 
  in 
  sulphuric 
  acid, 
  though 
  at 
  first 
  both 
  may 
  have 
  equally 
  

   affected 
  it, 
  as 
  judged 
  by 
  diminution 
  of 
  toughness. 
  It 
  may 
  be 
  thought 
  

   that 
  some 
  portion 
  of 
  the 
  less 
  volatile 
  sulphuric 
  acid 
  adhering 
  to 
  the 
  sur- 
  

   face 
  of 
  the 
  iron, 
  even 
  after 
  prolonged 
  washing, 
  will 
  account 
  for 
  it. 
  This 
  

   cannot 
  be 
  the 
  case 
  however 
  ; 
  for 
  the 
  wire 
  may 
  not 
  only 
  be 
  repeatedly 
  

   washed 
  in 
  water, 
  but 
  even 
  coated 
  with 
  lime-water, 
  dried, 
  and 
  finally 
  re- 
  

   duced 
  in 
  diameter 
  two 
  thirds 
  by 
  drawing 
  several 
  times 
  through 
  a 
  steel 
  

   die, 
  processes 
  which 
  must 
  surely 
  remove 
  any 
  surface-coating 
  of 
  acid 
  ; 
  and 
  

   yet 
  it 
  will 
  take 
  longer 
  to 
  recover 
  its 
  original 
  toughness 
  if 
  cleaned 
  in 
  

   sulphuric 
  acid 
  than 
  in 
  hydrochloric 
  acid. 
  

  

  2nd. 
  The 
  pieces 
  in 
  the 
  last 
  experiment 
  immersed 
  in 
  hydrochloric 
  acid 
  

   will 
  become 
  spotted 
  with 
  rust 
  on 
  the 
  surface 
  some 
  days 
  before 
  those 
  im- 
  

   mersed 
  in 
  sulphuric 
  acid. 
  

  

  If 
  the 
  supposition 
  that 
  acid 
  is 
  absorbed 
  by 
  the 
  iron 
  be 
  correct, 
  this 
  is 
  

   only 
  what 
  we 
  should 
  expect 
  ; 
  for 
  it 
  is 
  only 
  natural 
  to 
  suppose 
  that 
  the 
  most 
  

   volatile 
  acid, 
  viz. 
  hydrochloric, 
  will 
  come 
  to 
  the 
  surface 
  first. 
  

  

  We 
  know, 
  moreover, 
  that 
  water 
  can 
  by 
  great 
  pressure 
  be 
  forced 
  

   through 
  considerable 
  thicknesses 
  of 
  cast 
  iron. 
  Why, 
  then, 
  should 
  not 
  a 
  

   liquid 
  pass 
  into 
  the 
  pores 
  of 
  wrought 
  iron 
  ? 
  

  

  As 
  further 
  proof 
  of 
  the 
  presence 
  of 
  acid 
  in 
  iron, 
  I 
  have 
  found 
  that 
  

   blue 
  litmus-paper 
  was 
  slightly 
  reddened 
  when 
  moistened 
  by 
  a 
  drop 
  of 
  

   water 
  which 
  had 
  been 
  carefully 
  placed 
  on 
  the 
  fracture 
  of 
  a 
  piece 
  of 
  iron 
  

   •45" 
  in 
  diameter, 
  previously 
  immersed 
  in 
  sulphuric 
  acid 
  several 
  hours, 
  

   and 
  then 
  washed 
  in 
  water. 
  The 
  drop 
  of 
  water 
  in 
  this 
  case 
  did 
  not 
  

   moisten 
  the 
  sides 
  of 
  the 
  fracture, 
  where 
  sOme 
  trace 
  of 
  acid 
  might 
  be 
  

   present. 
  

  

  The 
  occlusion 
  of 
  hydrogen 
  by 
  iron 
  when 
  immersed 
  in 
  acid 
  solutions 
  

   enables 
  us 
  satisfactorily 
  to 
  account 
  for 
  some 
  of 
  the 
  difficulties 
  experienced 
  

   in 
  depositing 
  copper, 
  silver, 
  tin, 
  or 
  other 
  metals 
  from 
  their 
  solutions 
  in 
  

   acid 
  in 
  electrotyping 
  or 
  otherwise. 
  Generally 
  any 
  coat 
  of 
  appreciable 
  

   thickness 
  slowly 
  shells 
  off, 
  leaving 
  the 
  surface 
  of 
  the 
  iron 
  bare 
  in 
  places, 
  

   and 
  so 
  making 
  the 
  coating 
  of 
  no 
  avail 
  as 
  a 
  preventive 
  of 
  oxidation. 
  The 
  

   cause 
  is 
  obvious 
  ; 
  the 
  hydrogen 
  occluded 
  whilst 
  the 
  iron 
  is 
  being 
  cleaned 
  

   in 
  a 
  bath 
  of 
  vitriol 
  or 
  hydrochloric 
  acid, 
  and 
  subsequently 
  imprisoned 
  by 
  

   the 
  coat 
  of 
  metal, 
  must 
  escape, 
  and 
  in 
  so 
  doing 
  forces 
  its 
  way 
  out, 
  loosen- 
  

   ing 
  or 
  carrying 
  away 
  some 
  portion 
  of 
  the 
  superficial 
  covering 
  of 
  the 
  metal. 
  

   If, 
  however, 
  the 
  iron, 
  after 
  being 
  cleaned 
  in 
  acid, 
  is 
  boiled 
  in 
  caustic-soda 
  

   solution, 
  a 
  process 
  which 
  effectually 
  expels 
  the 
  occluded 
  hydrogen, 
  a 
  coat- 
  

   ing 
  of 
  copper 
  or 
  other 
  metal 
  may 
  then 
  be 
  electrically 
  deposited 
  which 
  will 
  

   not 
  shell 
  off 
  in 
  the 
  least. 
  This 
  is 
  actually 
  being 
  done 
  in 
  practice 
  ; 
  and 
  

  

  