﻿320 
  Scientific 
  Intelligence. 
  

  

  pletely 
  covered 
  by 
  the 
  mercury 
  and 
  the 
  amalgam 
  respectively. 
  

   On 
  the 
  mercury, 
  place 
  a 
  layer 
  one 
  cm. 
  thick 
  of 
  the 
  zinc 
  and 
  mer- 
  

   curous 
  sulphate 
  paste 
  described 
  in 
  5. 
  Both 
  this 
  paste 
  and 
  the 
  

   zinc 
  amalgam 
  must 
  then 
  be 
  covered 
  with 
  a 
  layer 
  of 
  the 
  neutral 
  

   zinc 
  sulphate 
  crystals 
  one 
  cm. 
  thick. 
  The 
  whole 
  vessel 
  must 
  then 
  be 
  

   filled 
  with 
  the 
  saturated 
  zinc 
  sulphate 
  solution, 
  and 
  the 
  stopper 
  

   inserted 
  so 
  that 
  it 
  shall 
  just 
  touch 
  it, 
  leaving, 
  however, 
  a 
  small 
  

   bubble 
  to 
  guard 
  against 
  breakage 
  when 
  the 
  temperature 
  rises. 
  

  

  Before 
  finally 
  inserting 
  the 
  glass 
  stopper, 
  it 
  is 
  to 
  be 
  brushed 
  

   round 
  its 
  upper 
  edge 
  with 
  a 
  strong 
  alcoholic 
  solution 
  of 
  shellac 
  

   and 
  pressed 
  firmly 
  in 
  place. 
  (For 
  details 
  of 
  filling 
  the 
  cell, 
  See 
  

   Note 
  B.) 
  

  

  Notes 
  to 
  the 
  Specifications. 
  

  

  (A.) 
  The 
  Mercurous 
  Sulphate. 
  The 
  treatment 
  of 
  the 
  mer- 
  

   curous 
  sulphate 
  has 
  for 
  its 
  object 
  the 
  removal 
  of 
  any 
  mercuric 
  

   sulphate 
  which 
  is 
  often 
  present 
  as 
  an 
  impurity. 
  

  

  Mercuric 
  sulphate 
  decomposes 
  in 
  the 
  presence 
  of 
  water 
  into 
  an 
  

   acid 
  and 
  a 
  basic 
  sulphate. 
  The 
  latter 
  is 
  a 
  yellow 
  substance 
  — 
  turpeth 
  

   mineral 
  — 
  practically 
  insoluble 
  in 
  water; 
  its 
  presence, 
  at 
  any 
  rate 
  

   in 
  moderate 
  quantities, 
  has 
  no 
  effect 
  on 
  the 
  cell. 
  If, 
  however, 
  it 
  

   be 
  formed, 
  the 
  acid 
  sulphate 
  is 
  also 
  formed. 
  This 
  is 
  soluble 
  in 
  

   water 
  and 
  the 
  acid 
  produced 
  affects 
  the 
  electromotive 
  force. 
  

   The 
  object 
  of 
  the 
  washings 
  is 
  to 
  dissolve 
  and 
  remove 
  this 
  acid 
  

   sulphate 
  and 
  for 
  this 
  purpose 
  the 
  three 
  washings 
  described 
  in 
  the 
  

   specification 
  will 
  suffice 
  in 
  nearly 
  all 
  cases. 
  If, 
  however, 
  much 
  of 
  

   the 
  turpeth 
  mineral 
  be 
  formed, 
  it 
  shows 
  that 
  there 
  is 
  a 
  great 
  

   deal 
  of 
  the 
  acid 
  sulphate 
  present 
  and 
  it 
  will 
  then 
  be 
  wiser 
  to 
  

   obtain 
  a 
  fresh 
  sample 
  of 
  mercurous 
  sulphate, 
  rather 
  than 
  to 
  try 
  by 
  

   repeated 
  washings 
  to 
  get 
  rid 
  of 
  all 
  the 
  acid. 
  

  

  The 
  free 
  mercury 
  helps 
  in 
  the 
  process 
  of 
  removing 
  the 
  acid, 
  

   for 
  the 
  acid 
  mercuric 
  sulphate 
  attacks 
  it, 
  forming 
  mercurous 
  

   sulphate. 
  

  

  Pure 
  mercurous 
  sulphate, 
  when 
  quite 
  free 
  from 
  acid, 
  shows 
  on 
  

   repeated 
  washing 
  a 
  faint 
  yellow 
  tinge, 
  which 
  is 
  due 
  to 
  the 
  

   formation 
  of 
  a 
  basic 
  mercurous 
  salt 
  distinct 
  from 
  the 
  turpeth 
  

   mineral, 
  or 
  basic 
  mercuric 
  sulphate. 
  The 
  appearance 
  of 
  this 
  

   primrose 
  yellow 
  tint 
  may 
  be 
  taken 
  as 
  an 
  indication 
  that 
  all 
  the 
  

   acid 
  has 
  been 
  removed 
  ; 
  the 
  washing 
  may 
  with 
  advantage 
  be 
  

   continued 
  until 
  this 
  tint 
  appears. 
  

  

  (B.) 
  Filling 
  the 
  Cell. 
  After 
  thoroughly 
  cleaning 
  and 
  drying 
  

   the 
  glass 
  vessel 
  place 
  it 
  in 
  a 
  hot 
  water 
  bath. 
  Then 
  pass 
  through 
  

   the 
  neck 
  of 
  the 
  vessel 
  a 
  thin 
  glass 
  tube 
  reaching 
  to 
  the 
  bottom 
  

   to 
  serve 
  for 
  the 
  introduction 
  of 
  the 
  amalgam. 
  This 
  tube 
  should 
  

   be 
  as 
  large 
  as 
  the 
  glass 
  vessel 
  will 
  admit. 
  It 
  serves 
  to 
  protect 
  the 
  

   upper 
  part 
  of 
  the 
  cell 
  from 
  being 
  soiled 
  with 
  the 
  amalgam. 
  To 
  

   fill 
  in 
  the 
  amalgam, 
  a 
  clean 
  dropping 
  tube 
  about 
  io 
  cms 
  long, 
  

   drawn 
  out 
  to 
  a 
  fine 
  pointy 
  should 
  be 
  used. 
  Its 
  lower 
  end 
  is 
  

   brought 
  under 
  the 
  surface 
  of 
  the 
  amalgam 
  heated 
  in 
  a 
  porcelain 
  

   dish, 
  and 
  some 
  of 
  the 
  amalgam 
  is 
  drawn 
  into 
  the 
  tube 
  by 
  means 
  

  

  