﻿318 
  Scientific 
  Intelligence. 
  

  

  The 
  current 
  used 
  in 
  this 
  experiment 
  must 
  be 
  obtained 
  from 
  a 
  

   battery 
  and 
  not 
  from 
  a 
  dynamo, 
  especially 
  when 
  the 
  instrument 
  

   to 
  be 
  calibrated 
  is 
  an 
  electrodynamometer. 
  

  

  Specification 
  B. 
  — 
  The 
  Volt. 
  

  

  Definition 
  and 
  Properties 
  of 
  the 
  Cell. 
  The 
  cell 
  has 
  for 
  its 
  posi- 
  

   tive 
  electrode, 
  mercury, 
  and 
  for 
  its 
  negative 
  electrode, 
  amalga- 
  

   mated 
  zinc 
  ; 
  the 
  electrolyte 
  consists 
  of 
  a 
  saturated 
  solution 
  of 
  

   zinc 
  sulphate 
  and 
  mercurous 
  sulphate. 
  The 
  electromotive 
  force 
  

   is 
  1-434 
  volts 
  at 
  15° 
  C, 
  and 
  between 
  10° 
  C. 
  and 
  25° 
  C, 
  by 
  the 
  

   increase 
  of 
  1° 
  C. 
  in 
  temperature, 
  the 
  electromotive 
  force 
  decreases 
  

   by 
  -00115 
  of 
  a 
  volt. 
  

  

  1. 
  Preparation 
  of 
  the 
  Mercury. 
  To 
  secure 
  purity 
  it 
  should 
  be 
  

   first 
  treated 
  with 
  acid 
  in 
  the 
  usual 
  manner 
  and 
  subsequently 
  dis- 
  

   tilled 
  in 
  vacuo. 
  

  

  2. 
  Preparation 
  of 
  the 
  Zinc 
  Amalc/ahi. 
  — 
  The 
  zinc 
  designated 
  in 
  

   commerce 
  as 
  " 
  commercially 
  pure" 
  can 
  be 
  used 
  without 
  further 
  

   preparation. 
  For 
  the 
  preparation 
  of 
  the 
  amalgam 
  one 
  part 
  by 
  

   weight 
  of 
  zinc 
  is 
  to 
  be 
  added 
  to 
  nine 
  (9) 
  parts 
  by 
  weight 
  of 
  mer- 
  

   cury 
  and 
  both 
  are 
  to 
  be 
  heated 
  in 
  a 
  porcelain 
  dish 
  at 
  100° 
  C. 
  with 
  

   moderate 
  stirring 
  until 
  the 
  zinc 
  has 
  been 
  fully 
  dissolved 
  in 
  the 
  

   mercury. 
  

  

  3. 
  Preparation 
  of 
  the 
  Mercurous 
  Sulphate. 
  Take 
  mercurous 
  

   sulphate, 
  purchased 
  as 
  pure, 
  mix 
  with 
  it 
  a 
  small 
  quantity 
  of 
  pure 
  

   mercury 
  and 
  wash 
  the 
  whole 
  thoroughly 
  with 
  cold 
  distilled 
  water 
  

   by 
  agitation 
  in 
  a 
  bottle 
  ; 
  drain 
  off 
  the 
  water 
  and 
  repeat 
  the 
  

   process 
  at 
  least 
  twice. 
  Alter 
  the 
  last 
  washing, 
  drain 
  off 
  as 
  much 
  

   of 
  the 
  water 
  as 
  possible. 
  (For 
  further 
  details 
  of 
  purification, 
  See 
  

   Note 
  A.) 
  

  

  4. 
  Preparation 
  of 
  the 
  Zinc 
  Sulphate 
  Solution. 
  Prepare 
  a 
  

   neutral 
  saturated 
  solution 
  of 
  pure 
  re-crystallized 
  zinc 
  sulphate, 
  

   free 
  from 
  iron, 
  by 
  mixing 
  distilled 
  water 
  with 
  nearly 
  twice 
  its 
  

   weight 
  of 
  crystals 
  of 
  pure 
  zinc 
  sulphate 
  and 
  adding 
  zinc 
  oxide 
  

   in 
  the 
  proportion 
  of 
  about 
  2 
  per 
  cent, 
  by 
  weight 
  ot 
  the 
  zinc 
  

   sulphate 
  crystals 
  to 
  neutralize 
  any 
  free 
  acid. 
  The 
  crystals 
  

   should 
  be 
  dissolved 
  with 
  the 
  aid 
  of 
  gentle 
  heat, 
  but 
  the 
  tempera- 
  

   ture 
  to 
  which 
  the 
  solution 
  is 
  raised 
  must 
  not 
  exceed 
  30° 
  C. 
  Mer- 
  

   curous 
  sulphate, 
  treated 
  as 
  described 
  in 
  3, 
  shall 
  be 
  added 
  in 
  the 
  

   proportion 
  of 
  about 
  12 
  per 
  cent 
  by 
  weight 
  of 
  the 
  zinc 
  sulphate 
  

   crystals 
  to 
  neutralize 
  the 
  free 
  zinc 
  oxide 
  remaining, 
  and 
  then 
  the 
  

   solution 
  filtered, 
  while 
  still 
  warm, 
  into 
  a 
  stock 
  bottle. 
  Crystals 
  

   should 
  form 
  as 
  it 
  cools. 
  

  

  5. 
  Preparation 
  of 
  the 
  Mercurous 
  Sulphate 
  and 
  Zinc 
  Sulphate 
  

   Paste. 
  For 
  making 
  the 
  paste, 
  two 
  or 
  three 
  parts 
  by 
  weight 
  of 
  

   mercurous 
  sulphate 
  are 
  to 
  be 
  added 
  to 
  one 
  by 
  weight 
  of 
  

   mercury. 
  If 
  the 
  sulphate 
  be 
  dry, 
  it 
  is 
  to 
  be 
  mixed 
  with 
  a 
  paste 
  

   consisting 
  of 
  zinc 
  sulphate 
  crystals 
  and 
  a 
  concentrated 
  zinc 
  

   sulphate 
  solution, 
  so 
  that 
  the 
  whole 
  constitutes 
  a 
  stiff 
  mass, 
  which 
  

   is 
  permeated 
  throughout 
  by 
  zinc 
  sulphate 
  crystals 
  and 
  globules 
  

  

  