﻿466 
  Prof. 
  0. 
  W. 
  Bichardson 
  on 
  the 
  

  

  at 
  least 
  not 
  to 
  be 
  carried 
  away 
  as 
  rapidly 
  when 
  the 
  magnetic 
  

   field 
  is 
  turned 
  off. 
  The 
  pressure 
  as 
  recorded 
  on 
  the 
  McLeod 
  

   gauge 
  in 
  the 
  above 
  experiments 
  was 
  a 
  little 
  under 
  O001 
  mm. 
  

   Although 
  the 
  above 
  results 
  serve 
  to 
  outline 
  some 
  of 
  the 
  

   main 
  features 
  of 
  the 
  emission 
  of 
  negative 
  ions 
  from 
  calcium 
  

   iodide 
  they 
  do 
  not 
  cover 
  the 
  whole 
  story. 
  In 
  carrying 
  out 
  

   the 
  experiments 
  I 
  was 
  often 
  surprised 
  to 
  find 
  the 
  magnitude 
  

   of 
  the 
  emission 
  to 
  be 
  very 
  different 
  under 
  what 
  were 
  appar- 
  

   ently 
  the 
  same 
  conditions 
  on 
  different 
  occasions. 
  This 
  

   happened 
  both 
  on 
  returning 
  to 
  examine 
  a 
  specimen 
  which 
  

   had 
  previously 
  been 
  tested 
  and 
  also 
  in 
  examining 
  different 
  

   specimens 
  which 
  were 
  about 
  equal 
  in 
  amount 
  and 
  which 
  

   seemed 
  to 
  have 
  been 
  treated 
  in 
  the 
  same 
  way. 
  One 
  cause 
  of 
  

   this 
  difference 
  seems 
  to 
  be 
  that 
  the 
  salt 
  loses 
  its 
  power 
  of 
  

   emission 
  when 
  it 
  is 
  left 
  cold 
  in 
  a 
  vacuum 
  (at 
  about 
  0*001 
  mm.) 
  

   and 
  can 
  recover 
  it 
  when 
  it 
  is 
  left 
  exposed 
  to 
  air 
  at 
  a 
  higher 
  

   pressure. 
  At 
  any 
  rate 
  a 
  number 
  of 
  experiments 
  which 
  were 
  

   made 
  agreed 
  with 
  this 
  conclusion, 
  although 
  I 
  am 
  not 
  clear 
  

   as 
  to 
  what 
  the 
  nature 
  of 
  the 
  change 
  which 
  occurs 
  may 
  be. 
  

   The 
  recovery 
  occurred 
  in 
  dried 
  air 
  as 
  well 
  as 
  in 
  the 
  ordinary 
  

   air 
  of 
  the 
  laboratory. 
  The 
  nature 
  of 
  these 
  changes 
  is 
  

   exemplified 
  by 
  the 
  following 
  observations 
  which 
  were 
  made 
  

   on 
  a 
  single 
  specimen 
  of 
  calcium 
  iodide 
  at 
  523° 
  C. 
  

  

  When 
  tested 
  immediately 
  after 
  setting 
  up, 
  the 
  air 
  being 
  

   pumped 
  out 
  as 
  quickly 
  as 
  possible, 
  curves 
  similar 
  to 
  those 
  in 
  

   fig. 
  4 
  were 
  obtained. 
  The 
  maximum 
  current, 
  which 
  was 
  

   reached 
  in 
  15 
  minutes, 
  was 
  1*24 
  XlO 
  -9 
  amp.; 
  it 
  fell 
  to 
  

   4-8 
  x 
  10 
  ~ 
  10 
  amp. 
  in 
  100 
  minutes. 
  After 
  heating 
  for 
  5 
  hours 
  

   altogether, 
  when 
  the 
  current 
  was 
  still 
  comparable 
  with 
  the 
  

   last 
  value, 
  it 
  was 
  left 
  cold 
  for 
  2 
  hours 
  in 
  the 
  vacuum. 
  On 
  

   subsequently 
  testing 
  at 
  523° 
  C. 
  the 
  current 
  rose 
  to 
  a 
  maximum 
  

   of 
  only 
  2*1 
  x 
  10 
  -11 
  amp. 
  and 
  then 
  fell 
  off. 
  After 
  heating 
  for 
  

   one 
  hour 
  it 
  was 
  left 
  all 
  night 
  in 
  air 
  at 
  20 
  cm. 
  pressure. 
  The 
  

   maximum 
  at 
  523° 
  C. 
  was 
  then 
  found 
  to 
  be 
  1*63 
  x 
  10 
  ~ 
  9 
  amp., 
  

   or 
  rather 
  larger 
  than 
  that 
  given 
  by 
  the 
  fresh 
  salt. 
  After 
  

   heating 
  for 
  5 
  hours 
  it 
  was 
  left 
  cold 
  in 
  air 
  at 
  atmospheric 
  

   pressure 
  for 
  6 
  hours. 
  The 
  maximum 
  at 
  523° 
  C 
  was 
  then 
  

   found 
  to 
  be 
  2*4 
  x 
  10~ 
  9 
  amp. 
  After 
  heating 
  continuously 
  for 
  

   about 
  24 
  hours 
  the 
  emission 
  had 
  only 
  fallen 
  to 
  1*13 
  x 
  10~ 
  9 
  

   amp., 
  although 
  no 
  doubt 
  this 
  was 
  due 
  to 
  air 
  having 
  leaked 
  in 
  

   through 
  the 
  low-pressure 
  side 
  of 
  the 
  Gaede 
  pump 
  to 
  about 
  

   half 
  an 
  atmosphere. 
  The 
  value 
  of 
  1*13 
  x 
  13~ 
  9 
  amp., 
  which 
  

   was 
  obtained 
  after 
  the 
  air 
  had 
  been 
  pumped 
  out 
  and 
  the 
  

   original 
  temperature 
  of 
  523° 
  0. 
  re-established, 
  remained 
  

   practically 
  constant 
  for 
  over 
  one 
  hour. 
  The 
  salt 
  was 
  then 
  

   left 
  cold 
  in 
  the 
  vacuum 
  for 
  3J 
  hours. 
  It 
  was 
  tested 
  at 
  

  

  