﻿750 
  Prof. 
  McLennan 
  and 
  Mr. 
  McLeod 
  : 
  Measurements 
  on 
  

  

  that 
  the 
  radiation 
  coming 
  from 
  the 
  air 
  above 
  the 
  lake 
  did 
  not 
  

   contribute 
  any 
  more 
  to 
  the 
  ionization 
  in 
  the 
  electrometer 
  

   than 
  about 
  "16 
  ion 
  per 
  c.c. 
  per 
  sec. 
  

  

  In 
  attempting 
  to 
  ascertain 
  the 
  contribution 
  made 
  to 
  the 
  

   ionization 
  in 
  a 
  closed 
  vessel 
  by 
  radiation 
  coining 
  from 
  

   active 
  matter 
  in 
  the 
  air 
  above 
  the 
  land, 
  measurements 
  are 
  

   complicated 
  by 
  the 
  effect 
  of 
  radiation 
  from 
  active 
  matter 
  

   in 
  the 
  soil 
  and 
  from 
  active 
  matter 
  deposited 
  on 
  the 
  surface 
  

   of 
  the 
  ground 
  from 
  the 
  atmosphere. 
  It 
  would 
  seem, 
  however, 
  

   since 
  the 
  measurements 
  described 
  above 
  were 
  made 
  at 
  a 
  

   point 
  only 
  about 
  2 
  kilometres 
  from 
  the 
  shore, 
  that 
  the 
  con- 
  

   tribution 
  made 
  to 
  the 
  ionization 
  in 
  air 
  in 
  a 
  zinc 
  vessel 
  by 
  

   radiation 
  from 
  the 
  active 
  matter 
  in 
  the 
  air 
  over 
  the 
  land 
  near 
  

   Toronto 
  cannot 
  be 
  very 
  far 
  from 
  0*2 
  ion 
  per 
  c.c. 
  per 
  sec. 
  

  

  VI. 
  Residual 
  Ionization. 
  

  

  From 
  the 
  measurements 
  which 
  have 
  been 
  described 
  above, 
  

   it 
  would 
  appear 
  that 
  the 
  lowest 
  value 
  obtainable 
  for 
  the 
  

   ionization 
  in 
  a 
  closed 
  zinc 
  receiver, 
  at 
  points 
  near 
  the 
  surface 
  

   of 
  the 
  earth 
  where 
  the 
  intensity 
  of 
  the 
  penetrating 
  radiation 
  

   may 
  be 
  considered 
  to 
  have 
  a 
  minimum 
  value, 
  approximates 
  

   to 
  4 
  ions 
  per 
  c.c. 
  per 
  sec. 
  This, 
  it 
  will 
  be 
  remembered, 
  was 
  

   the 
  mean 
  value 
  obtained 
  for 
  q 
  from 
  the 
  measurement 
  made 
  

   on 
  the 
  s.s. 
  'Hesperian 
  ' 
  on 
  Sept. 
  21st, 
  1912, 
  and 
  it 
  was 
  also 
  

   practically 
  the 
  value 
  found 
  for 
  " 
  q" 
  by 
  Simpson 
  and 
  Wright 
  * 
  

   on 
  a 
  skiff 
  on 
  the 
  Southern 
  Atlantic. 
  

  

  To 
  what, 
  then, 
  is 
  this 
  residual 
  ionization 
  due 
  ? 
  As 
  an 
  

   attempt 
  to 
  answer 
  this 
  question 
  the 
  following 
  experiments 
  

   may 
  be 
  of 
  interest. 
  In 
  these 
  the 
  electrometer 
  was 
  placed 
  in 
  

   the 
  aluminium-bronze 
  box 
  used 
  in 
  the 
  experiment 
  under 
  

   water 
  on 
  the 
  lake, 
  and 
  this 
  box 
  was 
  suspended 
  in 
  a 
  moderate- 
  

   sized 
  tank 
  which 
  could 
  be 
  filled 
  with 
  water 
  and 
  which 
  was 
  

   situated 
  in 
  the 
  attic 
  of 
  the 
  Physical 
  Laboratory 
  at 
  Toronto. 
  

   Readings 
  were 
  taken 
  of 
  the 
  ionization 
  in 
  the 
  electrometer 
  

   when 
  the 
  tank 
  was 
  empty 
  and 
  -also 
  when 
  it 
  was 
  rilled 
  with 
  

   water. 
  With 
  the 
  tank 
  empty 
  the 
  reading 
  was 
  8"05 
  ions 
  per 
  

   c.c. 
  per 
  sec, 
  and 
  with 
  it 
  filled 
  the 
  reading 
  was 
  5'82 
  ions 
  

   per 
  c.c. 
  per 
  sec. 
  The 
  dimensions 
  of 
  the 
  box 
  were 
  36 
  X 
  22 
  x 
  

   18 
  c. 
  cm., 
  and 
  those 
  of 
  the 
  mass 
  of 
  water 
  surrounding 
  it 
  were 
  

   121 
  x 
  90*7 
  x 
  83 
  c. 
  cm. 
  This, 
  it 
  will 
  be 
  seen, 
  gave 
  roughly 
  a 
  

   mean 
  of 
  36*5 
  cm. 
  as 
  the 
  thickness 
  of 
  the 
  water-screen 
  

   surrounding 
  the 
  box. 
  

  

  Assuming 
  the 
  water 
  to 
  contain 
  no 
  radioactive 
  matter, 
  we 
  

  

  * 
  Loc. 
  cit. 
  

  

  