﻿512 
  Prof. 
  P. 
  Lewis 
  on 
  the 
  Role 
  oj 
  

  

  The 
  theory 
  of 
  the 
  experiment 
  does 
  not 
  admit 
  of 
  any 
  accu- 
  

   rate 
  calculation 
  of 
  the 
  permeability 
  (//,) 
  from 
  the 
  results, 
  as 
  

   the- 
  depth 
  in 
  the 
  wire 
  at 
  which 
  the 
  magnetizing 
  force 
  has 
  

   sunk 
  to 
  an 
  insignificant 
  fraction 
  of 
  its 
  value 
  at 
  the 
  surface 
  

   itself 
  involves 
  fi. 
  An 
  attempt 
  was, 
  however, 
  made 
  to 
  work 
  

   out 
  an 
  approximate 
  value 
  for 
  jul 
  from 
  the 
  measurements 
  ob- 
  

   tained; 
  and 
  the 
  mean 
  of 
  some 
  ten 
  values 
  gave 
  //,= 
  110 
  for 
  a 
  

   certain 
  specimen 
  of 
  soft 
  iron 
  for 
  about 
  10,000 
  oscillations 
  

   per 
  second, 
  which 
  is 
  just 
  about 
  the 
  value 
  obtained 
  by 
  

   Klemencic 
  under 
  similar 
  conditions 
  by 
  a 
  totally 
  different 
  

   method, 
  but 
  it 
  is 
  much 
  smaller 
  than 
  that 
  got 
  by 
  St. 
  John 
  for 
  

   much 
  more 
  rapid 
  oscillations. 
  

  

  In 
  conclusion, 
  may 
  I 
  offer 
  my 
  best 
  thanks 
  to 
  Prof. 
  F. 
  Braun 
  

   of 
  Strassburg, 
  in 
  whose 
  laboratory 
  the 
  experiments 
  were 
  

   carried 
  out, 
  and 
  to 
  Prof. 
  M. 
  Cantor, 
  for 
  their 
  many 
  kindnesses 
  

   during 
  the 
  progress 
  of 
  the 
  research. 
  

  

  Cavendish 
  Laboratory, 
  Cambridge, 
  

   March 
  190*1. 
  

  

  LTX. 
  The 
  Hole 
  of 
  Water 
  Vapour 
  in 
  Gaseous 
  Conduction. 
  

   By 
  Professor 
  Percival 
  Lewis*. 
  

  

  IN 
  a 
  recent 
  number 
  of 
  this 
  Magazine 
  f 
  Professor 
  Trowbridge 
  

   expresses 
  his 
  conviction 
  that 
  pure 
  gases, 
  hydrogen 
  in 
  

   particular, 
  are 
  perfect 
  insulators, 
  and 
  that 
  the 
  presence 
  of 
  a 
  

   trace 
  of 
  water-vapour 
  or 
  oxygen 
  is 
  necessary 
  to 
  produce 
  

   the 
  dissociation 
  or 
  other 
  conditions 
  upon 
  which 
  conduction 
  

   depends. 
  When 
  we 
  consider 
  the 
  more 
  or 
  less 
  analogous 
  

   phenomena 
  of 
  electrolytic 
  conduction 
  in 
  solutions, 
  this 
  view 
  

   does 
  not 
  seem 
  improbable 
  ; 
  but 
  some 
  facts 
  may 
  make 
  us 
  

   hesitate 
  to 
  accept 
  the 
  conclusion 
  that 
  water- 
  vapour 
  is 
  so 
  

   essential 
  an 
  agent 
  in 
  discharges 
  through 
  gases. 
  

  

  In 
  the 
  first 
  place, 
  some 
  of 
  the 
  reasons 
  given 
  by 
  Professor 
  

   Trowbridge 
  for 
  believing 
  pure 
  hydrogen 
  to 
  be 
  an 
  insulator 
  do 
  

   not 
  seem 
  to 
  me 
  conclusive. 
  He 
  cites 
  the 
  fact, 
  discovered 
  by 
  

   Schumann 
  J, 
  that 
  hydrogen 
  at 
  atmospheric 
  pressure 
  seems 
  per- 
  

   fectly 
  transparent 
  .to 
  ultra-violet 
  radiation, 
  as 
  demonstrating 
  

   that 
  pure 
  hydrogen 
  is 
  an 
  insulator 
  under 
  all 
  conditions. 
  

   It 
  may 
  be 
  that 
  hydrogen 
  has 
  a 
  finite 
  but 
  very 
  small 
  absorption- 
  

   coefficient, 
  no 
  more 
  noticeable 
  within 
  the 
  limits 
  of 
  a 
  laboratory 
  

   than 
  the 
  absorption 
  of 
  carbon 
  dioxide 
  for 
  visible 
  radiation 
  

   under 
  the 
  same 
  conditions 
  ; 
  but 
  whether 
  this 
  be 
  so 
  or 
  not 
  does 
  

  

  ■•*- 
  Communicated 
  by 
  the 
  Author. 
  

   t 
  Trowbridge, 
  Phil 
  Mag. 
  [6] 
  ii. 
  p. 
  379 
  (1901). 
  

   \ 
  Schumann, 
  Ann. 
  der 
  Physik 
  (1901). 
  

  

  