﻿801 
  Mr. 
  II. 
  L. 
  P. 
  Jolly 
  on 
  the 
  

  

  gases 
  is 
  related 
  to 
  the 
  current-density 
  in 
  a 
  manner 
  repre- 
  

   sented 
  at 
  least 
  roughly 
  by 
  a 
  linear 
  law. 
  Probably 
  it 
  is 
  

   an 
  essential 
  condition 
  for 
  the 
  observance 
  of 
  this 
  that 
  the 
  

   tube 
  be 
  not 
  too 
  narrow 
  and 
  the 
  pressure 
  not 
  too 
  high. 
  In 
  

   other 
  words, 
  the 
  current-density 
  must 
  not 
  be 
  too 
  great. 
  

   A 
  large 
  increase 
  of 
  pressure 
  makes 
  it 
  necessary 
  to 
  employ 
  a 
  

   large 
  capacity 
  in 
  conjunction 
  with 
  a 
  spark-gap 
  in 
  order 
  to 
  

   obtain 
  a 
  luminous 
  discharge. 
  These 
  are 
  exactly 
  the 
  con- 
  

   ditions 
  for 
  obtaining 
  a 
  large 
  current-density 
  for 
  very 
  short 
  

   intervals 
  of 
  time, 
  and 
  a 
  proportionality 
  between 
  current- 
  

   strength 
  and 
  radiation 
  would 
  not 
  be 
  expected. 
  In 
  any 
  case 
  

   these 
  large 
  momentary 
  values 
  of 
  the 
  current 
  are 
  extremely 
  

   difficult 
  to 
  measure. 
  It 
  seems 
  probable 
  that 
  for 
  current- 
  

   densities 
  below 
  a 
  certain 
  value 
  the 
  proportionality 
  law 
  holds. 
  

   (Nutting 
  & 
  Tugman 
  assert 
  this 
  as 
  an 
  experimental 
  fact.) 
  

   Thus 
  it 
  is 
  noticeable 
  that 
  all 
  the 
  observers 
  who 
  put 
  forward 
  

   the 
  law 
  worked 
  either 
  with 
  low 
  currents 
  (Angstrom, 
  Ferry, 
  

   Berndt, 
  Waetzmann) 
  or 
  with 
  wide 
  tubes 
  (Geiger). 
  

  

  Such 
  a 
  law 
  is 
  attractive 
  because 
  of 
  its 
  simplicity, 
  but 
  there 
  

   can 
  be 
  no 
  doubt 
  that 
  at 
  higher 
  pressures 
  and 
  larger 
  current- 
  

   densities 
  no 
  such 
  simple 
  conditions 
  occur. 
  The 
  nature 
  of 
  

   the 
  causes 
  producing 
  radiation, 
  then, 
  presents 
  a 
  problem 
  as 
  

   yet 
  quite 
  unsolved. 
  As 
  to 
  the 
  exact 
  form 
  of 
  the 
  relation 
  

   between 
  current-density 
  and 
  radiation 
  when 
  no 
  direct 
  pro- 
  

   portionality 
  exists, 
  some 
  such 
  relation 
  as 
  that 
  suggested 
  

   by 
  Nutting 
  & 
  Tugman 
  may 
  represent 
  the 
  facts, 
  though 
  

   photometric 
  measurements 
  are 
  susceptible 
  of 
  no 
  great 
  

   accuracy. 
  

  

  It 
  may 
  be 
  that 
  no 
  definite 
  relation 
  exists 
  unless 
  other 
  

   variables, 
  such 
  as 
  pressure, 
  width 
  of 
  tube, 
  cathode 
  fall 
  of 
  

   potential, 
  potential 
  gradient, 
  &c, 
  be 
  included 
  in 
  the 
  formula. 
  

   The 
  important 
  point 
  on 
  which 
  to 
  obtain 
  information 
  is 
  the 
  

   relative 
  change 
  in 
  energy 
  of 
  different 
  parts 
  of 
  the 
  spectrum. 
  

  

  Pfliiger 
  (also 
  Ladenburg) 
  showed 
  conclusively 
  that 
  ab- 
  

   sorption 
  in 
  hydrogen 
  depends 
  on 
  the 
  gas 
  being 
  ionized, 
  or 
  at 
  

   least 
  on 
  its 
  being 
  in 
  a 
  state 
  associated 
  with 
  the 
  passage 
  of 
  an 
  

   electric 
  current 
  through 
  it. 
  The 
  appearance 
  of 
  a 
  continuous 
  

   spectrum 
  at 
  higher 
  pressures 
  and 
  larger 
  current-densities 
  is 
  

   suggestive 
  of 
  a 
  temperature 
  radiation. 
  That 
  it 
  is 
  not 
  a 
  

   purely 
  temperature 
  effect 
  was 
  shown 
  by 
  Pfliiger, 
  in 
  the 
  case 
  

   of 
  mercury 
  vapour. 
  

  

  Nearly 
  all 
  of 
  the 
  work 
  mentioned 
  in 
  the 
  previous 
  summary 
  

   deals 
  with 
  measurements 
  made 
  on 
  the 
  spectrum 
  as 
  a 
  whole. 
  

  

  Mention 
  should 
  also 
  be 
  made 
  of 
  some 
  interesting 
  work 
  on 
  

   slightly 
  different 
  lines 
  by 
  Lunkenheimer 
  (Ann. 
  tier 
  Phys, 
  

   vol. 
  xxxvi. 
  p. 
  134, 
  1911) 
  and 
  a 
  very 
  comprehensive 
  series 
  of 
  

  

  