﻿Helium 
  from 
  the 
  Primary 
  Radio- 
  Elements. 
  525 
  

  

  seconds. 
  All 
  observations 
  must 
  be 
  carried 
  out 
  very 
  quickly, 
  

   but 
  with 
  experience 
  a 
  single 
  glance 
  is 
  usually 
  sufficient; 
  and 
  

   one's 
  first 
  judgment, 
  even 
  with 
  the 
  smallest 
  quantities 
  o£ 
  

   helium, 
  is 
  rarely 
  reversed 
  by 
  subsequent 
  observations. 
  

  

  One 
  other 
  very 
  important 
  consideration 
  must 
  be 
  mentioned. 
  

   The 
  fact 
  that 
  the 
  argon 
  from 
  a 
  cubic 
  millimetre 
  of 
  air 
  is 
  

   sufficient 
  to 
  spoil 
  the 
  testis 
  in 
  reality 
  a 
  very 
  great 
  advantage, 
  

   and 
  constitutes 
  its 
  main 
  claim 
  to 
  suitability 
  and 
  safety 
  in 
  the 
  

   present 
  work. 
  If, 
  for 
  example, 
  the 
  method 
  of 
  Sir 
  James 
  Dewar, 
  

   of 
  absorbing 
  non-helium 
  gases 
  by 
  cold 
  charcoal, 
  were 
  em- 
  

   ployed, 
  since 
  that 
  method 
  absorbs 
  argon 
  also, 
  there 
  never 
  

   could 
  be 
  any 
  complete 
  certainty, 
  when 
  helium 
  was 
  observed, 
  

   that 
  it 
  was 
  not 
  derived 
  from 
  atmospheric 
  air 
  which 
  had 
  been 
  

   left 
  in 
  the 
  solution, 
  or 
  had 
  leaked 
  in. 
  With 
  the 
  method 
  em- 
  

   ployed 
  the 
  slightest 
  leak 
  of 
  air 
  is 
  fatal 
  in 
  itself, 
  and 
  there 
  is 
  

   no 
  possibility 
  if 
  helium 
  is 
  observed 
  that 
  it 
  is 
  derived 
  from 
  the 
  

   air. 
  When 
  first 
  devised, 
  it 
  was 
  thought 
  the 
  method 
  would 
  

   make 
  the 
  work 
  comparatively 
  easy; 
  but 
  afterwards, 
  when 
  

   the 
  numerous 
  failures 
  due 
  to 
  residual 
  argon 
  were 
  being 
  ex- 
  

   perienced, 
  it 
  seemed 
  as 
  though 
  little 
  had 
  been 
  gained. 
  In 
  

   the 
  work 
  with 
  radium 
  the 
  difficulty 
  had 
  been 
  nitrogen 
  ; 
  

   whereas 
  here 
  exactly 
  the 
  same 
  precautions 
  had 
  to 
  be 
  taken 
  

   to 
  exclude 
  air 
  as 
  if 
  the 
  calcium 
  had 
  not 
  been 
  able 
  to 
  absorb 
  

   nitrogen. 
  Its 
  ability 
  to 
  absorb 
  the 
  hydrogen 
  and 
  hydro- 
  

   carbon 
  gases 
  always 
  normally 
  present 
  in 
  work 
  of 
  this 
  

   character, 
  is, 
  however, 
  its 
  real 
  advantage. 
  

  

  Calibration 
  Experiments. 
  

  

  Everything 
  depends, 
  if 
  the 
  results 
  are 
  to 
  have 
  any 
  theo- 
  

   retical 
  significance, 
  upon 
  the 
  smallest 
  amount 
  of 
  helium 
  that 
  

   can 
  be 
  detected 
  by 
  the 
  method 
  described 
  being 
  accurately 
  and 
  

   definitely 
  known; 
  and 
  this 
  part 
  of 
  the 
  investigation 
  has 
  been 
  

   carried 
  out 
  with 
  great 
  care. 
  The 
  results 
  have 
  been 
  gratifying 
  

   in 
  that 
  they 
  have 
  shown 
  that 
  the 
  limit 
  of 
  visibility 
  of 
  the 
  

   D 
  3 
  line 
  constitutes 
  a 
  perfectly 
  sharp 
  and 
  definite 
  quantitative 
  

   test 
  for 
  helium. 
  Some 
  preliminary 
  experiments 
  on 
  this 
  

   question 
  were 
  given 
  in 
  the 
  original 
  paper 
  describing 
  the 
  

   method. 
  Ordinary 
  Pliicker 
  tubes 
  were 
  employed 
  and 
  the 
  appa- 
  

   ratus 
  was 
  not 
  filled 
  with 
  mercury, 
  but 
  the 
  minimum 
  quantitv 
  

   detectable 
  was 
  arrived 
  at 
  by 
  dividing 
  the 
  total 
  quantity 
  added 
  

   by 
  the 
  ratio 
  of 
  the 
  total 
  volume 
  to 
  that 
  of 
  the 
  spectrum-tube 
  

   employed. 
  The 
  present 
  data 
  show 
  that 
  there 
  is 
  an 
  error 
  some- 
  

   where 
  in 
  these 
  calculations, 
  owing 
  probably 
  to 
  the 
  limiting 
  

   partial 
  pressure 
  of 
  helium 
  detectable 
  in 
  an 
  ordinary 
  Pliicker 
  

   tube 
  being 
  less 
  than 
  in 
  a 
  tube 
  of 
  minimum 
  volume. 
  The 
  

   minimum 
  detectable 
  quantity 
  of 
  helium, 
  r 
  even 
  in 
  a 
  tube 
  of 
  the 
  

  

  