﻿Helium 
  from 
  the 
  Primary 
  Radio- 
  Elements. 
  523 
  

  

  lowered 
  and 
  the 
  contents 
  of 
  E 
  removed 
  by 
  the 
  pump. 
  The 
  

   tap 
  F 
  alone 
  calls 
  for 
  more 
  detailed 
  description. 
  It 
  is 
  worked 
  

   by 
  connecting 
  the 
  tap 
  d 
  to 
  a 
  mechanical 
  air-pump. 
  It 
  is 
  made 
  

   slow-acting 
  by 
  means 
  of 
  the 
  construction 
  (<?). 
  Obviously 
  

   the 
  bulb 
  (c) 
  must 
  be 
  below 
  the 
  level 
  of 
  the 
  junction 
  of 
  the 
  

   limbs 
  (g) 
  and 
  (A) 
  ; 
  so 
  that 
  if 
  the 
  tap 
  (d) 
  were 
  left 
  open 
  to 
  

   the 
  air 
  when 
  h 
  is 
  also 
  open 
  to 
  the 
  air, 
  the 
  mercury 
  would 
  be 
  

   blown 
  out 
  of 
  the 
  tap 
  into 
  the 
  flask. 
  This 
  is 
  the 
  sole 
  danger 
  to 
  

   guard 
  against. 
  If 
  the 
  tap 
  (d) 
  is 
  closed 
  in 
  time, 
  even 
  a 
  sudden 
  

   break 
  anywhere 
  on 
  the 
  h 
  side 
  of 
  the 
  mercury 
  tap 
  does 
  no 
  harm. 
  

   The 
  small 
  bulbs 
  a 
  and 
  b 
  take 
  up 
  the 
  inertia 
  of 
  the 
  moving- 
  

   mercury, 
  and 
  the 
  slow 
  flow 
  of 
  mercury 
  into 
  c 
  increases 
  the 
  

   pressure 
  of 
  the 
  air 
  there 
  above 
  the 
  atmospheric 
  so 
  that 
  the 
  

   mercury 
  remains 
  in 
  the 
  tap 
  permanently 
  above 
  the 
  level 
  /. 
  

   The 
  dimensions 
  of 
  the 
  tap 
  are 
  as 
  follows 
  : 
  — 
  

  

  Height 
  over 
  all, 
  107 
  cms. 
  

  

  Distance 
  between 
  the 
  bulbs 
  a 
  and 
  b, 
  85 
  cms. 
  

  

  Diameter 
  of 
  bulb 
  c, 
  40 
  mms. 
  

  

  Diameter 
  of 
  barometer-tubes 
  g 
  and 
  7i, 
  3 
  mms. 
  

  

  Diameter 
  of 
  bulbs 
  a 
  and 
  b, 
  26 
  mms. 
  

  

  What 
  is 
  very 
  important 
  is 
  that 
  these 
  dimensions 
  have 
  suf- 
  

   ficient 
  margin 
  to 
  allow 
  of 
  the 
  slight 
  excess 
  of 
  pressure 
  required 
  

   for 
  glass-blowing 
  operations 
  on 
  the 
  A-limb 
  of 
  the 
  tap 
  while 
  

   there 
  is 
  a 
  vacuum 
  in 
  the 
  other. 
  It 
  was 
  the 
  failure 
  to 
  allow 
  

   for 
  this 
  contingency 
  which 
  caused 
  the 
  accident 
  to 
  the 
  second 
  

   series 
  of 
  experiments. 
  

  

  The 
  height 
  of 
  the 
  mercury 
  in 
  the 
  limb 
  g 
  indicates 
  the 
  

   pressure 
  of 
  gas 
  in 
  the 
  flask, 
  which 
  is 
  never 
  allowed 
  to 
  exceed 
  

   a 
  few 
  cms. 
  of 
  mercury. 
  Soon 
  after 
  setting 
  up 
  the 
  thorium 
  

   and 
  uranium 
  flasks, 
  some 
  of 
  the 
  mercury 
  in 
  the 
  trap 
  D 
  

   found 
  its 
  way 
  into 
  the 
  solutions 
  during 
  the 
  operations, 
  con- 
  

   taminating 
  them 
  and 
  causing 
  oxides 
  of 
  nitrogen 
  to 
  be 
  present 
  

   in 
  the 
  removed 
  gases. 
  This 
  does 
  no 
  great 
  harm, 
  but 
  in 
  the 
  

   later 
  experiments 
  the 
  trap 
  Z 
  has 
  been 
  added 
  to 
  avoid 
  this 
  

   contingency. 
  It 
  is 
  probable 
  no 
  further 
  modification 
  of 
  this 
  

   part 
  of 
  the 
  apparatus 
  will 
  be 
  called 
  for. 
  

  

  What 
  may 
  be 
  called 
  the 
  engineering 
  of 
  so 
  unwieldy 
  an 
  

   apparatus 
  may 
  be 
  seen 
  from 
  the 
  Plate 
  (XX.). 
  On 
  the 
  small 
  

   wood 
  base 
  supporting 
  the 
  water-bath 
  is 
  fixed 
  a 
  vertical 
  bar 
  

   of 
  T-iron 
  to 
  which 
  the 
  flask, 
  condenser, 
  voltameter, 
  and 
  

   mercury 
  tap 
  are 
  fastened. 
  The 
  voltameter 
  is 
  between 
  the 
  

   flask 
  and 
  mercury 
  tap, 
  and 
  is 
  not 
  seen 
  in 
  the 
  photographs 
  

   except 
  by 
  reflexion 
  in 
  the 
  circular 
  mirror 
  placed 
  behind 
  it. 
  

   The 
  apparatus, 
  when 
  sealed 
  off 
  from 
  the 
  furnace, 
  is 
  self- 
  

   contained, 
  and 
  can 
  if 
  necessary 
  be 
  moved. 
  When 
  not 
  in 
  use, 
  

  

  