﻿Radioaetive 
  Minerals 
  in 
  Common 
  Rocks. 
  ^11 
  

  

  charcoal, 
  the 
  tube 
  was 
  found 
  to 
  contain 
  argon 
  only. 
  No 
  

   lines 
  but 
  those 
  due 
  to 
  argon 
  could 
  be 
  observed 
  in 
  the 
  spectrum 
  

   of 
  the 
  gas. 
  The 
  volume 
  of 
  the 
  argon 
  was 
  measured 
  by 
  the 
  

   McLeod 
  gauge 
  and 
  found 
  to 
  be 
  3 
  cubic 
  mm., 
  a 
  quantity 
  

   which 
  could 
  be 
  well 
  accounted 
  for 
  by 
  contamination 
  with 
  air 
  

   in 
  the 
  course 
  of 
  transference 
  from 
  one 
  vessel 
  to 
  another. 
  

  

  As 
  krypton 
  is 
  very 
  easily 
  absorbed 
  by 
  charcoal 
  in 
  liquid 
  

   air, 
  it 
  was 
  thought 
  advisable 
  to 
  make 
  sure 
  that 
  none 
  had 
  

   remained 
  in 
  the 
  large 
  bulb 
  used 
  in 
  the 
  first 
  part 
  of 
  the 
  

   investio^ation. 
  This 
  bulb 
  was 
  maintained 
  at 
  a 
  temperature 
  of 
  

   220° 
  C. 
  for 
  about 
  6 
  hours 
  by 
  means 
  of 
  an 
  electric 
  heater, 
  and 
  

   the 
  expelled 
  gas 
  collected. 
  The 
  gas 
  was 
  practically 
  all 
  carbon 
  

   dioxide, 
  and 
  after 
  absorbing 
  most 
  of 
  it 
  with 
  hydrated 
  potassium 
  

   hydroxide, 
  the 
  residue 
  was 
  introduced 
  into 
  the 
  vacuum-tube 
  

   and 
  the 
  discharge 
  passed 
  until 
  all 
  trace 
  of 
  carbon 
  dioxide 
  was 
  

   removed. 
  The 
  spectrum 
  of 
  the 
  remaining 
  gas 
  was 
  compared 
  

   with 
  that 
  of 
  krypton, 
  but 
  no 
  agreement 
  was 
  found 
  in 
  any 
  of 
  

   the 
  lines. 
  

  

  The 
  amount 
  of 
  helium 
  given 
  ofE 
  by 
  the 
  mineral 
  was 
  in 
  

   round 
  numbers 
  100 
  c.cs. 
  Had 
  there 
  been 
  present 
  with 
  it 
  ^^ 
  

   of 
  a 
  c.c. 
  of 
  krypton, 
  the 
  spectrum 
  of 
  the 
  latter 
  would 
  have 
  

   been 
  conspicuous 
  when 
  the 
  residue 
  was 
  examined 
  : 
  hence 
  

   the 
  experiment 
  points 
  to 
  the 
  conclusion 
  that 
  krypton, 
  if 
  

   present 
  at 
  all, 
  which 
  is 
  indeed 
  doubtful, 
  must 
  amount 
  to 
  

   less 
  than 
  one 
  part 
  in 
  a 
  million 
  of 
  the 
  helium 
  content. 
  As 
  

   opportunity 
  affords 
  it 
  is 
  intended 
  to 
  carry 
  on 
  the 
  investi- 
  

   gation 
  with 
  respect 
  to 
  other 
  strongl}^ 
  radioactive 
  minerals, 
  

   as 
  it 
  is 
  not 
  to 
  be 
  expected 
  that 
  definite 
  results 
  can 
  be 
  

   drawn 
  from 
  the 
  behaviour 
  of 
  one 
  mineral. 
  

  

  In 
  conclusion 
  I 
  beg 
  to 
  express 
  my 
  thanks 
  to 
  Professor 
  

   Strutt 
  for 
  suggesting 
  this 
  research 
  and 
  for 
  his 
  valuable 
  advice 
  

   during 
  its 
  procedure. 
  

  

  LXVIII. 
  Radioactive 
  Minerals 
  in 
  Common 
  Rocks. 
  By 
  

   J. 
  W. 
  Waters, 
  B.Sc.^ 
  Imperial 
  College 
  of 
  Science^ 
  South 
  

   Kensington* 
  . 
  

  

  IT 
  has 
  been 
  shown 
  by 
  Rutherford 
  and 
  Strutt 
  that 
  a 
  know- 
  

   ledge 
  of 
  the 
  amount 
  of 
  uranium 
  and 
  also 
  the 
  amount 
  of 
  

   helium 
  contained 
  in 
  a 
  rock 
  or 
  mineral 
  enables 
  us 
  to 
  form 
  a 
  

   minimum 
  estimate 
  of 
  its 
  age. 
  In 
  making 
  accurate 
  measure- 
  

   ments 
  of 
  these 
  quantities 
  it 
  is 
  much 
  more 
  convenient 
  to 
  

   experiment 
  upon 
  substances 
  comparatively 
  rich 
  in 
  radio- 
  

   active 
  constituents. 
  Prof. 
  Strutt 
  has 
  found 
  that 
  the 
  

   phosphatic 
  nodules 
  and 
  phosphatized 
  bones 
  which 
  occur 
  in 
  

   * 
  Communicated 
  by 
  the 
  Hon. 
  R. 
  J. 
  Strutt, 
  F.R.S. 
  

  

  