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  Radioactive 
  Minerals 
  in 
  Common 
  Rocks. 
  

  

  sedimentary 
  rocks 
  afford 
  suitable 
  material 
  for 
  the 
  purpose. 
  

   The 
  object 
  of 
  the 
  following 
  investigation 
  was 
  to 
  find 
  in 
  what 
  

   minerals 
  the 
  radio-activity 
  of 
  igneous 
  rocks 
  is 
  most 
  concen- 
  

   trated, 
  and, 
  if 
  possible, 
  to 
  separate 
  them 
  with 
  a 
  view 
  to 
  their 
  

   use 
  as 
  material 
  for 
  finding 
  the 
  age 
  of 
  the 
  rocks 
  in 
  which 
  they 
  

   occur. 
  

  

  The 
  first 
  material 
  that 
  was 
  treated 
  was 
  rotted 
  Cornwall 
  

   granite 
  from 
  which 
  kaolin 
  and 
  mica 
  had 
  been 
  removed 
  at 
  a 
  

   China-Clay 
  Works. 
  This 
  was 
  passed 
  through 
  a 
  fine 
  sieve 
  

   and 
  then 
  washed 
  with 
  a 
  gold-miner^s 
  pan 
  to 
  reject 
  the 
  lighter 
  

   constituents, 
  such 
  as 
  quartz 
  and 
  felspar. 
  When 
  the 
  bulk 
  

   was 
  sufficiently 
  reduced 
  the 
  lighter 
  minerals 
  which 
  remained 
  

   were 
  removed 
  by 
  means 
  of 
  a 
  heavy 
  liquid 
  (bromoform, 
  spec. 
  

   grav. 
  = 
  2'9), 
  those 
  minerals 
  whose 
  specific 
  gravity 
  exceeds 
  

   2'\) 
  sinking 
  in 
  the 
  bromoform 
  while 
  the 
  lighter 
  ones 
  float 
  on 
  

   the 
  surface. 
  

  

  The 
  heavy 
  and 
  light 
  constituents 
  were 
  tested 
  for 
  radio- 
  

   activity 
  by 
  finding 
  their 
  efiect 
  on 
  the 
  rate 
  of 
  leak 
  of 
  a 
  gold- 
  

   leaf 
  electroscope 
  when 
  spread 
  over 
  a 
  tray 
  placed 
  inside 
  the 
  

   electroscope. 
  

  

  Normal 
  rate 
  of 
  leak 
  = 
  4*0 
  scale 
  divs. 
  per 
  hour. 
  

  

  Increase 
  due 
  to 
  heavy 
  minerals 
  = 
  2'5 
  „ 
  „ 
  

  

  %ht 
  „ 
  =0 
  

  

  It, 
  thus 
  appears 
  that 
  the 
  radioactivity 
  is 
  concentrated 
  in 
  

   the 
  heavier 
  minerals 
  ; 
  the 
  lighter 
  constituents 
  were 
  therefore 
  

   rejected 
  and 
  further 
  separation 
  of 
  the 
  heavier 
  minerals 
  

   attempted. 
  A 
  little 
  magnetite 
  was 
  removed 
  with 
  a 
  perma- 
  

   nent 
  magnet 
  and 
  found 
  to 
  be 
  non-radioactive. 
  The 
  residue 
  

   was 
  mainly 
  tourmaline. 
  This 
  was 
  removed 
  by 
  a 
  compara- 
  

   tively 
  powerful 
  magnetic 
  field 
  produced 
  by 
  an 
  electro- 
  

   magnet. 
  The 
  first 
  part 
  of 
  this 
  that 
  was 
  extracted 
  was 
  not 
  

   active, 
  but 
  on 
  removing 
  the 
  last 
  traces 
  of 
  magnetic 
  matter 
  

   by 
  a 
  very 
  powerful 
  field 
  a 
  powder 
  was 
  obtained 
  in 
  which 
  the 
  

   ladioactivity 
  appeared 
  to 
  be 
  concentrated. 
  By 
  concentrating 
  

   the 
  feebly 
  magnetic 
  mineral 
  by 
  magnetic 
  fractionation 
  a 
  

   powder 
  was 
  obtained 
  for 
  which 
  the 
  corrected 
  rate 
  of 
  leak 
  

   was 
  16 
  scale 
  divs. 
  per 
  hour. 
  On 
  examination 
  under 
  the 
  

   microscope 
  this 
  was 
  seen 
  to 
  be 
  distinguished 
  by 
  the 
  presence 
  

   of 
  a 
  number 
  of 
  small 
  opaque 
  fragments 
  in 
  some 
  of 
  which 
  an 
  

   hexagonal 
  shape 
  could 
  be 
  detected. 
  The 
  other 
  minerals 
  

   present, 
  being 
  of 
  density 
  not 
  much 
  greater 
  than 
  2*9, 
  were 
  

   washed 
  away 
  by 
  agitating 
  the 
  powder 
  in 
  bromoform. 
  

   Advantage 
  was 
  also 
  taken 
  of 
  the 
  small 
  size 
  of 
  the 
  particles 
  

   to 
  obtain 
  the 
  active 
  mineral 
  pure 
  by 
  sifting. 
  

  

  About 
  i 
  gram 
  of 
  the 
  mineral 
  was 
  obtained 
  from 
  2 
  or 
  3 
  lb. 
  

  

  