﻿Radioactivity 
  of 
  certain 
  Lavas. 
  579 
  

  

  The 
  method 
  pursued 
  in 
  these 
  experiments, 
  depending 
  upon 
  

   that 
  variable 
  property, 
  the 
  " 
  emanating 
  power,'^ 
  is 
  liable 
  to 
  

   give 
  results 
  lesser 
  or 
  greater 
  according 
  to 
  the 
  ultimate 
  com- 
  

   pactness 
  or 
  porosity 
  of 
  the 
  material 
  under 
  examination. 
  

   This 
  renders 
  its 
  indications 
  uncertain. 
  These 
  observations 
  

   seem, 
  however, 
  to 
  point 
  to 
  an 
  abnormally 
  high 
  radioactivitv 
  

   and 
  there 
  are 
  some 
  indications 
  o£ 
  the 
  rise 
  in 
  radioactivity 
  of 
  

   the 
  more 
  recent 
  lavas. 
  On 
  the 
  point 
  of 
  the 
  lavas 
  gaining 
  in 
  

   radioactivity 
  after 
  solidification, 
  if 
  this 
  gain 
  is 
  supposed 
  to 
  

   be 
  rapid, 
  my 
  experiments 
  are 
  not 
  relevant 
  ; 
  as 
  in 
  every 
  case 
  

   the 
  materials 
  which 
  I 
  examined 
  were 
  some 
  years 
  old. 
  But 
  

   if 
  the 
  gain 
  is 
  supposed 
  to 
  take 
  place 
  over 
  centuries 
  my 
  results 
  

   do 
  not 
  support 
  such 
  a 
  view. 
  In 
  fact, 
  we 
  would 
  expect 
  to 
  

   find 
  the 
  more 
  anciently 
  erupted 
  lavas 
  the 
  most 
  radioactive, 
  

   whereas 
  it 
  is 
  the 
  other 
  way 
  according 
  to 
  my 
  experiments. 
  

  

  In 
  the 
  course 
  of 
  my 
  work 
  on 
  the 
  Yesuvian 
  rocks 
  additional 
  

   security 
  against 
  error 
  was 
  obtained 
  by 
  systemmatically 
  inter- 
  

   polating 
  experiments 
  on 
  materials 
  from 
  other 
  localities. 
  

   These 
  were 
  in 
  all 
  respects 
  prepared 
  under 
  like 
  conditions 
  and 
  

   examined 
  in 
  the 
  same 
  way. 
  Of 
  the 
  lavas 
  listed 
  below 
  the 
  

   following 
  were 
  so 
  interpolated 
  : 
  — 
  Etna, 
  Pantellaria, 
  Stromboli, 
  

   Chimborazo, 
  Krakatao 
  ash. 
  In 
  addition 
  to 
  these, 
  specimens 
  

   of 
  Yesuvian 
  biotite 
  and 
  leucite 
  were 
  examined 
  ; 
  and 
  (for 
  

   other 
  work) 
  Moff'at 
  Shale 
  (3*1) 
  and 
  Keuper 
  Sandstone 
  (4-0). 
  

   It 
  will 
  be 
  seen 
  further 
  on 
  that 
  some 
  of 
  these 
  are 
  abnormal 
  only 
  

   in 
  the 
  sense 
  of 
  being 
  low. 
  The 
  experiments 
  were 
  in 
  some 
  cases 
  

   repeated 
  upon 
  fresh 
  material, 
  and 
  in 
  every 
  case 
  interior 
  chips 
  

   were 
  taken 
  from 
  the 
  specimens. 
  The 
  usual 
  precautious 
  us 
  

   to 
  purity 
  of 
  chemicals, 
  etc. 
  were 
  observed. 
  The 
  standard 
  of 
  

   comparison 
  is 
  uraninite 
  of 
  Joachimsthal 
  containing, 
  according 
  

   to 
  a 
  recent 
  analysis 
  made 
  here, 
  65 
  per 
  cent, 
  of 
  uranium. 
  

  

  More 
  recently 
  I 
  have 
  determined 
  the 
  thorium 
  content 
  of 
  

   most 
  of 
  the 
  materials 
  dealt 
  with. 
  In 
  some 
  cases 
  this 
  deter- 
  

   mination 
  was 
  carried 
  out 
  upon 
  the 
  same 
  solutions 
  used 
  for 
  

   measuring 
  the 
  radium 
  ; 
  in 
  some 
  a 
  fresh 
  solution 
  was 
  prepared. 
  

   In 
  the 
  course 
  of 
  these 
  experiments, 
  which 
  were 
  carried 
  out 
  

   by 
  a 
  method 
  recently 
  described 
  by 
  me 
  (Phil. 
  Mag. 
  May 
  

   1909), 
  a 
  test 
  of 
  the 
  reliability 
  of 
  the 
  method 
  was 
  made 
  which 
  

   may 
  with 
  advantage 
  be 
  referred 
  to. 
  

  

  The 
  amount 
  of 
  thorium 
  in 
  the 
  Krakatao 
  pumice 
  listed 
  in 
  

   the 
  table 
  below 
  was 
  found 
  to 
  be 
  b'2x 
  10"^ 
  gram 
  per 
  gram. 
  

   To 
  10 
  grams 
  of 
  the 
  powdered 
  pumice 
  I 
  added 
  1'2 
  milligram 
  

   of 
  a 
  specimen 
  of 
  thorianite 
  kindly 
  given 
  to 
  me 
  by 
  Professor 
  

   Dunstan 
  (Phil. 
  Mag. 
  July 
  1909, 
  p. 
  140). 
  The 
  whole 
  was 
  

   carefully 
  mixed 
  with 
  the 
  fusion 
  mixture 
  of 
  alkaline 
  carbonates 
  

   (25 
  grams) 
  and 
  fused 
  in 
  the 
  usual 
  manner. 
  The 
  melt 
  was 
  

   leached 
  with 
  distilled 
  water 
  and 
  filtered 
  cold. 
  The 
  residue. 
  

  

  