﻿578 
  Prof. 
  J. 
  Joly 
  on 
  the 
  

  

  in 
  common 
  with 
  all 
  the 
  Vesuvian 
  lavas 
  — 
  rich 
  in 
  leucite, 
  and 
  

   is, 
  apparently, 
  a 
  leucite-tephrite. 
  

  

  An 
  examination 
  of 
  these 
  rocks 
  afforded 
  results 
  in 
  harmony 
  

   with 
  the 
  indications 
  of 
  those 
  already 
  obtained. 
  The 
  recent 
  

   lava 
  was 
  considerably 
  richer 
  in 
  radium 
  than 
  any 
  of 
  the 
  

   others 
  and 
  the 
  ancient 
  dyke-rock 
  was 
  much 
  the 
  poorest 
  in 
  

   radium 
  of 
  any 
  yet 
  examined 
  ; 
  in 
  fact, 
  it 
  was 
  rather 
  below 
  

   the 
  normal 
  of 
  igneous 
  rocks. 
  

  

  Among 
  prior 
  observations 
  upon 
  the 
  lavas 
  of 
  Vesuvius, 
  I 
  

   am 
  acquainted 
  with 
  only 
  one, 
  that 
  made 
  by 
  the 
  Hon. 
  R. 
  J.. 
  

   Strutt, 
  effected 
  in 
  the 
  same 
  manner 
  ; 
  that 
  is, 
  by 
  solution 
  of 
  

   the 
  material 
  and 
  boiling 
  off 
  the 
  emanation 
  *. 
  This 
  obser- 
  

   vation 
  revealed 
  no 
  abnormality. 
  After 
  what 
  has 
  been 
  said^ 
  

   however, 
  I 
  do 
  not 
  think 
  any 
  contradiction 
  with 
  the 
  present 
  

   results 
  need 
  be 
  supposed 
  to 
  exist. 
  Thus 
  the 
  oldest 
  leucite- 
  

   tephrite 
  I 
  examined 
  resembled 
  many 
  feebly 
  radioactive 
  lavas, 
  

   and 
  it 
  is 
  probable 
  that 
  among 
  the 
  earlier 
  lavas 
  similar 
  rocks 
  

   abound. 
  Experiments 
  have 
  also 
  been 
  made 
  upon 
  the 
  lavas 
  

   of 
  1906 
  by 
  Nasini 
  and 
  Levi 
  (Atti 
  R. 
  Ace, 
  Lincei, 
  ser. 
  5, 
  

   Rendic. 
  xv. 
  sem. 
  2, 
  p. 
  391, 
  1906) 
  and 
  on 
  the 
  lava 
  of 
  1904 
  

   by 
  Tommasina 
  {Pliys. 
  Zeit. 
  vi. 
  1905, 
  p. 
  707). 
  In 
  both 
  cases 
  

   the 
  investigation 
  was 
  carried 
  out 
  by 
  the 
  dispersion-method 
  ; 
  

   the 
  powdered 
  rock 
  being 
  introduced 
  into 
  the 
  electroscope 
  

   and 
  the 
  increased 
  current 
  observed. 
  With 
  regard 
  to 
  the 
  

   lava 
  of 
  1906 
  Nasini 
  and 
  Levi 
  found 
  the 
  greatest 
  effect 
  from 
  

   the 
  lapilli 
  and 
  dust 
  ; 
  obtaining 
  (under 
  certain 
  conditions) 
  a 
  

   saturation 
  current 
  of 
  33*4 
  X 
  10"^^ 
  amp. 
  On 
  the 
  other 
  hand, 
  

   they 
  concluded 
  that 
  the 
  solidified 
  lavas 
  and 
  scoria 
  were 
  

   inactive. 
  They 
  examined 
  the 
  ejecta 
  of 
  some 
  other 
  outflows. 
  

   The 
  lapilli 
  and 
  dust 
  of 
  1872 
  gave 
  19*2 
  x 
  10-^^ 
  amp. 
  under 
  

   the 
  same 
  conditions, 
  and 
  the 
  lava 
  13 
  x 
  10~^^ 
  amp. 
  For 
  

   ejecta 
  of 
  various 
  years 
  the 
  effects 
  upon 
  the 
  electroscope, 
  to 
  

   the 
  same 
  order 
  of 
  magnitude 
  as 
  the 
  above 
  results, 
  were 
  : 
  — 
  

  

  1631 
  17-0 
  

  

  1769 
  19-7 
  

  

  1858 
  14-7 
  

  

  1868 
  23-2 
  

  

  1895-99 
  4-9 
  

  

  The 
  authors 
  conclude 
  that 
  a 
  lava 
  inactive 
  immediately 
  

   after 
  solidification 
  may 
  slowly 
  acquire 
  radioactivity. 
  

  

  Tommasina 
  found, 
  respecting 
  the 
  lava 
  of 
  1904, 
  that 
  while 
  

   obsidian 
  from 
  Lipari 
  showed 
  no 
  determinable 
  effect 
  upon 
  

   the 
  electroscope 
  the 
  lava 
  of 
  Vesuvius 
  increased 
  the 
  rate 
  of 
  

   fall 
  of 
  potential 
  from 
  a 
  normal 
  10 
  volts 
  per 
  hour 
  to 
  20 
  volts, 
  

   and 
  again 
  the 
  rate 
  of 
  loss 
  of 
  potential 
  might 
  even 
  be 
  increased 
  

   rather 
  more 
  than 
  three-fold. 
  

  

  * 
  Proc. 
  Koy. 
  Soc. 
  A. 
  vol. 
  Ixxvii. 
  p. 
  472. 
  

  

  