﻿198 
  Dr. 
  J. 
  0. 
  McLennan 
  on 
  Radioactivity 
  

  

  shaken 
  in 
  the 
  tube 
  so 
  that 
  fresh 
  surfaces 
  might 
  be 
  presented 
  

   to 
  the 
  rays. 
  

  

  This 
  method 
  o£ 
  exciting 
  the 
  salts 
  was 
  somewhat 
  tedious, 
  

   as 
  it 
  necessitated 
  the 
  exhaustion 
  of 
  the 
  discharge- 
  tube 
  each 
  

   time 
  an 
  exposure 
  was 
  made. 
  

  

  It 
  was 
  found, 
  however, 
  on 
  trial 
  that 
  a 
  radioactivity, 
  similar 
  

   and 
  equal 
  in 
  intensity 
  to 
  that 
  obtained 
  with 
  cathode 
  rays, 
  was 
  

   imparted 
  to 
  the 
  salts 
  by 
  simply 
  exposing 
  them 
  in 
  air 
  at 
  

   atmospheric 
  pressure 
  to 
  the 
  spark-discharge 
  of 
  an 
  induction- 
  

   coil 
  or 
  Wimshurst 
  machine. 
  Short 
  thick 
  sparks 
  were 
  more 
  

   effective 
  in 
  exciting 
  radioactivity 
  than 
  long 
  thin 
  ones; 
  and 
  

   therefore, 
  in 
  most 
  cases 
  in 
  which 
  this 
  method 
  was 
  adopted, 
  a 
  

   leyden-jar 
  was 
  inserted 
  in 
  parallel 
  with 
  the 
  terminals 
  of 
  the 
  

   induction-coil 
  during 
  the 
  exposures. 
  

  

  Various 
  metals 
  were 
  used 
  in 
  turn 
  as 
  terminals 
  lor 
  the 
  coil, 
  

   but 
  no 
  difference 
  in 
  their 
  efficiency 
  was 
  observed. 
  

  

  In 
  order 
  to 
  see 
  whether 
  the 
  excited 
  radioactivity 
  was 
  due 
  

   to 
  the 
  presence 
  of 
  ultra-violet 
  light 
  in 
  the 
  spark-discharge, 
  a 
  

   quantity 
  of 
  one 
  of 
  the 
  salts 
  which 
  could 
  be 
  made 
  radio- 
  

   active 
  was 
  placed 
  in 
  a 
  shallow 
  tray 
  beneath 
  the 
  spark-gap 
  and 
  

   close 
  to 
  it. 
  The 
  sparks 
  were 
  made 
  to 
  pass 
  between 
  terminals 
  

   of 
  aluminium, 
  and 
  a 
  thin 
  quartz 
  plate 
  was 
  placed 
  between 
  the 
  

   salt 
  and 
  the 
  spark-gap. 
  

  

  With 
  this 
  arrangement 
  no 
  radioactivity 
  was 
  imparted 
  to 
  

   the 
  salt. 
  , 
  On 
  removing 
  the 
  quartz 
  plate, 
  however, 
  so 
  that 
  

   nothing 
  but 
  air 
  intervened 
  between 
  the 
  salt 
  and 
  the 
  spark- 
  

   discharge, 
  the 
  salt 
  became 
  radioactive 
  with 
  the 
  usual 
  exposure. 
  

  

  From 
  this 
  experiment 
  it 
  seemed 
  clear 
  that 
  the 
  observed 
  

   radioactivity 
  could 
  not 
  be 
  due 
  to 
  the 
  action 
  of 
  the 
  ultra- 
  

   violet 
  light, 
  but 
  was 
  in 
  all 
  probability 
  excited 
  by 
  a 
  peculiar 
  

   kind 
  of 
  radiation 
  shown 
  by 
  E. 
  Wiedemann 
  * 
  to 
  be 
  emitted 
  

   by 
  the 
  spark-discharge 
  and 
  called 
  by 
  him 
  " 
  Entladung- 
  

   strablen/' 
  

  

  * 
  Zeitschrift 
  fur 
  J2lectroche?nie, 
  vol. 
  ii. 
  p. 
  159 
  (1895). 
  

  

  