﻿196 
  Dr. 
  J. 
  C. 
  McLennan 
  on 
  Radioactivity 
  

  

  faint 
  impression 
  on 
  a 
  photographic 
  plate. 
  The 
  action 
  was 
  

   very 
  weak, 
  and 
  an 
  exposure 
  of 
  eight 
  days 
  was 
  necessary 
  in 
  

   order 
  to 
  make 
  the 
  effect 
  visible. 
  

  

  This 
  latter 
  case 
  appears 
  to 
  be 
  the 
  only 
  example, 
  hitherto 
  

   recorded, 
  of 
  a 
  substance 
  normally 
  inactive 
  being 
  made 
  radio- 
  

   active 
  by 
  exposing 
  it 
  to 
  cathode 
  rays. 
  

  

  In 
  the 
  following 
  paper 
  an 
  account 
  is 
  given 
  of 
  an 
  investiga- 
  

   tion 
  in 
  which 
  several 
  salts, 
  showing 
  no 
  radioactivity 
  as 
  

   ordinarily 
  prepared, 
  were 
  found 
  to 
  exhibit 
  a 
  very 
  marked 
  radio- 
  

   activity 
  on 
  being 
  gently 
  heated 
  after 
  being 
  exposed 
  to 
  cathode 
  

   rays. 
  With 
  several 
  of 
  the 
  salts 
  a 
  temperature 
  of 
  100° 
  G. 
  

   sufficed 
  to 
  exhibit 
  the 
  effect, 
  though 
  with 
  still 
  higher 
  tem- 
  

   peratures 
  the 
  radioactivity 
  was 
  more 
  pronounced. 
  

  

  The 
  radiation 
  emitted 
  by 
  these 
  salts 
  was 
  found 
  to 
  discharge 
  

   positively 
  charged 
  bodies, 
  but 
  not 
  those 
  negatively 
  electrified. 
  

   In 
  no 
  case 
  did 
  it 
  appear 
  to 
  impart 
  a 
  charge 
  to 
  an 
  unelectrified 
  

   body. 
  

  

  II. 
  Measuring 
  System. 
  

  

  In 
  order 
  to 
  study 
  this 
  radiation 
  the 
  system 
  devised 
  by 
  

   C. 
  T. 
  R. 
  Wilson* 
  was 
  used. 
  A 
  thin 
  strip 
  of 
  brass, 
  A 
  (fig. 
  1), 
  

   was 
  suspended 
  vertically 
  in 
  a 
  small 
  metallic 
  box, 
  B, 
  by 
  means 
  

   of 
  an 
  insulating 
  bead 
  of 
  sulphur, 
  C. 
  To 
  this 
  strip 
  there 
  was 
  

   attached 
  a 
  gold 
  leaf 
  whose 
  deflexions, 
  observed 
  with 
  a 
  

   micrometer 
  microscope 
  through 
  a 
  glass 
  window 
  in 
  the 
  

   apparatus, 
  gave 
  a 
  measure 
  of 
  the 
  potential 
  of 
  the 
  brass 
  strip. 
  

  

  The 
  capacity 
  of 
  the 
  whole 
  measuring 
  system 
  was 
  small, 
  

   and 
  at 
  most 
  did 
  not 
  exceed 
  two 
  centimetres. 
  

  

  A 
  brass 
  rod, 
  D, 
  which 
  supported 
  the 
  measuring 
  system 
  

   passed 
  through 
  an 
  ebonite 
  plug, 
  E, 
  and 
  was 
  connected 
  to 
  one 
  

   of 
  the 
  terminals 
  of 
  a 
  battery 
  of 
  small 
  storage-cells, 
  the 
  other 
  

   terminal 
  of 
  the 
  battery 
  being 
  joined 
  to 
  the 
  case 
  of 
  the 
  

   electroscope. 
  

  

  ' 
  A 
  fine 
  steel 
  wire 
  which 
  was 
  attached 
  at 
  its 
  upper 
  end 
  to 
  the 
  

   rod, 
  D, 
  extended 
  below 
  the 
  sulphur 
  bead, 
  and 
  was 
  there 
  bent 
  

   into 
  a 
  loop 
  surrounding 
  the 
  brass 
  strip. 
  This 
  steel 
  wire 
  could 
  

   be 
  readily 
  drawn 
  into 
  contact 
  with 
  the 
  strip, 
  A, 
  by 
  means 
  of 
  

   a 
  small 
  magnet, 
  and 
  the 
  measuring 
  system 
  by 
  this 
  operation 
  

   was 
  raised 
  from 
  time 
  to 
  time 
  to 
  any 
  desired 
  potential. 
  

   » 
  Besides 
  having 
  a 
  small 
  capacity, 
  this 
  measuring 
  system 
  

   possessed 
  the 
  advantage 
  of 
  being 
  entirely 
  free 
  from 
  leakage 
  

   along 
  its 
  supports. 
  In 
  practice 
  the 
  conducting 
  rod, 
  D, 
  

   was 
  maintained, 
  throughout 
  any 
  measurement, 
  at 
  the 
  initial 
  

   potential 
  of 
  the 
  gold 
  leaf, 
  and 
  consequently, 
  when 
  the 
  latter 
  

  

  * 
  Proc. 
  Roy. 
  Soc. 
  vol. 
  lxviii. 
  p. 
  154. 
  

  

  