﻿200 
  Dr. 
  J. 
  C. 
  McLennan 
  on 
  Radioactivity 
  

  

  The 
  sulphates 
  of 
  calcium, 
  strontium, 
  barium, 
  and 
  potassium 
  

   were 
  found 
  to 
  differ 
  but 
  little 
  in 
  their 
  radioactive 
  powers, 
  

   while 
  the 
  effect 
  obtained 
  with 
  beryllium 
  sulphate 
  was 
  quite 
  

   small. 
  In 
  the 
  case 
  of 
  the 
  sulphides 
  the 
  radioactivity 
  was 
  

   found 
  to 
  be 
  only 
  about 
  one-fifth 
  that 
  of 
  the 
  sulphates 
  of 
  the 
  

   same 
  elements. 
  

  

  It 
  is 
  of 
  interest 
  to 
  note 
  in 
  this 
  connexion 
  that 
  although 
  

   calcium 
  chloride 
  could 
  not 
  be 
  made 
  radioactive, 
  still 
  calcium 
  

   sulphate 
  prepared 
  from 
  this 
  salt 
  became 
  quite 
  radioactive 
  on 
  

   being 
  exposed 
  to 
  cathode 
  rays. 
  

  

  All 
  the 
  salts 
  examined 
  rapidly 
  lost 
  their 
  radioactivity 
  

   on 
  being 
  heated. 
  From 
  three 
  to 
  five 
  minutes' 
  exposure 
  in 
  

   the 
  electroscope 
  to 
  the 
  flame 
  of 
  a 
  Bunsen 
  burner 
  generally 
  

   sufficed 
  to 
  drive 
  off 
  completely 
  every 
  trace 
  of 
  radioactivity. 
  

  

  The 
  active 
  salts 
  also 
  gradually 
  lost 
  their 
  radioactivity 
  with 
  

   the 
  lapse 
  of 
  time 
  without 
  being 
  heated. 
  With 
  a 
  specimen 
  of 
  

   calcium 
  sulphate 
  tested 
  twenty 
  hours 
  after 
  exposure 
  to 
  

   cathode 
  rays, 
  the 
  leak 
  obtained 
  was 
  faint 
  though 
  still 
  quite 
  

   marked. 
  

  

  In 
  a 
  series 
  of 
  comparative 
  measurements 
  with 
  active 
  

   calcium 
  sulphate, 
  in 
  air 
  at 
  pressures 
  varying 
  from 
  480 
  mms. 
  

   down 
  to 
  1 
  mm. 
  of 
  mercury, 
  no 
  difference 
  either 
  in 
  the 
  rate 
  

   or 
  the 
  total 
  amount 
  of 
  the 
  positive 
  leak 
  was 
  observed. 
  At 
  no 
  

   pressure 
  was 
  any 
  leak 
  of 
  the 
  negative 
  charge 
  obtained, 
  and 
  

   the 
  gold 
  leaf, 
  when 
  initially 
  unelectrified, 
  did 
  not 
  acquire 
  any 
  

   charge 
  from 
  the 
  radiation. 
  

  

  With 
  the 
  same 
  salt 
  (calcium 
  sulphate) 
  a 
  magnetic 
  field 
  of 
  

   3000 
  C.G.S. 
  units 
  was 
  found 
  to 
  slightly 
  decrease 
  the 
  leak. 
  

  

  In 
  order 
  to 
  obtain 
  an 
  approximate 
  estimate 
  of 
  the 
  lowest 
  

   temperature 
  at 
  which 
  the 
  radioactivity 
  appeared 
  in 
  the 
  case 
  

   of 
  calcium 
  sulphate, 
  a 
  jet 
  of 
  steam 
  was 
  directed 
  against 
  the 
  

   base 
  of 
  the 
  electroscope. 
  The 
  temperature 
  so 
  obtained 
  was 
  

   sufficient 
  to 
  bring 
  on 
  the 
  discharge, 
  but 
  the 
  rate 
  of 
  leak 
  

   was 
  much 
  smaller 
  than 
  when 
  the 
  electroscope 
  was 
  heated 
  

   with 
  the 
  flame 
  of 
  a 
  Bunsen 
  burner. 
  

  

  Each 
  of 
  the 
  salts 
  given 
  in 
  Table 
  I. 
  was 
  carefully 
  tested 
  

   for 
  radioactivity 
  at 
  the 
  temperature 
  of 
  the 
  laboratory 
  (about 
  

   18° 
  C), 
  both 
  before 
  being 
  exposed 
  to 
  the 
  cathode 
  rays 
  and 
  

   after 
  bombardment 
  ; 
  but 
  with 
  the 
  exception 
  of 
  potassium 
  

   sulphate 
  none 
  of 
  them 
  produced 
  a 
  leak 
  under 
  these 
  circum- 
  

   stances. 
  With 
  potassium 
  sulphate, 
  a 
  small 
  leak 
  of 
  positive 
  

   electricity 
  occurred 
  if 
  the 
  test 
  was 
  made 
  immediately 
  after 
  

   exposure 
  to 
  cathode 
  rays. 
  But 
  this 
  effect 
  soon 
  passed 
  ofrj 
  

   and 
  was 
  followed 
  on 
  the 
  application 
  of 
  heat 
  to 
  the 
  electroscope 
  

   by 
  the 
  usual 
  larger 
  leak 
  obtained 
  with 
  the 
  other 
  active 
  salts. 
  

  

  None 
  of 
  the 
  salts 
  examined 
  exhibited 
  any 
  radioactivity 
  

  

  