﻿Notations 
  employed 
  in 
  Theories 
  of 
  Crystal- 
  structure, 
  203 
  

  

  two 
  obvious 
  ways 
  of 
  explaining 
  this 
  peculiar 
  radiation. 
  One 
  

   of 
  these 
  is 
  to 
  regard 
  the 
  effect 
  as 
  due 
  to 
  ultra-violet 
  light 
  

   produced 
  by 
  heating 
  the 
  excited 
  salt, 
  this 
  light 
  acting 
  on 
  

   the 
  negatively-charged 
  salt 
  and 
  thereby 
  causing 
  a 
  leak 
  from 
  

   its 
  surface. 
  

  

  To 
  test 
  this 
  view, 
  an 
  amalgamated 
  zinc 
  sphere, 
  Z 
  (fig.l), 
  

   was 
  attached 
  to 
  the 
  lower 
  end 
  of 
  the 
  measuring 
  system. 
  With 
  

   this 
  arrangement, 
  though 
  the 
  zinc 
  sphere 
  was 
  within 
  four 
  

   millimetres 
  of 
  the 
  radioactive 
  salt, 
  no 
  indication 
  of 
  a 
  discharge 
  

   of 
  negative 
  electricity 
  from 
  the 
  gold 
  leaf 
  was 
  observed. 
  

  

  It 
  seems 
  scarcely 
  possible 
  that 
  an 
  ultra- 
  vjolet 
  radiation, 
  

   capable 
  of 
  producing 
  the 
  effects 
  observed, 
  could 
  be 
  absorbed 
  

   completely 
  in 
  passing 
  through 
  air 
  at 
  15 
  mins. 
  pressure 
  for 
  a 
  

   distance 
  of 
  only 
  four 
  millimetres. 
  

  

  ' 
  Probably 
  the 
  simpler 
  view 
  to 
  take 
  is 
  that 
  the 
  salts 
  on 
  being 
  

   heated 
  emit 
  a 
  stream 
  of 
  negatively-charged 
  particles 
  or 
  cor- 
  

   puscles 
  which 
  are 
  drawn 
  to 
  the 
  positively-charged 
  gold 
  leaf 
  

   by 
  the 
  electrostatic 
  field, 
  and 
  thus 
  discharge 
  it. 
  If 
  this 
  view 
  

   be 
  correct, 
  these 
  negatively-charged 
  particles 
  must 
  be 
  sent 
  

   out 
  from 
  the 
  salt 
  with 
  exceedingly 
  small 
  velocities, 
  otherwise 
  

   we 
  would 
  expect 
  the 
  gold 
  leaf, 
  when 
  initially 
  unelectrified, 
  

   to 
  acquire 
  at 
  least 
  a 
  small 
  negative 
  charge. 
  

  

  The 
  experiments 
  described 
  in 
  this 
  paper 
  were 
  conducted 
  in 
  

   the 
  Cavendish 
  Laboratory, 
  and 
  my 
  sincere 
  thanks 
  are 
  due 
  to 
  

   Professor 
  Thomson 
  for 
  many 
  valuable 
  suggestions 
  and 
  much 
  

   encouragement 
  given 
  throughout 
  the 
  investigation. 
  

  

  XXI. 
  A 
  Comparison 
  of 
  various 
  Sotatioiis 
  emp>loyed 
  in 
  

   " 
  Theories 
  of 
  Crystal-structure,'" 
  and 
  a 
  Revision 
  of 
  the 
  230 
  

   Groups 
  of 
  Mo 
  cements. 
  By 
  Harold 
  Hilton, 
  Magdalen 
  

   College, 
  Oxford*. 
  

  

  SINCE 
  the 
  determination 
  of 
  the 
  possible 
  groups 
  of 
  move- 
  

   ments 
  must 
  hold 
  an 
  important 
  position 
  in 
  any 
  future 
  

   theories 
  of 
  crystal 
  structure 
  ; 
  and 
  since 
  the 
  mathematical 
  

   theory 
  of 
  these 
  groups 
  has 
  been 
  practically 
  brought 
  to 
  a 
  

   completion; 
  it 
  seemed 
  advisable 
  to 
  draw 
  up 
  tables 
  com- 
  

   paring 
  the 
  different 
  notations 
  of 
  the 
  various 
  workers 
  on 
  the 
  

   subject, 
  with 
  the 
  hope 
  that 
  at 
  the 
  same 
  time 
  a 
  judgment 
  

   might 
  be 
  formed 
  on 
  the 
  correctness 
  of 
  the 
  statement 
  that 
  the 
  

   number 
  of 
  groups 
  of 
  movements 
  which 
  are 
  applicable 
  to 
  

   crystallography 
  is 
  230. 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  