﻿]875.] 
  

  

  Repulsion 
  resulting 
  from 
  Radiation. 
  

  

  375 
  

  

  very 
  good. 
  A 
  ray 
  of 
  sunlight 
  allowed 
  to 
  fall 
  once 
  on 
  the 
  pendulum 
  will 
  

   immediately 
  set 
  it 
  swinging. 
  

  

  The 
  form 
  of 
  apparatus 
  is 
  next 
  described 
  which 
  the 
  author 
  has 
  finally 
  

   adopted, 
  as 
  combining 
  the 
  greatest 
  delicacy 
  with 
  facility 
  of 
  obtaining 
  

   accurate 
  observations, 
  and 
  therefore 
  of 
  getting 
  quantitative 
  as 
  well 
  as 
  

   qualitative 
  results. 
  It 
  consists 
  of 
  a 
  glass 
  apparatus 
  in 
  the 
  shape 
  of 
  an 
  

   inverted 
  T, 
  and 
  containing 
  a 
  horizontal 
  glass 
  beam 
  suspended 
  by 
  a 
  very 
  

   fine 
  glass 
  thread. 
  At 
  the 
  extremities 
  of 
  the 
  beam 
  are 
  attached 
  the 
  sub- 
  

   stances 
  to 
  be 
  experimented 
  on, 
  and 
  at 
  the 
  centre 
  of 
  the 
  beam 
  is 
  a 
  small 
  

   mirror 
  from 
  which 
  a 
  ray 
  of 
  light 
  is 
  reflected 
  on 
  to 
  a 
  graduated 
  scale. 
  The 
  

   advantage 
  which 
  a 
  glass 
  thread 
  possesses 
  over 
  a 
  cocoon-fibre 
  is 
  that 
  the 
  

   index 
  always 
  comes 
  accurately 
  back 
  to 
  zero. 
  In 
  order 
  to 
  keep 
  the 
  

   luminous 
  index 
  at 
  zero, 
  except 
  when 
  experiments 
  are 
  being 
  tried, 
  extreme 
  

   precautions 
  must 
  be 
  taken 
  to 
  keep 
  all 
  extraneous 
  radiation 
  from 
  acting 
  on 
  

   the 
  torsion-balance. 
  The 
  whole 
  apparatus 
  is 
  closely 
  packed 
  all 
  round 
  

   with 
  a 
  layer 
  of 
  cotton-wool 
  about 
  6 
  inches 
  thick, 
  and 
  outside 
  this 
  is 
  

   arranged 
  a 
  double 
  row 
  of 
  Winchester 
  quart 
  bottles 
  filled 
  with 
  water, 
  

   spaces 
  only 
  being 
  left 
  for 
  the 
  radiation 
  to 
  fall 
  on 
  the 
  balance 
  and 
  for 
  the 
  

   index 
  ray 
  of 
  light 
  to 
  get 
  to 
  the 
  mirror. 
  

  

  However 
  much 
  the 
  results 
  may 
  vary 
  when 
  the 
  vacuum 
  is 
  imperfect, 
  

   with 
  an 
  apparatus 
  of 
  this 
  kind 
  they 
  alwa} 
  r 
  s 
  agree 
  among 
  themselves 
  

   when 
  the 
  residual 
  gas 
  is 
  reduced 
  to 
  the 
  minimum 
  possible 
  ; 
  and 
  it 
  is 
  of 
  

   no 
  consequence 
  what 
  this 
  residual 
  gas 
  is. 
  Thus, 
  starting 
  with 
  the 
  appa- 
  

   ratus 
  full 
  of 
  various 
  vapours 
  and 
  gases, 
  such 
  as 
  air, 
  carbonic 
  acid, 
  water, 
  

   iodine, 
  hydrogen, 
  ammonia, 
  &c.,.at 
  the 
  highest 
  rarefaction, 
  there 
  is 
  not 
  

   found 
  any 
  difference 
  in 
  the 
  results 
  which 
  can 
  be 
  traced 
  to 
  the 
  residual 
  

   gas. 
  A 
  hydrogen- 
  vacuum 
  appears 
  the 
  same 
  as 
  a 
  water- 
  or 
  an 
  iodine- 
  

   vacuum. 
  

  

  "With 
  this 
  apparatus 
  the 
  effect 
  of 
  exposing 
  a 
  torsion-balance 
  to 
  a 
  con- 
  

   tinuous 
  radiation 
  is 
  described, 
  and 
  the 
  results 
  are 
  shown 
  graphically. 
  The 
  

   effect 
  of 
  a 
  short 
  (11*3 
  seconds) 
  exposure 
  to 
  radiation 
  is 
  next 
  described, 
  

   and 
  the 
  results 
  are 
  given 
  in 
  the 
  form 
  of 
  a 
  Table. 
  

  

  In 
  another 
  Table 
  is 
  given 
  the 
  results 
  of 
  experiments 
  in 
  which 
  a 
  constant 
  

   source 
  of 
  radiation 
  was 
  allowed 
  to 
  act 
  upon 
  one 
  end 
  of 
  the 
  torsion-beam 
  

   at 
  a 
  distance 
  of 
  140 
  or 
  280 
  millims., 
  various 
  substances 
  being 
  interposed. 
  

   The 
  sensitiveness 
  of 
  this 
  apparatus 
  to 
  heat-rays 
  appears 
  to 
  be 
  greater 
  

   than 
  that 
  of 
  an 
  ordinary 
  thermo-multiplier. 
  Thus 
  the 
  obscure 
  heat-rays 
  

   from 
  copper 
  at 
  100°, 
  passing 
  through 
  glass, 
  produce 
  a 
  deflection 
  on 
  the 
  

   scale 
  of 
  3*25, 
  whilst 
  under 
  the 
  same 
  circumstances 
  no 
  current 
  is 
  detected 
  

   in 
  the 
  thermo-pile. 
  The 
  following 
  substances 
  are 
  used 
  as 
  screens, 
  and 
  

   the 
  deflections 
  produced 
  (when 
  the 
  source 
  of 
  radiation 
  is 
  magnesium 
  

   wire, 
  a 
  standard 
  candle, 
  copper 
  at 
  400°, 
  and 
  copper 
  at 
  100°) 
  are 
  tabu- 
  

   lated 
  : 
  — 
  

  

  Bock-salt, 
  20 
  millims. 
  thick 
  ; 
  rock-crystal, 
  42 
  millims. 
  thick 
  ; 
  dark 
  

   smoky 
  talc 
  ; 
  plate 
  glass 
  of 
  various 
  thicknesses, 
  both 
  white 
  and 
  green 
  ; 
  a 
  

  

  