﻿82 
  Scientific 
  Intelligence. 
  

  

  densation 
  and 
  reflection 
  of 
  gas 
  molecules. 
  The 
  advantage 
  of 
  

   cadmium 
  over 
  mercury 
  consists 
  in 
  the 
  fact 
  that 
  films 
  of 
  the 
  

   former 
  metal 
  remain 
  solid 
  and 
  in 
  situ 
  at 
  room 
  temperature 
  

   whereas 
  films 
  of 
  the 
  latter 
  liquefy 
  and 
  form 
  globules 
  at 
  tempera- 
  

   tures 
  convenient 
  for 
  making 
  observations. 
  We 
  shall 
  omit 
  the 
  

   experiments 
  relating 
  to 
  the 
  cosine 
  law 
  of 
  reflection 
  because 
  they 
  

   are 
  not 
  as 
  delicate 
  as 
  those 
  of 
  Knudsen 
  (vide 
  itifra). 
  

  

  A 
  long 
  glass 
  tube 
  was 
  sealed 
  off 
  at 
  one 
  end 
  and 
  blown 
  out 
  into 
  

   a 
  relatively 
  large 
  bulb 
  at 
  the 
  opposite 
  end. 
  Near 
  the 
  bulb 
  the 
  

   tube 
  was 
  constricted 
  for 
  a 
  length 
  sufficient 
  to 
  ensure 
  the 
  trans- 
  

   mission 
  of 
  an 
  approximately 
  one-dimensional 
  flow 
  of 
  vapor 
  mole- 
  

   cules 
  into 
  the 
  bulb. 
  The 
  cadmium 
  was 
  placed 
  in 
  the 
  small 
  end 
  of 
  

   the 
  glass 
  system 
  and 
  was 
  heated 
  by 
  a 
  gas-flame 
  3 
  or 
  4 
  nims 
  high, 
  

   burning 
  at 
  the 
  orifice 
  of 
  a 
  glass 
  tube 
  drawn 
  down 
  to 
  a 
  fine 
  capil- 
  

   lary 
  The 
  tube 
  and 
  associated 
  bulb 
  were 
  kept 
  in 
  communication 
  

   with 
  a 
  Gaede 
  pump 
  during 
  the 
  experiments, 
  since 
  a 
  good 
  vacuum 
  

   is 
  necessary. 
  

  

  When 
  the 
  bulb 
  is 
  kept 
  at 
  room 
  temperature 
  no 
  trace 
  of 
  a 
  

   deposit 
  appears, 
  but 
  when 
  the 
  wall 
  of 
  the 
  bulb 
  opposite 
  the 
  inlet 
  

   is 
  cooled 
  with 
  a 
  pad 
  of 
  cotton 
  wet 
  with 
  liquid 
  air 
  a 
  small 
  deposit 
  

   of 
  metal 
  begins 
  to 
  form 
  at 
  once. 
  If 
  the 
  degree 
  of 
  exhaustion 
  is 
  

   sufficiently 
  high, 
  prolonging 
  the 
  experiment 
  for 
  15 
  or 
  20 
  minutes 
  

   causes 
  no 
  sensible 
  increase 
  in 
  the 
  diameter 
  of 
  the 
  circular 
  zone 
  of 
  

   condensed 
  metal, 
  whereas 
  if 
  the 
  vacuum 
  is 
  inferior 
  the 
  diameter 
  

   of 
  the 
  spot 
  increases 
  due 
  to 
  the 
  deflection 
  of 
  the 
  cadmium 
  vapor 
  

   by 
  the 
  residual 
  gas 
  molecules. 
  It 
  is 
  evident, 
  therefore, 
  that 
  with 
  

   a 
  high 
  vacuum 
  the 
  molecules 
  of 
  the 
  vapor 
  shoot 
  across 
  the 
  bulb 
  

   without 
  spreading 
  lateral^. 
  If, 
  at 
  the 
  beginning 
  of 
  the 
  experi- 
  

   ment 
  the 
  cold 
  pad 
  is 
  placed 
  against 
  the 
  bulb 
  at 
  any 
  point 
  90° 
  

   from 
  the 
  inlet 
  a 
  large 
  deposit 
  of 
  irregular 
  shape 
  forms 
  immedi- 
  

   ately, 
  showing 
  that 
  the 
  bulb 
  is 
  filled 
  with 
  cadmium 
  vapor 
  having 
  

   three-dimensional 
  motion. 
  Even 
  if 
  the 
  cold 
  cotton 
  wad 
  is 
  held 
  

   against 
  the 
  bulb 
  for 
  only 
  a 
  second 
  or 
  two 
  the 
  deposit 
  starts 
  to 
  

   form 
  and 
  continues 
  to 
  build 
  up 
  indefinitely 
  after 
  the 
  pad 
  has 
  been 
  

   removed. 
  Hence, 
  the 
  chance 
  of 
  reflection 
  at 
  room 
  temperature 
  

   of 
  cadmium 
  vapor 
  from 
  a 
  cadmium 
  surface 
  is 
  zero, 
  that 
  is, 
  con- 
  

   densation 
  occurs 
  at 
  the 
  first 
  collision. 
  If 
  the 
  surface 
  of 
  the 
  bulb 
  

   opposite 
  the 
  inlet 
  is 
  subsequently 
  cooled 
  with 
  liquid 
  air 
  the 
  small 
  

   circular 
  spot 
  mentioned 
  above 
  forms 
  at 
  once 
  and 
  increases 
  in 
  

   thickness 
  after 
  removal 
  of 
  the 
  cotton 
  wad. 
  No 
  further 
  deposi- 
  

   tion 
  of 
  cadmium 
  molecules 
  occurs 
  on 
  the 
  metal 
  at 
  the 
  side 
  of 
  the 
  

   bulb, 
  for 
  the 
  polar 
  deposit, 
  when 
  once 
  formed, 
  serves 
  as 
  a 
  trap 
  for 
  

   the 
  incident 
  parallel 
  stream 
  of 
  molecules. 
  

  

  The 
  fact 
  that 
  molecules 
  of 
  cadmium 
  vapor 
  can 
  experience 
  a 
  

   large 
  number 
  of 
  reflections 
  from 
  glass 
  without 
  condensation 
  was 
  

   neatly 
  shown 
  in 
  the 
  following 
  manner. 
  A 
  glass 
  tube 
  w 
  T 
  as 
  bent 
  at 
  

   right 
  angles 
  in 
  a 
  number 
  of 
  places 
  and 
  blown 
  out 
  as 
  a 
  thin-walled 
  

   bulb 
  at 
  the 
  end 
  remote 
  from 
  the 
  source 
  of 
  vapor. 
  Condensation 
  

   took 
  place 
  in 
  the 
  bulb 
  when 
  it 
  was 
  cooled 
  with 
  liquid 
  air. 
  Tubes 
  

   with 
  more 
  than 
  a 
  dozen 
  bends 
  were 
  used, 
  proving 
  that 
  a 
  large 
  

  

  