﻿Chemistry 
  and 
  Physics. 
  83 
  

  

  number 
  of 
  reflections 
  are 
  possible, 
  since 
  the 
  number 
  of 
  bends 
  gives 
  

   the 
  minimum 
  number 
  of 
  reflections 
  suffered 
  by 
  a 
  molecule 
  before 
  

   reaching 
  the 
  condensation 
  bulb. 
  The 
  greater 
  the 
  number 
  of 
  

   bends 
  the 
  longer 
  was 
  the 
  time 
  required 
  to 
  produce 
  in 
  the 
  bulb 
  a 
  

   film 
  of 
  definite 
  thickness. 
  Consequently 
  a 
  certain 
  fraction 
  of 
  the 
  

   total 
  number 
  of 
  molecules 
  in 
  the 
  column 
  of 
  vapor 
  must 
  have 
  

   fallen 
  by 
  the 
  wayside. 
  In 
  a 
  particular 
  tube 
  deposits 
  of 
  equal 
  

   density 
  were 
  obtained 
  in 
  the 
  bulb, 
  at 
  a 
  chosen 
  bend, 
  and 
  at 
  a 
  

   bend 
  still 
  nearer 
  the 
  source, 
  in 
  15 
  mins., 
  2 
  mins,, 
  and 
  10 
  sees., 
  

   respectively. 
  Although 
  the 
  density 
  of 
  the 
  cadmium 
  vapor 
  is 
  con- 
  

   siderably 
  decreased 
  by 
  its 
  passage 
  along 
  the 
  bent 
  tube, 
  no 
  visible 
  

   deposit 
  can 
  be 
  detected 
  on 
  the 
  uncooled 
  walls. 
  Microscopic 
  exami- 
  

   nation 
  of 
  a 
  bulb 
  which 
  had 
  been 
  kept 
  at 
  room 
  temperature 
  and 
  in 
  

   communication 
  with 
  the 
  source 
  of 
  vapor 
  for 
  40 
  mins. 
  showed 
  dis- 
  

   crete 
  aggregates 
  of 
  metal 
  which 
  seemed 
  to 
  be 
  clusters 
  of 
  very 
  minute 
  

   crystals. 
  The 
  temperature 
  necessary 
  for 
  the 
  formation 
  on 
  glass 
  

   of 
  a 
  homogeneous 
  film 
  of 
  cadmium 
  was 
  found 
  experimentally 
  to 
  

   be 
  in 
  the 
  neighborhood 
  of 
  — 
  10()°O. 
  The 
  corresponding 
  " 
  critical 
  " 
  

   temperatures 
  for 
  iodine 
  and 
  mercury 
  are 
  roughly 
  — 
  60° 
  and 
  — 
  140°, 
  

   respectively. 
  — 
  Phil. 
  Mag., 
  xxxii, 
  p. 
  364, 
  October, 
  1916. 
  

  

  h. 
  s. 
  it. 
  

  

  8. 
  The 
  Cosine 
  Law 
  in 
  the 
  Kinetic 
  Theory. 
  — 
  From 
  his 
  earlier 
  

   experiments 
  on 
  the 
  flow 
  of 
  gases 
  through 
  capillary 
  tubes 
  Knurl- 
  

   sen 
  concluded 
  that 
  all 
  of 
  the 
  n 
  molecules 
  which 
  strike 
  an 
  element 
  

   of 
  surface 
  from 
  a 
  solid 
  angle 
  do 
  leave 
  this 
  surface 
  in 
  such 
  a 
  man- 
  

   ner 
  as 
  to 
  have 
  their 
  velocities 
  uniformly 
  distributed 
  over 
  every 
  

   azimuth. 
  More 
  precisely 
  dn 
  = 
  -K~ 
  l 
  n 
  cos^doi, 
  where 
  ^ 
  denotes 
  the 
  

   angle 
  between 
  the 
  normal 
  to 
  the 
  surface 
  and 
  the 
  direction 
  of 
  the 
  

   axis 
  of 
  do). 
  This 
  is 
  equivalent 
  to 
  putting 
  a 
  certain 
  fraction 
  f, 
  

   which 
  occurs 
  in 
  Maxwell's 
  original 
  theory, 
  equal 
  to 
  unity. 
  Later 
  

   simultaneous 
  and 
  independent 
  experiments 
  by 
  Knudsen 
  and 
  

   R. 
  W. 
  Wood 
  have 
  apparently 
  confirmed 
  Knudsen's 
  cosine 
  law. 
  

   Since, 
  however, 
  these 
  investigations 
  involved 
  the 
  comparison 
  of 
  

   different 
  surface 
  densities, 
  which 
  cannot 
  be 
  determined 
  with 
  ease 
  

   and 
  accuracy, 
  a 
  more 
  recent 
  and 
  conclusive 
  method 
  of 
  attacking 
  

   the 
  problem, 
  devised 
  and 
  employed 
  by 
  Martin 
  Knudsen, 
  deserves 
  

   notice. 
  

  

  The 
  innovation 
  consists 
  in 
  assuming 
  the 
  validity 
  of 
  the 
  cosine 
  

   law 
  and 
  then 
  finding 
  the 
  form 
  of 
  surface 
  over 
  which 
  the 
  distribution 
  

   of 
  condensed 
  vapor 
  would 
  be 
  uniform. 
  This 
  method 
  is 
  very 
  

   sensitive 
  because 
  any 
  appreciable 
  departure 
  from 
  constant 
  surface 
  

   density 
  can 
  be 
  readily 
  detected. 
  It 
  can 
  be 
  shown 
  by 
  very 
  simple 
  

   analysis 
  that 
  if 
  the 
  molecules 
  obey 
  the 
  cosine 
  law 
  and 
  are 
  reflected 
  

   from 
  one 
  portion 
  of 
  the 
  inner 
  surface 
  of 
  a 
  spherical 
  cavity 
  they 
  

   will 
  be 
  uniformly 
  distributed 
  over 
  the 
  rest 
  of 
  the 
  same 
  spherical 
  

   surface. 
  Accordingly 
  a 
  glass 
  bulb 
  was 
  blown 
  in 
  as 
  nearly 
  a 
  

   spherical 
  shape 
  as 
  possible 
  and 
  was 
  drawn 
  out 
  at 
  one 
  point 
  in 
  the 
  

   form 
  of 
  a 
  tube 
  which 
  communicated 
  with 
  the 
  source 
  of 
  mercury 
  

   vapor, 
  with 
  the 
  air 
  pump, 
  etc. 
  The 
  lateral 
  tube 
  and 
  the 
  auxiliary 
  

   apparatus 
  were 
  designed 
  in 
  such 
  a 
  way 
  as 
  to 
  cause 
  a 
  narrow 
  

   stream 
  of 
  mercury 
  vapor 
  to 
  enter 
  the 
  bulb 
  diagonally 
  and 
  

  

  