﻿the 
  Reflexion 
  of 
  the 
  X-rai/s. 
  22',) 
  

  

  The 
  positions 
  of 
  the 
  special 
  angles 
  are 
  indicated 
  on 
  the 
  

   curves 
  in 
  fig. 
  -. 
  The 
  five 
  angles 
  found 
  for 
  rocksalt 
  agree 
  in 
  

   position 
  with 
  the 
  three 
  Bound 
  by 
  Bragg. 
  His 
  slits 
  were 
  too 
  

   broad 
  to 
  separate 
  /3 
  from 
  <y 
  or 
  S 
  from 
  e. 
  If 
  the 
  way 
  in 
  which 
  

   the 
  atoms 
  are 
  packed 
  in 
  the 
  simple 
  cubical 
  crystal 
  rocksalt 
  

   is 
  known, 
  the 
  wave-lengths 
  of 
  these 
  five 
  characteristic 
  

   radiations 
  can 
  be 
  determined. 
  

  

  If 
  X 
  is 
  the 
  number 
  of 
  molecules 
  in 
  1 
  gram 
  mol., 
  M 
  the 
  

   molecular 
  weight, 
  and 
  a 
  the 
  density 
  of 
  the 
  rocksalt, 
  the 
  

  

  number 
  of 
  molecules 
  per 
  c.c. 
  is 
  -^-- 
  This 
  number 
  is, 
  

  

  K 
  . 
  

  

  however. 
  -«, 
  where 
  d 
  is 
  the 
  distance 
  between 
  successive 
  

   dr 
  

  

  la 
  vers 
  of 
  atoms 
  and 
  K 
  a 
  numerical 
  constant, 
  the 
  value 
  of 
  

  

  which 
  depends 
  on 
  the 
  arrangement 
  of 
  the 
  atoms 
  in 
  the 
  

  

  crystal. 
  Hence 
  

  

  \ 
  = 
  2d 
  sin 
  (9, 
  

  

  K 
  _ 
  No- 
  

   and 
  j3- 
  ¥ 
  f. 
  

  

  Xow 
  X 
  = 
  C>-22 
  xlO 
  23 
  , 
  M 
  = 
  58-4G, 
  and 
  a 
  was 
  found 
  to 
  be 
  

  

  2-167. 
  This 
  gives 
  d=K*x 
  3'513 
  x 
  10~ 
  8 
  cm. 
  

  

  It 
  follows 
  that 
  the 
  wave-lengths 
  of 
  the 
  five 
  radiations 
  a, 
  ft, 
  

  

  7 
  , 
  8, 
  e 
  are 
  1/642, 
  1*397, 
  1'375, 
  M94, 
  1-157 
  X 
  KJx 
  10" 
  8 
  cm. 
  

   It 
  is 
  probably 
  an 
  accident 
  that 
  the 
  first, 
  second, 
  and 
  fourth, 
  

   also 
  the 
  first, 
  third, 
  and 
  fifth 
  of 
  these 
  numbers 
  are 
  in 
  

   geometrical 
  progression. 
  These 
  values 
  for 
  the 
  wave-lengths 
  

   are 
  no 
  doubt 
  slightly 
  too 
  large, 
  as 
  crystals 
  are 
  apt 
  to 
  contain 
  

   minute 
  inclusions 
  of 
  gas 
  or 
  liquid, 
  which 
  would 
  make 
  the 
  

   experimental 
  value 
  of 
  a 
  slightly 
  too 
  low. 
  The 
  absorption 
  

   coefficient 
  of 
  the 
  characteristic 
  radiation 
  /3 
  in 
  aluminium 
  was 
  

   found 
  to 
  correspond 
  to 
  that 
  of 
  the 
  K 
  radiation 
  from 
  an 
  element 
  

   of 
  atomic 
  weight 
  about 
  7*55. 
  In 
  order 
  to 
  excite 
  this 
  radiation 
  

   a 
  cathode 
  particle 
  should 
  have 
  a 
  velocity* 
  about 
  755 
  X 
  10 
  9 
  cm. 
  

   c, 
  which 
  corresponds 
  to 
  an 
  energy 
  content 
  2*5 
  X 
  10~ 
  8 
  erg. 
  

   If 
  Planck's 
  formula 
  holds 
  good 
  the 
  energy 
  of 
  the 
  radiation 
  ft 
  

   should 
  be 
  given 
  by 
  hv. 
  where 
  // 
  is 
  6*455 
  x 
  10~ 
  27 
  erg. 
  sec. 
  f 
  

   and 
  v 
  is 
  the 
  frequency 
  of 
  /3. 
  Therefore 
  

  

  kv=h 
  ' 
  '- 
  = 
  1-37 
  x 
  K~* 
  x 
  10" 
  s 
  erg. 
  

  

  If 
  all 
  the 
  molecule- 
  are 
  arranged 
  in 
  a 
  simple 
  cubical 
  pattern 
  

  

  * 
  Whidding-ton, 
  Proc. 
  Roy. 
  Soc. 
  A. 
  Ixxxv. 
  p. 
  323 
  (1911). 
  

   t 
  Westphal, 
  Beutsch. 
  Phys. 
  Gesel. 
  xxi. 
  p. 
  «J87 
  (1912). 
  

  

  