﻿Chemistry 
  and 
  Physics. 
  147 
  

  

  SCIENTIFIC 
  INTELLIGENCE. 
  

  

  I. 
  Chemistey 
  and 
  Physics. 
  

  

  1. 
  On 
  the 
  Diffusion 
  of 
  Metals. 
  — 
  In 
  his 
  Bakerian 
  lecture 
  Rob- 
  

   erts-Austen 
  has 
  given 
  the 
  results 
  of 
  some 
  elaborate 
  experiments 
  

   showing 
  the 
  tendency 
  of 
  two 
  or 
  more 
  metals 
  to 
  mix 
  spontane- 
  

   ously 
  and 
  thus 
  to 
  form 
  a 
  homogeneous 
  mass. 
  Already 
  in 
  1883, 
  

   he 
  had 
  observed 
  that 
  " 
  while 
  molten 
  copper 
  and 
  antimony 
  inter- 
  

   penetrate 
  but 
  slowly, 
  the 
  mobility 
  of 
  gold 
  and 
  silver 
  in 
  molten 
  

   lead 
  is 
  comparatively 
  rapid." 
  In 
  his 
  later 
  experiments 
  molten 
  

   lead 
  and 
  bismuth 
  were 
  selected 
  as 
  the 
  fluids 
  into 
  which 
  the 
  diffu- 
  

   sion 
  of 
  the 
  other 
  metals 
  took 
  place. 
  The 
  tubes 
  containing 
  this 
  

   molten 
  metal, 
  which 
  were 
  about 
  200 
  mm 
  long 
  and 
  10 
  mm 
  internal 
  

   diameter, 
  were 
  arranged 
  in 
  an 
  air 
  bath 
  with 
  double 
  walls, 
  which 
  

   could 
  be 
  readily 
  maintained 
  at 
  fixed 
  temperatures, 
  determinable 
  

   accurately 
  by 
  means 
  of 
  thermoelectric 
  junctions. 
  To 
  avoid 
  con- 
  

   vection 
  currents 
  the 
  tubes 
  were 
  kept 
  hotter 
  at 
  top 
  than 
  at 
  bot- 
  

   tom. 
  After 
  a 
  suitable 
  time, 
  varying 
  from 
  six 
  hours 
  to 
  seven 
  

   days, 
  the 
  diffusion 
  tubes 
  were 
  removed, 
  cooled, 
  carefully 
  meas- 
  

   ured 
  and 
  cut 
  into 
  transverse 
  sections 
  ; 
  the 
  contents 
  of 
  each 
  sec- 
  

   tion 
  being 
  weighed 
  and 
  analyzed. 
  Kelvin, 
  as 
  a 
  deduction 
  from 
  

   Fourier's 
  theory 
  of 
  heat-conduction, 
  states 
  the 
  law 
  of 
  diffusion 
  as 
  

   follows 
  : 
  "The 
  rate 
  of 
  augmentation 
  of 
  the 
  'quality' 
  per 
  unit 
  of 
  

   time 
  is 
  equal 
  to 
  the 
  diffusivity 
  multiplied 
  into 
  the 
  rate 
  of 
  aug- 
  

   mentation 
  per 
  unit 
  of 
  space 
  of 
  the 
  rate 
  of 
  augmentation 
  per 
  unit 
  

   of 
  space 
  of 
  the 
  ' 
  quality.' 
  " 
  By 
  " 
  quality 
  " 
  is 
  here 
  meant 
  the 
  con- 
  

   centration 
  of 
  the 
  diffused 
  matter. 
  Hence 
  the 
  law 
  may 
  be 
  repre- 
  

   sented 
  by 
  the 
  differential 
  equation 
  

  

  dv/dt 
  = 
  k(d*v/dx*) 
  

  

  where 
  x 
  is 
  distance 
  in 
  the 
  direction 
  of 
  diffusion, 
  v 
  is 
  the 
  degree 
  of 
  

   concentration 
  of 
  the 
  diffusing 
  metal, 
  t 
  the 
  time, 
  and 
  k 
  the 
  diffu- 
  

   sion 
  constant; 
  i.e., 
  the 
  number 
  which 
  expresses 
  the 
  quantity 
  of 
  

   the 
  metal 
  in 
  grams 
  which 
  diffuses 
  through 
  unit 
  area 
  (one 
  sq. 
  cm.) 
  

   in 
  unit 
  time 
  (one 
  day) 
  when 
  unit 
  difference 
  of 
  concentration 
  (in 
  

   grams 
  per 
  cubic 
  cm.) 
  is 
  maintained 
  between 
  the 
  two 
  sides 
  of 
  a 
  

   layer 
  one 
  centimeter 
  thick. 
  Since 
  the 
  unit 
  of 
  diffusivity 
  has 
  the 
  

   dimensions 
  [L 
  2 
  T 
  _1 
  ] 
  the 
  diffusion 
  constants 
  may 
  be 
  expressed 
  in 
  

   square 
  centimeters 
  per 
  day. 
  Each 
  pair 
  of 
  metals 
  has 
  a 
  perfectly 
  

   definite 
  constant 
  of 
  diffusion 
  at 
  a 
  given 
  temperature. 
  On 
  tabu- 
  

   lating 
  the 
  results 
  obtained 
  it 
  appears 
  that 
  gold 
  diffuses 
  more 
  rap- 
  

   idly 
  in 
  bismuth 
  and 
  in 
  tin 
  than 
  in 
  the 
  heavier 
  metal, 
  lead, 
  as 
  also 
  

   does 
  platinum; 
  the 
  diffusion 
  of 
  both 
  being 
  about 
  equally 
  in- 
  

   creased 
  when 
  bismuth 
  is 
  replaced 
  by 
  lead. 
  Platinum 
  diffuses 
  

   more 
  slowly 
  in 
  lead 
  than 
  gold 
  does, 
  while 
  rhodium 
  diffuses 
  almost 
  

   as 
  rapidly 
  ; 
  suggesting 
  that 
  the 
  platinum 
  metals 
  are 
  molecularly 
  

   more 
  complex 
  than 
  gold 
  or 
  silver. 
  

  

  