﻿Becker 
  — 
  Some 
  Queries 
  on 
  Rock 
  Differentiation. 
  25 
  

  

  tain 
  ingredient 
  to 
  one 
  spot 
  from 
  the 
  adjacent 
  mass. 
  There 
  

   is 
  absolutely 
  no 
  theoretical 
  reason 
  why 
  such 
  processes 
  should 
  

   not 
  occur, 
  for 
  at 
  very 
  short 
  distances 
  molecular 
  flow 
  is 
  a 
  very 
  

   rapid 
  process, 
  as 
  will 
  be 
  explained 
  presently. 
  On 
  the 
  other 
  

   hand, 
  it 
  is 
  questionable 
  whether 
  masses 
  of 
  rock 
  of 
  hundreds 
  of 
  

   meters 
  in 
  thickness 
  could 
  be 
  thus 
  separated, 
  even 
  if 
  the 
  time 
  

   allowed 
  for 
  completion 
  of 
  the 
  process 
  were 
  equal 
  to 
  an 
  entire 
  

   geological 
  period. 
  

  

  Character 
  of 
  diffusion. 
  — 
  It 
  has 
  been 
  explained 
  above 
  that 
  

   all 
  the 
  processes 
  of 
  molecular 
  flow 
  are 
  reducible 
  to 
  the 
  same 
  

   elementary 
  action, 
  viz 
  : 
  movements 
  due 
  to 
  differences 
  of 
  

   osmotic 
  pressure. 
  This 
  kind 
  of 
  flow 
  is 
  most 
  simply 
  manifested 
  

   in 
  ordinary 
  diffusion, 
  and 
  it 
  is 
  also 
  in 
  the 
  ordinary 
  diffusion 
  of 
  

   concentrated 
  solutions 
  that 
  molecular 
  flow 
  takes 
  place 
  most 
  

   rapidly. 
  It 
  is 
  possible 
  to 
  bring 
  an 
  indefinitely 
  large 
  mass 
  of 
  

   an 
  absolutely 
  and 
  permanently 
  concentrated 
  liquid 
  in 
  contact 
  

   with 
  another 
  liquid 
  in 
  which 
  the 
  first 
  is 
  thoroughly 
  soluble. 
  

   Under 
  these 
  conditions, 
  the 
  resultant 
  osmotic 
  pressure 
  being 
  

   proportional 
  to 
  concentration, 
  must 
  have 
  its 
  highest 
  value, 
  and 
  

   molecular 
  flow 
  (measured 
  by 
  the 
  amount 
  of 
  substance 
  passing 
  

   through 
  a 
  given 
  area 
  in 
  a 
  given 
  time) 
  must 
  be 
  greater 
  than 
  it 
  

   otherwise 
  can 
  be. 
  Think, 
  for 
  example, 
  of 
  a 
  tall 
  vessel 
  at 
  the 
  

   bottom 
  of 
  which 
  is 
  a 
  layer 
  of 
  solid 
  sulphate 
  of 
  copper, 
  the 
  

   rest 
  of 
  the 
  vessel 
  being 
  full 
  of 
  pure 
  water. 
  Then 
  a 
  concen- 
  

   trated 
  solution 
  of 
  the 
  sulphate 
  will 
  form 
  in 
  contact 
  with 
  the 
  

   solid 
  sulphate 
  and 
  this 
  layer 
  will 
  continue 
  concentrated 
  until 
  

   solution 
  is 
  complete. 
  Diffusion 
  will 
  then 
  proceed 
  as 
  rapidly 
  as 
  

   it 
  can 
  do 
  at 
  the 
  temperature 
  of 
  the 
  experiment. 
  Under 
  such 
  

   conditions 
  the 
  amount 
  of 
  a 
  dissolved 
  substance 
  which 
  diffuses 
  

   through 
  an 
  area 
  of 
  one 
  square 
  centimeter 
  in 
  one 
  second, 
  when 
  

   the 
  gradient 
  of 
  concentration 
  (perpendicular 
  to 
  the 
  area) 
  is 
  one 
  

   gram 
  of 
  substance 
  per 
  cubic 
  centimeter 
  of 
  fluid 
  per 
  centimeter 
  

   of 
  distance, 
  is 
  a 
  constant 
  called 
  the 
  " 
  diffusivity 
  " 
  of 
  the 
  sub- 
  

   stance 
  in 
  water.* 
  In 
  the 
  case 
  of 
  gases 
  Maxwell 
  showed 
  that 
  a 
  

   simple 
  numerical 
  relation 
  exists 
  between 
  the 
  diffusivity 
  of 
  sub- 
  

   stance, 
  the 
  diffusivity 
  of 
  energy 
  or 
  heat, 
  and 
  the 
  diffusivity 
  of 
  

   momentum 
  which 
  gives 
  rise 
  to 
  viscosity. 
  f 
  In 
  the 
  case 
  of 
  

   fluids 
  the 
  a 
  "priori 
  determination 
  of 
  these 
  constants 
  is 
  not 
  yet 
  

   possible 
  and 
  they 
  must 
  be 
  found 
  by 
  experiment. 
  

  

  When 
  this 
  constant 
  called 
  diffusivity 
  of 
  substance 
  is 
  deter- 
  

   mined, 
  the 
  process 
  of 
  diffusion 
  can 
  be 
  accurately 
  predicted 
  

   under 
  uniform 
  conditions 
  of 
  temperature 
  and 
  pressure 
  at 
  least 
  

   for 
  weak 
  solutions. 
  In 
  1855 
  Prof. 
  A. 
  FickJ 
  advanced 
  the 
  

   hypothesis 
  that 
  the 
  time 
  rate 
  at 
  which 
  a 
  salt 
  diffuses 
  through 
  a 
  

  

  *Tait: 
  Prop, 
  of 
  Matter, 
  2d 
  ed., 
  1890, 
  p. 
  271. 
  

  

  f 
  Theory 
  of 
  Heat, 
  1894, 
  p. 
  332, 
  and 
  Nature, 
  vol. 
  viii. 
  

  

  X 
  P°gg' 
  Ann., 
  vol. 
  xciv, 
  1855, 
  p. 
  59. 
  

  

  