﻿Becker 
  — 
  Some 
  Queries 
  on 
  Rock 
  Differentiation. 
  28 
  

  

  In 
  this 
  case 
  any 
  one 
  layer 
  of 
  the 
  water 
  may 
  be 
  regarded 
  as 
  a 
  

   kind 
  of 
  septum 
  through 
  which 
  the 
  chloride 
  diffuses 
  at 
  a 
  higher 
  

   rate 
  than 
  does 
  the 
  sulphate. 
  There 
  are 
  many 
  septa 
  more 
  

   efficient 
  in 
  separating 
  solutions 
  than 
  is 
  water. 
  Especially 
  

   familiar 
  is 
  bladder, 
  through 
  which 
  one 
  class 
  of 
  compounds 
  (the 
  

   crystalloids) 
  passes 
  very 
  readily, 
  while 
  another 
  (the 
  colloids) 
  

   passes 
  with 
  difficulty. 
  Here 
  indeed 
  the 
  material 
  of 
  the 
  septum 
  

   perhaps 
  has 
  some 
  molecular 
  action 
  on 
  the 
  solutions, 
  and 
  if 
  so 
  

   the 
  explanation 
  is 
  thereby 
  complicated. 
  Nevertheless 
  osmosis 
  

   is 
  regarded 
  by 
  physicists 
  as 
  a 
  case 
  of 
  ordinary 
  diffusion 
  compli- 
  

   cated, 
  as 
  some 
  think, 
  by 
  the 
  molecular 
  action 
  of 
  the 
  septum.* 
  

  

  There 
  is 
  another 
  class 
  of 
  septa 
  which 
  seems 
  to 
  be 
  without 
  

   molecular 
  action 
  and 
  behave 
  as 
  mere 
  " 
  atomic 
  sieves." 
  They 
  

   are 
  wholly 
  impermeable 
  to 
  some 
  solutions 
  (bladder 
  is 
  not) 
  and 
  

   easily 
  permeated 
  by 
  others. 
  By 
  means 
  of 
  these 
  " 
  semi-perme- 
  

   able 
  " 
  septa, 
  which 
  are 
  produced 
  by 
  precipitation, 
  it 
  has 
  been 
  

   found 
  that 
  the 
  molecular 
  flow 
  of 
  a 
  gis 
  T 
  en 
  solution 
  continues 
  

   until 
  a 
  certain 
  definite 
  pressure 
  exists 
  on 
  one 
  side 
  of 
  the 
  sep- 
  

   tum, 
  this 
  pressure 
  being 
  characteristic 
  of 
  the 
  substance 
  experi- 
  

   mented 
  upon 
  and 
  independent 
  of 
  the 
  nature 
  of 
  the 
  membrane. 
  

   This 
  is 
  the 
  "osmotic" 
  pressure 
  of 
  the 
  dissolved 
  substance, 
  and 
  

   Mr. 
  van't 
  ELoff 
  has 
  shownf 
  that 
  it 
  is 
  equal 
  to 
  the 
  pressure 
  

   which 
  would 
  be 
  exerted 
  by 
  the 
  substance 
  in 
  a 
  gaseous 
  state 
  when 
  

   occupying 
  the 
  same 
  volume 
  at 
  the 
  same 
  temperature. 
  Evi- 
  

   dently 
  then 
  there 
  is 
  a 
  very 
  close 
  analogy 
  between 
  gases 
  and 
  

   substances 
  in 
  a 
  state 
  of 
  solution, 
  and 
  it 
  is 
  in 
  fact 
  now 
  well 
  

   recognized 
  that, 
  as 
  van't 
  Hoff 
  pointed 
  out, 
  they 
  obey 
  several 
  of 
  

   the 
  same 
  fundamental 
  laws 
  when 
  osmotic 
  pressure 
  and 
  simple 
  

   pressure 
  are 
  considered 
  as 
  interchangeable 
  terms.:): 
  

  

  Mr. 
  W. 
  Nernst§ 
  has 
  discussed 
  the 
  phenomena 
  of 
  simple 
  dif- 
  

   fusion 
  in 
  their 
  relation 
  to 
  osmotic 
  pressure 
  and 
  is 
  led 
  to 
  the 
  

   conclusion 
  that 
  osmotic 
  pressure 
  is 
  the 
  force 
  immediately 
  con- 
  

   cerned 
  in 
  the 
  diffusion 
  of 
  liquids, 
  just 
  as 
  the 
  pressure 
  exerted 
  

   by 
  a 
  gas 
  in 
  confinement 
  is 
  the 
  cause 
  of 
  the 
  diffusion 
  of 
  gases. 
  

  

  The 
  most 
  important 
  case 
  of 
  molecular 
  flow 
  for 
  the 
  purposes 
  

   of 
  this 
  discussion 
  arises 
  when 
  a 
  homogeneous 
  solution 
  is 
  heated 
  

   at 
  the 
  top. 
  Molecular 
  flow 
  then 
  takes 
  place 
  from 
  the 
  top 
  

  

  *Tait: 
  Prop, 
  of 
  Matter, 
  2d 
  ed., 
  p. 
  275. 
  It 
  is 
  probable 
  that 
  the 
  osmotic 
  action 
  

   of 
  animal 
  membranes 
  is 
  exactly 
  the 
  same 
  in 
  principle 
  as 
  that 
  of 
  precipitated 
  ones. 
  

   They 
  are 
  both 
  "atom 
  sieves," 
  only 
  the 
  "meshes" 
  are 
  of 
  a 
  different 
  size, 
  per- 
  

   haps. 
  If 
  any 
  " 
  molecular 
  action 
  " 
  (whatever 
  that 
  may 
  be) 
  exists 
  in 
  one 
  case, 
  it 
  

   probably 
  does 
  in 
  the 
  other 
  also. 
  The 
  evidence 
  of 
  such 
  action 
  is 
  not 
  distinct. 
  

  

  •fZeitschr. 
  phys. 
  Chem., 
  vol. 
  i, 
  1887, 
  p. 
  481. 
  

  

  % 
  The 
  osmotic 
  pressure 
  is 
  inversely 
  proportional 
  to 
  the 
  volume 
  of 
  the 
  fluid 
  in 
  a 
  

   given 
  space. 
  The 
  osmotic 
  pressure 
  at 
  constant 
  volume 
  is 
  proportional 
  to 
  the 
  

   absolute 
  temperature. 
  Solutions, 
  of 
  equal 
  volume 
  of 
  different 
  substances 
  which 
  

   contain 
  equal 
  numbers 
  of 
  molecules 
  at 
  equal 
  temperatures 
  exert 
  equal 
  osmotic 
  

   pressure. 
  These 
  laws 
  correspond 
  to 
  those 
  of 
  Boyle, 
  Gay-Lussac 
  and 
  Avogadro. 
  

  

  § 
  Zeitschr. 
  phys. 
  Chem., 
  vol. 
  ii, 
  1888, 
  p. 
  613. 
  

  

  