﻿The 
  Properties 
  of 
  Colloidal 
  Systems. 
  197 
  

  

  by 
  Swift, 
  which 
  was 
  found 
  to 
  admit 
  of 
  magnification 
  by 
  fairly 
  high-power 
  

   oculars, 
  such 
  as 
  No. 
  12 
  compensating 
  of 
  Zeiss. 
  Other 
  methods, 
  such 
  as 
  that 
  

   of 
  an 
  objective 
  as 
  sub-stage 
  condenser, 
  with 
  a 
  central 
  stop 
  in 
  the 
  observing 
  

   objective 
  to 
  cut 
  out 
  direct 
  rays, 
  were 
  tried 
  ; 
  but 
  the 
  paraboloid 
  was 
  found 
  to 
  

   be 
  the 
  best 
  on 
  the 
  whole. 
  

  

  In 
  practice, 
  the 
  observations 
  themselves 
  are 
  somewhat 
  trying, 
  on 
  account 
  

   of 
  the 
  extreme 
  brilliancy 
  of 
  the 
  larger 
  particles 
  when 
  they 
  come 
  into 
  the 
  

   field 
  of 
  view. 
  But 
  under 
  favourable 
  conditions, 
  and 
  with 
  sufficient 
  intensity 
  

   of 
  illumination, 
  it 
  is 
  not 
  difficult 
  to 
  see 
  that 
  the 
  clear 
  protoplasm 
  of 
  the 
  

   pseudopodia 
  contains 
  an 
  immense 
  number 
  of 
  very 
  minute 
  particles, 
  shown 
  

   by 
  their 
  bright 
  diffraction 
  images. 
  They 
  are 
  in 
  vigorous 
  Brownian 
  

   movement, 
  and 
  it 
  is 
  scarcely 
  possible 
  to 
  distinguish 
  separate 
  particles, 
  on 
  

   account 
  of 
  their 
  number. 
  The 
  general 
  appearance 
  is 
  that 
  of 
  a 
  shimmering 
  

   tremulous 
  movement 
  in 
  the 
  field 
  of 
  view. 
  

  

  This 
  Brownian 
  movement 
  is 
  one 
  of 
  the 
  most 
  convincing 
  proofs 
  of 
  the 
  

   liquid 
  nature 
  of 
  the 
  system. 
  A 
  simple 
  experiment 
  shows 
  the 
  fact. 
  If 
  

   a 
  cake 
  of 
  water-colour 
  gamboge 
  be 
  rubbed 
  in 
  a 
  drop 
  of 
  5 
  per 
  cent, 
  gelatine 
  

   solution 
  on 
  a 
  warmed 
  microscope 
  slide, 
  covered 
  and 
  immediately 
  examined 
  

   under 
  the 
  microscope, 
  the 
  particles 
  exhibit 
  Brownian 
  movement 
  in 
  the 
  usual 
  

   manner. 
  As 
  the 
  slide 
  cools 
  and 
  the 
  solution 
  sets 
  into 
  a 
  jelly, 
  the 
  movement 
  

   becomes 
  inert 
  and 
  finally 
  ceases, 
  the 
  particles 
  becoming 
  fixed 
  in 
  the 
  meshes 
  

   of 
  the 
  network. 
  On 
  warming, 
  the 
  movement 
  reappears. 
  The 
  fact 
  was 
  made 
  

   use 
  of 
  by 
  J. 
  Duclaux 
  (1908) 
  for 
  the 
  purpose 
  of 
  enumerating 
  the 
  particles 
  in 
  

   colloidal 
  ferric 
  hydroxide. 
  

  

  It 
  seems, 
  from 
  Kuhne's 
  description 
  of 
  his 
  valuable 
  observations, 
  that 
  he 
  

   was 
  unable 
  to 
  satisfy 
  himself 
  that 
  the 
  particles 
  which 
  were 
  visible 
  to 
  him 
  

   possessed 
  Brownian 
  movement, 
  or, 
  as 
  he 
  called 
  it, 
  " 
  molecular 
  movement," 
  as 
  

   distinct 
  from 
  the 
  translational 
  movements 
  due 
  to 
  currents 
  in 
  the 
  protoplasm. 
  

   If 
  he 
  had 
  been 
  able 
  to 
  use 
  dark-ground 
  illumination, 
  there 
  is 
  no 
  doubt 
  that 
  

   he 
  would 
  have 
  detected 
  them, 
  since 
  they 
  can 
  be 
  distinguished 
  by 
  the 
  

   vibratory 
  motion 
  of 
  the 
  particles 
  even 
  when 
  they 
  are 
  being 
  carried 
  along. 
  

   They 
  have, 
  indeed, 
  been 
  described 
  by 
  Gaidukov 
  (1910) 
  and 
  by 
  Price 
  (1914) 
  

   in 
  plant 
  cells, 
  by 
  Chambers 
  (1917) 
  in 
  various 
  animal 
  cells, 
  by 
  Mott 
  (1912) 
  

   and 
  by 
  Marinesco 
  (1912) 
  in 
  nerve 
  cells. 
  

  

  Protoplasm 
  belongs, 
  then, 
  to 
  the 
  class 
  of 
  colloidal 
  solutions 
  called 
  by 
  

   Graham 
  (1864) 
  " 
  hydrosols." 
  Now, 
  as 
  Graham 
  showed, 
  many 
  of 
  these 
  

   under 
  certain 
  conditions 
  change 
  their 
  state, 
  becoming 
  solid, 
  in 
  the 
  sense 
  

   that 
  they 
  become 
  fixed 
  in 
  shape. 
  This 
  phenomenon 
  is 
  familiar 
  in 
  the 
  

   " 
  setting 
  " 
  of 
  gelatin 
  when 
  cooled, 
  and 
  the 
  new 
  systems 
  are 
  called 
  " 
  hydrogels." 
  

   The 
  series 
  of 
  changes 
  taking 
  place 
  was 
  investigated 
  by 
  Hardy 
  (1900). 
  It 
  i3 
  

  

  