﻿in 
  Their 
  Thermodynamic 
  Relations. 
  171 
  

  

  is 
  already 
  too 
  rapid 
  to 
  admit 
  of 
  the 
  physical 
  measurement 
  of 
  

   its 
  successive 
  stages. 
  

  

  9. 
  The 
  appearance 
  of 
  the 
  cold 
  tube 
  when 
  cut 
  across 
  is 
  

   enlarged 
  in 
  figure 
  5 
  (with 
  fig. 
  4, 
  p. 
  170), 
  which 
  shows 
  the 
  

   external 
  and 
  the 
  original 
  internal 
  diameters, 
  and 
  the 
  diameter 
  

   of 
  the 
  core 
  of 
  water-glass 
  (the 
  latter 
  dotted). 
  Apart 
  from 
  the 
  

   hubbies 
  usually 
  connected 
  by 
  a 
  very 
  fine 
  capillary 
  canal, 
  the 
  

   tube 
  has 
  become 
  a 
  glass 
  stem. 
  On 
  holding 
  the 
  tube 
  in 
  a 
  suita- 
  

   ble 
  position, 
  the 
  core 
  of 
  water 
  glass 
  may 
  be 
  seen 
  sharply 
  dif 
  

   ferentiated 
  from 
  the 
  igneous 
  envelope. 
  In 
  other 
  respects 
  

   there 
  is 
  firm 
  coherence, 
  and 
  the 
  water 
  glass 
  appears 
  on 
  trial 
  

   about 
  as 
  hard 
  as 
  the 
  igneous 
  glass. 
  Slight 
  heating 
  melts 
  the 
  

   water 
  glass, 
  which 
  then 
  swells 
  up 
  enormously 
  and 
  exudes 
  from 
  

   the 
  tube, 
  like 
  pith. 
  

  

  10. 
  An 
  experiment 
  was 
  made 
  to 
  test 
  the 
  colloidal 
  character 
  

   of 
  the 
  reaction 
  by 
  replacing 
  the 
  thread 
  of 
  water 
  with 
  a 
  thread 
  

   of 
  solution 
  of 
  cobalt 
  nitrate. 
  Should 
  the 
  latter 
  turn 
  blue 
  at 
  

   210° 
  the 
  occurrence 
  of 
  chemical 
  reaction 
  would 
  be 
  indicated. 
  

   Not 
  only 
  was 
  this 
  not 
  the 
  case, 
  but 
  the 
  reaction 
  completing 
  

   itself 
  in 
  the 
  usual 
  way, 
  left 
  a 
  gray 
  granular 
  scum 
  of 
  decomposed 
  

   cobalt 
  salt 
  in 
  the 
  axis 
  of 
  the 
  tube. 
  Hence 
  all 
  water 
  enters 
  the 
  

   glass 
  by 
  diffusion, 
  the 
  glass 
  itself 
  performing 
  the 
  function 
  of 
  a 
  

   semi-permiable 
  membrane 
  of 
  the 
  energetic 
  character 
  specified. 
  

  

  Yet 
  the 
  reaction 
  as 
  a 
  whole 
  cannot 
  be 
  represented 
  as 
  a 
  mere 
  

   physical 
  diffusion 
  : 
  for 
  it 
  is 
  not 
  probable 
  that 
  the 
  reduction 
  of 
  

   the 
  concentration 
  gradient 
  from 
  the 
  axis 
  of 
  the 
  tube 
  outward, 
  

   conld 
  result 
  in 
  so 
  sharp 
  a 
  differentiation 
  of 
  water 
  glass 
  core 
  

   and 
  igneous 
  glass 
  envelope 
  as 
  is 
  observed. 
  Nor 
  am 
  I 
  aware 
  

   that 
  the 
  occurrence 
  of 
  contraction 
  in 
  a 
  system 
  undergoing 
  col- 
  

   loidal 
  solution, 
  has 
  been 
  definitely 
  observed 
  or 
  is 
  the 
  usual 
  

   rule. 
  Saturation 
  is 
  to 
  some 
  extent 
  given 
  by 
  the 
  swollen 
  solid 
  

   coagulate 
  only; 
  it 
  is 
  not 
  usually 
  observed 
  in 
  the 
  colloidal 
  

   solution 
  above 
  the 
  melting 
  point. 
  In 
  the 
  case 
  of 
  water 
  glass 
  a 
  

   definite 
  condition 
  of 
  saturation 
  at 
  the 
  close 
  of 
  the 
  reaction 
  

   must 
  be 
  inferred. 
  The 
  results 
  above 
  obtained 
  with 
  retorts 
  in 
  

   which 
  definite 
  quantities 
  of 
  water 
  were 
  needed 
  to 
  yield 
  a 
  clear 
  

   glass, 
  are 
  to 
  the 
  point. 
  

  

  Since 
  glass 
  at 
  ordinary 
  temperatures 
  does 
  not 
  swell 
  up 
  in 
  the 
  

   presence 
  of 
  water, 
  the 
  question 
  suggests 
  itself 
  whether 
  all 
  col- 
  

   loids 
  at 
  temperatures 
  sufficiently 
  below* 
  their 
  dissolving 
  points 
  

   will 
  lose 
  the 
  property 
  of 
  swelling. 
  

  

  If 
  the 
  colloid 
  be 
  regarded 
  as 
  consisting 
  of 
  aggregated 
  mole- 
  

   cules, 
  with 
  all 
  particles 
  of 
  nearly 
  the 
  same 
  specific 
  size 
  but 
  

   larger 
  than 
  the 
  molecular 
  diameter, 
  the 
  following 
  graphic 
  

   exhibit 
  of 
  the 
  phenomenon 
  of 
  solution 
  and 
  coagulation 
  is 
  ten- 
  

   able. 
  The 
  three 
  groups 
  of 
  forces 
  involved, 
  viz., 
  the 
  attraction 
  

   of 
  liquid 
  molecules 
  for 
  each 
  other, 
  of 
  solid 
  molecules 
  for 
  each 
  

  

  