﻿Molecular 
  Reactions 
  in 
  Heterogeneous 
  Systems. 
  575 
  

  

  In 
  Table 
  III. 
  b 
  the 
  same 
  selenite 
  is 
  investigated 
  in 
  

   two 
  different 
  directions 
  in 
  the 
  crystal 
  : 
  the 
  first 
  one 
  is 
  the 
  

   large 
  flat 
  surface 
  of 
  the 
  twin-crjstal 
  already 
  mentioned 
  

   before 
  (A 
  Selenite, 
  photo 
  11) 
  ; 
  the 
  second 
  is 
  a 
  surface 
  cut 
  

   vertically 
  to 
  the 
  surface 
  A, 
  as 
  in 
  the 
  annexed 
  

   y\ 
  figure. 
  I 
  cut 
  the 
  selenite 
  crystal, 
  as 
  in 
  the 
  

  

  ^ 
  ^ 
  _ 
  figure, 
  into 
  several 
  strips 
  and 
  connected 
  the 
  

   - 
  vertical 
  surfaces 
  to 
  one 
  surface, 
  the 
  cut-off 
  

   strips 
  separating 
  into 
  several 
  thin 
  plates, 
  and 
  I 
  

   made 
  up 
  a 
  block 
  connecting 
  the 
  same 
  by 
  means^ 
  

   of 
  paraffin 
  (see 
  photo 
  14). 
  The 
  surface 
  of 
  the 
  

   crystal 
  without 
  paraffin 
  was 
  49 
  mm. 
  in 
  length 
  and 
  49 
  mm, 
  

   in 
  widih; 
  through 
  the 
  paraffin 
  it 
  became 
  49 
  mm, 
  in 
  length 
  

   and 
  52 
  mm. 
  in 
  w^idth. 
  As 
  in 
  case 
  of 
  all 
  other 
  gypsums, 
  all 
  

   sides 
  were 
  covered 
  with 
  paraffin, 
  except 
  one 
  which 
  was 
  

   exposed 
  to 
  the 
  action 
  of 
  the 
  solution. 
  The 
  surface 
  was 
  

   perfectly 
  polished, 
  as 
  is 
  to 
  be 
  seen 
  also 
  from 
  photo 
  14 
  ; 
  the 
  

   velocity 
  constant 
  obtained 
  for 
  this 
  vertical 
  surface 
  is 
  about 
  

   3*376, 
  i»e. 
  about 
  6 
  times 
  as 
  large 
  as 
  the 
  velocity 
  constant 
  of 
  

   solution 
  of 
  the 
  other 
  surface 
  of 
  the 
  same 
  crystal 
  (which 
  is 
  

   •6378), 
  a 
  difference 
  which 
  is 
  truly 
  enormous, 
  and 
  cannot 
  

   possibly 
  be 
  attributed 
  either 
  to 
  the 
  difference 
  in 
  the 
  surface 
  

   or 
  to 
  the 
  method 
  employed. 
  This 
  experiment 
  proves 
  beyond 
  

   any 
  possible 
  doubt 
  that 
  the 
  same 
  crystal 
  dissolves 
  at 
  dif- 
  

   ferent 
  rates 
  in 
  different 
  directions 
  to 
  the 
  axis 
  of 
  the 
  crystal^ 
  

   and 
  is 
  at 
  the 
  same 
  time 
  an 
  absolute 
  proof 
  that 
  the 
  funda- 
  

   mental 
  assumptions 
  of 
  the 
  diffusion 
  theory 
  mentioned 
  above 
  

   are 
  thoroughly 
  wrong 
  and 
  belong 
  only 
  to 
  the 
  world 
  of 
  pure 
  

   imagination. 
  This 
  experiment 
  also 
  explains 
  the 
  reason, 
  why 
  

   the 
  different 
  modifications 
  of 
  gypsum 
  give 
  us 
  different 
  velocity 
  

   constants. 
  In 
  the 
  fibrous 
  gypsum 
  all 
  the 
  crystals 
  of 
  which 
  

   the 
  gypsum 
  is 
  formed 
  configurate 
  in 
  one 
  direction 
  and 
  not 
  

   in 
  the 
  other 
  ; 
  in 
  the 
  selenite 
  the 
  crystals 
  configurate 
  in 
  all 
  

   directions 
  of 
  the 
  twin-crystal 
  ; 
  while 
  in 
  alabaster, 
  precipitated 
  

   CaS04 
  + 
  2H20, 
  or 
  in 
  powdered 
  selenite 
  they 
  configurate 
  in 
  

   no 
  direction. 
  Owing 
  to 
  this 
  in 
  the 
  selenite 
  twin-crystal 
  in 
  

   the 
  large 
  side 
  of 
  the 
  same, 
  always 
  the 
  same 
  surfaces 
  of 
  all 
  

   the 
  crystals 
  (which 
  are 
  least 
  easily 
  dissolving) 
  are 
  exposed 
  to 
  

   solution, 
  in 
  the 
  fibrous 
  gypsum 
  the 
  surfaces 
  of 
  the 
  crystals 
  

   lying 
  in 
  two 
  directions 
  are 
  exposed 
  to 
  solution, 
  but 
  not 
  the 
  

   surfaces 
  in 
  the 
  third 
  direction, 
  which 
  combine 
  to 
  form 
  the 
  

   fibre, 
  while 
  in 
  the 
  alabaster 
  the 
  surfaces 
  of 
  the 
  crystal 
  in 
  all 
  

   three 
  directions 
  are 
  exposed 
  to 
  solution 
  in 
  equal 
  or 
  nearly 
  

   equal 
  shares. 
  So 
  we 
  find 
  for 
  S 
  = 
  20*4 
  cm.^ 
  the 
  velocity 
  

   constant 
  of 
  alabaster 
  is 
  about 
  2*6 
  ; 
  the 
  velocity 
  constant 
  of 
  

   selenite 
  in 
  powder 
  and 
  compressed 
  to 
  a 
  disk 
  is 
  for 
  the 
  same 
  

   surface 
  = 
  2'9, 
  while 
  the 
  velocity 
  constant 
  of 
  CaS042H2O 
  

  

  