﻿576 
  Molecular 
  Reactions 
  in 
  Heterogeneous 
  Systems. 
  

  

  precipitated 
  and 
  compressed 
  to 
  a 
  block 
  is 
  for 
  the 
  same 
  sur- 
  

   face 
  =3*0, 
  i.e. 
  is 
  only 
  a 
  little 
  different 
  (photograph 
  15 
  shows 
  

   block 
  of 
  powdered 
  selenite 
  after 
  solution 
  ; 
  the 
  corrosion 
  is 
  

   visible) 
  ; 
  which 
  seems 
  to 
  prove 
  that 
  when 
  the 
  crystal 
  has 
  

   no 
  special 
  configuration 
  and 
  all 
  its 
  surfaces 
  are 
  equally 
  

   exposed 
  to 
  solution, 
  the 
  speed 
  of 
  solution 
  is 
  practically 
  

   the 
  same. 
  

  

  It 
  should 
  be 
  noted 
  at 
  the 
  same 
  time 
  that 
  the 
  powdered 
  and 
  

   compressed 
  selenite 
  (see 
  Selenite) 
  and 
  the 
  fine 
  precipitated 
  

   and 
  compressed 
  CaS04 
  + 
  2H20 
  seem 
  to 
  give 
  a 
  very 
  good 
  

   constant 
  from 
  beginning 
  to 
  the 
  end, 
  without 
  any 
  essential 
  

   increase 
  in 
  the 
  same. 
  

  

  (ii.) 
  CaS04 
  + 
  2H2O 
  and 
  CaSO^ 
  Anhydride. 
  — 
  If 
  a 
  substance 
  

   forms 
  a 
  saturated 
  solution 
  at 
  the 
  surface 
  of 
  the 
  solid 
  with 
  

   infinitely 
  great 
  speed, 
  and 
  if 
  it 
  passes 
  from 
  the 
  surface 
  of 
  

   the 
  solid 
  to 
  the 
  rest 
  of 
  the 
  solution 
  only 
  by 
  diffusion, 
  then 
  

   CaS04 
  + 
  2H20 
  must 
  dissolve 
  with 
  greater 
  speed 
  than 
  an- 
  

   hydride 
  CaS04. 
  Because 
  the 
  CaS04 
  must 
  either 
  first 
  form 
  

   CaS04 
  + 
  2H20 
  in 
  the 
  solution 
  at 
  the 
  surface 
  of 
  the 
  solid, 
  before 
  

   it 
  diffuses 
  into 
  the 
  rest 
  of 
  the 
  solution 
  in 
  which 
  we 
  know 
  it 
  

   only 
  as 
  0aS04-f 
  2H2O, 
  not 
  as 
  CaS04, 
  or 
  CaS04 
  + 
  2H20 
  must 
  

   be 
  formed 
  first 
  on 
  the 
  solid 
  CaS04 
  itself, 
  as 
  a 
  thin 
  solid 
  layer, 
  

   under 
  the 
  action 
  of 
  water, 
  and 
  then 
  as 
  such 
  pass 
  into 
  the 
  

   solution. 
  Whether 
  this 
  reaction 
  of 
  transformation 
  of 
  CaS04 
  

   into 
  CaS04 
  + 
  2H20 
  be 
  quick 
  or 
  slow, 
  whether 
  it 
  takes 
  place 
  

   at 
  the 
  solid 
  or 
  in 
  solution, 
  the 
  passing 
  of 
  CaS04 
  into 
  the 
  

   whole 
  of 
  the 
  solution 
  by 
  diffusion 
  can 
  only 
  be 
  longer, 
  but 
  not 
  

   shorter 
  than 
  in 
  the 
  case 
  of 
  CaS04H-2H20, 
  if 
  the 
  fundamental 
  

   assumptions 
  of 
  the 
  diffusion 
  theory 
  be 
  true, 
  and 
  a 
  reverse 
  

   phenomenon 
  should 
  not 
  he 
  possible. 
  Table 
  III. 
  C 
  gives 
  the 
  

   speed 
  of 
  solution 
  of 
  anhydride 
  CaS04, 
  N'ova 
  Scotia 
  (photo 
  16 
  

   gives 
  Nova 
  Scotia 
  after 
  solution), 
  which 
  was 
  turned 
  to 
  a 
  disk 
  

   of 
  50 
  mm. 
  diameter. 
  The 
  velocity 
  constant 
  of 
  the 
  same 
  is 
  

   about 
  '64: 
  for 
  its 
  surface 
  ; 
  on 
  the 
  other 
  hand, 
  we 
  found 
  the 
  

   Selenite 
  CaS04 
  + 
  2H2O 
  gave 
  for 
  the 
  same 
  surface 
  

   X 
  = 
  20'4 
  cm.^, 
  and 
  under 
  all 
  other 
  identical 
  conditions 
  of 
  

   temperature, 
  stirring, 
  volume 
  of 
  solution, 
  &c., 
  a 
  constant 
  of 
  

   about 
  *49 
  ; 
  an 
  inspection 
  of 
  the 
  two 
  blocks 
  Selenite 
  and 
  Nova 
  

   Scotia 
  after 
  they 
  were 
  dissolving 
  in 
  solution, 
  also 
  show 
  a 
  

   greater 
  corrosion 
  in 
  the 
  last, 
  which 
  is 
  indicative 
  of 
  a 
  greater 
  

   speed 
  of 
  solution, 
  than 
  in 
  the 
  case 
  of 
  Selenite. 
  

  

  Conclusion. 
  

  

  I 
  have 
  thus 
  proved, 
  both 
  theoretically 
  and 
  experimentally, 
  

   that 
  the 
  explanation 
  of 
  my 
  equation 
  for 
  molecular 
  reactions 
  

   in 
  case 
  of 
  solution 
  of 
  salts, 
  as 
  a 
  diffusion 
  equation, 
  and 
  all 
  

  

  