﻿and 
  Chemical 
  Reactions 
  in 
  Heterogeneous 
  Systems. 
  541 
  

  

  this 
  method 
  we 
  always 
  get 
  air-tubes 
  and 
  spaces 
  enclosed 
  in 
  

   the 
  acid, 
  and 
  this 
  cannot 
  be 
  avoided 
  even 
  by 
  removing 
  the 
  

   air 
  from 
  the 
  liquid 
  acid 
  in 
  vacuum 
  or 
  by 
  very 
  slow 
  cooling 
  of 
  

   the 
  acid 
  in 
  a 
  double-walled 
  air-bath. 
  The 
  substance 
  does 
  not 
  

   crystallise 
  as 
  one 
  uniform 
  mass. 
  Photo 
  (1) 
  (PI. 
  XVII.) 
  gives 
  

   the 
  surface 
  of 
  the 
  block 
  of 
  benzoic 
  acid 
  as 
  soon 
  as 
  the 
  top 
  thin 
  

   film 
  is 
  removed 
  during 
  the 
  reaction. 
  There 
  is 
  no 
  need 
  for 
  

   me 
  to 
  add 
  that 
  such 
  a 
  block 
  cannot 
  possibly 
  be 
  suitable 
  for 
  

   any 
  comparative 
  quantitative 
  measurements. 
  This 
  process 
  of 
  

   melting 
  can 
  naturally 
  find 
  only 
  a 
  limited 
  application, 
  cannot 
  

   e. 
  g. 
  be 
  applied 
  to 
  inorganic 
  substances 
  requisite 
  for 
  the 
  

   research. 
  From 
  among 
  the 
  last 
  Nernst 
  and 
  Brunner 
  chose 
  for 
  

   their 
  research 
  "magnesium 
  hydroxide." 
  We 
  are 
  told 
  that 
  they 
  

   " 
  took 
  advantage 
  of 
  the 
  property 
  of 
  calcined 
  MgO 
  mixed 
  with 
  

   water, 
  to 
  dry 
  to 
  a 
  thick 
  mass 
  and 
  to 
  become 
  hard.^' 
  As 
  a 
  

   matter 
  of 
  fact, 
  ordinary 
  analysis 
  shows 
  that 
  the 
  Mg(0H)2 
  

   always 
  contains, 
  under 
  their 
  conditions 
  of 
  work, 
  very 
  much 
  

   carbonate, 
  and 
  we 
  find 
  that 
  Mg(0II)2 
  cannot 
  stick 
  together 
  

   without 
  it. 
  I 
  find 
  that 
  if 
  we 
  mix 
  MgO 
  with 
  water 
  and 
  let 
  it 
  

   dry 
  in 
  an 
  ebonite 
  lid 
  with 
  vertical 
  rims 
  over 
  CaCl2 
  in 
  vacuum, 
  

   so 
  as 
  to 
  prevent 
  absorption 
  of 
  CO2 
  by 
  the 
  MgO 
  or 
  Mg(0II)2, 
  

   we 
  get 
  the 
  block 
  as 
  in 
  photo 
  (2); 
  if 
  we 
  dry 
  pure 
  Mg(0H)2 
  

   mixed 
  with 
  HoO 
  over 
  CaCl2 
  in 
  vacuum, 
  we 
  get 
  a 
  block 
  as 
  in 
  

   photo 
  (3) 
  (in 
  which 
  one 
  part 
  of 
  the 
  surface 
  has 
  been 
  polished). 
  

   Both 
  of 
  them 
  show 
  a 
  great 
  contraction 
  on 
  drying, 
  contain 
  

   numerous 
  air-holes, 
  before 
  and 
  after 
  their 
  surfaces 
  are 
  cut 
  

   away 
  with 
  a 
  razor, 
  contain 
  big 
  cracks 
  across, 
  and 
  fall 
  to 
  pieces 
  

   as 
  soon 
  as 
  they 
  come 
  into 
  contact 
  with 
  water, 
  as 
  is 
  to 
  be 
  seen 
  

   from 
  photo 
  (-I), 
  in 
  which 
  the 
  breaking 
  up 
  of 
  Mg(0H)2 
  under 
  

   water 
  is 
  shown. 
  If 
  we 
  mix 
  MgO 
  with 
  10 
  per 
  cent, 
  gum- 
  

   arabicum, 
  fill 
  with 
  it 
  the 
  ebonite 
  lid 
  and 
  dry 
  over 
  CaCl2, 
  we 
  

   get 
  a 
  disk, 
  which 
  sticks 
  together 
  as 
  in 
  photo 
  (5), 
  which 
  

   at 
  the 
  same 
  time 
  shows 
  the 
  enormous 
  contraction 
  of 
  the 
  

   mixture 
  ; 
  but 
  as 
  soon 
  as 
  we 
  bring 
  this 
  disk 
  in 
  contact 
  with 
  

   water 
  it 
  breaks 
  up 
  to 
  pieces 
  as 
  is 
  seen 
  from 
  photo 
  (6). 
  

   There 
  is 
  no 
  possible 
  way 
  of 
  getting 
  a 
  block 
  of 
  pure 
  Mg(0H)2 
  

   which 
  could 
  be 
  brought 
  into 
  contact 
  with 
  ivater 
  without 
  falling 
  

   at 
  once 
  to 
  pieces 
  ! 
  I 
  lost 
  quite 
  a 
  considerable 
  time 
  in 
  my 
  

   attempts 
  to 
  produce 
  such 
  a 
  block, 
  but 
  entirely 
  failed, 
  though 
  

   I 
  was 
  with 
  my 
  method 
  quite 
  successful 
  in 
  a 
  great 
  number 
  of 
  

   other 
  cases, 
  namely 
  : 
  I 
  compressed 
  the 
  mixture 
  of 
  MgO 
  and 
  

   H2O 
  directly 
  after 
  they 
  were 
  mixed, 
  and 
  the 
  mixture 
  MgO 
  and 
  

   H2O 
  after 
  standing 
  in 
  a 
  closed 
  up 
  vessel 
  for 
  about 
  two 
  weeks; 
  

   then 
  I 
  compressed 
  dry 
  Mg(0H)2, 
  as 
  well 
  as 
  Mg(0H)2 
  mixed 
  

   with 
  water 
  in 
  moulds, 
  under 
  a 
  pressure 
  of 
  several 
  thousand 
  

   atmospheres 
  ; 
  and 
  though 
  I 
  succeeded 
  with 
  a 
  conical 
  piston 
  in 
  

  

  