﻿Foote 
  and 
  Bradley 
  — 
  Solid 
  Solution 
  in 
  Minerals. 
  183 
  

  

  The 
  ratios, 
  calculated 
  from 
  the 
  averages 
  given 
  above, 
  are 
  in 
  

   Table 
  III. 
  

  

  Table 
  III. 
  

  

  Eatios 
  of 
  Analyses 
  given 
  in 
  Table 
  II. 
  

  

  I 
  II 
  III 
  

  

  SiO, 
  1-36 
  1'75 
  1-26 
  

  

  CuO 
  1-00 
  l'OO 
  1 
  00 
  

  

  H„0 
  2-25 
  T29 
  2-19 
  

  

  A1 
  2 
  3 
  .._ 
  '12 
  -02 
  

  

  CaO 
  '04 
  -08 
  

  

  So 
  far 
  as 
  leading 
  to 
  any 
  definite 
  formula 
  is 
  concerned, 
  these 
  

   ratios 
  are 
  just 
  as 
  unsatisfactory 
  as 
  those 
  in 
  the 
  longer 
  list 
  pre- 
  

   viously 
  given. 
  If 
  allowance 
  be 
  made 
  for 
  the 
  alumina 
  as 
  allo- 
  

   phane, 
  the 
  ratios 
  are 
  not 
  improved. 
  By 
  assuming 
  a 
  mechanical 
  

   mixture 
  with 
  a 
  hydrated 
  silica 
  or 
  opal 
  of 
  empirical 
  composi- 
  

   tion, 
  the 
  residues 
  in 
  Nos. 
  I 
  and 
  III 
  could 
  be 
  forced 
  to 
  agree 
  in 
  

   composition 
  with 
  the 
  formula 
  usually 
  assigned 
  to 
  chrysocolla, 
  

   but 
  this 
  could 
  not 
  be 
  done 
  in 
  No. 
  II, 
  where 
  silica 
  and 
  water 
  

   are 
  both 
  low. 
  

  

  It 
  has 
  seemed 
  to 
  the 
  authors 
  that 
  the 
  composition 
  of 
  this 
  

   mineral, 
  and 
  probably 
  of 
  many 
  other 
  amorphous 
  minerals, 
  may, 
  

   however, 
  be 
  regarded 
  in 
  a 
  very 
  simple 
  manner 
  by 
  classifying 
  

   them 
  with 
  the 
  artificial 
  hydrogels 
  or 
  gelatinous 
  precipitates 
  

   such 
  as 
  silicic 
  acid 
  or 
  ferric 
  hydroxide. 
  It 
  ^as 
  been 
  shown 
  in 
  

   numerous 
  articles 
  by 
  Van 
  Bemmelen 
  and 
  others, 
  that 
  these 
  

   substances 
  when 
  freed 
  from 
  that 
  part 
  of 
  the 
  water 
  which 
  is 
  

   present 
  as 
  a 
  mechanical 
  mixture, 
  show 
  values 
  for 
  vapor 
  pres- 
  

   sure 
  which 
  continually 
  diminish 
  as 
  the 
  substance 
  is 
  dehydrated. 
  

   In 
  other 
  words, 
  the 
  vapor 
  pressure 
  of 
  a 
  hydrogel 
  is 
  a 
  function 
  

   of 
  its 
  composition. 
  This 
  leads 
  to 
  the 
  conclusion 
  that 
  hydrogels 
  

   are 
  not 
  mechanical 
  mixtures 
  of 
  two 
  definite 
  hydrates, 
  but 
  the 
  

   material 
  is 
  a 
  homogeneous 
  phase 
  of 
  variable 
  composition, 
  com- 
  

   parable 
  in 
  this 
  respect 
  to 
  a 
  solution 
  of 
  salt 
  in 
  water. 
  If 
  a 
  

   hydrogel 
  were 
  a 
  mechanical 
  mixture 
  of 
  two 
  definite 
  hydrates, 
  

   its 
  vapor 
  pressure 
  at 
  a 
  given 
  temperature 
  would 
  be 
  constant, 
  

   independent 
  of 
  its 
  composition. 
  This 
  is 
  the 
  case, 
  as 
  is 
  well 
  

   known, 
  with 
  a 
  mixture 
  of 
  two 
  crystallized 
  hydrates 
  of 
  a 
  salt, 
  

   the 
  vapor 
  pressure 
  remaining 
  constant 
  as 
  the 
  mixture 
  is 
  dehy- 
  

   drated. 
  These 
  hydrogels, 
  which 
  appear 
  to 
  be 
  homogeneous 
  

   substances, 
  have 
  been 
  called 
  " 
  adsorption 
  compounds 
  " 
  by 
  Van 
  

   Bemmelen, 
  but 
  they 
  may 
  equalh 
  T 
  well 
  be 
  regarded 
  as 
  solid 
  

   solutions 
  of 
  water 
  in 
  the 
  oxide 
  or 
  in 
  some 
  lower 
  hydrate.* 
  

   The 
  composition 
  of 
  these 
  substances 
  is 
  not 
  fixed, 
  as 
  in 
  the 
  case 
  

   of 
  a 
  chemical 
  compound, 
  but 
  is 
  variable, 
  depending 
  on 
  the 
  

   conditions. 
  The 
  composition 
  of 
  an 
  artificial 
  silicic 
  acid, 
  for 
  

   * 
  Jour. 
  Amer. 
  Chem. 
  Soc, 
  xxx, 
  1388, 
  1908. 
  

  

  