﻿Hypothesis, 
  and 
  the 
  Periodic 
  Law. 
  411 
  

  

  The 
  data 
  on 
  dissociation 
  are 
  meagre. 
  Iodine 
  in 
  Group 
  YII 
  as 
  

   is 
  known 
  is 
  easily 
  melted 
  and 
  dissociated 
  by 
  heat, 
  and 
  probably 
  

   also 
  the 
  alkali 
  metals 
  of 
  Group 
  I, 
  while 
  such 
  bodies 
  as 
  C 
  and 
  Si 
  

   of 
  Group 
  IV 
  are 
  but 
  little 
  affected, 
  so 
  that 
  so 
  far 
  as 
  known 
  

   theory 
  is 
  again 
  complied 
  with 
  ; 
  but 
  more 
  satisfactory 
  conclusions 
  

   can 
  be 
  drawn 
  from 
  the 
  heats 
  of 
  formation 
  in 
  solution, 
  for 
  on 
  

   the 
  very 
  probable 
  theory, 
  that 
  in 
  aqueous 
  solutions 
  of 
  binary 
  

   salts 
  the 
  elements 
  are 
  almost 
  entirely 
  dissociated, 
  we 
  can 
  com- 
  

   pare 
  these 
  thermal 
  reactions 
  of 
  members 
  of 
  Series 
  3. 
  

  

  Na 
  9 
  Group 
  I 
  2(Na, 
  CI, 
  Aq) 
  = 
  193020 
  —« 
  _ 
  1Q 
  „ 
  Q 
  .„ 
  

  

  Mg 
  9 
  Group 
  II 
  2(Mg, 
  CI 
  2 
  , 
  Aq) 
  = 
  373860 
  £ 
  * 
  " 
  ~!™ 
  

  

  Al 
  2 
  Group 
  III 
  2(A1, 
  CI 
  3 
  , 
  Aq) 
  = 
  475650 
  Um 
  ' 
  ~ 
  1U17y(J 
  

   Si 
  2 
  -P 
  2 
  

  

  S 
  a 
  Group 
  VI 
  2(Na 
  2 
  , 
  S, 
  Aq) 
  = 
  208000 
  Diff 
  _ 
  

  

  CI 
  2 
  Group 
  VII 
  2(Na, 
  CI, 
  Aq) 
  = 
  193020 
  um 
  ' 
  ~ 
  + 
  i4980 
  

  

  As 
  the 
  chlorides 
  of 
  Groups 
  IV 
  and 
  V 
  decompose 
  instead 
  

   of 
  dissolve 
  in 
  water 
  the 
  series 
  is 
  broken. 
  Now 
  in 
  the 
  reaction 
  

   2(Xa, 
  CI, 
  Aq) 
  we 
  have 
  the 
  dissociation 
  of 
  Na 
  2 
  and 
  CI 
  2 
  and 
  in 
  

   2(Mg, 
  CI 
  2 
  , 
  Aq) 
  the 
  dissociation 
  of 
  Mg 
  2 
  and 
  CP 
  that 
  is 
  to 
  say 
  the 
  

   difference 
  is 
  the 
  difference 
  between 
  the 
  heat 
  of 
  dissociation 
  of 
  

   Na 
  a 
  and 
  Mg 
  2 
  plus 
  the 
  dissociation 
  heat 
  of 
  CI 
  2 
  . 
  This 
  term 
  CI 
  2 
  

   is 
  a 
  constant 
  addition 
  to 
  the 
  first 
  two 
  series 
  differences. 
  We 
  

   are 
  evidently 
  justified 
  in 
  concluding 
  that 
  in 
  Groups 
  I 
  and 
  VII 
  

   the 
  heat 
  of 
  dissociation 
  is 
  small 
  but 
  large 
  in 
  Groups 
  III 
  and 
  IV. 
  

   We 
  may 
  however 
  consider 
  this 
  series 
  of 
  heats 
  of 
  formation 
  

  

  Group 
  I 
  2(Na, 
  CI) 
  = 
  195380, 
  Group 
  IV 
  2 
  (Si, 
  CI 
  4 
  ) 
  = 
  315280, 
  

   Group 
  II 
  2(Mg, 
  CI 
  2 
  ) 
  = 
  302020, 
  Group 
  V 
  2(P, 
  CI 
  5 
  ) 
  = 
  209980'. 
  

   Group 
  III 
  2(A1, 
  CI 
  3 
  ) 
  = 
  321060, 
  

  

  Here 
  as 
  before 
  the 
  result 
  is 
  masked 
  by 
  a 
  constant 
  addition 
  

   depending 
  on 
  the 
  constant 
  increment 
  CI 
  2 
  , 
  but 
  still 
  the 
  maxi- 
  

   mum 
  values 
  plainly 
  are 
  attained 
  in 
  families 
  III 
  and 
  IV 
  and 
  

   the 
  minimum 
  in 
  family 
  I 
  as 
  the 
  theory 
  requires. 
  " 
  Sub- 
  

   stances," 
  says 
  Muir, 
  " 
  which 
  are 
  formed 
  with 
  the 
  disappearance 
  

   of 
  heat 
  are 
  generally 
  more 
  readily 
  decomposed 
  by 
  the 
  applica- 
  

   tion 
  of 
  outside 
  forces 
  than 
  substances 
  which 
  are 
  formed 
  with 
  

   the 
  production 
  of 
  heat 
  ;" 
  and 
  Mendeleeff 
  has 
  noted 
  the 
  fact 
  

   that 
  elements 
  of 
  large 
  atomic 
  volume 
  combine 
  easily 
  with 
  

   others, 
  and 
  explains 
  it 
  by 
  assuming 
  a 
  comparatively 
  large 
  dis- 
  

   tance 
  between 
  the 
  single 
  atoms 
  in 
  the 
  molecule. 
  

  

  Our 
  theory 
  requires 
  that 
  the 
  force 
  holding 
  the 
  atoms 
  together 
  

   be 
  large 
  compared 
  with 
  both 
  gravitation 
  and 
  the 
  (elastic) 
  force 
  

   of 
  repulsion. 
  And 
  there 
  is 
  evidence 
  that 
  this 
  is 
  so. 
  For 
  

   instance, 
  at 
  18° 
  Centigrade 
  2 
  grams 
  of 
  H 
  and 
  16 
  grams 
  of 
  O, 
  

   combined 
  by 
  the 
  electric 
  spark 
  into 
  18 
  grams 
  of 
  water, 
  give 
  off 
  

   68360 
  calories 
  of 
  heat 
  or 
  more 
  than 
  6 
  times 
  the 
  quantity 
  neces- 
  

   sary 
  to 
  raise 
  the 
  water 
  thus 
  produced 
  from 
  18° 
  to 
  100° 
  and 
  

  

  