Ammonia. 
387 
serted  by  me  in  this  Journal.  Still  it  was  said  gypsum  must  be 
a fixer  because  it  is  sulphate  of  lime,  and  if  sulphate  of  lime  be 
mixed  with  carbonate  of  ammonia,  the  sulphuric  acid  quits  the 
lime,  seizes  the  ammonia,  and  holds  it  fast  as  an  inodorous  salt. 
Accordingly,  gypsum  has  kept  its  character  to  this  hour  as  a 
“ fixer,”  and  farmers  have  been  much  blamed  for  not  using  it. 
Boussingault,  however,  throws  a new  light  on  the  matter,  though 
in  discussing  another  subject.*  He  says  that  gypsum  in  solution , 
as  in  a laboratory  experiment,  does  act  as  desired,  but  that  in 
a state  of  moist  powder  the  gypsum  is  indifferent  towards  am- 
monia ; nay,  more,  that  in  that  state  the  law  of  affinity  is  reversed, 
and  that  carbonate  of  lime,  chalk,  decomposes  sulphate  of  am- 
monia, actually  unfixes  it.  To  explain  this  contradiction  he 
quotes  Berthollet  and  the  following  singular  law.  If  two  saline 
solutions,  containing  between  them  an  insoluble  salt,  be  mixed, 
that  insoluble  salt  will  be  formed  : hut  if  two  salts,  containing 
between  them  a volatile  salt,  be  mixed  in  a moist  pulverulent  state, 
the  volatile  salt  will  be  produced.  Thus,  sulphate  of  lime  and 
carbonate  of  ammonia  in  solution  produce  carbonate  of  lime  inso- 
luble, leaving  sulphate  of  ammonia,  which  is  soluble,  though  not 
volatile.  But  carbonate  of  lime  mixed  with  sulphate  of  ammonia 
in  a state  of  moist  powder,  acting  by  an  opposite  interchange, 
produce  carbonate  of  ammonia,  a volatile  salt,  and  sulphate  of 
lime.  The  following  diagrams  will  show  at  a glance  the  contrary 
changes. 
Solutions  mixed. 
Sulphate  of  Lime 
— Gypsum 
Carbonate  of  Am- 
monia .... 
Sulphuric  Acid 
Lime 
Sulphate  of  Ammonia. 
Carbonate  of  Lime — 
Insoluble. 
Moistened  Powders  mixed. 
Carbonate  of  Ammo- 
nia— Volatile. 
Sulphate  of  Lime. 
Gypsum,  then,  unless  in  solution,  will  not  fix  ammonia.  But  it 
requires  to  dissolve  it  500  times  its  own  weight  of  water.  There- 
fore, when  it  is  proposed  that  “ every  farmer  should  use  a waggon- 
load of  gypsum  each  year,”  we  see  that  500  waggon-loads  of 
* Boussingault's  Rural  Economy,  p.  442,  Eng.  Edit.  ; see  also  Fownes’s  Element- 
ary Chemistry,  p.  195. 
Carbonate  of  LimeJ 
—Chalk  . 
Sulphate  of  Am- 
monia .... 
Carbonic  Acid 
( Lime 
Ammonia 
Sulphuric  Acid 
