372 
On  the  Power  of  Soils  to  absorb  Manure. 
be  supposed  to  exist  in  clay  (that  is,  soluble  in  acids  and  alkalies), 
has  a power  of  combining  with  ammonia  and  other  bases  in  a way 
not  hitherto  understood.  Silica  is  really  soluble  in  ammonia, 
although  to  a small  extent  only  ; when  placed  in  a solution  of  am- 
monia it  absorbs  it,  forming  a silicate  of  ammonia,  a part  of  which 
only  dissolves  in  the  liquid.  It  does  not  seem  at  all  consistent 
with  the  facts  to  believe  that  the  free  silica  of  clay  is  the  agent  in 
these  absorptions,  but  the  observation  that  silica  is  soluble  in 
ammonia,  and  that  when  the  solution  is  gently  evaporated  the 
ammonia  escapes  and  leaves  the  silica  in  thin  films  like  a varnish, 
which  break  off  from  the  containing  vessel  as  a glass  of  wonder- 
ful lightness  and  transparency,  may  one  day  help  to  explain  both 
the  manner  in  which  the  cereals  obtain  their  beautiful  silicious 
coating,  and  the  singular  loss  of  nitrogen  which  Mr.  Lawes  has 
shown  takes  place  in  the  growth  of  a crop  of  wheat. 
Practical  Conclusions. 
It  has  been  shown  in  the  course  of  this  paper  that  ordinary 
soils  possess  the  power  of  separating  from  solution,  and  of  retain- 
ing for  the  purposes  of  vegetation,  the  bases  of  the  different 
alkaline  salts  and  certain  animal  and  vegetable  substances,  and 
that  this  power  extends  to  all  those  substances  to  which  we  attach 
the  chief  value  as  manure.  Until  we  are  in  possession  of  a full 
knowledge  of  the  cause  of  this  peculiar  action,  and  the  circum- 
stances under  which  it  becomes  operative,  we  should  not  be 
justified  in  insisting  upon  any  general  application  of  the  principles 
in  question  to  practical  agriculture. 
Still,  however,  there  are  certain  logical  deductions  which  seem 
to  follow  as  a matter  of  necessity  from  a consideration  of  the 
facts,  quite  independent  of  the  laws  upon  which  they  depend. 
And  first  in  regard  to  manures  : — 
We  mix  sulphate  of  ammonia  with  a soil  either  in  the  solid 
state  or  as  a solution.  Sulphate  of  ammonia  is  a very  soluble  salt, 
and  when  subsequently  we  add  water  in  quantity  to  the  soil,  it 
may  naturally  be  expected  to  bring  away  with  it  some  sulphate  of 
ammonia,  but  it  does  not,  neither  is  ammonia  in  any  shape  dis- 
solved out.  Now  whatever  may  be  the  form  of  combination  in 
which  the  ammonia  is  retained  by  the  soil,  it  is  plainly  in  an 
insoluble  state.  Further,  the  sulphuric  acid  is  now  found  com- 
bined with  lime,  and  the  sulphate  of  lime  is,  as  usual,  in  the 
soluble  condition.  If,  instead  of  sulphate  of  ammonia,  we  have 
used  the  muriate  of  the  same  base,  we  find  in  the  liquid  muriate 
of  lime,  the  ammonia  being  retained  as  before.  Obviously  in 
both  instances  the  particular  salts  of  ammonia  cease  to  exist  as 
such  directly  they  are  mixed  with  the  soil.  Instead  of  the  sul- 
