On  the  Power  of  Soils  to  absorb  Manure.  315 
separating  ammonia  from  solution  in  water  is  evident,  since  he 
distinctly  states  that  the  alkali  so  absorbed  from  the  air  is  separated 
by  the  first  quantity  of  rain  that  falls,  and  rendered  available  for 
plants.  He  also  considers  that  carbonate  of  ammonia  in  the  soil 
is  liable  to  loss  by  evaporation,  and  attributes  the  value  of  gypsum 
as  manure  to  its  faculty  of  converting  the  volatile  carbonate  of 
ammonia  into  the  comparatively  fixed  sulphate  of  the  same  base. 
If  the  experiments  which  are  about  to  be  described  possess  any 
claim  to  consideration  this  last  supposition  must  be  fallacious, 
since  sulphate  of  ammonia  itself  will  be  found  subject  to  a most 
important  change  when  it  is  mixed  with  the  soil.  The  fertilizing 
effect  of  burnt  clay  is,  in  like  manner,  attributed  by  this  illustrious 
chemist  to  its  power  of  condensing  ammonia  : a fact  which  in 
itself  is  sufficient  proof  that  all  his  remarks  upon  this  head  were 
intended  to  apply  to  the  abstraction  of  ammonia  from  the  air,  and 
not  to  its  retention  in  the  presence  of  water.  For,  as  will  pre- 
sently be  seen,  the  burning  of  clay,  which  is  supposed  to  render 
it  more  active  in  the  sense  in  which  Professor  Liebig  regarded 
the  absorptive  power,  has  really  the  effect  of  destroying  wholly  or 
in  part  its  efficacy  as  an  absorbent  from  solution. 
This  short  explanation  is  necessary  to  show  that  the  property 
in  question  is  totally  different  from  the  well-known  power  of 
porous  substances  to  absorb  ammoniacal  and  other  gases.  Nor, 
if  it  were  the  same,  would  the  explanation  be  sufficient  to  account 
for  the  decomposition  by  the  soil  of  the  different  salts  of  ammonia, 
and  for  the  power  which  they  w ill  be  shown  to  possess  of  com- 
bining with  other  alkaline  bases,  such  as  potash,  magnesia,  &c., 
which  have  no  gaseous  character  whatever.  Neither  must  the 
property  which  we  are  about  to  study  be  confounded  with  that 
surface  attraction  to  which  the  name  of  capillarity  is  given. 
Liquids  have  a tendency  to  attach  themselves  to  the  smooth  sur- 
faces of  bodies  with  which  they  are  brought  in  contact,  and  this 
tendency  exhibits  itself  by  raising  them  in  opposition  to  the 
natural  law  of  gravitation.  It  is  best  exemplified  in  the  case  of 
very  minute  tubes,  open  at  both  ends,  which,  being  dipped  into 
any  liquid,  cause  it  to  ascend  in  them  to  a considerable  height 
above  the  level  of  the  surrounding  solution.  But  capillary  action 
is  equally  active,  whatever  be  the  form  of  the  surface,  so  that  the 
interstices  are  sufficiently  minute — thus  a piece  of  sugar  throwm 
into  the  bottom  of  a teacup  is  almost  instantly  moistened  throughout 
by  the  ascent  of  the  liquid,  which  only  touches  its  lower  surface. 
The  attraction  of  capillarity  is  not  the  same  for  all  liquids,  and 
consequently  it  will  affect  to  a different  extent  a solution  of  a salt 
and  pure  water ; it  is  a surface  attraction,  and  will  act  in  retaining 
a liquid  just  in  the  same  way  that  it  does  in  causing  it  to  attach 
itself. 
