324  On  the  Power  of  Soils  to  absorb  Manure. 
ordinary  soils,  and  the  further  examination  of  particular  speci- 
mens was  reserved  to  another  period  of  the  investigation. 
The  power  of  soils  to  separate  ammonia  from  solution  being 
beyond  a doubt,  it  next  became  a question  to  ascertain  to  what  in- 
gredient the  property  is  attributable.  An  ordinary  soil  consists 
principally  of  three  substances — sand,  clay,  and  vegetable  matter. 
To  these  may  be  sometimes  added  a fourth,  namely,  carbonate 
of  lime. 
In  the  soils  with  which  the  previous  experiments  were  made, 
only  a very  small  proportion  of  carbonate  of  lime  occurred  ; and 
the  inquiry  is  therefore  limited  to  sand,  clay,  and  vegetable 
matter.  The  following  experiments  were  made  with  sand. 
Experiment  14. — A tube  18  inches  long,  filled  to  within  two  or  three 
inches  of  the  top  with  well-washed  white  sand ; solution  of  ammonia  of 
the  former  strength  passed  through.  The  very  first  drops  that  passed 
were  found  to  be  strongly  pungent,  aud  on  comparing  the  first  half-ounce 
with  the  original  liquid  no  difference  could  be  observed. 
Experiment  15. — Sand  of  a much  finer  grain  wrell  washed  and  dried, 
4000  grains;  solution  of  ammonia  as  before.  Owing  to  the  fineness  of 
the  particles  the  solution  did  not  begin  to  drop  from  the  tube  in  less  than 
10  minutes.  The  first  portions  coming  through  appeared  in  no  way 
affected  by  the  filtration. 
Experiment  16. — As  the  last,  the  column  of  dried  sand  being  somewhat 
longer,  18  inches.  The  first  liquid,  which  came  through  in  a quarter  of 
an  hour,  smelt  of  ammonia,  but  it  was  thought,  when  compared  with  the 
ammoniacal  solution,  to  be  somewhat  weaker:  if,  however,  this  difference 
really  existed,  it  soon  disappeared  as  further  quantities  came  through  the 
filter. 
These  experiments  sufficiently  prove  that  the  power  is  not  to 
he  ascribed  to  sand.  The  last  result  was  thought  at  the  time  in 
favour  of  the  existence  of  a surface  attraction  of  a feeble  kind 
which  sand  and  other  granular  substances  might  exhibit  to  a 
solution  such  as  that  of  ammonia.  The  power  possessed  by  char- 
coal, and  some  other  bodies,  of  condensing  gases  and  vapours,  is 
plainly  in  relation  to  the  surface  exposed,  and  might  be  expected 
to  remain  active  although  the  gas  or  vapour  were  in  solution  in 
water.  In  the  same  way  sand  might  operate  a condensation  of 
the  ammonia  of  the  solution  on  its  surface,  and  the  liquid  coming 
away  would  be  correspondingly  diminished  in  strength.  Whether 
this  is  so  or  not,  it  will  be  abundantly  evident  as  we  proceed,  that 
the  absorptive  power  of  soils  is  something  entirely  different  from 
a surface  attraction. 
To  ascertain  whether  the  vegetable  portion  of  the  soil  had  any 
share  in  the  action,  the  soils  in  the  succeeding  experiments  were 
burnt  before  being  used  in  the  filtrations.  At  the  same  time  it 
was  thought  quite  possible  that  the  evidence  thus  obtained  would 
be  unsatisfactory,  inasmuch  as  burning  might  alter  the  character 
