Some  Secondary  Actions  of  Manures  upon  the  Soil.  25 
falls  to  the  bottom  of  the  jar  and  leaves  the  water  clear  above  ; 
the  finest  particles  of  clay  usually  keep  the  water  somewhat 
troubled  for  a day  or  two  before  they  settle  out  completely. 
Btrt  if  the  settlement  of  soil  from  the  unmanured  plots  or 
from  the  plots  receiving  ammonium  salts  was  complete  in  two 
days,  it  would  take  three  or  four  days  or  sometimes  an 
indefinite  time  to  bring  about  the  same  clearness  in  the  jars 
containing  soil  from  the  nitrated  plots.  Now  it  had  previously 
been  proved  that  there  were  fewer  of  the  finest  particles  in 
the  nitrated  soils,  so  that  they  should  settle  more  quickly  and 
more  completely  were  there  not  some  other  factor  at  work 
hindering  the  precipitation. 
The  settlement  of  clay  from  its  turbid  suspensions  in  water 
has  been  frequently  investigated,  because  it  finds  a good  many 
practical  applications  in  such  matters  as  the  texture  of  the 
soil,  the  fitness  of  clay  for  pottery  and  brick-making,  &c., 
and  the  important  facts  are  that  acids  or  certain  salts  hasten 
the  settlement  greatly,  whereas  alkalis  will  retard  or  even 
entirely  prevent  it.  It  is  well  known,  for  example,  how 
a little  alum  will  bring  about  the  clearing  of  turbid  water, 
and  a trace  of  acid  or  of  some  salt  of  lime  will  produce  the 
same  effect  almost  as  rapidly.  With  suitable  arrangements 
the  process  can  be  watched  under  the  microscope  ; as  soon  as 
the  acid  or  salt  is  introduced  the  very  fine  particles,  which 
before  were  moving  about  in  the  liquid  without  ever 
coalescing,  suddenly  rush  together  and  flocculate  or  coagulate 
into  comparatively  large  and  heavy  groups  which  will  fall 
rapidly  through  the  liquid.  In  clay  soils  that  are  in  good  tilth 
the  very  fine  particles  are  grouped  together  in  this  flocculated 
condition,  and  the  soil  in  consequence  behaves  as  if  it  was 
more  coarsely  grained,  drying  more  easily  and  into  a friable 
condition  ; whereas  if  the  clay  be  knocked  about  or  tempered 
when  it  is  wet,  the  groups  are  broken  up  and  the  clay  becomes 
deflocculated.  The  value  of  lime  in  improving  the  texture 
of  a soil,  in  rendering  it  dryer  and  more  workable,  is  due 
to  the  flocculating  power  of  the  lime  salts  which  begin  to 
wash  through  the  soil  ; chemical  flocculation  of  the  clay  is 
set  up  and  aids  very  greatly  the  mechanical  flocculation 
which  the  careful  cultivator  attains  by  exposing  his  soil  in 
a rough  state  to  the  wettings  and  dryings,  frosts  and  thaws  of 
the  winter. 
Returning  to  the  experiment,  it  was  pretty  clear  that  there 
must  be  some  substance  in  the  soils  from  the  nitrated  plots 
which  had  brought  them  into  a deflocculated  condition,  and 
this  substance  could  not  well  be  anything  else  than  a trace 
of  alkali.  On  testing,  the  soils  from  the  nitrated  plots 
were  found  to  be  slightly  alkaline,  probably  with  carbonate 
