326 
On  the  Power  of  Soils  to  absorb  Manure. 
The  first  three  of  these  experiments  (16,  17,  18)  are  rather 
unsatisfactory.  They  show,  indeed,  that  the  power  of  a soil  to 
combine  with  ammonia  is  greatly  diminished  by  burning  it ; but 
they  leave  it  in  doubt  whether  the  little  influence  which  remains 
is  to  be  ascribed  to  the  mineral  elements  of  the  soil  or  to  the 
charcoal,  which  is  known  to  possess  a similar  property.  The  ex- 
periments with  the  clay  are  more  decisive,  but  do  not  afford  con- 
clusive evidence.  It  would  seem,  however,  that  the  more  per- 
fectly (or  rather  strongly)  the  soils  were  burnt,  the  more  com- 
pletely was  their  absorptive  power  destroyed.  This  was  very  evi- 
dent in  the  last  experiment,  where  the  clay  was  almost  fused  by 
the  heat.  Acting  upon  this  impression,  the  next  step  in  the  in- 
quiry was  to  employ  filters  of  the  most  perfectly  burnt  clay,  and 
bricks  and  tobacco-pipes  appeared  likely  to  answer  this  character. 
Experiment  22. — A soft  red  brick  was  broken  in  a mortar,  and  the 
powder  passed  through  a sieve  of  20  holes  to  the  inch  ; a stratum  of  16 
inches  in  depth  was  used  as  the  filter  for  the  standard  solution  of  ammonia. 
The  first  drops  that  came  through  smelt  strongly  of  ammonia. 
Experiment  23. — The  same  brickdust  passed  through  a much  finer  sieve 
(of  60  holes  to  the  inch)  ; the  column  of  the  filter  and  the  solution  the 
same  as  in  No.  22.  The  smell  of  ammonia  was  perceptible  from  the  very 
first,  and  it  soon  became  quite  as  powerful  as  in  the  original  liquid.  The 
solution  became  very  yellow,  smelt  strongly  of  soot,  and  contained  sul- 
phate of  lime  : some  of  the  ammonia  in  it  was  in  the  state  of  sulphate. 
Experiment  24. — In  this  experiment  the  brickdust  and  solution  being  the 
same  as  in  No.  23,  only  so  much  of  the  solution  was  added  as  to  pass 
down  without  running  out  of  the  tube  ; the  materials  were  then  left  in 
contact  for  two  hours,  the  object  being  to  give  time  for  the  perfect  contact 
of  the  brickdust  and  the  ammoniacal  solution.  Upon  adding  more  liquid 
the  first  portions  coming  through  gave  with  potash  indications  of  ammonia, 
to  all  appearance  in  equal  quantity  with  the  original  solution. 
Experiment  24. — A repetition  of  the  last,  with  the  same  result. 
As  a variation  of  these  last  experiments,  a trial  was  made  of 
the  action  of  powdered  tobacco-pipes  upon  ammonia.  The  clay 
with  which  tobacco-pipes  are  made  is  of  a very  pure  character, 
being  almost  entirely  free  from  oxide  of  iron,  winch  colours  the 
red  bricks.  In  the  manufacture  of  pipes,  this  clay  is  also  very 
carefully  and  perfectly  burnt,  both  of  which  peculiarities  seemed 
to  fit  it  for  a trial  in  these  experiments. 
Experiment  25. — Tobacco-pipes  were  broken  in  an  iron  mortar  and  the 
powder  passed  through  a sieve  of  40  holes  to  the  inch.  The  standard 
solution  of  ammonia  was  poured  upon  a column  of  12  inches  of  this 
powder.  The  liquid  was  slow  in  filtering,  but  when  it  came  through 
neither  ammonia  nor  its  salts  could  be  detected. 
Experiment  26. — A repetition  of  the  previous  experiment,  the  pipe-dust 
being  mixed  with  its  own  weight  of  sand  to  hasten  the  filtration.  The 
liquid  coming  through  was  free  from  ammonia  or  its  salts. 
Here,  then,  we  have  a contradictory  result.  A specimen  of 
powdered  red  brick  appears  entirely  devoid  of  absorptive  power, 
