On  the  Power  of  Soils  to  absorb  Manure. 
327 
whilst  the  powder  of  tobacco-pipes,  equally  well  burnt  and  equally 
devoid  of  organic  matter  or  charcoal,  exhibits  the  property  in  con- 
siderable intensity.  The  next  experiment  was  with  pipe-dust  of 
tobacco-pipes  obtained  from  a different  locality,  and  in  all  pro- 
bability made  from  a different  bed  of  clay. 
Experiment  27. — Pipes  of  another  manufacture  broken  and  sifted  (40- 
hole  sieve)  ; the  powder  mixed  with  its  own  weight  of  sand,  and  a column 
of  18  inches  used  as  the  filter ; the  standard  solution  of  ammonia.  The 
liquid  began  to  come  through  in  one  hour.  The  first  few  drops  were  free 
from  ammonia,  which,  however,  was  quite  perceptible  before  a quarter  of 
an  ounce  had  passed. 
Experiment  28. — The  same  pipes  broken  and  sifted  through  a sieve  of 
20  holes  to  the  inch.  The  column  of  powder  used  being  14  inches,  with 
the  same  solution  of  ammonia.  The  liquid  came  through  in  about  two 
hours,  and  the  first  half-ounce  contained  no  ammonia  in  any  form. 
These  experiments  certainly  gave  the  impression  at  the  time 
that  different  specimens  of  clay  would  be  differently  influenced  in 
regard  to  their  absorptive  powers  by  the  process  of  burning.  It  is 
easy  to  conceive  that  at  a temperature  only  short  of  that  necessary 
for  the  fusion  of  the  materials  of  the  clay,  new  compounds  may 
be  formed  in  it,  and  previously  existing  combinations  destroyed  ; 
and  that  one  clay  varying  from  another  in  the  relative  quantities 
of  silica  and  alumina,  and  in  the  possession  of  small  quantities  of 
the  silicates  of  other  bases,  the  effect  of  heat  would  in  the  one 
case  be  very  different  from  that  in  another.  The  power  pos- 
sessed by  well-burnt  pipeclay,  although  much  inferior  to  that  of 
the  natural  clay,  is  yet  sufficiently  appreciable ; and  its  existence 
is  quite  irreconcilable  with  the  notion  that  the  absorption  of 
ammonia  by  soils  is  due  to  the  production  of  any  organic  com- 
pounds. But  on  this  subject  more  important  evidence  will  be 
forthcoming  as  we  proceed. 
It  seemed  important  at  this  stage  of  the  investigation  to  make 
use  of  a clay  as  pure  as  possible,  and  which  had  neither  been  ex- 
posed to  the  influence  of  the  air  nor  of  vegetation.  A clay  also 
free  from  oxide  of  iron  was  to  be  desired.  For  this  purpose  a 
quantity  of  white  clay,  of  the  plastic  clay  formation,  was  obtained. 
It  was  dug  from  a pit  at  about  20  feet  below  the  surface,  and 
contained  no  vegetable  matter.*  It  is  much  used  for  making 
pottery. 
Experiment  29. — White  clay  powdered,  1500  grains,  white  sand  an  equal 
quantity ; the  mixture  formed  a column  of  14  inches,  through  which  a 
strong  solution  of  ammonia  was  passed.  The  first  portions  were  free  from 
ammonia  or  its  salts,  but  it  soon  began  to  percolate. 
The  solution  of  ammonia  employed  in  this  case  was  too  strong. 
(It  was  one  of  the  early  experiments.)  The  next  are  more  satis- 
factory. 
For  analysis  of  this  clay  see  p.  320. 
