360  On  the  Power  of  Soils  to  absorb  Manure. 
wliich  it  may  be  mixed.  When  boiled  with  potash  the  silica  is 
dissolved  and  forms  a soluble  silicate  of  the  alkali.  In  the  ex- 
periment which  has  just  been  described  we  find  clay,  which  is 
usually  thought  to  contain  silica  in  a free  state,  not  only  refusing 
to  give  up  that  silica  to  potash,  but  actually  combining  with  and 
removing  from  solution  the  alkali.  It  should  be  understood, 
however,  that  this  result  is  affected  by  the  relative  proportions  of 
the  two  substances,  since  a stronger  solution  of  potash  will,  even 
in  the  case  of  clay,  dissolve  out  silica. 
The  experiment  is  of  interest,  however,  in  showing  how  power- 
ful is  the  tendency  of  soils  to  form  new  compounds  where  the 
opportunity  of  gratifying  that  tendency  may  be  offered  them. 
It  will  be  remembered  that  in  the  account  of  the  experiments 
with  ammonia,  clay  that  had  been  boiled  with  hydrochloric  acid 
was  found  to  absorb  nearly  as  much  ammonia  as  that  which  had 
not  been  so  treated  (see  Experiment  79).  The  following  is  a result 
obtained  by  digesting  the  acid-treated  clay  with  caustic  potash. 
Experiment  85. — White  Pottery  clay  boiled  in  Hydrochloric  acid,  &c., 
(see  page  354)  and  Caustic  Potash  of  1 per  cent. 
Acid  treated  Clay  . . 2000  grains  1 Boiled  together,  as  in  the  last  Experiment, 
Solution  of  Caustic  Potash  4000  , , j for  half  an  hour. 
1st  Analysis  of  the  resulting  Liquid — 
Grains.  Platinum  Salt.  Potash.  Per  Cent. 
200*05  gave  4*10  = *7914  or  -3938 
2nd  Analysis  — 
233-30  gave  4-78  = *5227  or  -3954 
1st  Analysis  ...  -3938  per  Cent. 
2nd  ,,  ...  -3954  , , 
Mean  . . . -3946  ,, 
100  grains  of  the  original  Solution  contained  . 1 -0023  grains  of  Potash. 
100  grains  of  the  resulting  Solution  contained  -3946  ,,  , 
Loss  by  each  100  grains  of  Solution  . . -6077  ,, 
The  whole  quantity  (4000  grains)  would  therefore  have  lost  24-308 
grains  of  potash,  which  has  been  absorbed  by  2000  grains  of  the  clay, 
making  the  absorption  1-2154  per  cent. 
The  absorption  by  the  acid  treated  is  therefore  slightly  more 
than  by  the  natural  clay,  so  slightly,  however,  as  to  justify  us  in 
believing  the  amount  the  same  in  both  cases.  This  agrees  with 
the  result  obtained  in  the  corresponding  experiment  with  am- 
monia, and  sufficiently  proves  that  the  absorptive  power  is  not 
due  to  any  substances,  such  as  free  alumina,  that  are  soluble  in 
boiling  mineral  acids. 
The  experiment  which  follows  exhibits  a greater  absorptive 
power  for  potash  in  a clay  after  being  boiled  with  caustic 
potash : — 
