365 
On  the  Power  of  Soils  to  absorb  Manure. 
acting  upon  putrid  human  urine,  upon  the  stinking  water  in  which 
flax  had  been  steeped,  and  upon  the  water  of  a London  sewer. 
That  the  power  of  the  soil  in  all  these  cases  is  due  to  the  clay 
contained  in  it  there  is  not  the  slightest  doubt ; many  similar 
trials  were  made  with  sand,  but  although  the  colour,  so  far  as  it 
Avas  due  to  suspended  matter,  was  in  some  degree  reduced,  the 
offensive  character  of  the  solutions  was  but  slightly  modified.  On 
the  other  hand,  the  pure  white  clay,  so  often  before  mentioned, 
proved  an  admirable  absorbent  both  of  colour  and  smell.  It  is 
worthy  of  remark,  that  by  merely  stirring  up  a portion  of  soil  or 
of  pure  clay  with  the  solution,  the  same  result  is  obtained,  though 
not  in  the  first  instance  so  perfectly.  If,  however,  the  liquid,  in 
great  measure  deprived  of  colour  and  taste,  is  poured  off,  and 
treated  with  a second  quantity  of  the  absorbing  soil,  the  effect 
is  equally  complete  as  when  a process  of  filtration  has  been 
adopted. 
Solutions  of  different  colouring  matters,  such  as  those  of  log- 
wood, sandal-wood,  cochineal,  litmus,  &c.,  when  filtered  through 
or  shaken  up  with  a portion  of  clay,  are  entirely  deprived  of 
colour.  Alumina  is  known  to  possess  an  extraordinary  power  to 
combine  with  colouring  matters,  and  is  extensively  used  in  dye- 
works  as  a mordant.  It  would,  therefore,  naturally  be  supposed 
that  the  decolorizing  power  of  clay  was  due  to  the  free  alumina; 
but  granting  even  that  free  alumina  exists  in  clay,  of  which  there 
is  no  sufficient  proof,  it  is  certain  that  other  compounds  of  alu- 
mina exist  in  the  clay  possessing  this  power.  Thus,  white  clay, 
when  boiled  with  hydrochloric  acid  (which  dissolves  out  the  alu- 
mina), and  subsequently  well  washed,  retains  apparently  in  undi- 
minished force  the  power  both  of  precipitating  the  colouring 
matters  of  logwood,  &c.,  and  also  of  decolorizing  and  deodo- 
rizing putrid  urine  and  other  offensive  animal  solutions;  and  we 
must  therefore  believe  that  there  are  compounds  of  alumina  with 
silica,  having  in  some  respects  the  same  chemical  properties  as 
alumina  itself.  In  what  form  of  combination  these  colouring 
matters  and  the  organic  substances  giving  smell  are  retained  by  the 
clay  it  is  very  difficult  to  say.  All  organic  matters  are  certainly 
not  abstracted  from  solution  to  the  same  extent.  Thus  a colour- 
less solution  of  sugar  will  pass  apparently  unaltered  through  a 
filter  of  clay  and  sand,  whereas  if  it  be  coloured  the  clay  will  act 
like  animal  charcoal,  completely  removing  the  colouring  matter. 
It  concerns  us  very  much  to  know  for  what  substances  and  to 
what  extent  this)  power  of  soils  to  unite  with  organic  matters 
is  operative.  So  far  as  the  experiments  have  at  present  been 
carried,  it  would  appear  that  there  is  a greater  power  in  the  soil 
for  retaining  the  products  of  the  decay  of  animal  matters  than  the 
animal  matters  themselves. 
Fresh  urine  is  purified — that  is  to  say,  deprived  of  animal 
