278  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1906,  PART    I. 
Uics"  states  that  fuller's  earth  probably  contains  much    hydroi 
silica,  the  inference  being  that  it  is  this  constituent  which  gives  it  it 
distinctive  qualities.     A  somewhat  similar  inference  is  perhaps  to  bj 
drawn   from   the  following  quotation,  referring  to   the    Florida  am 
Georgia  earths:  b 
This  leads  to  the  hypothesis  that  this  earth  probably  originated  in  ordinary  clay 
which  lias  received  from  interfiltering  solutions  an  additional  supply  of  silicic  acid, 
which  sometimes  combined  with  the  clay  and  occasionally  was  deposited  as  chert 
My  analyses  have  confirmed  Ries's  statement  that  fuller's  earth 
contains  appreciable  amounts  of  hydrous  silica.  (See  Table  3,  p.  272.1 
However,  it  is  evident  that  if  the  bleaching  power  of  fuller's  earth  is 
due  to  this  hydrous  silica,  treatment  of  the  earth  with  boiling  carbon] 
ate  or  hydrate  of  soda,  which  removes  the  silica,  should  destroy  this 
bleaching  power.  T  have  tried  this  experiment  i>\\  several  earths,  and 
the  result  will  be  found  in  Table  5  (p.  275).  It  will  be  seen  at  once  thai 
although  the  results  are  not  entirely  concordant,  yet  there  can  be  no 
doubt  that  fuller's  earth  retains  at  least  a  part  of  its  efficiency  after 
treatment  by  alkalies.  It  is  also  plain  that  carbonate  of  soda  has  a 
much  less  harmful  action  on  the  earth  than  the  hydrate.  As  Frej 
scnius  and  other  authorities  state  that  hydrous  and  amorphous  silica 
are  freely  soluble  in  hot  carbonate  of  soda,  as  well  as  in  sodium 
hydrate,  this  difference  can  hardly  he  due  to  nonsolution  of  the  silica  j 
by  the  carbonate,  but  is  almost  entirely  due  to  partial  decomposition 
of  the  earth  by  sodium  hydrate.  In  this  connection,  I  have  noticed 
that  sodium  hydrate  appeal's  to  extract  considerable  alumina  as  well 
as  silica,  but  the  carbonate  does  not.  Fuller's  earth  after  treatment 
with  sodium  hydrate  is  left  in  a  very  gelatinous  condition  and  is 
extremely  hard  to  filter  and  wash.  For  this  reason  it  is  possible  t  hat- 
sonic  of  the  samples  treated  retained  considerable  amounts  of  soluble 
salts  after  washing,  although  great  pains  were  taken  to  remove  themj 
and  tins  may  have  had  some  influence  on  the  results. 
Another  fact  which  might  be  used  as  an  argument  against  this 
theory  is  that  hydrous  silica  artificially  prepared  has  but  xovy  slighfj 
bleaching  powers.  (See  Table  5.)  These  results  are  naturally  not 
conclusive,  as  the  silicic  acid  occurring  naturally  may  differ  either 
physically  or  chemically,  or  both,  from  the  artificial  product. 
On  the  other  hand,  some  earths  still  retain  a  considerable  portion 
of  their  bleaching  power  after  decomposition  by  acids,  consisting 
then  of  80  per  cent  or  more  of  silica;  and  from  this  it  seems  probable 
that  hydrous  silica  when  prepared  in  certain  ways  may  have  some 
small  efficiency. 
In  conclusion,  I  think  I  am  justified  in  stating,  first,  that  hydrous 
silica  does  not  of  itself  possess  bleaching  power,  although  it  may  at 
"  Hies.  If.,  Fuller's  earth  of  South  Dakota:  Trans.  Am.  Inst.  Min.  Eng.,  vol.  27,  1898,  p.  333. 
'j  Twenty-first  Ann.  Rept.  U.  S.  Geol.  Survey,  pt.  6,  continued,  1901,  p.  591. 
