283 
2.  These  silicates  differ  in  chemical  composition. 
3.  They  are,  however,  similar  in  that  they  all  possess  an  amorphous 
colloidal  structure. 
4.  The  colloidal  structure  is  of  a  rather  persistent  form  and  is  not 
lost  on  drying  at  a  temperature  of  130°  C,  or  possibly  higher. 
5.  These  colloidal  silicates  possess  the  power  of  absorbing  and  retain- 
ing organic  coloring  matter,  thus  bleaching  oils  and  fats. 
I  have  used  the  word  colloidal  in  this  statement  in  its  broadest 
sense — to  cover  the  whole  range  of  conditions  expressed  by  the  words 
colloid,  pectoid,  and  hydrogel.  It  is  my  opinion  that  the  word  pec- 
toid  would  most  properly  express  the  condition  of  the  active  con- 
stituents of  fuller's  earth,  but  it  is  not  impossible  that  these  may  go 
into  partial  solution  in  oil  and  thus  become  true  colloids. 
It  will  be  seen  by  a  study  of  the  table  of  oil  tests  (p.  274)  that  the 
effect  of  acids  on  the  bleaching  efficiency  of  fuller's  earth  is  peculiar, 
some  earths  being  much  more  affected  than  others.  The  Eimer  & 
Amend  earth  is  evidently  the  least  affected,  and  the  Owl  the  most. 
I  find  it  difficult  to  explain  this  except  on  the  assumption  that  these 
earths  contain  several  different  minerals  varying  in  amount  and  in 
ease  of  decomposition  by  acids,  the  bleaching  power  also  being 
dependent  on  these  minerals.  I  have  noted  on  page  277  that  the  ease 
of  decomposition  by  acids  is  for  this  series  of  minerals  (the  hydrous 
aluminum  silicates)  probably  a  function  of  the  percentage  of  alumina 
and  water,  and  we  should  expect  to  find  this  approximately  true  of 
the  three  earths  tested  in  this  investigation.  An  inspection  of  the 
ultimate  analyses  shows  this  to  be  so  in  a  general  way,  and  the 
parallel  is  much  more  apparent  when  we  consider  that  the  Owl  earth 
contains  about  15  per  cent  of  lime  carbonate  as  an  impurity.  The 
analyses  of  the  residues  after  treatment  with  the  acids  and  alkalies 
also  show  that  the  constituent  silicates  of  the  earth  vary  in  resist- 
ance to  acids.  The  improvement  in  efficiency  of  certain  of  the 
earths  by  treatment  with  dilute  acids  I  attribute  to  the  removal  of 
limonite,  calcite,  and  possibly  some  hydrous  alumina,  which  may 
exert  a  clogging  effect  on  the  active  minerals.  This  treatment  may 
also  increase  the  amount  or  the  efficiency  of  the  colloidal  substance, 
as  Cushman  points  out a  that  certain  salts  and  other  substances 
possess  the  power  of  coagulating  clay  substance  and  increasing  its 
binding  power. 
The  wide  variation  in  ultimate  analysis  which  is  exhibited  in  Table 
3  (p.  272)  and  which  persists  even  after  recalculation  eliminating  impu- 
rities, is  readily  explained  by  this  assumption  of  a  series  of  similar 
minerals.  Variation  in  bleaching  power  of  various  earths  is  also 
thus  explained,  and  if  we  can  assume  that  these  minerals  have  vary- 
a  Cushman,  A.  S..  Effect  of  water  on  rock  powders.  Bull.  Bur.  Chemistry,  U.  S.  Dept.  Agr.,  No.  92, 
1905,  pp.  19,  22. 
