PROPERTIES    AND    TESTS    OF    FULLER 's    EARTH.  287 
slightest  trace  of  sintering  on  material  of  this  sort  would  close  up 
these  minute  pores,  and  no  doubt  this  is  the  cause  of  the  destruction 
of  the  decolorizing  power  by  ignition.  (See  tests  Nos.  47  and  51, 
Table  5,  p.  275.) 
The  mode  of  action  of  colloidal  substances  is  probably  very  similar, 
even  though  the  body  of  material  containing  the  pores  is  of  so  different 
a  nature.  As  before  noted,  many  investigators  have  found  that  pec- 
toidal  substances  possess  the  power  of  absorbing  the  basic  ions  of 
salts  into  their  "fine  submicroscopic  pores,"  and  there  is  no  reason  to 
think  that  the  ions  of  coloring  matter  are  held  in  any  different  way. 
However,  I  must  confess  that  I  do  not  feel  very  clear  in  my  mind 
as  to  the  exact  state  or  characteristics  of  what  I  have  called  the  effi- 
cient pectoids.  So  little  that  is  really  definite  is  known  on  the  subject 
that  it  is  very  difficult  to  draw  a  hard  and  fast  line  between  colloids 
and  amorphous  bodies.  Possibly  all  intermediate  states  between  the 
soluble  colloid  and  the  most  insoluble  amorphous  substance  exist.  If 
this  is  the  case,  it  will  lead  to  many  interesting  speculations.  For 
example^  it  is  possible  that  the  more  nearly  a  substance  approaches 
the  purely  colloidal  state  the  greater  affinity  it  will  have  for  coloring 
matter;  but  in  the  case  of  oil  bleaching  this  state  vvill  be  limited  by 
the  necessity  of  increased  water  in  the  softer  colloids.  At  some  inter- 
mediate point  the  maximum  efficiency  for  oils  would  be  found.  The 
difficulty  of  preparing  alumina  without  including  a  large  excess  of 
water  is  evidently  the  cause  of  my  failures  with  that  substance.  We 
know  that  alumina  has  a  strong  affinity  for  coloring  matter  in  aqueous 
solutions  and  very  probably  it  would  act  with  equal  efficiency  on  oil, 
if  it  could  be  prepared  without  the  water.  It  seems  likely  that  the 
drying  to  a  horny  mass  is  an  attribute  of  the  softer  and  more  highly 
hydrated  colloids.  Silicic-acid  jelly  when  dried  forms  such  a  mass, 
but  if  it  is  first  washed  with  alcohol  and  ether  (which  removes  the 
excess  of  water)  it  dries  to  a  white  pulverulent  powder,  still,  however, 
containing  much  combined  water.  m  It  will  be  noted  (see  test  No.  50) 
that  when  so  prepared  it  has  a  slight  efficiency.  Fuller's  earth  also 
when  treated  with  sodium  hydrate  swells,  softens,  and  apparently 
takes  up  much  water,  forming  a  softer  colloid.  On  drying  it  acquires 
the  hornlike  characteristics  of  dried  alumina  and  loses  considerable 
efficiency.  (See  also  p.  286.)  Another  possible  explanation  of  the 
action  of  dilute  acids  in  increasing  the  efficiency  of  some  earths  occurs 
to  me  here.  May  not  the  acids  decompose  and  remove  the  very  soft 
and  hydrous  colloids  and  thus  prevent  their  drying  up  and  clogging 
the  efficient  particles?  Another  query  in  the  same  line:  Might  not 
strong  drying  decrease  the  efficiency  of  some  earths  that  contain  a 
larger  proportion  of  the  softer  colloids,  and  would  not  a  considerable 
variation  in  resistance  to  heat  be  found  if  many  earths  were  tested? 
There  is  evidently  room  for  much  research  along  these  lines. 
