286  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,  PART    I. 
I  believe  that  the  reason  for  using  heal  in  bleaching  oils  is  that  the 
lessened  viscosity  of  the  oil  when  heated  allows  the  molecules  of  color 
to  come  into  better  contact  with  the  particles  of  earth.  Possibly, 
however,  the  beneficial  result  is  partially  due  to  the  expulsion  of 
water  or  air  from  the  submicroscopic  pores  of  the  pectoid  surface,  thus 
making  room  for  the  coloring  substance. 
The  table  of  oil  tests  (p.  275)  shows  that  pipe  clay,  which  is  nearly 
pure  kaolin,  has  some  slight  bleaching  power.  This  is  in  line  with 
the  colloidal  theory,  taken  in  connection  with  Cushman's  experiments 
on  kaolin,  in  which  he  found  that  the  kaolin  grains  were  coated  with 
a  thin  layer  of  pectoidal  substance.  This  coating  should  be  sufficient 
to  give  the  kaolin  the  slight  bleaching  power  it  was  found  to  have. 
Tests  Nos.  31  and  32  show  that  the  kaolin  is  improved  in  bleaching 
efficiency  by  drying  and  also  by  decomposition  with  sulphuric  acid. 
According  to  the  colloidal  theory,  the  improvement  on  drying  is  pos- 
sibly due  to  the  expulsion  of  the  excess  of  water  from  the  thin  pectoid 
film  on  the  surface  of  the  crystals.  The  treatment  by  sulphuric  acid, 
however,  entirely  decomposes  the  kaolin  and  leaves  an  amorphous 
silica,  the  excess  of  water  being  removed  by  the  dehydrating  action 
of  the  acid.  It  will  be  noted  that  the  silica  from  the  pipe  clay  does 
not  vary  much  in  efficiency  from  those  samples  prepared  from  fuller's 
earth  in  the  same  manner,  especially  when  the  latter  have  been  thor- 
oughly decomposed.  It  Is  also  of  interest  to  note  that  the  silica  from 
kaolin  giving  a  color  value  of  L6  does  not  differ  very  much  from  that 
prepared  by  precipital  ion  and  washing  with  alcohol  and  ether,  value  1  s. 
The  limestone  clay,  which  1  found  had  a  very  considerable  bleaching 
power,  was  probably  in  a  more  pectoidal  state  than  the  kaolin,  and  it 
is  evidently  to  be  expected  that  all  gradations  of  bleaching  power  will 
be  found  in  the  various  clays. 
This  same  idea  will  apply  to  the  line  mineral  powders  which  Wheeler 
discovered  to  have  a  slight  bleaching  action.  1  do  not  know  whether 
or  not  his  pi  >wders  w  ere  wet  ground.  If  so,  pectoidal  substances  were 
certainly  contained  in  them.  If  ground  (\v\\  their  efficiency  is  prob- 
ably due  to  the  increased  surface  action  of  amorphous  particles. 
At  this  point  it  seems  necessary  to  examine,  into  the  difference,  if 
any,  in  the  mode  of  action  of  colloids  and  amorphous  substances.  It 
has  been  pretty  well  proved  that  the  decolorizing  power  of  charcoal 
and  bone  char  (amorphous  substances)  is  due  to  their  exceedingly 
small  u submicroscopic  pores."  These  pores  seem  to  be  exceptional ly 
well  developed  by  certain  forms  of  carbonaceous  matter,  but  probably 
exist  to  a  greater  or  less  extent  in  all  forms  of  finely  divided  amor- 
phous matter.  This,  I  take  it,  explains  the  small  but  uniform  effi- 
ciency of  such  materials  as  precipitated  and  dried  silica  and  alumina, 
and,  as  before  noted,  may  also  explain  the  results  which  Wheeler 
obtained  with  very  finely  pulverized  materials.     It  is  evident  that  t  he 
