284  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
ing  preferences  for  different  classes  of  organic  colors,  we  have  a  pos- 
sible explanation  of  the  puzzling  fact  that  some  earths  which  are 
very  efficient  for  vegetable  oils  are  useless  on  mineral  oils,  and  vice 
versa.  As  bearing  on  the  probability  of  this  last  assumption,  Cramp- 
ton  and  Simons a  find  that  fuller's  earth  absorbs  much  more  color 
(caramel)  from  artificially  colored  spirits  than  from  the  natural, 
and  remark  that  it  appears  to  exert  a  selective  action  for  some 
coloring  matters. 
In  passing  I  wish  to  say  that  the  possibility  of  the  efficiency  of  fuller's 
earth  being  due  to  organic  colloids  has  occurred  to  me,  but  after 
due  consideration  I  decided  that  this  theory  was  untenable,  as  anj 
organic  matter  was  almost  certainly  destroyed  in  several  of  the  acid! 
treated  earths  which  gave  fairly  good  results  on  oil. 
As  the  recent  work  of  Cushman  and  others  has  practically  proved 
that  the  plasticity  and  binding  power  of  clays  and  rock  powders  is 
dependent  on  the  existence  of  pectoids,  the  nonexistence  of  plasticity 
in  fuller's  earth  would  certainly  be  a  stumbling  block  to  my  theory. 
However,  as  I  have  shown  on  page  '-'To,  the  earths  on  which  I  have 
worked  possess  a  most  decided  plasticity,  and  an  investigation  is 
needed  to  determine  the  correctness  of  the  numerous  statements  to 
the  contrary.  The  property  which  I  noted,  of  taking  up  very  large 
quantities  of  water  before  reaching  the  plastic  state,  is  certainly  an 
at  t  ribute  of  colloids,  and  the  extreme  greasy  or  soapiness  of  feel  is  not 
less  so. 
The  action  of  high  temperature  in  destroying  the  efficiency  of 
fuller's  earth  (see  oil  test  No.  6,  Table  5,  p.  274  ), although  itdoes  not  of 
itself  prove  the  presence  of  colloids,  at  least  agrees  well  with  that 
theory,  as  it  is*  well  known  that  the  colloidal  structure  is  destroyed  by 
heat,  whereas  an  amorphous  structure  is  unaffected. 
Fuller-  earth  in  suspension  in  water,  in  common  with  other  clays, 
is  ivadiix  coagulated  and  settled  by  traces  of  acid  or  larger  quantities 
of  salts.  Wiley6  calls  this  line  suspended  matter  colloidal  clay,  and 
it  probably  does  consist  of  kaolin  particles  which  are  more  or  less 
pectinized  on  the  surface.  I  have  noticed  m  washing  fuller's  earth 
after  it  has  been  treated  with  sodium  hydrate  that  it  settles  readily 
during  the  firsl  decantations ;  then,  as  the  amount  of  sodium  hydrate 
becomes  less,  a  part  refuses  to  settle,  and  that  which  does  settle 
becomes  more  gelatinous  and  swells  until  it  occupies  a  volume  several 
times  as  large  as  the  original.  On  adding  a  few  drops  of  acid  to 
neutralize  the  sodium  hydrate  the  earth  settles  readily.  On  drying, 
however,  the  earth  forms  a  hard,  hornlike  mass  similar  to  precipitated 
alumina.     I  explain  these  facts  as  follows:  It  is  well  known  that  alka- 
aCrampton,C  A.,  and  Simons,  F.  D.,  Detection' of  caramel  in  spirits  and  vinegar.  Jour.  Am.  Cliem. 
Soc,  vol.  21.  1SP9,  p.  356. 
&  Wiley,  II.  W'.,  Agricultural  Analysis,  vol.  I,  Soils,  p.  231. 
