PROPERTIES  AND  TESTS  OF  FULLER 's  EARTH.        285 
lies  prevent  flocculation  of  colloids  and  that  acids  and  neutral  salts 
promote  it.  This  explains  the  nonsettlement  of  the  earth.  As  for 
the  gelatinization,  it  is  evident  that  the  strong  alkali  has  the  power  of 
partially  decomposing  the  earth  and  of  converting  the  compact, 
Vesistant  pectoids  into  a  softer  form  that  requires  more  water  and  is 
perhaps  between  a  pectoid  and  a  soluble  colloid  and  similar  to  pre- 
cipitated alumina. 
Colloids  are  known  to  possess  the  power  of  taking  up  organic  colors 
from  solution  (see  p.  282),  and  the  analogy  with  fuller's  earth  is  so 
apparent  as  to  excite  surprise  that  it  has  not  been  studied  from  this 
point  of  view  before.  It  has  even  been  known  that  pectoids  could 
extract  colors  from  oils  and  resins  as  well  as  aqueous  solutions. 
This  I  regard  as  a  strong  point  in  favor  of  the  colloidal  theory, 
although  by  itself  it  does  not,  of  course,  constitute  a  proof. 
The  power  of  colloids  to  absorb  certain  salts,  or  at  least  the  basic 
ions  of  these  salts,  has  been  known  a  long  time.  It  is  also  possessed 
to  a  degree  by  certain  amorphous  substances  having  a  fine  porous 
structure,  such  as  charcoal  and  boneblack.  Fuller's  earth  has  this 
property  to  a  marked  degree.  In  fact,  its  use  has  been  proposed  to 
remove  the  lime  from  boiler  water. a  I  have  found  that  after  it  has 
absorbed  ammonia  or  salts  it  loses  a  great  part  of  its  efficiency  in 
bleaching  oils.  From  this  I  infer  that  the  bases  are  absorbed  in  a 
similar  manner  to  the  coloring  matter  of  oil,  and  occupy  the  pores 
which  otherwise  would  hold  the  color. 
It  has  been  pointed  out  that  these  absorbent  materials  have  a 
selective  action  on  the  salts,  absorbing  the  unlike  ions  and  discarding 
the  ions  of  ]jke  chemical  nature.  If  colors  in  oil  solution  are  absorbed 
in  the  same  manner  as  salts  in  aqueous  solution,  analogy  would  lead 
to  the  supposition  that  fuller's  earth  would  exert  a  selective  action 
for  certain  classes  of  coloring  matter  and,  moreover,  that  the  earths 
themselves  would  differ  in  their  selection  of  colors  according  as  they 
are  more  or  less  acidic  in  composition.  We  find  this  entirely  in 
accordance  with  the  facts. 
I  have  noticed  in  the  course  of  my  oil  tests  that  on  the  same  sample 
of  oil  different  materials  give  products  varying  greatly  in  shade,  the 
main  color  being  in  some  tests  of  a  yellow  and  in  others  of  a  green  tint. 
Is  it  not  likely  that  the  oil  contains  several  compounds  of  varying 
chemical  nature,  and  that  the  earths  or  other  materials  used  in  bleach- 
ing extract  them  in  ratios  proportional  to  their  own  basicity  or  acid- 
ity? I  have  been  unable  to  obtain  any  exact  data  on  this  point  from 
my  results,  owing  to  the  difficulty  of  following  these  slight  changes  in 
tint  with  the  unaided  eye.  A  tintometer  would  be  needed  if  this 
line  of  investigation  were  to  be  followed  out. 
a  Mineral  Industry,  vol.  8,  1900,  p.  219. 
Bull.  315—07 19 
