280         CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,   L906,   PART    T. 
which  seems  1o  be  anything  but  uniform  >  and  its  effecl  upon  various  coloring  nialtei 
should  receive  attention  and  would  doubtless  yield  interesting  and  valuable  result 
to  investigation.  If  any  work  has  been  done  along  this  Line,  no  results  have  bee 
published,  as  the  journals  make  little  or  uo  mention  even  of  the  fact  that  fuller 
earth  possesses  such  properties. 
COLLOIDS    AND     PKCTOIDS. 
A  colloid  is  defined  "  as  referring  generally- 
to  those  bodies  which  pass  into  solul  ion  in  water,  but  which  will  not  while  in  solutio 
pass  through  a  parchment  diaphragm,  as  substance-  in  true  molecular  solution  alwaji 
will  do.  The  word  hydrogel  given  by  Graham  is  used  to  refer  to  the  jellied  colloili 
as  they  originally  coagulate  from  the  hydrosols.  *    Ostwald  says:  "Colloids 
substances  exist  in  two  modifical  ions     i  ho  soluble  and  the  coagulated  or  '  poetini/.od/ 
We  ueed  another  word  to  express  this  latter  condition,  and  the  word  pectoid  is  true 
to  the  Creek  rool  {itrfKroi)  than  pectinized. 
A  little  later  on  1  shall  endeavor  to  show  thai  the  peculiar  prop 
erties  of  fuller's  earth  arc  probably  due  to  the  presence1  of  a  group  c 
aluminum  hydrosilicates  existing  in  the  form  of  pectoids,  as  ahoy 
denned,  and  with  this  object  in  view  I  will  now  give  a  short  diges' 
of  the  present  knowledge1  of  colloidal  substances  us  far  as  1  have  beqt 
able  to  find  it  in  the  literature  at  my  command. 
Colloids  and  also  pectoids  exist  among  both  organic  and  inorgani 
substance's,  ferric  and  aluminum  hydroxides  being  familiar  exaiB 
pies  of  the  latter.  With  regard  to  the  existence  of  these  substancii 
in  the  mineral  world,  the  Following  reierences  have  been  found: 
Ladd,6  quoting  Le  Chatelier,  says: 
It  may  be  said  to  be  well  established  that  there  are  various  hydrosilicates  of  alii 
minimi  which  differ  in  proportion,  form,  ami  structure,  some  being  colloidal  amo 
phi  ius,  and  si  ime  crystalline. 
Comeyc  speaks  of  halloysite  as  a  colloidal  clay. 
Riesd  found  colloidal  particles  in  some  clays,  and  advanced  tfc 
theory  that   plasticity  was  largely  due  to  them. 
Grimsley  '  speaks  of  the  colloidal  theory  as  explaining  1  he  plasticitlfl 
of  clays,  and  notes  that  mosl  clays  contain  from  I  to  5  per  cent  < 
grains  which  will  take  stain  from  methylene  blue,  gentian  viola 
eosin,  or  fuchsine.  lie  found  that  the  addition  of  0.08  per  cent  ( 
agar-agar  increased  the  plasticity  of  clay  very  markedly,  also  that  tr 
addition  of  precipitated   alumina  if  used  without  filtering  or  dryifl 
oCushman,  \.  s..  Effect  <>f  water  on  rock  powders:  Bull.  Bur.  Chemistry,  U.S.  Dept.  \.gr.,  No.) 
1905,  p.  12. 
b  Ladd,  <■.  l...  Preliminary  report  on  a  pari  of  the  clays  of  Georgia :  I'. nil.  Georgia  Geol.  Sun  ey  N 
6 A,  1898,  p.  15. 
cComey,  A.M.,  Dictionary  of  Solubilities,  1896,  p. 361. 
d  Ries,  H.,  Clays  and  clay  industries  of  New  Jersey:  Final  Repts.  New  Jersej  Ceo!.  Survey,  vol. 
1904,  p.  83. 
e  Grimsley,  G.  P.,  Clays  of  \\  est  Virginia:  West  Virginia  Geol.  Survey,  1906,  pp.37,  16. 
