CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
Cushman0  says: 
Linder  and  Picton  have  shown  that  when  a  coagulated  colloidal  precipitate  is  form* 
in  the  presence  of  an  electrolyte,  a  portion  of  the  basic  ions  of  the  electrolyte  is  at 
sorbed  by  or  adsorbed  on  the  pectoid  matter  in  the  form  of  hydroxide,  while  the  aci< 
ions  remain  free  in  solution.  Many  other  investigators  have  cheeked  this  aslonishin 
and  interesting  fact.  *  *  *  This  absorptive  power  is  in  no  sense  of  the  word  duet  j 
chemical  combination,  but  seems  to  depend  upon  the  fine  subniicroscopic  porosity  c  j 
the  pectoid. 
It  can  be  seen  that  as  a  result  of  this  power  of  pectoids,  it  is  in  e;en 
eral  impossible  to  wash  a  preparation  of  this  character  entirely  free 
from  the  salts  with  which  it  is  formed.     Linebarger  b  found  it  impossi 
ble  to  remove  all  the  chlorine  from  colloidal  ferric  hydrate  prepared 
from  ferric  chloride  by  dialysis. 
Colloid^  and  pectoids  are  destroyed  by  heat,  in  some  cases  prob 
ably  by  drying.  Data  on  these  points  seem  to  be  sadly  lacking,  anc 
there  is  also  lacking  a  closely  drawn  distinction  between  pectoid: 
and  those  bodies  which  are  amorphous  without  being  colloidal  h 
their  nature.  However,  as  a  certain  amount  of  water  is  essential  I 
the  colloidal  state,  heating  to  a  point  at  which  the  combined  wate: 
is  driven  off  must  in  all  cases  destroy  this  state.  Further,  it  is  prob- 
ably true  that  a  substance  is  colloidal  as  long  as  it  retains  its  power 
of  jellying  with  water  either  throughout  or  on  the  surface  only 
Thai  some  pectoids,  at  least,  retain  their  characteristic  properties 
after  drying  is  shown  by  Cushman's  experiments,  but  it  is  probable 
that  the  colloidal  nat  ure  of  alumina  and  silicic  acid  is  largely  destrovec 
by  drying  and  is  not  capable  of  restoration  by  subsequent  digestior 
with  water.  Whether  an  oil  could  take  the  place  of  water  in  pro- 
moling  tiie  colloidal  stale  in  mineral  substances  is  an  open  question 
hut  it  is  certain  that  organic  colloids  exist  in  oils,  and  it  does  nol 
seem  impossible  that  some  mineral  colloids  may  also. 
done-'  states  that  colloids  diffuse  very  slowly  as  compared  wit! 
crystalloids.  As  diffusion  is  due  to  osmotic  pressure,  this  mean* 
that  the  osmotic  pressure  of  substances  in  colloidal  solution  is  vem 
low.  Whal  hearing  this  interesting  fact  may  have  on  the  absorptfvl 
powers  of  colloids.  I  can  not  at  present  state,  but  it  seems  likely  thai 
they  may  he  in  some  way  connected. 
A    \KW    THEORY. 
The  theory  which  I  have  devised  to  explain  the  peculiar  properties 
of  fuller's  earth  may  be  Stated  as  follows: 
1.   Fullers  earth  has  for  its  base  a  series  of  hydrous  aluminum 
silicates. 
a  Cushman,  A.  S.,  Effect  of  water  on  rock  powders:  Bull.  Bur.  Chemistry,  U.S.  Dept.  Agr.,  No.  02. 
1905,  p.  15. 
b  Linebarger,  C  E.,  on  the  rate  of  diffusion  of  colloids:  Jour.  Am.  Chem.  Soc,  vol.  20,  1898,  p.  !7ii. 
c  Cushman,  A.  S..  Effect  of  water  on  rock  powders:  Bull.  Bur.  Chemistry,  U.  S.  Dept.  Agr.,  No.  02, 
1905,  p.  251. 
