PROPERTIES    AND    TESTS    OF    FULLER 's    EARTH.  281 
Id  the  same  effect,  but  if  air  dried,  no  such  increase  was  noted.  He 
Imarks  that  a  colloid  which  will  soften  in  water  after  drying  is  needed. 
I  Wiley a  speaks  of  a  colloidal  constituent  of  the  clay,  designating  by 
lat  term  that  portion  of  the  clay  which  remains  suspended  almost 
^definitely  in  pure  water.  He  states  that  it  is  precipitated  by  brine, 
Irming  a  gelatinous  mass  like  the  mixed  hydrates  of  iron  and  alumina. 
ICushman6  finds  that  "the  action  of  water  on  many  rock  powders 
Isults  in  the  formation  of  colloid  films  on  the  surface  of  the  particles," 
md  expresses  the  opinion  that  the  binding  power  of  rock  dust,  clays, 
lid  soils  is  due  to  the  formation  and  presence  of  these  films  on  the  sur- 
Ice  of  crystalline  or  amorphous  granules. 
I  It  will  be  noted  from  these  extracts  that  the  word  colloid  is  very 
losely  employed  by  various  writers,  many  of  them  using  it  where 
lydrogel  or  pectoid  would  be  more  correct,  while  some  seem  to  con- 
Ider  it  as  synonymous  with  amorphous. 
I  Colloids,  or  more  properly  pectoids,  possess  the  property  of  being 
fcadily  stained  by  most  organic  coloring  matters  and  of  holding  this 
plor  in  an  insoluble  form.  A  familiar  example  is  the  use  of  alumina 
Is  a  mordant  in  cotton  printing,  and  Cushmanc  states  that  orthoclase 
lowder  which  has  been  wet  ground  to  develop  the  pectoidal  films  takes 
Ip  a  brown  stain  from  the  Canada  balsam  used  in  mounting  it  for 
hicroscopic  examination.  On  the  other  hand,  it  is  well  known  that 
lertain  amorphous  substances,  not  pectoids,  also  possess  this  prop- 
rty — for  example,  charcoal  and  boneblack.  Grimsleyd  states  that 
[fresh  gelatinized  silicic  acid  will  easily  stain;  air-dried  silicic-acid 
jelly  will  take  a  stain  with  equal  readiness."  From  his  further  remark 
[hat  the  addition  of  the  air-dried  jelly  injures  the  plasticity  of  clay,  I 
pfer  that  this  silicic  acid  had  passed  from  the  pectoid  to  the  amor- 
phous state.  Samples  of  precipitated  silica  which  I  prepared  acted  in 
his  way  when  dried  in  air  or  by  washing  with  alcohol  and  ether. 
Soils  and  clays  have  long  been  known  to  possess  the  property  of 
absorbing  certain  materials,  both  gaseous  and  liquid.  Wiley e  quotes 
nany  experimenters  on  this  subject  and  from  his  work  the  following 
lotes  are  taken.  Clay  exhibits  a  selective  absorption  of  certain 
salts,  holding  the  basic  ions  and  rejecting  the  acidic  ions.  Way 
Succeeded  in  producing  an  artificial  hydrated  silicate  possessing 
ibsorptive  powers.  Eichhorn  found  that  natural  hydrated  silicates  or 
zeolites  acted  in  a  similar  manner,  and  others  have  shown  that  the 
ibsorptive  power  bears  a  close  relation  to  the  amount  of  soluble  sili- 
cates present. 
a  Wiley,  H.  W.,  Agricultural  Analysis,  vol.  1,  Soils,  1894.  p.  231. 
b  Cushman,  A.  S.,  Effect  of  water  on  rock  powders:  Bull.  Bur.  Chemistry,  U.  S.  Dept.  Agr.,  No.  92 
905,  p.  12. 
c  Op.  cit.,  p.  13. 
<i  Grimslcy,  G.  P.,  Clays  of  West  Virginia:  West  Virginia  Geol.  Survey,  1906,  p.  49. 
e  Wiley,  H.  W.,  Agricultural  Analysis,  vol.  1,  Soils,  1894,  p.  118. 
