PROPERTIES    AND    TESTS    OF    FULLER 's    EARTH. 
277 
To  return  now  to  Dana's  theory  of  fuller's  earth,  it  is  evident  that 
it  will  not  stand  sinee  the  discovery  of  American  deposits  havin°-  a 
comparatively  low  percentage  of  combined  water.  Such  earths  could 
not  possibly  have  as  their  base  either  smectite  or  malthacite,  although 
they  might  contain  very  small  quantities  of  these  minerals.  For 
example,  take  analysis  18  of  Table  2  (p.  271),  representing  a  sample  of 
earth  from  Fairburn,  S.  Dak.  Calculating  the  water  in  this  entirely  to 
smectite  gives  28.1  per  cent  smectite,  requiring  16  per  cent  silica  and 
3.8  per  cent  alumina,  and  leaving  a  large  balance  of  silica  and  alumina, 
as  well  as  impurities,  to  be  calculated  to  anhydrous  minerals. 
Again,  my  analyses  of  Fairbank  and  Owl  fuller's  earths  (Table  3, 
p.  272)  show  that  it  is  utterly  impossible  for  these  earths  to  contain 
even  moderate  amounts  of  smectite  or  malthacite,  in  view  of  the  small 
quantities  of  quartz  and  undecomposible  anlrydrous  silicates  shown. 
This  point  is  very  clearly  brought  out  when  the  calculation  of  a 
rational  composition  is  attempted.  Another  point  which  argues 
against  the  presence  of  smectite  and  malthacite  in  American  fuller's 
earths  is  the  fact  that  these  earths  retain  much  and  in  some  cases  all 
of  their  efficiency  after  treatment  with  strong  acids.  Although  I  have 
not  been  able  to  find  any  statement  that  smectite  and  malthacite  are 
decomposed  by  sulphuric  acid,  still  it  is  very  probable  that  they  are 
so  attacked,  owing  to  their  large  percentage  of  combined  water. 
It  is  interesting  to  note  in  passing  that  an  inspection  of  the  table  of 
minerals  shows  that  where  determined  the  ease  of  decomposition  by 
acids  of  the  hydrous  aluminum  silicates  is  probably  a  direct  function 
of  the  percentage  of  alumina  and  combined  water,  the  water  probably 
having  the  greater  effect. 
Although  it  may  be  regarded  as  settled  that  smectite  and  maltha- 
cite are  not  contained  to  any  extent  in  most  American  fuller's  earths, 
and  hence  are  not  essential  to  the  bleaching  power,  still  it  is  probably 
true  that  smectite  forms  the  basis  of  many  foreign  earths.  It  is  of 
course  impossible  to  calculate  rational  composition  with  any  certainty 
from  ultimate  analysis  alone.  However,  the  following  composition 
has  been  calculated  from  analysis  24  of  Table  2  (p.  271),  principally 
to  show  the  possibility  that  this  earth  contains  smectite  as  a  base : 
Rational  analysis  of  fuller's  earth  from  Reigate,  England. 
Total. 
Si02. 
A120,. 
H20. 
Fe203. 
Alkalies, 
CaO  and 
MgO. 
Smectite 
70 
12.6 
1.6 
6.5 
8.9 
.4 
40 
9.6 
20.4 
2.9 
.3 
9.7 
.9 
3.2 
8.9 
.4 
3.3 
.4 
