SILICEOUS  EARTH.
153
of  this  division  ;  and  the  nature  of  the  process  is  such,  that  the  earth
is  obtained  in  the  form  of  a  precipitate,  which  is  an  exceedingly  tender, ¬
  light,  and  spongy  earth.
The  properties  of  it  are  these  :
1mo,  It  is  not  diffolved  or  otherwise  affected  by  any  acid,  except
the  fluoric,  which,  when  applied  warm  to  it,  and  especially  in  the
form  of  vapour,  not  only  difsolves  this  earth,  but  volatilizes  it.
2do,  It  is  not  fusible  in  our  most  violent  fires.
3tio,  When  mixed  in  powder  with  half  its  weight  of  potaſh  or
foda,  or  the  carbonats  of  thoſe  alkalis,  the  mixture  can  be  melted  easily
in  a  strong  heat,  and  forms  a  perfect  glass.  It  is  even  in  some  meafure ¬
  diffolved  or  combined  with  potaſh,  by  boiling  in  a  strong  watery
solution  of  that  salt.  This  experiment  will  not  succeed  with  any  siliceous ¬
  substance  reduced  mechanically  to  a  powder,  however  subtile.
The  utmoſt  effects  of  mechanical  division  and  trituration  fall  infinitely
ſhort  of  the  ſubtile  division  and  attenuation  which  we  obtain  on  many
occasions  by  chemical  folution  and  precipitation.  And  this  earth,  in
order  to  make  it  combine  with  an  alkali  that  is  difsolved  in  water
must  either  be  in  this  state  of  the  most  subtile  and  tender  powder,  or
it  muſt  be  previously  combined  with  a  ſmall  proportion  of  alkali  in
the  dry  way,  or  by  fuſion.
This  experiment  and  others  enabled  me  to  understand  the  nature
of  an  earth  which  occurs  in  some  parts  of  Scotland,  in  the  form  of  a
sediment  or  mud,  at  the  bottom  of  lakes  of  fresh  water.  There  is  a
lake  in  Galloway  in  which  it  is  found.  Samples  of  it  were  brought
to  me  a  long  time  fince,  to  know  if  it  was  a  marle  :  I  quickly  perceived ¬
  that  it  was  not  a  marle  ;  but  the  properties  of  it  were  such,
that  for  a  considerable  time  I  was  at  a  loſs  to  give  it  a  name.  At  laſt
I  found  that  the  qualities  of  it  were  those  of  the  silex,  or  siliceous
earth,  and  that  it  was  principally  composed  of  this  earth.  It  may
therefore  be  named  limus  siliceus,  or  filex  limosa.  It  is  undoubtedly
deposited  in  those  lakes,  in  consequence  of  the  demolition  and  decomVoL. -
  II.