ROTTEN  STONE. 
463 
KOTTEN  STONE. 
This  is  a  useful  yellow-colored  substance  much  employed  in 
scouring  brass  and  tin  by  mixture  with  a  little  sweet  oil,  then 
finishing  off  with  some  dry  whiting.    Very  few  persons  know 
where  it  comes  from,  or  of  what  it  is  composed.    According  to 
Professor  Johnstone,  it  is  composed  of  silica,  alumina  and  car- 
bon.   It  is  obtained  from  a  ridge  in  Derbyshire,  England,  which 
is  covered  with  drift  10  or  20  feet  thick,  consisting  of  brown 
clay,  with  manes  of  black  marble,  chert,  and  rotten  stone.  The 
rotten-stone  is  so  soft  whilst  in  the  soil  that  the  spade  goes 
through  it  readily,  but  it  hardens  on  exposure  ;  the  holes  from 
which  it  is  dug  are  only  two  feet  deep  in  some  places ;  at  others, 
from  six  to  eight.  On  examining  a  series  of  specimens,  Professor 
Johnstone  found  that  while  some  were  homogeneous,  others  had 
nucleus  of  black  marble.    He  then  treated  specimens  of  the 
black  marble  with  weak  acid,  and  found  that  on  the  removal  of 
the  carbonate  of  lime,  there  remained  from  15  to  20  per  cent,  of 
a  silicious  substance  perfectly  like  the  natural  rotten-stone.  He 
concluded  that  there  existed  in  the  soil  some  acid  which  penetra- 
ted it  and  dissolved  out  the  calcareous  matter  of  the  rocks  below. 
The  agent  in  this  case  might  be  the  carbonic  acid  of  the  air, 
brought  down  by  rain  ;  but  there  were  instances  not  capable  of 
explanation  by  this  agency  alone,  and  attributable  to  other  acids, 
which  are  produced  under  certain  conditions,  and  exercise  a 
much  wider  influence.    The  bottoms  of  peat  bogs  present  very 
strong  evidence  of  the  action  of  acids;  the  stone  and  clay  are 
bleached  and  corroded,  only  silicious  and  colorless  materials  being 
left.    The  source  of  the  acid  here  the  same  as  in  the  former 
instance  ;  the  vegetable  matter  growing  on  the  surface  produces 
in  its  decay  substances  which  exert  a  chemical  action  on  the 
subsoil,  and  escape  by  subterranean  outlets,  carrying  away  the 
materials  dissolved  in  their  progress.  Another  instance  was  af- 
forded by  the  mineral  Pigotite,  formed  in  the  caves  of  Cornwall 
by  water  dripping  from  the  roof.     This  water  contains  a  pecu- 
liar organic  acid,  derived  from  the  soil  of  the  moors,  which  dis- 
solves the  alumina  of  the  granite  and  combines  with  it.  The 
organic  acids  are  very  numerous  and  different  in  composition, 
but  agree  in  producing  chemical  action  upon  rocks.  They 
