PURIFICATION  OF  WATER  SUPPLIED  TO  TOWNS.  431 
Carbonate  of  lime  itself  is  very  sparingly  soluble  in  water ; 
probably  5000  gallons  would  be  requisite  to  dissolve  one  pound 
avoirdupois.  But  when  combined  with  an  additional  amount  of 
carbonic  acid,  it  forms  bicarbonate  of  lime,  which  is  so  much 
more  soluble  in  water,  that  one  pound  of  carbonate  with  seven 
ounces  additional  of  carbonic  acid  would  dissolve  in  400  gallons 
of  water  ;  and  this  is  about  the  amount  present  in  well-water 
from  the  chalk  strata. 
The  carbonic  acid  may  be  separated  from  carbonate  of  lime 
by  heating,  as  in  the  ordinary  operation  of  lime-burning,  and 
the  lime  thus  obtained  is  still  more  soluble  in  water  than  the 
bicarbonate  of  lime ;  so  that  a  pound  of  carbonate  of  lime,  con- 
sisting of — 
Lime  -----  9  ounces, 
Carbonic  Acid  7  ounces, 
yields  a  quantity  of  lime  that  may  be  dissolved  in  40  gallons  of 
water. 
Thus  it  appears  that  carbonate  of  lime,  itself  scarcely  at  all 
soluble  in  water,  may  be  rendered  soluble  in  two  different  ways 
— either  by  being  deprived  of  its  carbonic  acid,  or  by  combining 
with  an  additional  quantity  of  carbonic  acid. 
It  is  by  the  latter  of  these  two  changes  that  water,  in  travers- 
ing chalk  strata,  becomes  so  highly  impregnated  with  carbonate 
of  lime  ;  for  carbonic  acid  is  always  substracted  from  the  atmos- 
phere by  water  during  its  condensation  as  rain,  &c,  and  a  further 
amount  is  frequently  dissolved  by  the  water  in  percolating  the 
vegetable  soil. 
To  separate  this  dissolved  carbonate  of  lime,  so  far  as  may  be 
practicable,  is  the  object  of  Dr.  Clark's  method  of  purification. 
It  is  based  upon  the  fact  that  when  a  solution  of  bicarbonate  of 
lime,  such  as  ordinary  water,  is  mixed  with  a  solution  of  lime, 
half  the  carbonic  acid  is  abstracted  from  the  bicarbonate,  and 
both  lime  and  bicarbonate  of  lime  are  converted  into  the  very 
sparingly  soluble  carbonate. 
When  this  operation  is  so  managed  that  the  lime  added  is  just 
sufficient  to  form  carbonate  with  the  surplus  carbonic  acid  in 
the  bicarbonate,  almost  the  whole  of  the  dissolved  carbonate 
will  be  removed  from  the  water,  and  only  so  much  will  remain 
