432         PURIFICATION  OF  WATER  SUPPLIED  TO  TOWNS- 
dissolved  as  corresponds  with  the  solubility  of  carbonate  of 
lime. 
Bicarbonate  of  lime  (  Carbonate  of  lime  16  oz.  "} 
in  400  gallons     \  Carbonic  acid     .    7  oz. )  L 
Lime  in  40  gallons  |  I— 16oz.  carb.  of  lime  f2  lbs 
of  water.  9  oz.  J  J 
This  residual  carbonate  of  lime  is  always  small  in  amount. 
Supposing  in  the  above  instance  the  440  gallons  contained  17j 
oz.  dissolved  carbonate  of  lime,  12  or  16  oz.  would  be  separated^ 
and  only  1|  oz.  be  left  in  solution.  The  water,  before  being 
softened,  would  destroy  35  oz.  of  soap  for  every  100  gallons ; 
after  being  softened,  the  same  quantity  would  destroy  only  5  oz. 
Most  water  contains,  besides  carbonate  of  lime,  calcareous 
and  magnesian  sulphates,  chlorides,  &c.  These  substances  com- 
municate hardness  to  water,  as  well  as  carbonate  of  lime ;  but 
there  is  the  difference — -that  the  hardness  owing  to  the  presence 
of  these  substances  is  not  removed  by  limeing.  This,  however, 
is  not  of  any  practical  importance,  so  far  as  regards  the  puri- 
fication of  the  water  supply  of  London  by  this  method ;  for  the 
hardness  of  the  water  round  London  is  chiefly  owing  to  carbon- 
ate of  lime. 
Without  perhaps  being  prejudicial  to  health,  the  disadvantages 
arising  from  the  presence  of  carbonate  of  lime  in  water  are  nu- 
merous and  considerable. 
1.  It  is  the  principal  cause  of  the  incrustation  of  steam-engine 
boilers. 
2.  It  causes  a  great,  and  at  the  same  time  useless  increase  in 
the  consumption  of  soap,  and  is  deposited  in  dirty  linen  in  such 
a  manner  as  to  fix  the  dirt,  and  prevent  its  being  rendered  white. 
3.  For  many  culinary  purposes  it  is  less  suitable  than  soft 
water. 
Dr.  Clark's  method  is  remarkable,  inasmuch  as  it  differs  from 
most  chemical  operations  in  not  introducing  any  other  substance 
into  the  water  in  place  of  the  carbonate  of  lime  separated  ;  and, 
moreover,  the  separation  is  effected  without  the  use  of  any  sub- 
stance foreign  to  the  water  in  its  natural  state. 
There  is  another  effect  produced  by  this  method  of  purifying 
water,  which  does  not  appear  to  have  been  at  first  anticipated 
by  Dr.  Clark.    It  is  the  removal  of  organic  substance. 
