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PURIFICATION  OF  WATER   SUPPLIED  TO  TOWNS. 
PURIFICATION  OF  WATER  SUPPLIED  TO  TOWNS,  ETC. 
At  a  recent  meeting  of  the  Society  of  Arts,  the  method  pro- 
posed by  Dr.  Clark  for  purifying  water  for  the  supply  of- towns 
was  described  by  him,  and  its  applicability  for  this  purpose  dis- 
cussed. 
The  substances  with  which  water  is  contaminated  may  be  in 
two  states — suspended  and  dissolved ;  both  may  contain  mineral 
and  organic  substance. 
Spring  water  contains  from  _JL  to  JL,  or  even  _L,  dissolved 
L  20000         1000  1000 
substance,  but  no  suspended  substance.  This  is  the  case  with 
many  kinds  of  water  in  and  around  London  ;  but,  when  collected 
at  the  surface  in  reservoirs,  and  exposed  to  light  and  air,  vegeta- 
tion commences,  and  is  succeeded  by  the  development  of  animal- 
cules. After  a  time,  both  the  plant  and  animal  organisms  pass 
into  a  state  of  putrefaction,  and  become  a  source  of  serious  con- 
tamination. 
The  water  of  rivers  generally  contains  less  dissolved  substance 
than  that  of  the  springs  in  the  same  district,  and  it  also  contains 
suspended  substances  of  various  kinds  that  are  washed  into  the 
rivers  from  the  banks  by  small  streams,  rivers,  &c. 
The  separation  of  suspended  substance  is  effected  either  by 
subsidence  or  by  filtration. 
The  nature  of  the  dissolved  substance  depends  upon  the  kinds 
of  strata  traversed  by  the  water ;  it  generally  consists,  for  the 
most  part,  calcareous  salts — sometimes  with  magnesian  salts — 
alkaline  salts,  ammoniacal  salts,  rarely,  and  in  small  amounts. 
The  calcareous  and  magnesian  salts  communicate  to  water  the 
character  of  hardness.  This  character  varies  considerably  in 
amount  in  different  kinds  of  water,  and  is  expressed  in  degrees, 
each  degree  of  hardness  being  as  much  hardness  as  a  grain  of 
chalk,  or  the  lime,  or  the  calcium,  in  a  grain  of  chalk,  would 
produce  in  a  gallon  of  water,  by  whatever  means  it  may  be 
dissolved. 
The  hardness  of  most  of  the  water  around  London  is  owing 
to  the  presence  of  dissolved  carbonate  of  lime.  The  amount  is 
so  large,  that  the  average  supply  of  water  to  a  single  family 
would  yield  in  eight  months  100  pounds  of  chalk,  and  in  100 
gallons  of  water  there  is  enough  to  destroy  35  ounces  of  soap. 
