434  PURIFICATION  OF  WATER  SUPPLIED  TO  TOWNS. 
water  from  nearly  the  same  part  of  the  Thames,  low  down,  where 
it  was  contaminated  with  town-drainage. 
The  method  of  purification  proposed  by  Dr.  Clark  not  only 
effects  the  separation  of  carbonate  of  lime,  which  as  regards  the 
wholesomeness  of  water  is  of  secondary  importance,  but  it  also 
separates  organic  substance.  At  the  print  works,  in  Manches- 
ter, it  is  applied  specially  for  this  purpose,  and  in  an  experiment 
made  upon  3,000,000  gallons  at  the  Chelsea  Water  Works  it  is 
stated  by  Dr.  Miller  that  the  amount  of  organic  substance  was 
reduced  to  one-third. 
Some  doubt  was  expressed  by  speakers  who  took  part  in  the 
discussion,  as  to  whether  the  organic  substance  removed  by  lime- 
ing  was  that  suspended  or  that  in  solution.  Both  are  in  fact  re- 
moved, but  it  does  not  appear  that  there  are  any  grounds  for 
regarding  the  one  more  prejudicial  to  health  than  the  other. 
The  removal  from  water  of  the  carbonate  of  lime  dissolved  by 
carbonic  acid,  has  also,  indirectly,  an  influence  upon  the  con- 
tamination with  organic  substance,  by  serving  as  a  preventive 
of  vegetation,  and  of  the  consequent  development  of  animal  or- 
ganism. 
When  chalk  spring  water  is  pumped  up  from  a  well  and  ex- 
posed to  light  and  air,  in  a  clean  glass  vessel,  capable  of  holding 
a  few  gallons,  with  a  glass  covering,  and  so  exposed  that  the 
changes  can  be  observed  as  they  take  place  from  day  to  day,  it 
will  be  seen  that  all  around  the  sides  and  bottom  a  green  vegeta- 
tion will  appear  in  summer  time  within  a  few  days.  In  process 
of  time  this  vegetation  tends  to  a  brown,  and  if  a  close  observa- 
tion be  made,  a  slight  incrustation  may  be  discovered,  partly  to 
float  on  the  surface  of  the  water,  and  partly  to  adhere  to  the 
sides  and  bottom  of  the  vessel.  This  incrustation  consists  of 
carbonate  of  lime,  slowly  precipitated  from  the  water  by  the 
separation  of  the  duplicate  dose  of  carbonic  acid  that  kept  the 
carbonate  of  lime  dissolved.  It  is  this  carbonic  acid  that  serves 
as  the  food  of  plants,  furnishing  carbon  to  them,  and  the  car- 
bonate of  lime  that  was  kept  in  solution  by  it  forms  the  mineral 
part  of  the  incrustation.  If  the  glass  vessel,  after  having  been 
exposed  as  described  for  several  weeks,  be  emptied,  a  dirty 
brownish  incrustation,  including  vegetable  substance,  may  be 
very  well  seen,  all  down  the  sides,  and  on  the  bottom.  This 
