Lm'Mayri909arm'}      Purification  of  Drinking  Water.  221 
contamination  is  ignored.  This  leaves  then,  for  our  consideration, 
the  soluble  organic  matter.  It  may  be  said  by  way  of  illustration 
that  this  matter  is  entirely  represented,  from  every  source,  by  four 
elements,  namely,  carbon,  hydrogen,  oxygen,  and  nitrogen.  How 
to  remove  this  matter  was  the  problem,  and  it  remained  to  the 
science  of  bacteriology  to  point  the  way.  When  dead  organic  matter 
is  exposed  to  proper  influences  it  begins  to  decay  at  once.  This 
decay  is  brought  about  by  the  growth  in  the  organic  matter  of 
minute  vegetable  organisms,  bacteria.  The  products  of  this  decay, 
when  the  decomposition  is  effected  in  the  presence  of  oxygen,  are 
the  simple  oxides  of  the  elements  that  enter  into  the  original 
material ;  that  is,  carbon  becomes  carbon  dioxide ;  hydrogen  becomes 
oxidized  to  water ;  and  nitrogen  first  becomes  ammonia,  which  is 
finally  oxidized  to  nitric  acid  or  its  compounds.  It  had  long  been 
known  that  when  water  is  allowed  to  slowly  percolate  through  a  bed 
of  sand  its  chemical  character  is  improved.  The  reasons  for  this 
were  not  understood  until  after  bacteriology  was  placed  on  a  scien- 
tific basis  by  the  discoveries  of  Koch,  and  later  by  many  others. 
Then  it  wTas  found  that  in  a  filtering  bed  of  sand  each  grain  of  sand 
became  coated  with  a  zooglcea  of  bacteria.  The  arrangement  of  a 
filtering  bed  of  sand  is  such  that  it  forms  innumerable  avenues 
through  which  water  passes  very  slowly,  carrying  the  organic  matter 
in  solution,  together  with  dissolved  oxygen.  The  organic  matter 
becomes  food  for  the  bacteria  living  within  the  filter  bed  attached 
to  the  grains  of  sand.  The  chemical  analysis  of  water  so  treated 
shows  a  distinct  change  from  that  which  was  applied  before  filtra- 
tion and  proves  the  theory  to  be  correct.  This,  then,  is  the  plan 
underlying  the  great  water  purification  project  of  the  city  of  Phila- 
delphia, around  which  is  clustered  so  much  of  vital  importance  to 
her  million  and  a  half  of  people. 
The  very  first  treatment  in  the  purification  of  water  is  that  of 
sedimentation.  This  is  accomplished  in  a  large  basin  open  to  air 
and  sunlight.  The  storage  capacity  is  equal  to  about  forty-eight 
hours'  supply.  The  basin  is  divided  into  two  equal  parts  by  a  heavy, 
broad  earth  and  concrete  partition.  This  operation  includes  some 
very  ingenious  handling.  Raw  water  is  entered  at  the  bottom  and 
at  the  far  side  of  basin  No.  1.  The  outlet  of  No.  1  is  through  a 
floating  drum,  which  is  attached  to  a  large  pipe,  hinged  at  the 
bottom  of  the  reservoir  to  the  large  main  that  conducts  the  water 
to  the  far  side  of  basin  No.  2.   In  this  basin  is  another  floating  drum 
