jo  Filtration  of  Drinking  Water.  {^ebv^im^' 
nature  carries  out  in  springs  under  the  most  favorable  conditions. 
For  convenience  of  illustration,  we  may  assume  that  the  bulk  of  or- 
ganic matter  is  embraced  within  the  elements  C,  H,  O,  N.  No  matter 
how  complex  the  molecules  may  be,  the  matter  is  ultimately  broken 
down  into  the  most  simple  compounds  of  the  elements,  namely,, 
C  into  C02;  H  and  O  into  water,  and  the  N  into  nitric 
acid  or  salts  of  the  same.  This  is  all  brought  about  through 
the  functioning  of  those  minute  vegetable  cells  called  bacteria, 
not  instantly,  but  progressively ;  not  necessarily  all  in  one  oper- 
ation, but  in  consecutive  changes,  proceeding  orderly  and  with> 
deliberation  until  that  which  was  organic  and  perhaps  toxic  becomes^ 
the  most  simple  inorganic  compounds  of  the  elements,  quite  harm- 
less and  ready  food  for  plant  assimilation.  This  is  all  carried  out  in 
a  slow  sand  filter,  and  the  object  is  to  cultivate  rather  than  destroy 
bacteria.  The  sand  is  not  the  filter,  the  sand  is  simply  the  bones 
upon  which  the  filter  grows.  Surrounding  each  individual  bacterium, 
under  the  microscope  may  be  seen  a  gelatinous  envelope,  when 
many  bacteria  are  joined  together  in  mass;  this  envelope  may  be 
seen  collectively  without  a  lens,  forming  a  jelly-like  mass  and  is  then 
called  a  Zoogloea.  In  a  sand  filter  this  Zoogloea  attaches  to  and 
covers  completely  each  grain  of  sand  in  the  filter.  The  grains  form 
fine  avenues  through  which  the  water  is  compelled  to  pass.  The 
bacteria  line  these  avenues.  The  water  carrying  its  organic  content 
brings  it  as  food  for  the  bacteria  in  the  Zoogloea.  As  the  water  passes 
along,  it  is  gradually  relieved  of  its  organic  matter,  because  it  is 
digested  by  the  bacteria,  and  in  its  place  bears  away  the  products  of 
the  decomposition.  Sublime  in  its  beautiful  simplicity  !  We  have 
chemical  and  biological  proof  of  each  change.  Those  just  men. 
tioned  are  indicators  of  every  step — C  into  C02;  H  and  O  into  water, 
N  into  nitric  acid  and  its  compounds  ;  but  the  proper  conditions 
must  be  maintained,  and  perhaps  the  most  important  factor,  aside 
from  the  bacteria,  in  the  operation  of  the  filter,  is  oxygen.  Without 
this  element  the  particular  kinds  of  bacteria  necessary  for  water- 
purification  cannot  perform  their  function.  The  oxygen  must  be  in 
solution  and  carried  along  with  the  water  into  the  filter,  where  it  is 
utilized  in  the  oxidation  changes. 
Winogradsky  has  shown  that  nitrifying  or  oxidizing  bacteria 
grow  upon  media  altogether  inorganic.  No  less  than  three  sepa- 
rate and  distinct  classes  of  organisms  are  concerned  in  the  transition 
of  nitrogenous  organic  matter  to  the  inorganic  state,  as  follows  : 
