68  Filtration  of  Drinking  Water.  {AFebJrOuarryP?ea02m' 
laws ;  yet  the  great  mass  remains  a  source  of  the  utmost  solicitude 
so  far  as  the  health  of  communities  is  concerned.  The  impurities  in 
water  may  be  separated  into  two  principal  divisions,  namely, 
Inorganic  and  Organic.  That  consisting  of  mineral  substances, 
such  as  salts  of  K,  Na,  Ca,  Ba,  Mg,  as  may  be  dissolved  by  the 
water  in  its  passage  through  the  earth,  is  of  the  first  class,  and 
from  a  hygienic  point  of  view  is  not  of  a  particularly  harmful 
character,  though  from  a  technical  point  of  view  it  presents  consid- 
erations of  most  serious  importance. 
In  the  second  class  is  embraced  every  substance  produced  during 
the  life-processes  of  plants  and  animals.  It  is  to  this  source  of  con- 
tamination that  our  attention  is  most  earnestly  directed  for  the 
purpose  of  providing  a  wholesome  and  economical  water-supply. 
To  this  kind  of  contamination  is  chargeable  all  of  the  diseases 
peculiar  to  drinking  water.  It  must  be  plainly  understood  that 
disease  is  not  necessarily  due -to  the  simple  presence  of  organic 
matter,  but  such  material  invariably  decays,  and  it  is  this  change 
that  causes  the  trouble  by  enabling  pathogenic  "organisms  to  pro- 
long their  existence. 
From  the  beginning  of  time  Nature  has  rid  herself  of  all  dead 
organized  bodies  in  but  one  way,  decay,  brought  about  by  the 
growth  of  bacteria  at  the  expense  of  the  dead  matter.  These 
minute  vegetable  organisms,  either  by  their  mechanical  presence 
or  through  their  excretory  products,  are  responsible  for  all  water- 
borne  diseases. 
With  such  facts  in  mind,  then,  it  is  but  natural  that  effort  is  being 
constantly  put  forth  to  remove  from  our  water-supplies  not  only  the 
bacteria,  but  the  pabulum  for  their  existence- — the  organic  matter. 
Nature  has  pointed  unerringly  the  way.  Springs  have  long  been 
held  in  popular  esteem  as  the  source  of  pure  drinking  water,  and 
not  without  good  reason,  for  in  the  majority  of  cases  springs 
sustain  this  reputation  after  chemical  and  biological  examination. 
It  has  long  been  known  that  impure  water  percolated  through  a  deep 
bed  of  sand  issues  greatly  improved  in  chemical  character,  but  the 
precise  nature  of  the  changes  were  not  thoroughly  understood  until 
Koch's  revelation  made  possible  the  isolation  and  study  of 
individual  species  of  bacteria.  Art  seeks  to  copy  the  changes 
so  long  carried  out  in  springs,  but  with  the  precise  care  of 
scientific  exactness  instead   of  the  haphazard  of  chance,  as  in 
