AVarch,T9ho4.rm'}        Methods  of  Water  Analysis.  113 
agar  is  of  uniform  composition;  and  offers  the  additional  advantage 
that  the  colonies  developing  on  it  never  spread  nor  grow  so  large  as 
to  obscure  their  smaller  neighbors.  The  plates  are  usually  counted 
on  the  ninth  or  tenth  day,  although  the  count  may  be  made  on  the 
fifth  or  sixth  day  without  any  great  error  being  introduced.  The 
colonies  developing  on  this  medium  are,  as  a  rule,  not  characteristic, 
but  chromogenesis  is  brought  out  remarkably  well.  A  plate  con- 
taining a  number  of  chromogenic  species  looks  like  a  field  bedecked 
with  early  spring  flowers.  It  is  quite  likely  that  this  medium  may 
prove  of  great  use  in  the  grouping  of  bacterial  species  according  to 
chromogenesis. 
But  even  if  in  NahrstofT  Heyden  agar  we  possess  a  medium  which 
will  show  all  the  bacteria  found  in  a  given  sample  of  water,  we  are 
still  unable  to  pass  definite  judgment  on  its  hygienic  quality.  After 
all,  the  number  of  bacteria  in  water  indicates  the  presence  of  bacterial 
food,  or  organic  matter,  but  does  not  reveal  to  us  the  nature  of  that 
organic  food,  whether  of  vegetable  or  animal  origin.  Therefore,  the 
same  objection  that  is  raised  against  the  chemical  analysis  of  water 
pertains  with  almost  equal  force  to  the  mere  counting  of  the  number 
of  bacteria.  To  remedy  this  defect,  bacteriologists  introduced  the 
presence  or  absence  of  B.  coli  communis,  a  normal  resident  of  the 
intestinal  tract  of  man  and  animals,  as  the  criterion  for  the  presence 
or  absence  of  fecal  pollution.  The  presence  of  the  B.  coli  communis 
indicates  the  presence  of  feces,  and  the  contamination  with  the  latter 
makes  it  possible  for  the  typhoid  bacilli  to  be  present.  Consequently, 
the  investigation  of  water  supposed  to  have  been  the  cause  of  a 
typhoid  epidemic  rests  on  the  presence  of  the  colon  bacillus  as  the 
indirect  but  certain  evidence.  However,  the  mere  presence  of  the 
colon  bacillus,  which  is  so  widely  spread  in  nature,  is  no  certain 
indication  of  fecal  pollution,  unless  the  number  of  B.  coli  is  large. 
Unfortunately,  the  methods  for  the  enumeration  of  this  micro- 
organism are  either  too  complicated  for  routine  work  or  inaccurate ; 
and,  besides,  bacteriologists  are  not  quite  agreed  as  to  what  con- 
stitutes a  genuine  B.  coli  communis,  there  being  a  number  of  species 
not  found  in  feces  which  closely  resemble  it. 
It  would  seem  from  the  foregoing  considerations  that  we  possess 
no  certain  means  of  detecting  dangerous  pollution  in  cases  in  which 
a  mere  sanitary  inspection  does  not  make  the  source  of  the  pollution 
evident  (proximity  of  privy,  discharge  of  sewage  into  the  stream,  etc.). 
