114  Methods  of  Water  Analysis.  {^mI^iwT™- 
Fortunately,  however,  this  is  not  the  case.  While  it  is  true  that  no 
single  factor  establishes  definitely  the  character  of  the  water  under 
suspicion,  a  combination  of  factors  with  their  proper  grouping  and 
interpretation  is  capable  of  forging  a  chain  of  evidence,  placing  the 
verdict  **  beyond  a  reasonable  doubt."  The  procedure,  which  will 
yield  satisfactory  results,  is  as  follows : 
The  water  is  subjected  to  a  chemical  analysis,  and  an  adequate 
portion,  I  c.c.  or  a  fraction  of  a  cubic  centimetre,  plated  in  gelatin, 
Nahrstoff  Heyden  agar,  litmus  lactose  agar,  carbolic  acid  lactose 
agar  and  neutral  red  lactose  bouillon.1  The  carbolic  acid  lactose  agar 
is  made  by  the  addition  to  5  c.c.  of  the  medium  of  0  05  —  O-i  c.c.  of 
Parietti's  solution  (hydrochloric  acid  4  c.c,  5  per  cent,  carbolic  acid 
solution  100  c.c).  The  neutral  red  lactose  bouillon  is  made  by 
adding  10  c.c  of  a  1  per  cent,  solution  of  neutral  red  to  1  litre  of 
lactose  bouillon  (1  per  cent.).  The  gelatin  and  NahrstofT  Heyden 
plates  are  kept  at  200  C,  and  the  others  at  370  C.  The  gelatin 
plates  are  counted  at  the  end  of  two  days,  the  Nahrstoff  Heyden 
agar  plate  at  the  end  of  nine  days,  the  litmus  lactose  agar  plate  at 
the  end  of  twenty-four  hours  and  the  carbolic  acid  lactose  agar  plate 
at  the  end  of  forty-eight  hours. 
Interpretation  of  Results. — By  using  these  several  media  we  aim 
to  demonstrate:  (1)  The  presence  of  organic  pollution  by  the  com- 
bined chemical  analysis  and  bacterial  count,  the  count  on  gelatin 
serving  as  a  comparate  with  the  counts  obtained  by  other  observers 
who  have  used  gelatin,  while  the  Nahrstoff  Heyden  agar  shows  the 
total  number  of  bacteria. 
(2)  The  presence  and  number  of  bacteria  which  develop  at  37 0  C. 
and  the  presence  and  number  of  red  colonies  which  may  be  either 
B.  coli  communis,  Houston's  streptococcus  or  some  other  sewage 
organism  producing  acid.  This  information  is  furnished  by  the 
litmus  lactose  agar  plate. 
(3)  The  presence  and  number  of  bacteria  which  resist  the  addition 
of  carbolic  acid,  as  B.  coli  communis  or  some  other  equally  resistant 
microorganism  which  could  not  be  an  ordinary  water  saprophyte. 
This  is  indicated  by  the  carbolic  acid  lactose  agar. 
^he  neutral  red  lactose  bouillon  was  suggested  by  Dr.  Stokes,  of  the  Balti- 
more City  Board  of  Health,  at  the  meeting  of  the  American  Public  Health 
Association,  at  Washington,  D.  C,  1903. 
