Am.  Jour.  Pliarm.") 
March,  190i.  / 
Methods  of  Water  Analysis. 
115 
(4)  The  absence  or  possible  presence  of  B.  coli  communis  as  indi- 
cated by  the  production  or  non-production  of  gas  and  characteristic 
reaction  with  the  neutral  red  dye. 
Given  a  water  which  shows  on  chemical  analysis  organic  pollution 
and  which  shows  a  large  number  of  bacteria  on  gelatin  and  a  con- 
siderable number  of  bacteria  on  the  litmus  lactose  agar  and  Parietti's 
solution  lactose  agar  plates  together  with  red  colonies  on  the  former 
and  production  of  gas  plus  characteristic  reaction  with  the  neutral 
red  in  the  neutral-red  lactose  bouillon,  such  a  water  may  be  pro- 
nounced polluted  with  sewage,  beyond  a  reasonable  doubt. 
The  practical  application  of  this  method  is  illustrated  in  the 
following  instance. 
Water  from  the  race  carrying  Brandywine  water  was  examined 
with  the  following  results  (in  1  c.c.) : 
No.  of  bacteria  on  gelatin,  second  day  count   1,000 
"    "       "        "  Nahrstoff  agar,  tenth  day  count   9,000 
"    "       <f         "  litmus  lactose  agar,  twenty-four  hours  ...  57 
"    "  red  colonies  (proved  to  be  B.  coli)   8 
Neutral  red  lactose  bouillon  (1  c.c.  of  water),  typical  reaction. 
At  the  same  time  the  water  from  the  Cool  Spring  Reservoir,  con- 
taining the  same  water,  was  examined  and  showed  (in  1  c.c.) : 
No.  of  bacteria  on  gelatin,  second  day  count  .........  3,400 
"    "       "        "  Nahrstoff  agar,  tenth  day  count  75, 000 
"  "  "  "  litmus  lactose  agar,  twenty-four  hours  ...  26 
"    "       "        "  red  colonies   1 
Neutral  red  lactose  bouillon  (1  c.c.  of  water)  showed  production  of  gas 
without  characteristic  change  of  color. 
Subsequent  study  of  the  single  red  colony  showed  that  it  belonged 
to  the  proteus  group. 
At  another  time  the  race-water  showed  (in  I  c.c.) : 
No.  of  bacteria  on  gelatin  ■  .  13,000 
"    "       "        "   lactose  litmus  agar    97 
"    "  red  colonies  (proved  to  be  B.  coli)    33 
"    "  bacteria  on  carbolized  lactose  agar   46 
Neutral -red  lactose  bouillon,  typical  reaction. 
The  water  from  the  reservoir  showed  (in  1  c.c.) : 
No.  of  bacteria  on  gelatin   16,350 
"    "       V        "  lactose  litmus  agar   37 
"    "  red  coloniesX proved  to  be  B.  coli )   7 
"    "  bacteria  on  carbolized  lactose  agar   37 
Neutral-red  lactose  bouillon,  typical  reaction . 
