112 
Methods  of  Water  Analysis. 
Am.  Jour.  Pharm. 
March,  1904. 
Gelatin  plates,  second  day  count 
2,150 
7,350 
2,050 
11,000 
sixth  " 
second  " 
sixth     1 ' 
The  committee  of  the  American  Public  Health  Association  on 
Standard  Laboratory  Methods  recommends  that  plates  be  counted 
on  the  second  day.  The  difficulty,  however,  of  establishing  a  uni- 
form practice  lies  in  the  variability  of  saprophytic  bacterial  species 
not  only  in  different  waters,  but  in  the  same  water  at  different  times. 
Thus  a  certain  water  may  contain  species  which  develop  rapidly 
at  20°  C,  while  another  water,  or  the  same  at  another  time,  may 
contain  species  which  develop  slowly  at  that  temperature.  As  a 
matter  of  fact,  waters  containing  large  numbers  of  bacteria  whose 
optimum  temperature  is  370  C.  (fecal  and  other  pathogenic  organ- 
isms) will  show  a  lower  count,  owing  to  the  fact  that  these  species 
develop  slowly  at  20°  C,  and  the  colonies  could  be  readily  overlooked 
on  the  second  day  count.  In  the  case  of  testing  the  efficiency  of  a 
filter,  the  difficulty  is  augmented  by  the  fact  that  the  raw  water  does 
not  contain,  in  point  of  numbers,  the  same  species  as  does  the 
effluent.  On  the  other  hand,  prolonging  the  final  count  to  the 
third  or  fourth  day  endangers  the  integrity  of  the  gelatin  plate,  which 
often  becomes  liquefied  at  the  end  of  the  second  day,  unless  kept  at  a 
temperature  lower  than  20°  C,  when  the  error,  occasioned  by  a  still 
lower  temperature,  is  introduced. 
The  investigations  of  others,  as  well  as  my  own,  demonstrate 
conclusively  that  Nahrstoff  Heyden  agar  permits  the  development 
of  the  maximum  number  of  bacteria,  very  likely  all  the  bacteria 
found  in  a  given  quantity  of  water.  This  medium,  according  to 
Hesse  and  Niedner,  who  were  the  first  to  suggest  it  (Zeitschr.f. 
Hyg  ,  Vol.  XXIX,  p.  454)  is  prepared  as  follows  : 
The  Nahrstoff  Heyden,  which  is  an  albumose,  is  dissolved  in 
water,  mixed  with  the  agar  and  the  whole  boiled  until  the  agar  is 
completely  dissolved.  It  may  then  be  filtered  through  absorbent 
cotton.  The  medium  requires  no  adjustment  of  reaction,  nor  any 
other  manipulations  which,  in  the  case  of  other  media,  interfere  with 
the  uniform  composition  of  the  finished  product.    Nahrstoff  Heyden 
Per  cent. 
Agar-agar   .  .  . 
Nahrstoff  Heyden 
Distilled  water  .  . 
1.25 
o.75 
98.00 
