AFebJr0UarryP?9a^m•}        Filtration  of  Drinking  Water.  75 
lean  beef  to  a  litre  of  water,  the  finished  product  having  an  acidity 
of  1 5  degrees.  This  means  that  a  litre  of  such  medium  would  require 
the  addition  of  15  c.c.  of  normal  sodium  hydrate  solution  to  bring; 
it  to  the  phenolphtalein  neutral  point.  It  has  been  repeatedly 
demonstrated  that  this  medium  gives  larger  counts  than  the  same 
material  made  more  acid  or  more  alkaline,  showing  the  very  marked 
influence  that  the  degree  of  acidity  exerts  upon  the  development  of 
the  bacteria  under  examination.  This  medium  is  the  one  now  used 
almost  universally  by  water  analysts. 
The  special  apparatus  used  in  quantitative  bacteriological  investi- 
gation is  quite  simple,  consisting  essentially  of  test-tubes  and  petri 
dishes.  The  petri  dish  is  simply  a  circular  glass  vessel  about  four 
inches  in  diameter,  with  raised  edge  from  three-eighths  to  one-half 
inch  high.  Two  of  these  make  a  complete  dish,  the  upper  one  fit- 
ting loosely  over  the  lower.  There  are  other  pieces  of  apparatus, 
but  those  named  are  most  in  evidence. 
Absolute  sterility  of  media  and  apparatus  is  the  only  condition? 
under  which  this  work  can  be  successfully  carried  out.  To  make  a 
plate,  the  sterilized  glass  dish  is  set  upon  the  level  table ;  a  tube  ol 
the  gelatin  medium,  previously  described,  containing  about  7  c.c,  is 
fused  by  gentle  heat,  and  when  cooled  to  about  blood  temperature^ 
a  measured  quantity  of  the  water  to  be  examined  is  introduced 
from  a  sterilized  graduated  pipette.  The  test-tube  is  shaken  to- 
thoroughly  incorporate  the  water  with  the  medium,  which  is  then 
poured  into  the  petri  dish  and  immediately  covered.  After  it  has 
solidified  it  is  placed  in  the  incubator,  where  it  remains  for  a  period 
of  forty-eight  hours  at  a  uniform  temperature  of  200  C,  being  the 
most  favorable  temperature  for  the  cultivation  of  water  bacteria. 
This  period  of  incubation  is  adopted,  because  it  not  only  indicates 
the  condition  as  well  as  a  longer  time  would  do,  but  gives  a  more 
speedy  notification  of  any  change  in  the  filters.  Counting  is  done 
with  the  aid  of  a  simple  lens,  and  refers  not  to  the  number  of  bac- 
teria on  the  plate,  but  to  the  number  of  colonies.  Each  colony  is 
supposed  to  be  the  progeny  of  one  original  bacterium,  and  the 
count  gives  the  relative  number  of  bacteria  in  the  water  at  the  time 
of  plating.  Determination  of  species  is  unnecessary  as  a  routine 
procedure,  though  frequent  search  is  made  for  the  Bacillus  Coli 
Communis,  as  this  organism  is  invariably  present  in  sewage  and 
serves  as  an  indicator  of  its  presence  in  the  water-supply. 
