AmMi°rch\f9M.rm'}    Refined  Methods  in  Water  Purification.  117 
species,  not  in  the  water  applied  to  the  filter,  but  in  the  effluent. 
The  presence  or  absence  of  the  organisms  here,  being  a  certain  indi- 
cator of  the  efficiency  of  the  filter. 
The  organism  sought  is  known  as  the  Bacillus  coli  communis, 
invariably  found  in  sewage.  This  organism  has  many  features  in 
common  with  the  Bacillus  typhosa,  and  it  is  almost  identical  with 
several  organisms  found  naturally  in  grain.  It  is  desirable  to  be 
possessed  of  a  speedy  certain  means  of  identifying  the  colon  bacillus. 
Several  plans  have  been  used,  based  upon  its  peculiarities.  Among 
its  cultural  characteristics  is  this  property,  when  grown  in  neutral 
milk,  containing  enough  blue  tr.  litmus  to  decidedly  color  the  milk, 
the  blue  color  becomes  red  and  a  firm  coagulum  occurs  in  the  test- 
tube,  after  eighteen  to  twenty-four  hours'  cultivation  in  the  incu- 
bator. This  red  color  is  due  to  a  change  in  the  reaction  of  the 
milk,  caused  by  the  transition  of  the  sugar  of  milk  to  lactic  acid 
through  the  agency  of  the  colon  bacillus.  Advantage  is  taken  of 
this  feature.  Plates  are  made  of  agar-agar,  containing  in  addition  to 
beef  bouillon,  sugar  of  milk,  and  strongly  colored  with  blue  litmus. 
The  plates  are  prepared  by  fusing  a  tube  of  the  media,  and  when 
cooled  to  blood  heat,  1  c.c.  of  the  filtered  water  is  added,  gently  but 
thoroughly  shaken  together  and  poured  into  the  plate.  After  it  is 
set  hard  it  is  placed  in  the  incubator  and  cultivated  at  37^°  C.  for 
eighteen  to  twenty-four  hours,  when  any  colon  bacilli  developed  will 
be  made  manifest  by  red  colonies  on  the  plate  with  a  considerable 
red  area  surrounding  them. 
Another  means  of  the  identification  of  the  colon  bacillus  is  found 
in  its  property  of  causing  fermentation  when  cultivated  in  a  fluid 
medium  containing  1  or  2  per  cent,  of  a  fermentable  carbohydrate, 
such  as  grape  sugar.  The  products  of  fermentation  are  conserved 
and  subjected  to  analysis.  The  operation  is  facilitated  and  best  car- 
ried out  in  a  special  device  known  as  the  Smith  fermentation  tub?, 
devised  by  Theobald  Smith. 
An  experiment  conducted  under  these  conditions  yields  a  gaseous 
product  of  from  30  to  50  per  cent,  of  the  volume  of  the  liquid  used. 
On  examination  of  this  gas  it  is  almost  uniformly  found  to  contain  I 
part  C02  and  2  parts  of  an  inflammable  gas  akin  to  hydrogen.  The 
two  features  briefly  described  were  formerly  the  principal  reliance 
for  the  identification  of  the  colon  bacillus,  but  it  has  been  so  fre- 
quently demonstrated  that  other  organisms  duplicated  these  phe- 
