%2;5m'}        Methods  of  Water  Analysis.  109 
drawn  affects  the  numerical  results,  also  whether  distilled  or  tap-water 
is  used  as  a  diluent.  The  method  I  employ  is  to  have  on  hand  50 
and  ioocx.  graduated  flasks  half-full  of  tap  water.  These  are  sterilized 
in  the  autoclave.  One  cubic  centimetre  of  the  water  is  added  to  either 
the  50  or  100  c.c.  flask,  and  contents  subjected  to  ten  vigorous 
shakes.  The  flask  is  then  filled  to  the  mark  with  sterile  tap-water 
and  inverted  twenty-five  times.  If  higher  dilutions  are  required 
portions  of  diluted  water  are  similarly  treated. 
(3)  Composition  of  Media. — That  the  constituents,  reaction  and 
character  of  the  medium  influence  the  number  of  bacteria  to  a  very 
great  extent  is  a  well-known  fact  attested  by  numerous  experimental 
data.  The  marked  variations  in  the  number  of  bacteria  in  the  same 
water  plated  on  different  media  is  shown  by  the  following  data 
obtained  by  Jordan  and  Irons  (/.  c.) : 
(i)     SURFACE  WATER  (UKE  MICHIGAN). 
Reaction  No.  of  Colonies. 
(Fuller's  scale).         Eight  days. 
Ordinary  Witte's  peptone  agar     ........  -f-10  50 
Ordinary  Witte's  peptone  gelatin  -fio  130 
Somatose  gelatin  (no  broth)                                     o  110 
Nahrstoff  Heyden  gelatin  (no  broth)                        o  460 
Somatose  agar  (no  broth)                                         o  470 
Nahrstoff  Heyden  agar  (no  broth)                            o  570 
(2)     SURFACE  WATER  (MISSISSIPPI  RIVER). 
Reaction  No.  of  Colonies. 
(Fuller's  scale).         Seven  days. 
Ordinary  Witte's  peptone  agar  +10  2°6 
Somatose  agar  (no  broth)   o  543 
Nahrstoff  Heyden  agar  (no  broth)   o  6  [2 
(3)     SEWAGE  (DILUTED  t^oo)- 
Reaction  No.  of  Colonies. 
(Fuller's  scale).  Ten  days. 
f  127 
Witte's  peptone  agar  ,  .  -f-10  <  ' 
S  127 
Somatose  agar   o  198 
Nahrstoff  Heyden  agar   o       •  j 
(4)     GROUND  WATER  (ARTESIAN  WEEE). 
Reaction  No.  of  Colonies. 
(Fuller's  scale).  Nine  days. 
Ordinary  Witte's  peptone  agar                             -\-\o  15 
Ordinary  Witte's  peptone  gelatin  +IQ  21 
Nahrstoff  Heyden  gelatin                                        o  700 
Somatose  agar                                                         o  584 
Nahrstoff  Heyden  agar                                      .      o  920 
