108  Methods  of  Water  Analysis.  {^Mareh,iSS.rm* 
ice,  the  bacteria  decrease  in  number  sometimes  to  a  very  marked 
degree.  This  is  clearly  shown  by  Jordan  and  Irons  {1  tans.  Am. 
Pub.  B.  Ass.,  Vol.  XXV,  1899)  in  the  following  table: 
A. 
B. 
C. 
0* 
O 
d 
a 
V 
No.  Bacteria 
per  1  c.c. 
0' 
U 
d 
a 
U 
{H 
No.  Bacteria 
per  1  c.c. 
0' 
O 
d 
a 
No.  Bacteria 
per  1  c.c. 
Immediately  after  collection    .  . 
2075 
176 
23"5 
950,000 
29 
385,000 
9 
123  . 
9 
510,000 
29 
"    6  "   
6 
93 
6 
90,000 
(4hrs.)  66 
130,000 
8 
87 
6 
430,000 
(8hrs.)  2 
210,000 
"24  "   
7 
72 
7 
380,000 
(22hrs.)  6 
136,000 
"3*  "   
8 
46 
8 
340,000 
4 
27 
429,000 
(46hrs.)  8 
305,000 
"72  "   
1 
39 
3 '5 
480,000 
This  marked  decrease  the  authors  ascribe  to  the  effect  of  sudden 
chilling. 
(2)  Dilution. — It  is  by  no  means  a  matter  of  indifference  whether 
the  water  is  plated  as  it. is  or  diluted;  also  the  degrees  of  the  dilu- 
tion employed  has  an  effect  on  the  number  of  bacteria  per  1  c.c,  as 
shown  by  Jordan  and  Irons  (/.  c.)  in  the  following  table : 
A. 
Undiluted   218 
Diluted  i-to  470 
C. 
Undiluted  i,5°o 
Diluted  1-10  4.34° 
Diluted  1-100  8,800 
B. 
Diluted  1-1,000  844,000 
Diluted  1-10,000   .  2,630,000 
Diluted  1-100,000  4,300,000 
D.  .•'•«- 
Diluted  1-1,000  479,000 
Diluted  1-10,000  1,123,000 
Diluted  1-100,000   1,300,000 
This  variation  in  number,  dependent  on  dilution,  is  due  to  the 
obscuration  of  colonies  through  inhibition  of  growth  when  undiluted 
water  is  plated.  In  the  matter  of  dilutions,  the  number  and  vigor 
of  shakes  to  which  the  vessel  is  subjected  before  the  I  c.c.  is  with- 
