CHAPTER 1%; 
BACTERIAL ANALYSIS OF WATER, MILK, AIR, 
AND SOIL. 
WATER ANALYSIS. 
THE biologic examination of water is for the 
purpose of determining the number and kinds of 
organisms present. It serves to supplement chem- 
ical analysis, and both are necessary to arrive at a 
sound conclusion as to potability. 
I. Quantitative Analysis of Water.—The num- 
ber of bacteria present in water varies within wide 
limits without affecting the value of the water; 
consequently no fixed standard can be of much 
value in determining the quality. The proper 
study of a water-supply should include the deter- 
mination of its normal mean number of bacteria at 
every season of the year. Any variation from the 
mean can then readily be determined, and its cause 
investigated. If simultaneous analyses of waters 
from various sources are made, the preferable water 
for drinking-purposes can easily be selected. 
Beyond this the value of the water can be deter- 
mined by quantitative analysis alone only within 
wide limits. Water containing less than 100 bac- 
teria per cubic centimeter is presumably from a 
deep source and uncontaminated by surface drain- 
age. It can usually be recommended for drinking- 
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