CHAPTER X. 
BACTERIA IN WATER. 
This subject is of great importance in the relation it bears to the 
water supplies of cities, and most of its important phases concern 
only the city water-supply. So far as relates to farm life the sub- 
ject has interest in two directions: i. The purity of the drinking- 
water. 2. The pollution of streams. 
ABUNDANCE OF BACTERIA IN WATER. 
All surface waters contain bacteria. We shall find them when- 
ever we examine the water of the ocean, the brook, the pool or the 
reservoir. Even rain water contains them, doubtless washed from 
the air, and the same is true of snow and hail. 
The number of bacteria in water is not exactly what would be 
expected in accordance with our ideas of pure water. The water in 
the running brook is commonly thought of as purer than that of the 
stagnant pond. But this is certainly not true; the brook contains 
more bacteria than the pond, and the supply streams always contain 
more bacteria than the water of the lake or reservoir. The reason 
for this is evident. The brooks form the drainage system of the 
country. The rains wash the whole surface of the land, and all the 
dirt and dust is carried into the brooks. In this dust will always be 
hosts of bacteria which are thus carried by the streams into the lake 
in great numbers. In the lake many of them soon die; others 
settle to the bottom; the water in the reservoir rapidly becomes 
purified, and it always contains fewer bacteria than the water 
brought into it by its supply streams. 
The number of bacteria in a body of water will depend upon the 
extent of the contamination which it receives from sources of active 
bacterial growth. The actual number is quite variable, ranging 
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