164 AGRICULTURAL BACTERIOLOGY. 



more bacteria than the pond. The supply streams of a lake 

 or reservoir always contain more bacteria than the water of the 

 lake. One reason for this is evident enough. The brooks 

 form the drainage system of the country. The rains wash 

 the whole surface of the land and all of 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. We might imagine that they would accumu- 

 late in the lake. But this does not happen. Many of them 

 die, for some reason, and others soon settle to the bottom, so 

 that the water in the reservoir rapidly tends to become puri- 

 fied, and is always found to contain fewer bacteria per c.c. than 

 the water brought into it by its supply streams. 



It will be readily understood that the number of bacteria in 

 a stream will depend upon the extent of the contamination 

 which it receives from sources of active bacterial growth. The 

 actual number found by bacteriologists is quite variable, rang- 

 ing from a score or more per c.c., in very pure waters, to a 

 few hundreds, in a moderately pure reservoir; and from this 

 number to many thousands in streams which are badly con- 

 taminated with sewage. 



DANGER OF SEWAGE CONTAMINATION. 



It is common to regard water in which the bacteria are 

 reckoned by thousands per c.c. as badly contaminated, as sus- 

 picious and unsafe for drinking purposes. While this may be 

 a proper procedure it is very necessary to guard against a 

 misunderstanding. The fact does not indicate that it is unsafe 

 to swallow such large numbers of bacteria. We have most 

 excellent proof that such is not the case. Ordinary milk, as we 

 shall see in a later chapter, contains many more bacteria than 

 this. Indeed, the milk of our cities frequently contains bacteria 

 which are reckoned by millions rather than thousands, and 

 some of the best milk has several hundred thousands of bac- 



