128 BACTERIA IN WATER. 



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 contaminated with sewage. 



I. THE PURITY OF DRINKING-WATERS. 



In determining the purity of drinking-water we are not so much 

 concerned with the number of bacteria it contains as with the kinds. 

 Very large numbers may be present and yet the water may be 

 perfectly wholesome, while, on the other hand, with only a small 

 number present the water may be deadly. As we shall see in a 

 later chapter, the bacteria in milk may be reckoned by the millions 

 per c.c., and yet the milk may be perfectly healthful; and at the same 

 time bacteriologists regard with suspicion water that contains them 

 only in thousands. The reason for this difference is simple. When 

 milk contains these large numbers they are almost sure to be harmless 

 types; but if water contains even a few thousands, the typhoid 

 bacillus is likely to be among them. It is impossible to condemn 

 any sample of water simply from the number of bacteria which 

 it contains; still the number serves as a useful measure of purity 

 for the following reason: Water that is fairly puie and contains 

 only the bacteria liable to come from ordinary sources seldom contains 

 more than a few hundreds of bacteria per c.c.; it is only water 

 that is receiving contamination from sewage or some other source 

 of decaying filth that contains large numbers of bacteria. Hence, 

 the finding of large numbers of bacteria in a water-supply suggests 

 sewage contamination and the water at once becomes suspicious. 



SEWAGE CONTAMINATION. 



Water may receive hosts of bacteria from various sources, but 

 the one great and almost only source of real danger is from sewage 

 contamination. Most of the types of bacteria found in nature 

 and in natural water are perfectly harmless, so that it makes little 

 difference whether they are abundant or few in the water we drink. 

 But this is not true of the types likely to be found in sewage. Sew- 



