490 SA CTEBIOL OGY. 



detected, still, waters in which such pollutions are pos- 

 sible are a constant source of menace to the health of 

 those who use them for domestic purposes. 



A sudden variation from the normal, mean number 

 of bacteria, or from the normal chemical composition of 

 a water, calls at once for a thorough inspection of the 

 supply, while at the same time the characters of the 

 organisms present are to be subjected to the most care- 

 ful study. 



The Qualitative Bacteriological Analysis 

 OF Water. — The qualitative bacteriological analysis 

 of water entails much labor, as it requires not only that 

 all the different species of organism found in the water 

 should be isolated, but that each representative should 

 be subjected to systematic study, and its pathogenic or 

 non-pathogenic properties determined. 



For this purpose a knowledge of the methods for the 

 isolation of individual species, which have already been 

 described, and of the means of studying these species 

 when isolated, is indispensable. 



For this analysis certain precautions essential to 

 accuracy are always to be observed. 



The sample is to be collected under the most rigid 

 precautions that will exclude organisms from sources 

 other than that under consideration. If drawn from a 

 spigot, it should never be collected until the water has 

 been flowing for fifteen to twenty minutes in a full 

 stream. If obtained from a stream or a spring, it 

 should be collected, not from the surface, but rather 

 from about one foot beneath the surface. 



It should always be collected in vessels which have 

 previously been thoroughly freed from all dirt and 

 organic particles, and then sterilized; and the plates 



