BACTEEIOLOGY OF WATEB AND SEWAGE' 433 



Tip of all of the dissolved oxygen, the organic matter then 

 under these conditions undergoes putrefaction and malodor- 

 ous gases may be thrown off. 



Methods of Determining Purity of "Water 



The degree of desirable purity in water varies according 

 to circumstances. A water which is to be used for drinking 

 purposes must naturally have a much higher degree of 

 purity than the water of some stream in which it is neces- 

 sary only to prevent undesirable odors and putrefaction, 

 that is, the development of a nuisance. The methods for 

 determining whether or not water is suitable for drinking 

 purposes and the methods for determining whether or not 

 water is so heavily contaminated as to injure the water 

 fauna such as fish and mussels or create a nuisance are 

 quite different. They will be considered in order. 



Methods of Determining the Potability of Water. — 

 When methods of determining numbers of bacteria by 

 means of plate counts were first introduced, it was assumed 

 that this would give a reliable test as to the suitability of 

 water for drinking purposes. In course of time, however, 

 it developed that there is not always a direct relationship 

 between the numbers of bacteria present and the potability 

 of the water. The amount of food material required by 

 microorganisms is very small indeed in some cases. The 

 water from a well, for example, which contains not more 

 than 8 or 10 bacteria per cubic centimeter, when allowed 

 to stand in a warm place for twenty-four hours or more,, 

 may have bacteria present by the thousands. These micro- 

 organisms may all be harmless and do not seriously injure 

 it for drinking purposes. Although the determination of 

 numbers is therefore not a reliable test, it is in many cases, 

 very helpful. In general it is true that natural waters do- 

 not contain large numbers of bacteria unless they contain 

 considerable amounts of organic material. Any natural 



