1072 FRESH-WATER BIOLOGY 



ents, and much attention has been given to the study of these 

 growths during recent years. This study has resulted in success- 

 ful measures for destroying the organisms by the use of chemicals 

 and for removing the organisms and the odors produced by them 

 by aeration and filtration. Very little success, however, has been 

 achieved in preventing the organisms from growing. 



Chemically pure water is free from taste and odor. Water con- 

 taining certain substances, as for example sugar or salt, may have 

 a decided taste but no odor. On the other hand water may con- 

 tain substances, like vanilla, that have a strong odor but no taste. 

 The two senses, though distinct, are closely related to each other. 

 Most of the bad tastes observed in drinking waters are due to 

 organisms that produce odors rather than tastes. 



Most surface waters contain some organic matter and have a 

 vegetable or earthy odor. When decomposing organic matter is 

 present the odors may be foul and disagreeable. These odors may 

 be classified in three general groups: (i) those caused by organic 

 matter other than living organisms; (2) those caused by the de- 

 composition of organic matter; and (3) those caused by living 

 organisms. 



Observation of Odor. The odor of cold water is best observed by 

 shaking a partly filled bottle of the water and immediately remov- 

 ing the stopper and applying the nose. The odor of hot water is ob- 

 tained by heating a portion of it in a tall beaker covered with a watch 

 glass to a point just short of boiling. When sufficiently cooled 

 the cover is slipped aside and the observation quickly made. The 

 intensity of odors is commonly indicated by numbers as follows; - 

 o, no odor; i, a very faint odor that would not be ordinarily de- 

 tected by a person drinking the water; 2, a faint odor that might 

 be detected by the consumer but that would not attract any spe- 

 cial attention; 3, a distinct odor that would be readily detected; 



4, a decided odor, strong enough to make the water unpalatable^ 



5, a very strong odor that would make the water unfit for use. 

 The character of the odor is usually indicated by a letter which 

 stands for a descriptive adjective. For purposes of record the two 

 are combined. Thus 3f indicates a distinct fishy odor; 2v, a faint 

 vegetable odor; 4m, a decided moldy odor. Heating usually 



