142 THE AMERICAN MONTHLY [May, 



be present in such large numbers that a bottle of the 

 water when held towards the light has a silvery, glisten- 

 ing appearance. 



The Asterionella is usually the most troublesome of 

 the diatoms. A comparatively small number, say 1000 

 per c. c, is often sufficient to impart a decidedly unpleas- 

 ant taste to the water. This taste is a very character- 

 istic one, a person soon becoming so familiar with it that 

 the presence of Asterionella in a sample of water, when 

 concentrated for examination according to the Sedgwick- 

 Raf ter method, may often be detected by tasting it. The 

 taste is usually described as resembling the odor of a 

 sweet geranium. At times it is decidedly oily. 



Tabellaria, Diatoma and Stephanodiscus are also to be 

 noted as taste producers. Synedra and Melosira usually 

 give no trouble. 



Diatoms usually flourish best in ponds having muddy 

 bottoms. They are not found equally abundant at all 

 seasons of the year, but their growth is confined chiefly 

 to the spring and fall. There seems to be a considerable 

 difference between the seasonal distribution of diatoms 

 in deep and shallow ponds. In ponds more than about 

 30 feet deep there are usually two periods of maximum 

 growth, one in the spring and one in the fall ; while in 

 ponds less than 30 feet deep the fall growth is usually 

 slight or entirely absent. During the summer diatoms 

 are absent from deep ponds, but in shallow^ ponds they 

 sometimes attain a considerable growth. 



The cause of this seasonal distribution is to be found 

 in the " phenomena of stagnation and circulation," 



It is well known that in ponds which are more than 25 

 or 30 feet deep the temperature of the water at the bot- 

 tom remains quite constant during the summer, while 

 the temperature of the surface water rises and falls with 

 the tempei'ature of the air. The consequence is that the 



