BIOLOGICAL WATER ANALYSIS 115 



blue-green algae of the Nostoc and Oscillaria groups. Various 

 more or less futile attempts are made by the water companies to 

 correct these conditions. In order to reduce the growth of algae the 

 reservoirs are roofed over (the algae requiring sunlight for their 

 development), forgetting that while one evil is thus in a measure 

 corrected, another and perhaps greater, is encouraged by such pro- 

 cedure, namely, the growth and development of bacteria which 

 thrive best in the absence of sunlight. Numerous desmids, di- 



FIG. 36. S. cholera very highly magnified, showing flagellae. (MacNeal, from 

 Kotte and Schiirmann.) 



atoms and blue-green algae in drinking water, indicate the presence of 

 dead and decaying organic matter in comparatively large amount. 

 Diatoms are especially abundant in water supplies from old 

 wooden tanks and wood-lined reservoirs. Nostoc and Oscillaria 

 are especially abundant in water supplies fed from soil drainage. 

 Bacteria are present in all soil and sewage contaminated waters. 

 The well water of the farms may be contaminated with all manner 

 of organisms, such as sewage organisms and disease germs, includ- 



9 



