72 Bacteria in' Relation to Country Life 



before they can be seen. When properly magnified, they 

 may be observed rapidly moving about in the drop of 

 water and stirring up currents in it by means of which 

 the bacteria are drawn into the mouth of these animal- 

 cules. Their transparent bodies show the bacteria 

 within them in a more or less advanced state of de- 

 composition. The development of these animalcules, 

 designated as protozoa, is affected, of course, by the 

 prevailing temperature, and it is quite apparent, there- 

 fore, that in the summer months their consumption of 

 bacteria is a factor to be reckoned with, whereas, in the 

 cold months of the winter their activities are practically 

 suspended. 



Sedimentation. — The solid particles suspended in 

 water tend to sink to the bottom when the velocity of 

 the current is reduced. As the stream becomes more 

 sluggish, the diameter of the particles still in suspension 

 becomes smaller and smaller until only the finest frag- 

 ments of silt, clay and organic matter remain floating 

 in the water. When the water is at a standstill, a large 

 proportion of even these finest particles fall to the bot- 

 tom. This gradual settling out of the solid matter in 

 suspension is known as sedimentation, a process which 

 plays an important role in reducing the number of bac- 

 teria in rivers, lakes and reservoirs. 



The decrease in the number of bacteria is encouraged 

 by sedinientation in a twofold manner. In the first 

 place, the settling^ out of the suspended matter, which 

 includes; also; a large portion of the organic substances, 

 reduces the quantity of food available to the bacteria 

 and their growth is thereby hindered. In the second 



