94 Bacteria in Relation to Country Life 



attack of animalcules which devour them in great 

 numbers; hence the decrease. 



Deep wells, improperly protected from surface wash- 

 ing, may also contain considerable numbers of bacteria. 

 It has been observed that the first portions of the water 

 pumped out of wells contain large numbers of bacteria, 

 whereas the portions brought up later contain compara- 

 tively few. The phenomenon in question is explained 

 by the rapid multipHcation of the bacteria in the slimy 

 layer covering the walls of the well, and their washing 

 down into the surface water. Hence, when the water 

 in the well had been left undisturbed for some hours, the 

 accumulation of the bacteria in the surface layer may 

 be quite considerable. Later on their numbers will 

 tend to decrease again, owing to the action of sedimen- 

 tation. The influence of sedimentation in this direction 

 is made evident in the comparison of the bacteria in 

 the undisturbed and the disturbed water. When the 

 bottom layers of the well-water are thus disturbed, the 

 number of bacteria per cc. may increase to a very 

 striking extent. 



Springs. — The bacterial nature of spring-water does 

 not differ much from that of deep-well-water. When 

 contamination from the surroundings is excluded, it is 

 not unusual to find the samples germ-free, or contain- 

 ing a few organisms per cubic centimeter. 



Driven wells. — Wells of this class have the advantage 

 over open wells in that they are better protected against 

 pollution from surface drainage. Under certain condi- 

 tions, however, surface washings may pass downward 

 along the tube until the ground-water is reached. 



