108 BACTERIOLOGY. 



after it reaches the ground the number of bacteria is 

 greatly increased from the soil. As it drains from the 

 surface of the earth or percolates through it, it is 

 classed either as surface water, of which ponds, lakes, 

 or rivers are examples, or as ground water, which 

 feeds wells. Surface water always contains large 

 numbers of bacteria, but the water in wells contains 

 only a few on account of the filtering action of the 

 soil. While the number of bacteria in surface water 

 is large, there is going on constantly processes of puri- 

 fication which keep the number in check. 

 Natural First, there is sedimentation or the sinking of 



methods . 



of pun- impurities by reason of their weight. The effect of 



fication * 



sedimentation can be seen after floods, where the mud 

 and dirt is found over the flooded areas. Sedimenta- 

 tion takes place slowly ; so in streams that are flowing 

 fast it cannot be relied upon to remove much of the 

 impurities. Aeration is another factor. This means 

 the mixing of water with air, as takes place, for ex- 

 ample, in water-falls. It does not destroy the bacteria 

 but it removes objectionable odors. Sunlight exerts a 

 powerful destructive action on the bacteria in water, 

 provided the depth of the water is not too great for 

 the sunlight to penetrate. Unfortunately, the pene- 

 trating power of sunlight is not great ; so its action is 

 - limited to the upper layers of the water. The ground 

 water is purified by the filtering action of the soil, 

 which is very efficient, provided the amount of water 

 to be filtered is not too great and it is not required to 

 work continuously. 



