140 MANUAL OF BACTERIOLOGY. 



of mud, upon the growth of the ordinary water-plants and pro- 

 tozoa, upon the exhaustion of the food-supply by the growth of 

 bacteria themselves, upon the destructive influence of direct 

 sunlight, and the dilution of the matter added with a large 

 volume of water.* It is not usually to be relied upon as a 

 means of freeing the water-supply from pathogenic bacteria. 



Storage of Water. When water is kept in large reservoirs, 

 the solid particles in it, including bacteria, tend to fall to the 

 bottom. The number of bacteria in a water-supply may be 

 considerably reduced in this way by allowing sedimentation to 

 take place and using the upper portion of the water. 



Filtration. Filtration on a large scale has been more com- 

 monly in use in the cities of Europe than elsewhere until lately. 

 But filtration-plants now exist in several cities of the United 

 States. By this method 98 per cent, to 99- per cent, of the 

 bacteria in water may be removed. 



Slow Sand Filtration.^ The filter consists of successive layers 

 of stones, coarse and fine gravel. The uppermost layers are 

 of fine sand. The whole filter is from i to 2 meters thick. 

 The sand should be 60 cm. to 1.2 meters in thickness. The 

 accumulated deposit from the water and a little of the fine sand 

 must be removed from time to time, but the layer of fine sand 

 must never be allowed to become less than 30 cm. in thickness. 

 The first water coming from the filter is discarded. The 

 actual filtration is done largely by the slimy sediment which 

 collects on the surface of the layer of fine sand. The filter- 

 beds may be several acres in extent, and in cold climates 

 should be protected by arches of brick or storie. They re- 

 quire renewal occasionally. This kind of filtration has come 

 largely into use since the cholera epidemic of 1892-93, and 

 it appears to be very effective. It is often advisable to use 



*See Jordan. Journal of Experimental Medicine. Vol. V., p. 271. 

 fFor a full discussion see Journal American Medical Association. Oct. 3, to 

 3i, 1903- 



