138 MANUAL OF BACTERIOLOGY. 



may exist long enough to infect individuals habitually 

 drinking the water. Many epidemics of cholera and typhoid 

 fever have been traced to water polluted with the discharges 

 from cases of these diseases. 



By self-purification of water is meant the removal through 

 natural processes of contaminating organisms such as might 

 occur from the discharge of sewage into it. It depends 

 upon the sedimentation of the contaminating material, in 

 the form of mud, upon the growth of the ordinary water- 

 plants and protozoa, 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. 1 It is not 

 usually to be relied upon as a means of freeing the water- 

 supply from pathogenic bacteria. 



Storage of water. AYhen water is kept in large reser- 

 voirs, 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. 



Filtration. Filtration on a large scale has been more 

 commonly in use in the cities of Europe than elsewhere, until 

 lately. Similar filtration-plants now exist in several cities 

 of the United States. 



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. in thickness. The 

 upper layers may be removed from time to time, the re- 

 mainder not becoming 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 are protected by arches of 

 brick or stone. They require renewal occasionally. This 



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



