BACTERIOLOGY OF WATER 147 



miles. Such water is apt to be excellent, provided proper 

 care is taken to prevent pollution at the reservoirs, but 

 for most large towns and cities such a supply is un- 

 available, and they must often take their supply from 

 lakes or rivers. In these cases the danger from pollu- 

 tion is very great, especially if the country around the 

 body of water is thickly settled, and town after town 

 empties its sewers into the same. The water of some 

 rivers is practically dilute sewage, and polluting material 

 is added so fast that the natural means of purification 

 are entirely unavailing. 



Natural Purification of Water. — The most important of 

 the natural agents of purification are sedimentation, oxida- 

 tion, and the disinfecting action of sunlight. By sedimen- 

 tation the large foreign bodies suspended in the water 

 carry with them to the bottom many bacteria, and other 

 bacteria are killed through the life-activities of certain 

 water plants. Sunlight does not act to any great depth, 

 but probably kills many bacteria in the water at the 

 surface. Freezing mechanically frees water from a certain 

 percentage of impurities, including bacteria, by squeezing 

 them out, but low temperatures do not kill all germs; 

 hence the danger of using ice from polluted lakes and 

 streams. 



Water-borne Diseases. — The two most important 

 water-borne diseases are typhoid fever and Asiatic cholera, 

 and the latter disease has furnished a classic example of 

 sewage contamination of drinking-water and its conse- 

 quences, as well as the most striking proof of the efficiency 

 of filtration. The two towns of Hamburg and Altona are 

 situated closely together, at the mouth of the river Elbe, 

 and both draw upon that river for their water supply. 



