BUBBLING FOUNTAIN 343 



filter beds of coarse gravel and by seepage through them loses 

 most of the larger particles suspended in it. It then passes 

 through beds of fine sand and gravel and emerges from the last 

 bed clear and pure. The separate filtration beds are often an 

 acre in area and 5 or 6 feet deep. Filtration beds soon become 

 choked with impurities and require frequent cleaning. When 

 the beds are to be cleaned, the openings to the city outlet pipes 

 are closed and water from a hose is directed against the sand. 

 This stirs up the mud and washes the impurities to the top ; 

 there they drain off into sewage pipes through openings made 

 for the purpose. If continued use of a powerful hose for sev- 

 eral hours does not clean the soil, fresh soil is substituted. 



The purification of water by filtration is due largely to 

 beneficial bacteria which develop in the sand of the filter. 

 Newly built filters or freshly cleaned filters do not satisfac- 

 torily purify the contaminated water which flows through them. 

 Why not ? Because soil bacteria have not had time to develop 

 in them. But a scum of gelatin-like matter slowly forms on the 

 upper surfaces and clings to the sand grains. This slimy cover- 

 ing is the product of living active bacteria in the soil. After 

 sufficient of it has formed, the bacteria present in the water 

 are entrapped in it and prevented from passing through the 

 filter. The outflowing water is then satisfactorily purified or 

 filtered. Bacteriologists make frequent daily tests of the water 

 coming from filter beds and do not allow it to pass into the local 

 supply tanks unless it is reasonably free of bacteria. Usually 

 several days are required for the growth of the bacterial scum 

 and for satisfactory working of the filter beds. 



Bubbling fountain. Many disease germs are spread from 

 person to person through carelessness. When a person with 

 diphtheria or tuberculosis drinks from a cup, he leaves germs 

 on it that act as a source of disease to the next person who 

 uses the cup. Every community should demand bubbling 



