PUBLIC HYGIENE, OR GENERAL SANITATION 365 



by some of the various methods now in use for freeing it 

 from the unwholesome matters almost always present. It is 

 true that running streams and lakes exposed to the action 

 of sun and wind are purified to a considerable extent by 

 natural influences, and where they do not receive an 

 excessive amount of the waste of towns and factories, 

 may usually be safely used. But very often the filth of 

 a city or of several cities is poured continually into the 

 lake or river which supplies the inhabitants with drinking 

 water, the amount of poison being far more than nature is 

 able to destroy. In such cases many and complicated 

 devices are used for getting rid of the injurious sub- 

 stances. The water may be drawn off into large reser- 

 voirs and allowed to settle. Then, the coarser filth having 

 been deposited, the water may be drawn into other reser- 

 voirs and treated with certain chemicals which will cause 

 the precipitation of other substances as sediment. Some 

 chemical substances destroy organic matter by oxidizing 

 it, and sometimes the same result is obtained by forcing 

 air through the contaminated water. 



Water is also partly purified by filtering through vari- 

 ous porous materials large beds of sand, gravel, and 

 broken stone, for instance. The filtering body itself soon 

 becomes clogged with filth, and must be often renewed 

 or cleansed. 



None of these processes destroys all the dangerous germs 

 to be found in impure water, and it is a wise precaution 

 for every family to boil for half an hour all water for 

 drinking which comes from a source liable to contamina- 

 tion. The water may then be placed in glass cans, tightly 

 closed, and cooled in a refrigerator, or in a cellar. By 

 this course much illness and death in large towns, where 

 typhoid fever and other water-borne diseases are always 

 MACY'S PHYS. 23 



