342 



THE WATER PROBLEM OF A LARGE CITY 



water is not as pleasant as that of raw or uncooked water, be- 

 cause the gases which give taste to raw water are expelled by 

 boiling. 



We know that fresh water contains dissolved gases because 

 if a tumbler of water freshly drawn from the faucet is left 

 on the table for a few minutes, gas bubbles collect on the 

 sides of the tumbler. Warm water does not dissolve or hold in 

 solution as much gas as cold water ; and water loses air in the 

 process of boiling. 'Although boiled water can never be as 

 tasty as ordinary water, it can be improved by slowly pouring it 

 from one vessel to another and allowing it to dissolve air from 

 its surroundings. 



(b) Water is purified by filtration ; that is, by passage through 

 porcelain or other porous substances which allow the passage 

 of water, but hold back foreign particles suspended in it. The 

 filters used in ordinary dwellings are of sand, asbestos, or char- 

 coal ; but they are often 

 worse than useless be- 

 cause they soon become 

 choked and cannot be 

 properly cleaned. Since 

 the health of a com- 

 munity depends on its 

 drinking water, it is the 

 business of the city to 

 provide good water and 

 most cities filter the water before it is sent to the residents. 

 This is usually done by flowing the water over sand and 

 gravel beds. The principle of filtration is shown in Figure 210. 

 When impure water is poured into the tank, it seeps slowly 

 through the layers of soil and issues from the bottom free 

 from foreign particles. In the filtration plants owned and 

 operated by large cities, water from rivers or lakes passes over 



FIG. 210. Filtration. 



